- Thomas A. Hegna
Department of Geology and Environmental Sciences
SUNY Fredonia
203 Jewett Hall
280 Central Avenue
Fredonia, NY 14063
USA
https://sites.google.com/site/thehegnalab/
http://www.wiu.edu/cas/geology/
https://www.researchgate.net/profile/Thomas_Hegna2/?ev=hdr_xprf
- SUNY Fredonia, Geology, Faculty MemberWestern Illinois University, Deparment of Earth, Atmospheric, and Geographic Information Sciences, Department Member, and 2 moreadd
- Branchiopoda, Morphological evolution, Evolutionary Developmental Biology, Phylogenetic studies, Invertebrate Biology, Paleobiology, and 17 moreEvolutionary Biology, Paleontology, Zoology, Carcinology, Philosophy of Science, X-ray Tomography, Trilobita, Taxonomy, Systematics (Taxonomy), Taphonomy, Ichnology, Philosophy of Systematics, Philosophy of Biology, Soft Tissue Preservation, Cladocera, Cladoceran Ecology, and Ecdysozoaedit
- My research tries to use the fossil record of arthropods as a lens to bring phylogenetic relationships into focus.edit
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Amphipods are extremely diverse malacostracan crustaceans that have conquered many environments from oceanic abysses to the terrestrial realm. Despite their impressive modern diversity and abundance, they are particularly rare in the... more
Amphipods are extremely diverse malacostracan crustaceans that have conquered many environments from oceanic abysses to the terrestrial realm. Despite their impressive modern diversity and abundance, they are particularly rare in the fossil record. Herein, we describe the exceptionally preserved Gammaroidorum yooling sp. nov. from the Late Neogene of Xiaobai Formation near Pandao village, Shanxi, China. Due to the limitation of the preservation, namely the non-preservation of setae, we could not establish a precise generic assignment, so used the collective group Gammaroidorum Jarzembowski, Chény, Fang & Wang, 2020. Although there are still doubts on the precise placement of this new species within amphipods, these fossils are important since they represent the first known fossil occurrence of an amphipod in China. Besides, these fossils have an unusual preservation, being almost transparent in most cases, and being most easily seen by reflecting light on the fossil.
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The Weeks Formation (House Range, Utah) is an important Cambrian (Guzhangian stage) site for its fauna of well-preserved and diverse, articulated trilobites. The articulated trilobites are only part of the story—there are also trilobite... more
The Weeks Formation (House Range, Utah) is an important Cambrian (Guzhangian stage) site for its fauna of well-preserved and diverse, articulated trilobites. The articulated trilobites are only part of the story—there are also trilobite sclerites that are preserved in silica and recoverable via acid digestion. The acid residues derived from the Weeks Formation contain a diverse set of juvenile trilobites. Matching these juvenile stages with their corresponding articulated adult stages has the potential to provide a suite of new characters that will be useful in deciphering the evolutionary relationships of Cambrian trilobites.
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Research Interests: Geology and Paleontology
Research Interests: Geology and Fossil record
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A new notostracan crustacean, Strudops goldenbergi gen. et sp. nov., is described from the well-preserved terrestrial arthropod fauna of the Upper Devonian of Strud, Belgium. The fossil notostracan bears a close resemblance to modern... more
A new notostracan crustacean, Strudops goldenbergi gen. et sp. nov., is described from the well-preserved terrestrial arthropod fauna of the Upper Devonian of Strud, Belgium. The fossil notostracan bears a close resemblance to modern notostracans in possessing a large, simple head shield covering almost half of the whole body, a set of phyllopodous thoracic appendages and a legless posterior abdomen with a telson bearing a caudal furca. The differentiation and relative size of mouthparts and limbs suggest that these specimens are all adults. The notostracans described herein are the earliest clear members of the total group Notostraca
Changes in the online availability of 3D models of skeletal elements and 3D printer technology have made it feasible to develop a fossil or osteological collection from scratch. Many online sites have models that can be freely downloaded... more
Changes in the online availability of 3D models of skeletal elements and 3D printer technology have made it feasible to develop a fossil or osteological collection from scratch. Many online sites have models that can be freely downloaded and 3D printed. Other sites have CT-scan movies online—which can be downloaded and turned into printable 3D models by using the free software suite SPIERS. Post-processing of the printed models (sanding, painting, etc.) can make them almost visually indistinguishable from real specimens.
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Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans; likely crown-group representatives have been in existence since the Devonian Period (over 360 mya). Branchiopoda consists of several... more
Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans; likely crown-group representatives have been in existence since the Devonian Period (over 360 mya). Branchiopoda consists of several groups: the anostracans, the notostracans, the paraphyletic conchostracans and the cladocerans. They have a very good fossil record that has been largely overlooked and never placed in a phylogenetic context. Re-evaluation of the branchiopod fossil record has yielded interesting new insights into the morphological evolution and diversification of this clade. Notostracans have often been referred to as living fossils. Rather than being emblematic of stasis, they have a dynamic evolutionary history. Their fossil record suggests that they had a ‘bivalved’ posture ancestrally and that the differentiation of their anterior thoracic limbs was a relatively late development. Fossil conchostracans are more difficult to reconcile with a modern phylogenetic understanding of the group. Fossil conchostracans typically preserve only the carapace and its ornamentation, but systematics of modern conchostracansignores these attributes. Documentation of the carapace ornamentation of modern species will enable testing of the phylogenetic utility of those features in the fossil representatives. Conchostracan growth lines are a much more ambiguous character—they may be a basal feature of diplostracans rather than of spinicaudatans. Inclusion of fossil taxa in a phylogenetic analysis based on morphological characters does not alter the inter-relationships of extant taxa, but it does alter our perception of character evolution within the clade. Understanding the course of character evolution within Branchiopoda may be useful for understanding the origin of Hexapoda.
" The phrase ‘stem-ward slippage’ was coined by Sansom and colleagues (2010) to refer to the relationship between morphological decay and phylogenetic interpretation in chordates. Namely, derived characters tend to decay first,... more
" The phrase ‘stem-ward slippage’ was coined by Sansom and colleagues (2010) to refer to the relationship between morphological decay and phylogenetic interpretation in chordates. Namely, derived characters tend to decay first, causing an organism to essentially recapitulate its phylogenetic history as decays. A derived chordate, after significant decay, would be interpreted as a basal chordate based on the remaining preserved characters. Sansom and colleagues suggest that stem-ward slippage may represent a general rule for other organisms. Decay experiments on arthropods suggest that this is not the case. The decay-resistant arthropod exoskeleton masks the decay of the internal anatomy. Post-mortem character loss in arthropods is driven not by decay, but by fragmentation. This often means that multiple characters are lost in fragmentation events; instead of the step-wise pattern of character loss observed by Sanson and colleagues, arthropod decay yields a tiered pattern with organisms falling to increasingly basal polytomies. Progressive decay of arthropods does not yield predictable phylogenetic placements as in chordates. Taphonomy and decay can be confounding effects on fossil interpretation, and the ambiguity they introduce is often intractable. Two recently published pancrustacean fossils serve as examples of this. Strudiella devonica, a putative Devonian insect from Belgium, and Ebullitiocaris elatus, a putative Carboniferous anomopod cladoceran from northern England, illustrate this point. Based on comparisons with notostracan decay experiments, a fossilized notostracan decay series from the Mesozoic of China, and notostracan fossils from the same Devonian locality in Belgium, the identity of Strudiella is consistent with an interpretation as the remains of a notostracan after significant decay. Ebullitiocaris is reinterpreted after comparison with branchiopod fragmentation patterns and is found to be inconsistent with a cladoceran identity. "
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"Preservation in amber yields animals that appear so exquisitely preserved . . . so exquisitely preserved that they appear as if they could walk right out of their amber prison if only someone would let them out. This perception is... more
"Preservation in amber yields animals that appear so exquisitely preserved . . . so exquisitely preserved that they appear as if they could walk right out of their amber prison if only someone would let them out. This perception is completely understandable when studying amber, and has led, for example, to the premise in the movie “Jurassic Park”—where perfectly preserved insects in amber still have perfectly preserved innards that contain perfectly preserved dinosaur blood. Such a scenario is, sadly, too perfect to be true. Unfortunately, without early replacement by stable minerals, all things are fated to decay—albeit at different rates. Amber is no exception. However, without slicing into the beautiful amber-preserved fossil, it is impossible to know exactly what is going on inside. Micro-CT scanning provides an excellent way to see inside the fossil without destroying it. Without the amber stabilizing the internal tissue, it is subject to the forces of microbial decay, dehydration, and resin infiltration. Using fossil amphipods (Crustacea, Amphipoda) from the Chiapas Amber of Mexico, we show how a superficially perfect specimen can have a surprising lack of correspondence between the preserved insides and the original internal anatomy. Micro-CT scanning offers a new way to think about the microenvironment inside of an amber-preserved fossil. "
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Clam shrimp ('conchostracans') are a diverse group of phyllopod crustaceans with modern representatives that occur on every continent except Antarctica. Restricted to freshwater systems, the majority of modern taxa inhabit... more
Clam shrimp ('conchostracans') are a diverse group of phyllopod crustaceans with modern representatives that occur on every continent except Antarctica. Restricted to freshwater systems, the majority of modern taxa inhabit lakes, ponds and ephemeral water bodies. Although well-...
... Three examples of Isotelus are known to preserve evidence of the unmineralized ventral exoskeleton in at least two different preservational modes ... There appears to be a differentiated smaller pair of appendages (2 nd pair?)... more
... Three examples of Isotelus are known to preserve evidence of the unmineralized ventral exoskeleton in at least two different preservational modes ... There appears to be a differentiated smaller pair of appendages (2 nd pair?) positioned under the inner margin of the hypostome. ...
