Abstract The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton ... more Abstract The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occurs in vertebrate taxa such as the fossil group Placodermi and the Chondrichthyes (Holocephali, Batoidea). Although the synarcual varies morphologically in these groups, it represents the first indication, phylogenetically, of a differentiation of the vertebral column into separate regions. Among the placoderms, the synarcual of Cowralepis mclachlani Ritchie, 2005 (Arthrodira) shows substantial changes during ontogeny to ...
5th International Palaeontological Congress, poster, 2018
Six arthrodiran egg cases were reported from the Cleveland Shale (Famennian) of Ohio, U.S.A. Usin... more Six arthrodiran egg cases were reported from the Cleveland Shale (Famennian) of Ohio, U.S.A. Using scanning electron microscopy (SEM), the recognition of obliquely arranged layers of collagen fibers formed the basis for assignment to the vertebrates. The search for a producer of these cases was based on the distinct morphology of the cases compared to the known morphotypes among the chondrichthyans and a single specimen possessing a tiny fragment of apparent tuberculated bone within the egg case. Suggesting an arthrodiran producer, the nature of the tubercles is distinct from the surface ornament in other vertebrates, but a common feature among the arthrodires within the fauna. Samples of the bone were not extracted for further analysis. The purpose of this study is to test the hypothesis for the origin of these egg cases. To this end, the morphology of the specimens is being described to provide a comparison to known chondrichthyan egg cases. The ultrastructure and composition are being analyzed using mi-croXRD using TEM to determine the nature of phosphatization, thin sectioning for both light microscopy and SEM analysis, and energy dispersive spectroscopy (EDS) to evaluate the elemental variations within individual laminae.
Six enigmatic fossils from the Famennian (Devonian) Cleveland Shale in Ohio, U.S.A., are interpre... more Six enigmatic fossils from the Famennian (Devonian) Cleveland Shale in Ohio, U.S.A., are interpreted here as arthrodiran (Placodermi) egg cases. Recognition as egg cases is confirmed based on the observation of layered collagen fibers. The presence of a tuberculated bone fragment preserved within one case confirms a vertebrate source. The nature of the tubercles and the unique morphology of the egg cases supports the interpretation of an arthrodiran source. Reports of Devonian egg cases are limited to either assumed chondrichthyan producers or a putative ‘egg sac’ with a morphology atypical for any vertebrate. The Cleveland Shale egg cases thus represent the first record for a non-chondrichthyan producer. Among placoderms, behaviors of a pelagic life style with obligate nesting sites, reef fishes with live birth, and estuarine and fluvial nurseries, along with eggcase oviparity testifies to the diversity of reproductive strategies. As with modern fishes these strategies may be ecol...
The Cleveland Shale fauna represents a unique view of the time after a major Devonian extinction ... more The Cleveland Shale fauna represents a unique view of the time after a major Devonian extinction event (Frasnian–Famenian) with the recovery of arthrodires (Placodermi) best represented by this most specious North American fauna. This time was followed by an additional event (Hangenberg Biocrisis) leading to the extinction of arthrodires (and all other placoderms). An understanding of the diversity and interrelationships of North American arthrodires can aid our understanding of this critical time in vertebrate evolution. A new aspinothoracid arthrodire Hlavinichthys jacksoni gen. et sp. nov. is described from the Late Devonian of northern Ohio, U.S.A., which adds to our knowledge of this group. It provides a point of comparison to other members of the fauna whose interrelationships are poorly known. A phylogenetic analysis supports an assignment of Hlavinichthys jacksoni gen. et sp. nov. among the aspinothoracid arthrodires. This work has drawn attention to the continued need for ...