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Material and Methods
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Generic presence/absence data matrix used for the similarity analyses of the Cambrian exceptionally-preserved faunas of Utah.
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Supplementary tables: compositions of the lower and upper Weeks faunas, structure of the upper Weeks fauna, and results of the similarity analyses.
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Fossils are critically important for evolutionary studies as they provide the link between geological ages and the phylogeny of life. The Pancrustacea are an incredibly diverse clade, representing over 800,000 described extant species,... more
Fossils are critically important for evolutionary studies as they provide the link between geological ages and the phylogeny of life. The Pancrustacea are an incredibly diverse clade, representing over 800,000 described extant species, encompassing a variety of familiar and unfamiliar forms, such as ostracods, tongue worms, crabs, lobsters, shrimps, copepods, barnacles, branchiopods, remipedes, and insects. Having colonized nearly every environment on Earth, from hydrothermal vents to terrestrial habitats, they have a diverse fossil record dating back to the Cambrian (540–485 Ma). The quality of the fossil record of each clade is variable and related to their lifestyle (e.g., free-living versus parasitic, benthic versus pelagic) and the degree of mineralization of their cuticle. We review the systematics, morphology, preservation, and paleoecology of pancrustacean fossils; each major clade is discussed in turn, and, where possible, fossil systematics are compared with more recent da...
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Clam shrimp (the paraphyletic assemblage of spinicaudatans, laevicaudatans, cyclestherids and the extinct leaiins) are small, bivalved branchiopod crustaceans that specialize in ephemeral freshwater habitats. They have a long fossil... more
Clam shrimp (the paraphyletic assemblage of spinicaudatans, laevicaudatans, cyclestherids and the extinct leaiins) are small, bivalved branchiopod crustaceans that specialize in ephemeral freshwater habitats. They have a long fossil record (Devonian onward) that has often been overlooked. Here we briefly review the fossil record of the major groups of clam shrimp and clear up some misconceptions in the literature as to their origin. The dominant group of clam shrimp in the fossil record is the Spinicaudata, which have a diverse fossil record beginning in the Devonian. The clam shrimp suborder Laevicaudata are known from the Permian, with possible soft-part preservation from the Jurassic. However, owing the character-poor nature of these fossils, it is impossible to tell if they represent crown group or stem group laevicaudatans. In contrast, the total group Spinicaudata have a rich record of mostly carapace fossils- the earliest from the Early Devonian. The leaiins are an enigmatic ...
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After a symposium and special issue devoted to the study of clam shrimp, it is tempting to ask what is next... where is the study of clam shrimp going? Rather than try to read the tea leaves to predict the future, we will instead offer... more
After a symposium and special issue devoted to the study of clam shrimp, it is tempting to ask what is next... where is the study of clam shrimp going? Rather than try to read the tea leaves to predict the future, we will instead offer some closing thoughts on where the study of clam shrimp should go and what areas are ripe for investigation. Many of these ideas integrate both fossil and modern clam shrimp to get at a more complete view of their evolution and ecology.
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This special volume of Zoological Studies is the result of a symposium entitled "Fossil and Modern Clam Shrimp" held at the midyear meeting of The Crustacean Society in May of 2019. This symposium is the first ever focusing on... more
This special volume of Zoological Studies is the result of a symposium entitled "Fossil and Modern Clam Shrimp" held at the midyear meeting of The Crustacean Society in May of 2019. This symposium is the first ever focusing on clam shrimp, and the first conference where both palaeontologists and biologists specialising in these animals were able to come together. The papers presented here provide insight into the palaeontology, biology, ecology, taxonomy and phylogeny of the clam shrimp. This chapter introduces the symposium, its aims, and the resulting research, presented in the subsequent chapters. In addition, in this symposium we celebrate our great friend Brian V. Timms, who has mentored so many of us, brought us on various excursions across Australia, and has done more to advance Australian branchiopod studies than anyone else in history.
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Abstract Orthestheria shupei comb. nov. is a small clam shrimp from the Red Branch Member of the Woodbine Formation in Texas, USA, originally described as belonging to the living genus, Cyzicus. The redescription herein reveals its... more
Abstract Orthestheria shupei comb. nov. is a small clam shrimp from the Red Branch Member of the Woodbine Formation in Texas, USA, originally described as belonging to the living genus, Cyzicus. The redescription herein reveals its ornamentation pattern consisting of straight lirae formed perpendicular to the growthband, with rare shorter intercalating lirae. This is one of only two known records of clam shrimp from the Cretaceous of North America. The genus, Orthestheria, is also known from China and South America, and it is used in a biostratigraphic scheme in China. Orthestheria is very similar to another genus, Nemestheria, which is classified in a separate subfamily (Jilinestheriidae vs Fushunograptidae). The diagnoses for both are useless for telling the genera apart—both diagnoses exploit vague language that allow the admission of a broad array of taxa within a narrow stratigraphic range. Deciphering clam shrimp phylogeny will only progress with meaningful diagnoses that circumscribe monophyletic groups. Despite the higher level taxonomic problems, the age of the Woodbine Formation is broadly consistent with the age predicted from the Asian biostratigraphic scheme using Orthestheria.
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Clam shrimps constitute a paraphyletic group of bivalved branchiopod crustaceans that includes orders Laevicaudata, Spinicaudata, and Cyclestherida. The unique mineral content of the carapace, formed by molt retention in Spinicaudata and... more
Clam shrimps constitute a paraphyletic group of bivalved branchiopod crustaceans that includes orders Laevicaudata, Spinicaudata, and Cyclestherida. The unique mineral content of the carapace, formed by molt retention in Spinicaudata and Cyclestherida, has been variously ascribed to calcium carbonate or calcium phosphate. We analyzed the composition of modern carapaces from one laevicaudatan species, thirteen spinicaudatan species (including cyzicids, leptestheriids, and limnadiids), one cyclestherid species, and two species of the notostracan TriopsSchrank, 1803 (as an outgroup comparison within Branchiopoda) via Raman spectroscopy. The results were surprisingly variable. The outgroup Triops species varied in either having no mineral content to having a slight amount of calcium phosphate. The laevicaudatans likewise had a minor calcium phosphate peak; the leptestheriid spinicaudatan had strong calcium phosphate and calcium carbonate peaks; the limnadiid spinicaudatans were variable...
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Thylacocephalans are an extinct group of arthropods of an uncertain systematic position. Originally considered phyllocarid crustaceans, they have since been classified within their own class, the Thylacocephala Pinna, Arduini, Pesarini... more
Thylacocephalans are an extinct group of arthropods of an uncertain systematic position. Originally considered phyllocarid crustaceans, they have since been classified within their own class, the Thylacocephala Pinna, Arduini, Pesarini and Teruzzi, 1982 on the basis of the exceptionally preserved Lower Jurassic (Sinemurian) species Ostenocaris cypriformis from Osteno, Italy. Since that time, the membership of the Thylacocephala has only grown as a number of new species have been discovered, as well as previously known species moved into the group.
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Abstract Amber from the Campo La Granja mine in Chiapas, Mexico, is distinct from other sources of amber in Chiapas. Campo La Granja amber has distinct layers created by successive flows of resin with thin layers of sand on most surfaces.... more
Abstract Amber from the Campo La Granja mine in Chiapas, Mexico, is distinct from other sources of amber in Chiapas. Campo La Granja amber has distinct layers created by successive flows of resin with thin layers of sand on most surfaces. Aquatic and semi-aquatic arthropods are commonly found. Together these pieces of evidence suggest an estuarine environment similar to modern mangrove communities. The aquatic crustaceans are the most intriguing aspect of the biota. A large number of ostracods have been found in the amber—many with their carapaces open, suggesting that they were alive and submerged in water at the time of entombment. The only known examples of brachyuran crabs preserved in amber are found in the Campo La Granja amber. Amphipods, copepods, isopods, and tanaids are also members of the crustacean fauna preserved in amber.
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Preservation in amber yields animals that appear so exquisitely preserved . . . so exquisitely preserved that they appear as if they could walk right out of their amber prison if only someone would let them out. This perception is... more
Preservation in amber yields animals that appear so exquisitely preserved . . . so exquisitely preserved that they appear as if they could walk right out of their amber prison if only someone would let them out. This perception is completely understandable when studying amber, and has led, for example, to the premise in the movie “Jurassic Park”—where perfectly preserved insects in amber still have perfectly preserved innards that contain perfectly preserved dinosaur blood. Such a scenario is, sadly, too perfect to be true.
Unfortunately, without early replacement by stable minerals, all things are fated to decay—albeit at different rates. Amber is no exception. However, without slicing into the beautiful amber-preserved fossil, it is impossible to know exactly what is going on inside.
Micro-CT scanning provides an excellent way to see inside the fossil without destroying it. Without the amber stabilizing the internal tissue, it is subject to the forces of microbial decay, dehydration, and resin infiltration. Using fossil amphipods (Crustacea, Amphipoda) from the Chiapas Amber of Mexico, we show how a superficially perfect specimen can have a surprising lack of correspondence between the preserved insides and the original internal anatomy. Micro-CT scanning offers a new way to think about the microenvironment inside of an amber-preserved fossil.
Unfortunately, without early replacement by stable minerals, all things are fated to decay—albeit at different rates. Amber is no exception. However, without slicing into the beautiful amber-preserved fossil, it is impossible to know exactly what is going on inside.
Micro-CT scanning provides an excellent way to see inside the fossil without destroying it. Without the amber stabilizing the internal tissue, it is subject to the forces of microbial decay, dehydration, and resin infiltration. Using fossil amphipods (Crustacea, Amphipoda) from the Chiapas Amber of Mexico, we show how a superficially perfect specimen can have a surprising lack of correspondence between the preserved insides and the original internal anatomy. Micro-CT scanning offers a new way to think about the microenvironment inside of an amber-preserved fossil.
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Two specimens of the Carboniferous whip spider Graeophonus (Order Amblypygi) are presented as high-resolution microCT scan reconstructions. The fossils are preserved within siderite nodules from the Middle Coal Measures of Coseley,... more
Two specimens of the Carboniferous whip spider Graeophonus (Order Amblypygi) are presented as high-resolution microCT scan reconstructions. The fossils are preserved within siderite nodules from the Middle Coal Measures of Coseley, Staffordshire, UK. The new 3D reconstructions of Graeophonus yield additional morphological information not revealed by surface examinations. Of particular interest is the form of the pedipalps, which are revealed to have a more complex armature. Compared to most modern species, the armature of the pedipalps belonging to Graeophonus is relatively meager, but it supports previous assertions of a close relationship between Graeophonus and the most primitive living species, Paracharon.
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The first Mesozoic thylacocephalans from the western hemisphere are documented from the Muhi Quarry plattenkalk in the La Negra facies of the El Doctor Formation (Albian-Cenomanian), Zimapán Area, northwestern Hidalgo, central Mexico.... more
The first Mesozoic thylacocephalans from the western hemisphere are documented from the Muhi Quarry plattenkalk in the La Negra facies of the El Doctor Formation (Albian-Cenomanian), Zimapán Area, northwestern Hidalgo, central Mexico. They all are found in a single horizon at the base of the type section. These specimens represent two new genera and species, and a third undetermined species which likely also represents a new genus and species. Victoriacaris muhiensis is a shorted-bodied thylacocephalan with concave anterior and posterior gapes as well as a beveled anterior spine. Polzia eldoctorensis is trapezoidal in outline with a rounded posteroventral corner. The third undetermined species is known only from one broken specimen, but has distinctive marginal spines that are not possessed by any other post-Cambrian thylacocephalan. The occurrence of such a diverse fauna of thylacocephalans markedly increases the diversity and geographic range of Cretaceous thylacocephalans, but does not offer any insights into the tempo of thylacocephalan extinction at the end of the Cretaceous.
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"Lerosey-Aubril, R., R. Gaines, T. A. Hegna, J. Ortega-Hernández, L. E. Babcock, B. Lefebvre, C. Kier, E. Bonino, Q. Sahratian & J. Vannier. 2013. The Weeks Formation Lagerstätte (Cambrian; Utah, USA): a unique insight into the evolution... more
"Lerosey-Aubril, R., R. Gaines, T. A. Hegna, J. Ortega-Hernández, L. E. Babcock, B. Lefebvre, C. Kier, E. Bonino, Q. Sahratian & J. Vannier. 2013. The Weeks Formation Lagerstätte (Cambrian; Utah, USA): a unique insight into the evolution of soft-bodied metazoans during the late Cambrian. Geological Society of America Annual Meeting, Denver, CO. [talk]
Cambrian Lagerstätten offer extraordinary insight into the early evolution of metazoans and their organization into complex ecosystems. Recent discoveries in the Ordovician question the traditional view of an abrupt transition between the Cambrian and Paleozoic Evolutionary Faunas. This emerging picture of a more gradual evolution of metazoan communities during the early Paleozoic stresses the need for more data on Cambrian Series 3 to Furongian Series non-biomineralized animals. Here we present some preliminary results of our investigations of the Weeks Formation Konservat Lagerstätte (Series 3: Guzhangian Stage, Cedaria Zone) of the House Range, Utah.
The Weeks Formation comprises a 300-m-thick sequence of thin-bedded lime mudstones, wackestones, and grainstones with variable amounts of shale. It represents an open-shelf marine environment and a late stage in the filling of the House Range Embayment. Two types of exceptional preservation of fossils are recognized in the Weeks Formation: 1, pyritization of major morphological details with subsequent coatings of chlorite and oxidization to iron oxides; and 2, phosphatization of digestive tracts of arthropods.
The Weeks Formation yields taxa characteristic of Cambrian Epochs 2-3, but also includes a mix of taxa representative of older and younger Konservat-lagerstätten. The biota can be considered as a transitional one that lends support to the hypothesis of more gradual evolution of metazoans during the early Paleozoic. The Weeks Formation has yielded a rich trilobite assemblage, abundant inarticulate brachiopods and sponge spicules, and rare hyolithids and echinoderms. The upper Weeks has also produced non-biomineralized and weakly biomineralized organisms. This exceptionally preserved biota is dominated by arthropods and worms (paleoscolescids, priapulids), but it also includes sponges, a possible comb jelly, and other organisms. The arthropod fauna is rich in aglaspidids, a group best known from Guzhangian-Furongian deposits. These are associated with animals characteristic of Cambrian Series 2-3 (Anomalocaris, leanchoiliids) and Ordovician (Tremaglaspis) biotas. The importance of the non-trilobite arthropod fauna is also highlighted by the presence of several species of uncertain affinities.
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Cambrian Lagerstätten offer extraordinary insight into the early evolution of metazoans and their organization into complex ecosystems. Recent discoveries in the Ordovician question the traditional view of an abrupt transition between the Cambrian and Paleozoic Evolutionary Faunas. This emerging picture of a more gradual evolution of metazoan communities during the early Paleozoic stresses the need for more data on Cambrian Series 3 to Furongian Series non-biomineralized animals. Here we present some preliminary results of our investigations of the Weeks Formation Konservat Lagerstätte (Series 3: Guzhangian Stage, Cedaria Zone) of the House Range, Utah.
The Weeks Formation comprises a 300-m-thick sequence of thin-bedded lime mudstones, wackestones, and grainstones with variable amounts of shale. It represents an open-shelf marine environment and a late stage in the filling of the House Range Embayment. Two types of exceptional preservation of fossils are recognized in the Weeks Formation: 1, pyritization of major morphological details with subsequent coatings of chlorite and oxidization to iron oxides; and 2, phosphatization of digestive tracts of arthropods.
The Weeks Formation yields taxa characteristic of Cambrian Epochs 2-3, but also includes a mix of taxa representative of older and younger Konservat-lagerstätten. The biota can be considered as a transitional one that lends support to the hypothesis of more gradual evolution of metazoans during the early Paleozoic. The Weeks Formation has yielded a rich trilobite assemblage, abundant inarticulate brachiopods and sponge spicules, and rare hyolithids and echinoderms. The upper Weeks has also produced non-biomineralized and weakly biomineralized organisms. This exceptionally preserved biota is dominated by arthropods and worms (paleoscolescids, priapulids), but it also includes sponges, a possible comb jelly, and other organisms. The arthropod fauna is rich in aglaspidids, a group best known from Guzhangian-Furongian deposits. These are associated with animals characteristic of Cambrian Series 2-3 (Anomalocaris, leanchoiliids) and Ordovician (Tremaglaspis) biotas. The importance of the non-trilobite arthropod fauna is also highlighted by the presence of several species of uncertain affinities.
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Research Interests: Trilobites and Cambrian
How does one interpret an ambiguous fossil? Whenever a paleontologist approaches a bizarre fossil, the interpretive steps that are taken become even more important—and often include answering questions that are trivial with more... more
How does one interpret an ambiguous fossil? Whenever a paleontologist approaches a bizarre fossil, the interpretive steps that are taken become even more important—and often include answering questions that are trivial with more straightforward remains. Does the fossil represent one individual or several? A whole organism, or only a part? Are there repeated organs or structures? What is the symmetry of the organism? What does the present state of the remains suggest about their original composition?
Some unusual fossils from the Cambrian Weeks Formation Konservat-Lagerstätte in Utah provide an opportunity to discuss these interpretive steps. The most complete specimen is inferred to be a single organism, based on the presence of a possible plane of symmetry. The repeated structures on both sides of this plane are suggestive of paired arthropod limbs: two pairs of antennae-like structures and six robust, grasping-type appendages. The second specimen is inferred to be a single appendage, similar to the grasping-type appendages mentioned above. However, it also exhibits some similarities with the grasping appendages of some better-known euarthropods (e.g. anomalocaridids), and therefore its identity remains obscure.
Some unusual fossils from the Cambrian Weeks Formation Konservat-Lagerstätte in Utah provide an opportunity to discuss these interpretive steps. The most complete specimen is inferred to be a single organism, based on the presence of a possible plane of symmetry. The repeated structures on both sides of this plane are suggestive of paired arthropod limbs: two pairs of antennae-like structures and six robust, grasping-type appendages. The second specimen is inferred to be a single appendage, similar to the grasping-type appendages mentioned above. However, it also exhibits some similarities with the grasping appendages of some better-known euarthropods (e.g. anomalocaridids), and therefore its identity remains obscure.
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Pancrustaceans (= crustaceans + hexapods) undergo some of the most radical ontogenetic changes seen in the Metazoa. The spectacular upper Cambrian ‘Orsten’-type faunas preserve phosphatized fossil larvae, including putative stem- and... more
Pancrustaceans (= crustaceans + hexapods) undergo some of the most radical ontogenetic changes seen in the Metazoa. The spectacular upper Cambrian ‘Orsten’-type faunas preserve phosphatized fossil larvae, including putative stem- and crown-group pancrustaceans with amazing developmental sequences. The putative presence of adult stages remains a source of debate. This causes spurious placements in morphological analyses. We introduce a new method of coding ontogenetic data where each semaphoront (discrete larval or adult stage) is considered an OTU. This decreases the reliance on continuous timing of developmental ‘events’, avoiding a priori assumptions of heterochrony. Characters and states are carefully defined to identify specific putative homologies across taxa, as well as changes in morphology throughout ontogeny. Exemplar taxa from Pancrustacea are included (direct and indirect developers). Phylogenetic analyses of semaphoronts produced relationships of each Orsten fossil to the crown-group clade expected from morphology shared with extant larvae. Bredocaris is a member of the stem lineage of Thecostraca and/or Copepoda, and Yicaris and Rehbachiella are members of the stem lineage of Branchiopoda and/or Cephalocarida. The position of Phosphatocopina remains unresolved. A result consistent with fossil morphology was produced, suggesting this method may have broader applications to other phylogenetic problems that rely on ontogentically variable homology statements.