The Famennian fish fauna from the Cleveland Shale Member (Ohio Shale Formation) represents a 160-... more The Famennian fish fauna from the Cleveland Shale Member (Ohio Shale Formation) represents a 160-year collection effort starting in the mid-19th Century and continuing today. Three periods of increased research include 1857-1909 with such notable names as Newberry, Claypole, Dean, and Hussakof. This was followed in the 1930-1947 period with the work of Dunkle and Bungart. Since 1964 there has been a steadily increasing interest in the Cleveland Shale fauna. Hlavin (1976) completed the first comprehensive review of the fauna and noted 40 vertebrate taxa. This number has grown to 66 taxa (Carr & Jackson, 2008), more than double the next-most speciose Famennian fauna (32 species, Tafilalt, Morocco; Lelièvre, 2003), and our knowledge of the Cleveland Shale fauna continues to grow. This growth includes a new genus and species of selenosteid arthrodire, other new species in the shoreward laterally-equivalent Chagrin Shale Member (Ohio Shale Formation), and the preparation of new material for taxa that are only know from limited or incomplete specimens. The discovery of placoderm egg cases in the Cleveland Shale (Carr, 2010a) demonstrates the equal antiquity of oviparity and viviparity among the basal gnathostomes. The pelagic nature of the large arthrodires (Carr, 2010b) associated with these egg cases suggests the possibility of arthrodiran hatcheries in the eastward shallower parts of the Appalachian Basin. An interesting note regarding the diversity of the Cleveland Shale fauna is the lack of any analysis of microvertebrates, although bone beds and lag deposits have been identified in the field. The Cleveland Shale was considered at one time to represent the latest Devonian with the overlying Bedford Shale and Berea Sandstone Formations representing the earliest Mississippian (e.g., Lewis, 1988). This entire sequence, found in the Cleveland, Ohio area and NW Pennsylvania, is now considered to represent the latest Devonian (Brezinski et al., 2010; Baird et al., 2011; Baird, 2013) and is equivalent to the Hangenberg shales and sandstones of the European Hangenberg Biocrisis (Kaiser et al., 2016). Associated with the biocrisis is an episode of end-Devonian glaciation. This is documented in the eastern Appalachian Basin by a succession of glacigenic tillites (Brezinski et al., 2010). In Kentucky, a 3-ton granitic dropstone is embedded within the Cleveland Shale (Lierman et al., 2009) suggesting ice rafting from eastern glaciated regions. The correlation between the Ohio and NW Pennsylvania sequences with those in the glaciated regions of the eastern Appalachian Basin remains unclear. However, the presence of a series of unit-boundary bone beds and bone-containing lag deposits within the Cleveland Shale offer the opportunity to correlate these units, to tie the Appalachian events to those of Europe, and to document the pattern of extinction for the vertebrates in Cleveland Shale fauna. Sallan & Coates (2010) report both a loss of diversity and a complete restructuring of the vertebrate worldwide communities associated with the Hangenberg Biocrisis; however, they note a lack of
The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occur... more The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occurs in vertebrate taxa such as the fossil group Placodermi and the Chondrichthyes (Holocephali, Batoidea). Although the synarcual varies morphologically in these groups, it represents the first indication, phylogenetically, of a differentiation of the vertebral column into separate regions. Among the placoderms, the synarcual of Cowralepis mclachlani Ritchie, 2005 (Arthrodira) shows substantial changes during ontogeny to produce an elongate, spoolshaped structure with a well-developed dorsal keel. Because the placoderm synarcual is covered in perichondral bone, the ontogenetic history of this Cowralepis specimen is preserved as it developed anteroposteriorly, dorsally and ventrally. As well, in the placoderm Materpiscis attenboroughi Long et al., 2008 (Ptyctodontida), incomplete fusion at the posterior synarcual margin indicates that both neural and haemal arch vertebral elements are added to the synarcual. A survey of placoderm synarcuals shows that taxa such as Materpiscis and Cowralepis are particularly informative because perichondral ossification occurs prior to synarcual fusion such that individual vertebral elements can be identified. In other placoderm synarcuals (e.g. Nefudina qalibahensis Lelièvre et al., 1995; Rhenanida), cartilaginous vertebral elements fuse prior to perichondral ossification so that individual elements are more difficult to recognize. This ontogenetic development in placoderms can be compared to synarcual development in Recent chondrichthyans; the incorporation of neural and haemal elements is more similar to the holocephalans, but differs from the batoid chondrichthyans. Keywords Vertebral fusion Á Synarcual Á Placodermi Á Chondrichthyes Á Holocephali Á Batoidea Á Vertebral column Communicated by A. Schmidt-Rhaesa.
A new species of the actinolepidoid arthrodire Bryantolepis Camp, Welles, and Green, 1949, is des... more A new species of the actinolepidoid arthrodire Bryantolepis Camp, Welles, and Green, 1949, is described from the Water Canyon Formation of northern Utah and southern Idaho, where it is characteristic of the uppermost part of the Lower Devonian in the Grassy Flat Member. Bryantolepis williamsi n. sp. is about 60% larger than the only other previously recognized species, B. brachycephala (Bryant, 1932), from the Early Devonian Beartooth Butte Formation of Wyoming, but is otherwise very similar to it. The new material provides information on the endocranial morphology showing that it is very similar to that of Kujdanowiaspis Stensiö , 1942 and Lehmanosteus Goujet, 1984, the only other actinolepidoids for which the endocranium is known. The parasphenoid is described for the first time in this genus, as is the anterior superognathal. The superognathal is only the second to be definitely attributed to an actinolepidoid and is shown to be a crushing rather than a shearing element.
is being tested at the hominin rich site of Malapa, South Africa. There are several advantages to... more is being tested at the hominin rich site of Malapa, South Africa. There are several advantages to incorporating a virtual preparation technique. First, this considerably shortens overall preparation time, making more efficient use of a given number of preparation hours. Exact positions of fossils within the hard matrix can be communicated to preparators before extraction begins. Second, this method provides an opportunity to finish particularly delicate preparations virtually, which saves specimens from damage during risky extractions. Copies of specimens can be produced using 3D printing technology. Third, the method removes an additional element of chance from discovery by allowing more systematic searching of matrix for fossils (i.e., fossils buried completely in blocks of matrix are no longer invisible). This procedure also reduces the potential for damaging unknown specimens that are encountered as a surprise during preparation of known specimens. Results are scanner-specific and the choice of scan parameters will produce varying results. Thus, there appears to be no single 'best practice', although similar protocols may be applicable for blocks from similar sedimentary units. Formalizing the process by creating reporting forms to aid in the prioritization of preparation, as well as involving technicians in the process by showing the location, direction and position of fossils within the matrix have produced excellent results. For example, the methods employed have resulted in the discovery and identification of early hominid fossils that were not visible on the surface of individual blocks, thereby enhancing our search capabilities for typically rare fossils, and by further minimizing destructive extractive and preparation techniques.