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The phrase ‘stem-ward slippage’ was coined by Sansom and colleagues (2010) to refer to the relationship between morphological decay and phylogenetic interpretation in chordates. Namely, derived characters tend to decay first, causing an... more
The phrase ‘stem-ward slippage’ was coined by Sansom and colleagues (2010) to refer to the relationship between morphological decay and phylogenetic interpretation in chordates. Namely, derived characters tend to decay first, causing an organism to essentially recapitulate its phylogenetic history as decays. A derived chordate, after significant decay, would be interpreted as a basal chordate based on the remaining preserved characters.
Sansom and colleagues suggest that stem-ward slippage may represent a general rule for other organisms. Decay experiments on arthropods suggest that this is not the case. The decay-resistant arthropod exoskeleton masks the decay of the internal anatomy. Post-mortem character loss in arthropods is driven not by decay, but by fragmentation. This often means that multiple characters are lost in fragmentation events; instead of the step-wise pattern of character loss observed by Sanson and colleagues, arthropod decay yields a tiered pattern with organisms falling to increasingly basal polytomies. Progressive decay of arthropods does not yield predictable phylogenetic placements as in chordates.
Taphonomy and decay can be confounding effects on fossil interpretation, and the ambiguity they introduce is often intractable. Two recently published pancrustacean fossils serve as examples of this. Strudiella devonica, a putative Devonian insect from Belgium, and Ebullitiocaris elatus, a putative Carboniferous anomopod cladoceran from northern England, illustrate this point. Based on comparisons with notostracan decay experiments, a fossilized notostracan decay series from the Mesozoic of China, and notostracan fossils from the same Devonian locality in Belgium, the identity of Strudiella is consistent with an interpretation as the remains of a notostracan after significant decay. Ebullitiocaris is reinterpreted after comparison with branchiopod fragmentation patterns and is found to be inconsistent with a cladoceran identity.
Sansom and colleagues suggest that stem-ward slippage may represent a general rule for other organisms. Decay experiments on arthropods suggest that this is not the case. The decay-resistant arthropod exoskeleton masks the decay of the internal anatomy. Post-mortem character loss in arthropods is driven not by decay, but by fragmentation. This often means that multiple characters are lost in fragmentation events; instead of the step-wise pattern of character loss observed by Sanson and colleagues, arthropod decay yields a tiered pattern with organisms falling to increasingly basal polytomies. Progressive decay of arthropods does not yield predictable phylogenetic placements as in chordates.
Taphonomy and decay can be confounding effects on fossil interpretation, and the ambiguity they introduce is often intractable. Two recently published pancrustacean fossils serve as examples of this. Strudiella devonica, a putative Devonian insect from Belgium, and Ebullitiocaris elatus, a putative Carboniferous anomopod cladoceran from northern England, illustrate this point. Based on comparisons with notostracan decay experiments, a fossilized notostracan decay series from the Mesozoic of China, and notostracan fossils from the same Devonian locality in Belgium, the identity of Strudiella is consistent with an interpretation as the remains of a notostracan after significant decay. Ebullitiocaris is reinterpreted after comparison with branchiopod fragmentation patterns and is found to be inconsistent with a cladoceran identity.
Whip spiders (Order Amblypygi) are a relatively small group of living arachnids that have a fossil record extending back to scraps of cuticle from the middle Devonian. They are arachnids with a flattened body, ‘clasp-knife’ chelicerae,... more
Whip spiders (Order Amblypygi) are a relatively small group of living arachnids that have a fossil record extending back to scraps of cuticle from the middle Devonian. They are arachnids with a flattened body, ‘clasp-knife’ chelicerae, and a pair of long, antenniform anteriormost legs. Graeophonus, from the Middle Coal Measures of Coseley, Staffordshire, UK, represents the first articulated whip spider in the fossil record. Its preservation as a void within a siderite nodule allowed the specimen to be CT-scanned, digitally dissected from the rock, and reconstructed. Previously, our ability to examine fossil-bearing siderite nodules was limited by the way the nodule cracked to reveal the specimen. CT-scanning reveals fine details of anatomy that have never been previously observed.
Preserved color markings in Paleozoic fossils are extremely rare and have been hypothesized to reflect muscular attachment scars, diagenetic artifacts unrelated to biological features or the altered remains of biochromes (organic... more
Preserved color markings in Paleozoic fossils are extremely rare and have been hypothesized to reflect muscular attachment scars, diagenetic artifacts unrelated to biological features or the altered remains of biochromes (organic pigments) or sclerochromes (structural colors) embedded in fossilized skeletal remains. More than 25 exceptionally well-preserved phacopid trilobites with spotted patterns are described from the Middle Devonian western and central New York. The small (~0.24 mm) circular markings appear either brown on a lighter cuticle, or white on a darker cuticle. Spots are non-randomly distributed on the entire dorsal exoskeleton and the regions of occurrence are bilaterally symmetrical and are not correlated with the locations of tubercles or with other surface features (e.g., frontal auxiliary impressions, or facial sutures). Thin section, SEM imaging, and EDX and WDS elemental analyses show spots to be spheres embedded within the primary layer of the cuticle below the prismatic layer and composed of microcrystalline low-Mg calcite. Surrounding cuticle (also low-Mg calcite) exhibits coarser crystallinity, lamellar structures, pore canals, and likely organic matrix as expected for unaltered trilobite skeletons. Potential diagenetic mineralogies or microstructures were not observed making diagenesis an unlikely explanation. Spot distribution, morphology and position in cuticle rule out a relationship with sites of musculature attachment/insertion. We suggest that the spots represent original biologic structures manifest as either crystallographic/optical loci resulting in structural color spots or potentially clear spots embedded in the cuticle that contrasted with a pigmented exoskeleton and may have served as windows to an underlying epidermis. The spots likely served as camouflage by providing a disruptive pattern to their dorsal surfaces serving to break-up the trilobite’s visual outline and also to reduce the starkness of shadows caused by surface relief.
Studies of ontogeny have a long history, but only recently have ontogenetic data been explicitly incorporated into phylogenetic analyses. However, defining homologous stages across disparate taxa remains problematic. Pancrustaceans (=... more
Studies of ontogeny have a long history, but only recently have ontogenetic data been explicitly incorporated into phylogenetic analyses. However, defining homologous stages across disparate taxa remains problematic. Pancrustaceans (= crustaceans + hexapods) undergo some of the most radical ontogenetic changes seen in the Metazoa. The spectacular upper Cambrian ‘Orsten’ fauna preserves phosphatized fossil larvae, including putative stem- and crown-group pancrustaceans with amazingly complete developmental
sequences. The putative presence and nature of adult stages remains a source of debate. We introduce a new method of coding ontogenetic data where each semaphoront (discrete larval or adult stage) is considered an OTU. This decreases the reliance on continuous timing of developmental ‘events’, and permits a theory-free identification of ontogenetic similarity. Characters and their states are carefully defined to identify specific putative homologies across taxa, as well as changes in morphology throughout ontogeny. Exemplar taxa covering most of Pancrustacea are included (both direct and indirect developers). We draw morphological data mainly from the rich ontogenetic and embryological literature, augmented with personal observations. Diverse early larval semaphoronts are found to group together rather than with their conspecific later stages, indicating a source of bias. To overcome this, two parsimony-based tree building methods are introduced.
sequences. The putative presence and nature of adult stages remains a source of debate. We introduce a new method of coding ontogenetic data where each semaphoront (discrete larval or adult stage) is considered an OTU. This decreases the reliance on continuous timing of developmental ‘events’, and permits a theory-free identification of ontogenetic similarity. Characters and their states are carefully defined to identify specific putative homologies across taxa, as well as changes in morphology throughout ontogeny. Exemplar taxa covering most of Pancrustacea are included (both direct and indirect developers). We draw morphological data mainly from the rich ontogenetic and embryological literature, augmented with personal observations. Diverse early larval semaphoronts are found to group together rather than with their conspecific later stages, indicating a source of bias. To overcome this, two parsimony-based tree building methods are introduced.
Trilobites, like all arthropods, possessed an exoskeleton composed mainly of chitin but with a spatially variable mineral content. This exoskeleton included a dorsal mineralized portion (of articulating plates or sclerites) and a ventral... more
Trilobites, like all arthropods, possessed an exoskeleton composed mainly of chitin but with a spatially variable mineral content. This exoskeleton included a dorsal mineralized portion (of articulating plates or sclerites) and a ventral unmineralized portion (often referred to as ‘soft-parts’). The much lower fossilization potential of the unmineralized exoskeleton means that we have relatively little information on its morphology or how the two portions functioned as a whole.
The common Ordovician trilobite Isotelus reflects this contrast: its mineralized exoskeleton is well-known, while its soft-parts and unmineralized exoskeleton are rarely preserved. Three examples of Isotelus are known to preserve evidence of the unmineralized ventral exoskeleton in at least two different preservational modes (pyrite and calcite). Two of the three specimens are preserved in three dimensions, revealing how the body parts fit together and interacted.
Contrary to previous reports, no gut tract is preserved. In one specimen, the entire body cavity is filled with sparry calcite, as observed in trilobites from the Ordovician Walcott-Rust Quarry in upstate New York. In the pygidial region, only the axial region accommodated significant amounts of tissue. The broad pleural areas were covered ventrally by a ‘soft’ exoskeleton preserved as a thin layer of transparent spar. Head appendages are evident in only one specimen. There appears to be a differentiated smaller pair of appendages (2nd pair?) positioned under the inner margin of the hypostome. The presence of these limbs supports an earlier hypothesis of a differentiated feeding mechanism in asaphid trilobites related to their distinctive hypostome morphology. Thus, the ventral morphology of trilobites may not have been as uniform as previously thought.
The common Ordovician trilobite Isotelus reflects this contrast: its mineralized exoskeleton is well-known, while its soft-parts and unmineralized exoskeleton are rarely preserved. Three examples of Isotelus are known to preserve evidence of the unmineralized ventral exoskeleton in at least two different preservational modes (pyrite and calcite). Two of the three specimens are preserved in three dimensions, revealing how the body parts fit together and interacted.
Contrary to previous reports, no gut tract is preserved. In one specimen, the entire body cavity is filled with sparry calcite, as observed in trilobites from the Ordovician Walcott-Rust Quarry in upstate New York. In the pygidial region, only the axial region accommodated significant amounts of tissue. The broad pleural areas were covered ventrally by a ‘soft’ exoskeleton preserved as a thin layer of transparent spar. Head appendages are evident in only one specimen. There appears to be a differentiated smaller pair of appendages (2nd pair?) positioned under the inner margin of the hypostome. The presence of these limbs supports an earlier hypothesis of a differentiated feeding mechanism in asaphid trilobites related to their distinctive hypostome morphology. Thus, the ventral morphology of trilobites may not have been as uniform as previously thought.
Studies of ontogeny have a centuries-long history pioneered by the likes of von Baer, Haeckel, and Darwin. Only in the past 15 years, however, have ontogenetic data been explicitly incorporated into cladistic analyses. However, defining... more
Studies of ontogeny have a centuries-long history pioneered by the likes of von Baer, Haeckel, and Darwin. Only in the past 15 years, however, have ontogenetic data been explicitly incorporated into cladistic analyses. However, defining homologous stages across disparate taxa remains problematic. Pancrustaceans undergo some of the most radical ontogenetic changes seen in the Metazoa. They are especially suited to studies of ontogeny as their larval stages are discretely partitioned as moults. The spectacular upper Cambrian ‘Orsten’ fauna (and similar faunas throughout the Phanerozoic) preserve phosphatized fossil larvae, including putative stem- and crown-group pancrustaceans with amazingly complete developmental sequences. The putative presence and nature of adult stages is a source of debate. We introduce a new method of coding ontogenetic data where each semaphoront (discrete larval or adult stage) is considered to be an OTU. This decreases the reliance on continuous timing of developmental ‘events’, and permits a theory-free identification of ontogenetic similarity. Characters and their states are carefully defined to identify specific putative homologies across taxa, as well as changes in morphology throughout ontogeny. We focus in particular on the ontogeny of branchiopods, ostracods, and thecostracans, as some of the best-known Orsten fossils (Rehbachiella, phosphatocopines, and Bredocaris, respectively) have been classified as members of these groups. Exemplar taxa covering most of Pancrustacea were also included. We draw our morphological data mainly from the rich ontogenetic and embryological literature, augmented with some personal observations. This is the first attempt to integrate developmental data from hexapods with other crustaceans, which may provide clues about hexapod affinities and the pattern of character change and ontogenetic evolution leading to terrestrialization. The result demonstrates the complex relationship between ontogeny and phylogeny and sheds light on life cycle evolution.
Though branchiopod crustaceans are characterized as possessing primitively homonymous trunk limbs, the typical notostracan has a unique pattern of trunk limb differentiation. This differentiation is manifest in the first thoracic limb by... more
Though branchiopod crustaceans are characterized as possessing primitively homonymous trunk limbs, the typical notostracan has a unique pattern of trunk limb differentiation. This differentiation is manifest in the first thoracic limb by a much-reduced endopod and greatly elongated, pseudoannulated 4th-5th endites. Other anterior thoracic limbs display gradational differentiation, but by the 5th thoracic limb, the distal endites and endopod are roughly equant. Only one, poorly-known modern species, [‘Lepidurus’ batesoni] (Kazakhstan), lacks this anterior limb differentiation. Furthermore, all fossil notostracans lack this thoracic limb differentiation. This suggests that virtually the entire fauna of extant notostracans had a relatively recent (≥65 mya) origin. Perhaps more interesting is the fact that this limb differentiation suggests a significant change in body patterning—perhaps related to a shift in Hox gene expression domains, or another downstream regulatory network. Notostracans are also characterized by another unique feature of their limb differentiation: after the 11th thoracic limb, the correspondence between dorsal and ventral segmentation is lost. Dorsally, the trunk maintains its even, ring-like segmentation, but ventrally, the limbs decrease in size and increase in number per segment. This means that there are more limbs per segment posteriorly along the body axis. The genetic mechanisms of this segmental mismatch are not yet understood, but it is suggestive of a divergent body axis. No other branchiopod clade possesses this feature. All fossil notostracans with preserved limbs display this feature—perhaps as early as the Devonian (≤360 mya). These two examples show that the integration of fossil forms into studies of arthropod development can help identify previously unrecognized problems in body patterning and add an extra temporal dimension to their study. Incorporation of these features into a phylogenetic analysis helps clarify the relationship of extant notostracans with their extinct relatives and the enigmatic fossil kazacharthrans.
Clam shrimp (‘conchostracans’) are a diverse group of phyllopod crustaceans with modern representatives that occur on every continent except Antarctica. Restricted to freshwater systems, the majority of modern taxa inhabit lakes, ponds... more
Clam shrimp (‘conchostracans’) are a diverse group of phyllopod crustaceans with modern representatives that occur on every continent except Antarctica. Restricted to freshwater systems, the majority of modern taxa inhabit lakes, ponds and ephemeral water bodies. Although well-represented in the fossil record, the vagaries of preservation have forced paleontologists to rely on a completely different set of characters (those of the carapace) than those used by modern taxonomists (characters based on the thinly sclerotized organism within the carapace). The separation of fossil and modern clam shrimp taxonomy is further complicated by three separate fossil taxonomic systems that have arisen independently. Together, these factors all prevent a phylogenetic synthesis of the group and an understanding of their evolutionary ecology.
In an attempt to resolve these long standing issues, we have begun a multi-faceted research program that tackles the growth, variation and taphonomy of clam shrimp. Most fossil clam shrimp are represented by the carapace only. Features of the carapace, like shape and ornamentation, have been dismissed or ignored by taxonomists who work on living species, but their utility has never been tested. Preliminary work involving the application of contemporary morphometric methods has shown that the shape of the carapace can be quantified and implemented in the identification of inter- and intra-specific (sex & ontogenetic stages) morphotypes. Meanwhile, extensive study of the ornamentation suggests that, contrary to common wisdom, it is phylogenetically useful. Although patterns of ornamentation may vary through ontogeny, they vary in predictable ways.
Furthermore, unlike most other crustacea, clam shrimp undergo incomplete molting, which means they retain a complete ontogenetic record in the morphology of the carapace. To understand what these characters means, we need to have an understanding of a) how the characters are generated, b) how they grow and c) how they are affected by taphonomic processes. By conducting taphonomic and rearing experiments, we aim to assess the factors that contribute to the condition of clam shrimp remains in the fossil record. With this data, we will discover how best to interpret them and begin to formulate a much-needed comprehensive review of the specifics of clam shrimp paleontology.
In an attempt to resolve these long standing issues, we have begun a multi-faceted research program that tackles the growth, variation and taphonomy of clam shrimp. Most fossil clam shrimp are represented by the carapace only. Features of the carapace, like shape and ornamentation, have been dismissed or ignored by taxonomists who work on living species, but their utility has never been tested. Preliminary work involving the application of contemporary morphometric methods has shown that the shape of the carapace can be quantified and implemented in the identification of inter- and intra-specific (sex & ontogenetic stages) morphotypes. Meanwhile, extensive study of the ornamentation suggests that, contrary to common wisdom, it is phylogenetically useful. Although patterns of ornamentation may vary through ontogeny, they vary in predictable ways.
Furthermore, unlike most other crustacea, clam shrimp undergo incomplete molting, which means they retain a complete ontogenetic record in the morphology of the carapace. To understand what these characters means, we need to have an understanding of a) how the characters are generated, b) how they grow and c) how they are affected by taphonomic processes. By conducting taphonomic and rearing experiments, we aim to assess the factors that contribute to the condition of clam shrimp remains in the fossil record. With this data, we will discover how best to interpret them and begin to formulate a much-needed comprehensive review of the specifics of clam shrimp paleontology.
Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans (dating from at least the Devonian). Branchiopoda consists of several groups: the anostracans, the notostracans, the paraphyletic... more
Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans (dating from at least the Devonian). Branchiopoda consists of several groups: the anostracans, the notostracans, the paraphyletic ‘conchostracans’ and the cladocerans. Their thin, lightly mineralized exoskeleton requires special conditions for fossilization. Nonetheless, branchiopods have a very good fossil record that has been largely overlooked and never placed in a phylogenetic context. Re-evaluation of the branchiopod fossil record has yielded interesting new insights into the morphological evolution and diversification of this clade.
Notostracans, rather than being emblematic of stasis, have a dynamic evolutionary history. Their fossil record suggests that they had a ‘bivalved’ posture ancestrally and that the differentiation of their anterior thoracic limbs was a relatively late development. The extinct sister group to notostracans, the geographically isolated kazacharthrans, do not seem to represent a monophyletic clade at all — they are more likely a part of the Notostraca stem group. Fossil ‘conchostracans’ are much more difficult to reconcile with the modern phylogenetic understanding of the group. Fossil ‘conchostracans’ typically preserve only the ‘bivalved’ carapace and its ornamentation, but the systematics of modern ‘conchostracans’ essentially ignores these attributes. Documentation of the carapace ornamentation of modern species will enable testing of the phylogenetic utility of those features in the fossil representatives. ‘Conchostracan’ growth lines may be a basal feature of diplostracans rather than of spinicaudatans. These new discoveries all contribute to a rich, emerging picture of branchiopod evolution. Furthermore, they demonstrate the utility of integrating fossil forms into the phylogenies of modern animals in order to constrain ancestral states.
Notostracans, rather than being emblematic of stasis, have a dynamic evolutionary history. Their fossil record suggests that they had a ‘bivalved’ posture ancestrally and that the differentiation of their anterior thoracic limbs was a relatively late development. The extinct sister group to notostracans, the geographically isolated kazacharthrans, do not seem to represent a monophyletic clade at all — they are more likely a part of the Notostraca stem group. Fossil ‘conchostracans’ are much more difficult to reconcile with the modern phylogenetic understanding of the group. Fossil ‘conchostracans’ typically preserve only the ‘bivalved’ carapace and its ornamentation, but the systematics of modern ‘conchostracans’ essentially ignores these attributes. Documentation of the carapace ornamentation of modern species will enable testing of the phylogenetic utility of those features in the fossil representatives. ‘Conchostracan’ growth lines may be a basal feature of diplostracans rather than of spinicaudatans. These new discoveries all contribute to a rich, emerging picture of branchiopod evolution. Furthermore, they demonstrate the utility of integrating fossil forms into the phylogenies of modern animals in order to constrain ancestral states.
Crustaceans are rarely preserved in amber and their discovery warrants special attention. Several new specimens of amphipod are reported from the Lower Miocene amber of Chiapas, Mexico. A combination of standard light microscopy and... more
Crustaceans are rarely preserved in amber and their discovery warrants special attention. Several new specimens of amphipod are reported from the Lower Miocene amber of Chiapas, Mexico. A combination of standard light microscopy and photography, together with microCT scanning, allows the animals to be documented in exquisite detail. Although conventional microscopy often provides finer resolution than microCT scanning, scanning can resolve hidden features (e.g., obscured by inclusions or superimposition of limbs) and internal morphology that cannot be imaged using other nondestructive methods.
The specimens (one male and several females) represent a new genus and species of sexually dimorphic amphipod. The most notable feature of the new species is that it is blind (suggestive of a cavernicolous ecology). It differs significantly from the other amphipod known from the Chiapas amber, Transitroides morsei, and more closely resembles the extant genus Cerrorhestia, which today is known only from the cloud and rain forests of Costa Rica and Panama.
The scans revealed internal features that appear to correspond to parts of the digestive tract and other internal structures. Animals in amber are not pristine: internal features are the result of a complex interplay of resin infiltration, dehydration, and decay driven by the gut flora – agents that are poorly understood. MicroCT scans and traditional microscopy can be used in concert to reveal the morphology, taphonomy, and even internal anatomy of fossil amphipods in greater detail than ever before.
The specimens (one male and several females) represent a new genus and species of sexually dimorphic amphipod. The most notable feature of the new species is that it is blind (suggestive of a cavernicolous ecology). It differs significantly from the other amphipod known from the Chiapas amber, Transitroides morsei, and more closely resembles the extant genus Cerrorhestia, which today is known only from the cloud and rain forests of Costa Rica and Panama.
The scans revealed internal features that appear to correspond to parts of the digestive tract and other internal structures. Animals in amber are not pristine: internal features are the result of a complex interplay of resin infiltration, dehydration, and decay driven by the gut flora – agents that are poorly understood. MicroCT scans and traditional microscopy can be used in concert to reveal the morphology, taphonomy, and even internal anatomy of fossil amphipods in greater detail than ever before.
Our perceptions of the arthropod fossil record have been shaped by both exceptionally preserved faunas and taphonomy experiments. The latter still have considerable potential inform how we interpret fossil arthropods. In order to evaluate... more
Our perceptions of the arthropod fossil record have been shaped by both exceptionally preserved faunas and taphonomy experiments. The latter still have considerable potential inform how we interpret fossil arthropods. In order to evaluate the effect of taphonomy on specific arthropod characters, fossil notostracans were investigated with parallel decay experiments on their living relatives. These crustaceans are of special interest because of their supposed ‘primitive’ nature, and the status of Triops cancriformis as the oldest living species (with putative occurrences from the Permian). Experiments focused on evaluating the persistence of specific features relevant to the taxonomy of living species: the shape and position of the eyes, dorsal organ and mandibular grooves as well as the morphology of the differentiated first thoracic limbs. Decay and disarticulation were monitored for carcasses and exuviae, while the effect of desiccation on morphology was investigated experimentally.
The dorsal features of the notostracan carapace persisted throughout decay; in fact, after six weeks of decay, a carcass was still largely indistinguishable from an exuvia. Furthermore, the elongate, ‘antennaform’ first thoracic limb, present in almost all living notostracans, remained identifiable. This is in contrast to all known fossil notostracans, where the first thoracic limb is like the rest of the thoracic limbs. This suggests that the presence of an elongate first thoracic appendage may be a useful in defining a monophyletic group of living notostracans including T. cancriformis. The status of T. cancriformis as a living fossil, based on its supposed identity with Permian and Mesozoic fossils, should therefore be reconsidered.
The dorsal features of the notostracan carapace persisted throughout decay; in fact, after six weeks of decay, a carcass was still largely indistinguishable from an exuvia. Furthermore, the elongate, ‘antennaform’ first thoracic limb, present in almost all living notostracans, remained identifiable. This is in contrast to all known fossil notostracans, where the first thoracic limb is like the rest of the thoracic limbs. This suggests that the presence of an elongate first thoracic appendage may be a useful in defining a monophyletic group of living notostracans including T. cancriformis. The status of T. cancriformis as a living fossil, based on its supposed identity with Permian and Mesozoic fossils, should therefore be reconsidered.
The forked morphology of the hypostome of asaphid (Asaphidae) trilobites is enigmatic. Although the trilobite hypostome is analogous to the labrum in other arthropods, the hypostome of Isotelus lacks an obvious modern counterpart. The... more
The forked morphology of the hypostome of asaphid (Asaphidae) trilobites is enigmatic. Although the trilobite hypostome is analogous to the labrum in other arthropods, the hypostome of Isotelus lacks an obvious modern counterpart. The Isotelus hypostome has
closely-spaced terrace lines on a greatly thickened inner surface of the fork, the scarp of the terrace facing anteroventrally. This is compatible with a grinding function, suggesting possible limb differentiation to complement this structure. The inner face of the fork is
also unique in that is has a microstructure perpendicular to the surface of the sclerite. Macropredatory and filter-feeder roles are ruled out, and previous characterizations of the hypostome as knife-like or serrated are rejected. It is concluded that the hypostome of Isotelus is unique and lacks modern analogues. The features of the asaphid-type hypostome are compared with those of other trilobites with forked hypostomes and with the labrum of other arthropods. Its function is incompatible with that of other non-asaphid trilobites with forked hypostomes, like the remopleuridid Hypodicranotus.
closely-spaced terrace lines on a greatly thickened inner surface of the fork, the scarp of the terrace facing anteroventrally. This is compatible with a grinding function, suggesting possible limb differentiation to complement this structure. The inner face of the fork is
also unique in that is has a microstructure perpendicular to the surface of the sclerite. Macropredatory and filter-feeder roles are ruled out, and previous characterizations of the hypostome as knife-like or serrated are rejected. It is concluded that the hypostome of Isotelus is unique and lacks modern analogues. The features of the asaphid-type hypostome are compared with those of other trilobites with forked hypostomes and with the labrum of other arthropods. Its function is incompatible with that of other non-asaphid trilobites with forked hypostomes, like the remopleuridid Hypodicranotus.
Ptychaspididae is a family of trilobites known from the Upper Cambrian (Furongian, Sunwaptan) of North America and Asia. Conventionally, ptychaspidid trilobites have been split into three subfamilies: the Euptychaspidinae, the... more
Ptychaspididae is a family of trilobites known from the Upper Cambrian (Furongian, Sunwaptan) of North America and Asia. Conventionally, ptychaspidid trilobites have been split into three subfamilies: the Euptychaspidinae, the Macronodinae, and the Ptychaspidinae (see Adrain and Westrop, 2005). Though a smaller analysis of euptychaspidine and macronodine trilobites has been conducted (Adrain and Westrop, 2001), a complete analysis of the Ptychaspididae has never been undertaken. Monophyly of the Ptychaspidinae was assumed but never fully tested.
Previously unreported silicified trilobite faunas occur in a narrow stratigraphic interval of the Upper Cambrian (Sunwaptan) St. Charles Formation in the Bear River Range of southeastern Idaho. The faunas occur in four closely spaced... more
Previously unreported silicified trilobite faunas occur in a narrow stratigraphic interval of the Upper
Cambrian (Sunwaptan) St. Charles Formation in the Bear River Range of southeastern Idaho. The faunas occur in four closely spaced rudstones and trilobite packstones indicating deposition in a shallow subtidal setting above storm wave base. At least 23 species are represented, included two undescribed genera and several undescribed species. The faunas are notable for their high trilobite abundance and pervasive silicification. Most coeval faunas have been described on the basis of small numbers of "crack-out" specimens, and the new material reveals many details of anatomy, including knowledge of most exoskeletal
sclerites. The four trilobite-yielding beds contain markedly different taxon-abundance profiles, yet most species are shared between them. This suggests multiple, taphonomically-controlled samples of a similar underlying distribution, though true ecological variation cannot be discounted. Because the rocks can be physically cracked, to yield trilobites in traditional mechanical fashion, a comparison of "crack-out" versus silicified taxon-abundance profiles reveals strong bias in mechanical sampling toward large, smooth taxa.
Cambrian (Sunwaptan) St. Charles Formation in the Bear River Range of southeastern Idaho. The faunas occur in four closely spaced rudstones and trilobite packstones indicating deposition in a shallow subtidal setting above storm wave base. At least 23 species are represented, included two undescribed genera and several undescribed species. The faunas are notable for their high trilobite abundance and pervasive silicification. Most coeval faunas have been described on the basis of small numbers of "crack-out" specimens, and the new material reveals many details of anatomy, including knowledge of most exoskeletal
sclerites. The four trilobite-yielding beds contain markedly different taxon-abundance profiles, yet most species are shared between them. This suggests multiple, taphonomically-controlled samples of a similar underlying distribution, though true ecological variation cannot be discounted. Because the rocks can be physically cracked, to yield trilobites in traditional mechanical fashion, a comparison of "crack-out" versus silicified taxon-abundance profiles reveals strong bias in mechanical sampling toward large, smooth taxa.
Over the past 30 years, several major studies have concluded that Cambrian and Ordovician trilobite assemblages on the Laurentian paleocontinent were arranged in discrete biofacies along environmental gradients, with water depth the chief... more
Over the past 30 years, several major studies have concluded that Cambrian and Ordovician trilobite assemblages on the Laurentian paleocontinent were arranged in discrete biofacies along environmental gradients, with water depth the chief control. Biofacies were originally determined qualitatively, but more recent work has focused on q- and r-mode hierarchical cluster analyses using taxon-abundance data. Studies have typically identified 4-6 coeval biofacies occurring on a proximal to distal transect at any one time.
The end-Ordovician mass extinction saw the reduction of global trilobite taxic diversity by about half. Recent work has demonstrated that Silurian trilobite within-habitat (alpha) diversity was largely unaffected, suggesting that there must have been serious reductions in the beta (between habitat) and gamma (geographic) components of Silurian diversity.
While the Silurian has long been known as a time of increased cosmopolitanism, the potential effects of the end-Ordovician extinctions on ecological partitioning have not been quantitatively examined. We compiled all available literature and field based taxon-abundance information on Llandovery, Wenlock, and lower Ludlow trilobite faunas of Laurentia and carried out q- and r-mode cluster analyses. The results demonstrate a greatly reduced range of biofacies development in Silurian trilobite faunas. Nearshore and basinal biofacies remain distinct, but there is little environmental differentiation of assemblages occuring in normal marine settings between these extremes.
Trilobite taxa which survived the end-Ordovician extinction typically had relatively distinct biofacies occurrence during the Ordovician, but their ecological range was greatly expanded following the extinction. Although loss of gamma diversity likely played a significant, if thus far unquantified, role in the fall in global clade diversity, our results indicate that breakdown in between habitat diversity was also a major factor.
The end-Ordovician mass extinction saw the reduction of global trilobite taxic diversity by about half. Recent work has demonstrated that Silurian trilobite within-habitat (alpha) diversity was largely unaffected, suggesting that there must have been serious reductions in the beta (between habitat) and gamma (geographic) components of Silurian diversity.
While the Silurian has long been known as a time of increased cosmopolitanism, the potential effects of the end-Ordovician extinctions on ecological partitioning have not been quantitatively examined. We compiled all available literature and field based taxon-abundance information on Llandovery, Wenlock, and lower Ludlow trilobite faunas of Laurentia and carried out q- and r-mode cluster analyses. The results demonstrate a greatly reduced range of biofacies development in Silurian trilobite faunas. Nearshore and basinal biofacies remain distinct, but there is little environmental differentiation of assemblages occuring in normal marine settings between these extremes.
Trilobite taxa which survived the end-Ordovician extinction typically had relatively distinct biofacies occurrence during the Ordovician, but their ecological range was greatly expanded following the extinction. Although loss of gamma diversity likely played a significant, if thus far unquantified, role in the fall in global clade diversity, our results indicate that breakdown in between habitat diversity was also a major factor.
Abundant trilobite sclerites occur as molds in weathered blocks of the early Llandovery (Rhuddanian) Bowling Green Dolomite (Edgewood Formation) in a section near Clarksville, Missouri. The trilobite fauna is entirely undescribed, and... more
Abundant trilobite sclerites occur as molds in weathered blocks of the early Llandovery (Rhuddanian) Bowling Green Dolomite (Edgewood Formation) in a section near Clarksville, Missouri. The trilobite fauna is entirely undescribed, and includes at least ten species belonging to seven families, including Encrinuridae (49% of the total 233 specimens), Calymenidae (25%), Phacopidae (11%), Odontopleuridae (5%), Proetidae (4%), Aulacopleuridae (3%), and Illaenidae (3%). The complete absence of other common Silurian families, such as Cheiruridae and Lichidae, is unusual. The shelly fauna is dominated by articulate brachiopods, and the unit was likely deposited in a deep, subtidal, outer shelf environment, below storm wave base. The trilobite fauna is important due to the extremely limited global sampling of Rhuddanian assemblages. The recorded diversity corroborates recent work indicating that the end-Ordovician mass extinction, which it immediately post-dates, had little or no effect on trilobite alpha (within-habitat) diversity. In addition, although many Silurian trilobite faunas from the Eastern United States have been recorded, few have received modern systematic treatment. The Bowling Green fauna can be used to test hypotheses of lineage survivorship across the end-Ordovician extinction. The Transcontinental Arch exerted considerable influence on Early Silurian Laurentian trilobites, dividing the continent into northern and southern geographic realms. The Bowling Green fauna indicates that southern genus-level endemism was entrenched very early during the Silurian.
Pre-college education provides a crucial foundation for geoscience education at higher levels. Out-of-school-time programs are a powerful tool to equip and motivate K-12 students to consider further academic studies and careers in the... more
Pre-college education provides a crucial foundation for geoscience education at higher levels. Out-of-school-time programs are a powerful tool to equip and motivate K-12 students to consider further academic studies and careers in the sciences. GEODES (Gearing Educational Opportunities toward Diversity in the Earth Sciences) is a partnership between the Yale Peabody Museum's after school program ‘Evolutions' and Yale geoscience faculty. The aim of GEODES is to supplement the formal school experiences of diverse Grades 9 – 12 students in order to develop their future aspirations in the geosciences and in the sciences more generally. The program promotes geoscience literacy by expanding each student's knowledge base while fostering a better understanding of post-secondary academics and improving college preparation.
The program was developed in collaboration with the urban New Haven Public School system. Eighty high school students come to the Museum each week throughout the academic year for seminars from practicing scientists, career research, fieldwork, college field trips, laboratory visits and behind-the-scenes collection tours. Twenty students take part in paid internships undertaking research with Yale faculty. The culminating event is the production of a multi-media museum exhibition that goes on display in the Museum public galleries. The program is designed to address factors that particularly influence college and career choices of underrepresented groups including peer pressure, the need for representative role-models, the perceived lack of interesting career pathways, and the importance of family involvement. Among other things, independent evaluation has demonstrated that the program increases students' interest and knowledge of the geosciences and geoscience careers, and that the program's combination of learning and social opportunities impacted students' attitudes to and visions of science. The strength of this program is in providing meaningful geoscience learning experiences within an established museum-based college and career-focused after school program.
The program was developed in collaboration with the urban New Haven Public School system. Eighty high school students come to the Museum each week throughout the academic year for seminars from practicing scientists, career research, fieldwork, college field trips, laboratory visits and behind-the-scenes collection tours. Twenty students take part in paid internships undertaking research with Yale faculty. The culminating event is the production of a multi-media museum exhibition that goes on display in the Museum public galleries. The program is designed to address factors that particularly influence college and career choices of underrepresented groups including peer pressure, the need for representative role-models, the perceived lack of interesting career pathways, and the importance of family involvement. Among other things, independent evaluation has demonstrated that the program increases students' interest and knowledge of the geosciences and geoscience careers, and that the program's combination of learning and social opportunities impacted students' attitudes to and visions of science. The strength of this program is in providing meaningful geoscience learning experiences within an established museum-based college and career-focused after school program.
Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans (in existence since at least the Devonian). Though they often co-occur with fossil insects, study of the two groups has largely been... more
Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans (in existence since at least the Devonian). Though they often co-occur with fossil insects, study of the two groups has largely been divorced from one another. In fact, the paleobiology of branchiopod crustaceans impinges in several key ways on the paleobiology of hexapods. Molecular phylogenetic work over the past fifteen years has not only suggested that crustaceans and hexapods are closely related to one another, but that branchiopods may, in fact, be the sister group of hexapods (making Crustacea paraphyletic). The fossil record provides a perfect way to test this hypothesis by looking for animals that possess synapomorphies linking the two groups. In addition, many insect-bearing fossil localities represent freshwater environments—often also containing fossil branchiopods. Thus, study of fossil branchiopods can contribute to he study of their paleoecology and paleobiogeography as well.
Perhaps more interesting are the unanswered questions about the evolution of branchiopods themselves. There are vexing gaps in the fossil record for many of the clades—gaps where, based on phylogenetic predictions, we would expect them to be present. For ‘conchostracans’ in particular, we have no way at present to reconcile the fossil taxonomy with modern phylogenetic work. We do not yet understand the morphological transitions involved in the evolution of the group; in particular, the evolution of the carapace into univalved, bivalved, and growth line bearing forms. Furthermore, we do not understand how or why branchiopods acquired their peculiar ecological strategy of inhabiting ephemeral freshwater environments and producing desiccation-resistant reproductive cysts. A paradox observed by other specialists is that this ecology may have contributed to their long-term morphological (but not taxonomic) stasis in that ephemeral freshwater environments are very persistent over geological timescales.
Perhaps more interesting are the unanswered questions about the evolution of branchiopods themselves. There are vexing gaps in the fossil record for many of the clades—gaps where, based on phylogenetic predictions, we would expect them to be present. For ‘conchostracans’ in particular, we have no way at present to reconcile the fossil taxonomy with modern phylogenetic work. We do not yet understand the morphological transitions involved in the evolution of the group; in particular, the evolution of the carapace into univalved, bivalved, and growth line bearing forms. Furthermore, we do not understand how or why branchiopods acquired their peculiar ecological strategy of inhabiting ephemeral freshwater environments and producing desiccation-resistant reproductive cysts. A paradox observed by other specialists is that this ecology may have contributed to their long-term morphological (but not taxonomic) stasis in that ephemeral freshwater environments are very persistent over geological timescales.
The early Cretaceous Jehol Biota of northeastern China contains a diverse group of ‘notostracans’, including two new genera: Chenops and Jeholops. Chenops is characterized by the combination of an ovate carapace, narrow anal plate, and... more
The early Cretaceous Jehol Biota of northeastern China contains a diverse group of ‘notostracans’, including two new genera: Chenops and Jeholops. Chenops is characterized by the combination of an ovate carapace, narrow anal plate, and equate distal endites on the anterior thoracic limbs. In addition to the new species, C. yixianensis, the genus also provisionally includes Prolepidurus oblongus Oleynikov, 1968 and the living species Lepidurus batesoni Longhurst, 1955. Jeholops, however, is monotypic, represented by the new species J. hongi. It is characterized by a combination of kazacharthran and notostracan features that does not fit into any known taxon. Both new genera are provisionally placed in the taxon Notostraca, but the difficulties in placing fossil notostracans into a phylogenetic framework are discussed. More detailed work exploring the morphology of exceptionally preserved branchiopod crustaceans is needed.
Aquatic crustaceans are rare inclusions in amber. The recent investigation of a unique piece of Chiapas amber (lower Miocene) identified (among other arthropods) amphipod and ostracod crustaceans. The occurrence of these kinds of... more
Aquatic crustaceans are rare inclusions in amber. The recent investigation of a unique piece of Chiapas amber (lower Miocene) identified (among other arthropods) amphipod and ostracod crustaceans. The occurrence of these kinds of crustaceans in amber not only presents a rare window into their evolutionary history, but also offers insight into the processes by which they become trapped in amber. This study focused on a unique piece of amber from the lower Miocene of Chiapas, Mexico. Organic and inorganic debris are concentrated within the amber in two discrete, essentially parallel layers—suggesting two separate flows of resin. While both layers contain sand and indeterminate organic debris (including insect debris), only one layer contains ostracods. The other layer contains an amphipod and, among other things, an indeterminate dipteran and orthopteran.
Combining standard light microscopy and photography with microCT scanning allows for documentation of the animals in exquisite detail. Though the resolution of conventional microscopy is often finer than microCT scanning; the microCT scanning can resolve hidden and internal features that would be unrecoverable using other nondestructive methods.
The crustaceans are the best-exposed animals in the studied piece of amber. There are two apparent ostracode morphs, based on carapace shape. The ostracod carapaces contain finely preserved appendages that can be revealed from within the carapace by the microCT scans. The amphipod, known from multiple specimens, is a new species (likely representing a new genus as well). It is closely related to Transitroides morsei, which was previously described from Chiapas amber. The most notable detail about the new species of amphipod is that it is blind. MicroCT scans and traditional microscopy work in concert to reveal the morphology and internal anatomy of a fossil amphipod in greater detail than ever before.
It is not a simple question to ask how all of these arthropods came to be preserved in the same piece of amber. By far, the capture of aquatic crustaceans is the most puzzling. The presence of sand grains suggests that the original resin reached the ground, rather than capturing the crustaceans in an arboreal pool of water (i.e. ‘tank-forming’ bromeliads). Tree resin is usually insoluble in water, and the differences in surface tension and viscosity between water and resin would make it difficult for the resin to capture the aquatic crustaceans directly from the water. The parallel nature of the two debris streams in the amber argue against the resin ‘dripping’ into a pool to capture the animals. Furthermore, the aquatic crustaceans do not seem to show any signs of significant desiccation or decay—making it unlikely that the resin flow covered them in a dried out pool.
Combining standard light microscopy and photography with microCT scanning allows for documentation of the animals in exquisite detail. Though the resolution of conventional microscopy is often finer than microCT scanning; the microCT scanning can resolve hidden and internal features that would be unrecoverable using other nondestructive methods.
The crustaceans are the best-exposed animals in the studied piece of amber. There are two apparent ostracode morphs, based on carapace shape. The ostracod carapaces contain finely preserved appendages that can be revealed from within the carapace by the microCT scans. The amphipod, known from multiple specimens, is a new species (likely representing a new genus as well). It is closely related to Transitroides morsei, which was previously described from Chiapas amber. The most notable detail about the new species of amphipod is that it is blind. MicroCT scans and traditional microscopy work in concert to reveal the morphology and internal anatomy of a fossil amphipod in greater detail than ever before.
It is not a simple question to ask how all of these arthropods came to be preserved in the same piece of amber. By far, the capture of aquatic crustaceans is the most puzzling. The presence of sand grains suggests that the original resin reached the ground, rather than capturing the crustaceans in an arboreal pool of water (i.e. ‘tank-forming’ bromeliads). Tree resin is usually insoluble in water, and the differences in surface tension and viscosity between water and resin would make it difficult for the resin to capture the aquatic crustaceans directly from the water. The parallel nature of the two debris streams in the amber argue against the resin ‘dripping’ into a pool to capture the animals. Furthermore, the aquatic crustaceans do not seem to show any signs of significant desiccation or decay—making it unlikely that the resin flow covered them in a dried out pool.
Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans; likely crown-group representatives have been in existence since the Devonian Period (over 360 mya). Branchiopoda consists of several... more
Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans; likely crown-group representatives have been in existence since the Devonian Period (over 360 mya). Branchiopoda consists of several groups: the anostracans, the notostracans, the paraphyletic conchostracans and the cladocerans. They have a very good fossil record that has been largely overlooked and never placed in a phylogenetic context. Re-evaluation of the branchiopod fossil record has yielded interesting new insights into the morphological evolution and diversification of this clade. Notostracans have often been referred to as living fossils. Rather than being emblematic of stasis, they have a dynamic evolutionary history. Their fossil record suggests that they had a ‘bivalved’ posture ancestrally and that the differentiation of their anterior thoracic limbs was a relatively late development. Fossil conchostracans are more difficult to reconcile with a modern phylogenetic understanding of the group. Fossil conchostracans typically preserve only the carapace and its ornamentation, but systematics of modern conchostracansignores these attributes. Documentation of the carapace ornamentation of modern species will enable testing of the phylogenetic utility of those features in the fossil representatives. Conchostracan growth lines are a much more ambiguous character—they may be a basal feature of diplostracans rather than of spinicaudatans. Inclusion of fossil taxa in a phylogenetic analysis based on morphological characters does not alter the inter-relationships of extant taxa, but it does alter our perception of character evolution within the clade. Understanding the course of character evolution within Branchiopoda may be useful for understanding the origin of Hexapoda.
Though branchiopod crustaceans are often thought of as possessing primitively homonymous trunk limbs, the typical notostracan has a unique pattern of trunk limb differentiation. This differentiation is manifest in the first thoracic limb... more
Though branchiopod crustaceans are often thought of as possessing primitively homonymous trunk limbs, the typical notostracan has a unique pattern of trunk limb differentiation. This differentiation is manifest in the first thoracic limb by a much-reduced endopod and greatly elongated, pseudoannulated 4th and 5th endites. Anterior thoracic limbs also show some limited differentiation, but by the 5th thoracic limb, the endites and endopod are roughly equant. Only one, poorly-known living species, ‘Lepidurus’ batesoni from Kazakhstan lacks this anterior limb differentiation. Furthermore, all fossil notostracans lack this thoracic limb differentiation. This suggests that virtually the entire fauna of extant notostracans had a relatively recent (≥65 mya) origin. Perhaps more interesting is the fact that this limb differentiation suggests a significant change in body patterning—perhaps related to a shift in Hox gene expression domains, or another downstream regulatory network. Notostracans are also characterized by another unique feature of their limb differentiation: after the 11th thoracic limb, the correspondence between dorsal and ventral segmentation is lost. Dorsally, the trunk maintains its even, ring-like segmentation, but ventrally, the limbs decrease in size and increase in number per segment. This means that there are more limbs per segment posteriorly along the body axis. The genetic mechanisms of this segmental mismatch are not yet understood, but it is suggestive of a divergent body axis. No other branchiopod clade possesses this feature. However, all fossil notostracans with preserved limbs display this feature—going back as far as the Devonian (≤360 mya). These two examples show that the integration of fossil forms into studies of arthropod development can help identify previously unrecognized problems in body patterning and add an extra temporal dimension to their study.
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Hegna, T.A. 2015. Book review of Coram, R.A. & Jepson, J.E. 2012. Fossil Insects of the Purbeck Limestone Group of Southern England: Palaeoentomology from the Dawn of the Cretaceous. Monograph Series Volume 3. Siri Scientific Press,... more
Hegna, T.A. 2015. Book review of Coram, R.A. & Jepson, J.E. 2012. Fossil Insects of the Purbeck Limestone Group of Southern England: Palaeoentomology from the Dawn of the Cretaceous. Monograph Series Volume 3. Siri Scientific Press, Manchester, UK, 144 pp. Priscum, Jan. 2015.