The Cleveland Member (Late Famennian) fish fauna represents one of the most speciose and well-col... more The Cleveland Member (Late Famennian) fish fauna represents one of the most speciose and well-collected faunas from the Late Devonian; however, our understanding of the fauna's paleoecology is limited. Published interpretations of placoderm paleoecology typically suggest that most species are obligate bottom-living forms or are tied closely to life on the bottom. Dunkleosteus terrelli (Newberry, 1873) (Placodermi, Arthrodira), however, was a pelagic organism. This interpretation is based on an analysis of the nature of the distal Appalachian Basin depositional habitat and the distribution of this species' remains within the basin. The species presence as fossils represents the remains of organisms that lived within the water column in the basin rather than an allochthonous accumulation of floating carcasses. Thus, the disarticulation seen in specimens of Dunkleosteus terrelli is a result of local post-mortem flotation and possible scavenging within the water column. This hyp...
Remarkably preserved specimens of Cowralepis mclachlani Ritchie, 2005 (Proc Linn Soc NSW 126:215-... more Remarkably preserved specimens of Cowralepis mclachlani Ritchie, 2005 (Proc Linn Soc NSW 126:215-259) (Phyllolepida, Placodermi) represent a unique ontogenetic sequence adding to our understanding of anatomy, function, and phylogeny among basal jawed vertebrates (gnathostomes). A systematic review demonstrates that the Phyllolepida are a subgroup of the Arthrodira. Consideration of visceral and neurocranial characters supports the hypothesis that placoderms are the sister group to remaining gnathostomes. Placoderms possess, as adult plesiomorphic features, a number of characters that are only seen in the development of extant gnathostomes-a peramorphic shift relative to placoderms. Developmental evidence in vertebrates leads to a revised polarity of character transitions. These include 1) hyomandibula-neurocranium and ventral parachordal-palatoquadrate articulations (vertebrate synapomorphies); 2) jointed pharynx, paired basibranchials, anterior ethmoidal-palatoquadrate articulation, short trabeculae cranii, and anterior and posterior neurocranial fissures (gnathostome synapomorphies); and 3) fused basibranchials, dorsal palatoquadrate-neurocranium articulation, loss of the anterior neurocranial fissure, elongated trabeculae cranii, and transfer of the ventral parachordal-palatoquadrate articulation to the trabeculae (crown group gnathostomes). The level of preservation in C. mclachlani provides the basis for a reinterpretation of phyllolepid anatomy and function. Cowralepis mclachlani possesses paired basibranchials allowing the reinterpretation of the visceral skeleton in other placoderms. Mandible depression in C. mclachlani follows an osteichthyan pattern and the ventral visceral skeleton acts as a functional unit. Evidence for hypobranchial musculature demonstrates the neural crest origin of the basibranchials and that Cowralepis was a suction feeder. Finally, the position of the visceral skeleton relative to the neurocranium in placoderms parallels the condition in selachians and osteichthyans, but differs in the elongation of the occiput. The cucullaris fossa of placoderms (interpreted as a site of muscle attachment) is shown to represent, in part, the parabranchial chamber.
Abstract The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton ... more Abstract The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occurs in vertebrate taxa such as the fossil group Placodermi and the Chondrichthyes (Holocephali, Batoidea). Although the synarcual varies morphologically in these groups, it represents the first indication, phylogenetically, of a differentiation of the vertebral column into separate regions. Among the placoderms, the synarcual of Cowralepis mclachlani Ritchie, 2005 (Arthrodira) shows substantial changes during ontogeny to ...
5th International Palaeontological Congress, poster, 2018
Six arthrodiran egg cases were reported from the Cleveland Shale (Famennian) of Ohio, U.S.A. Usin... more Six arthrodiran egg cases were reported from the Cleveland Shale (Famennian) of Ohio, U.S.A. Using scanning electron microscopy (SEM), the recognition of obliquely arranged layers of collagen fibers formed the basis for assignment to the vertebrates. The search for a producer of these cases was based on the distinct morphology of the cases compared to the known morphotypes among the chondrichthyans and a single specimen possessing a tiny fragment of apparent tuberculated bone within the egg case. Suggesting an arthrodiran producer, the nature of the tubercles is distinct from the surface ornament in other vertebrates, but a common feature among the arthrodires within the fauna. Samples of the bone were not extracted for further analysis. The purpose of this study is to test the hypothesis for the origin of these egg cases. To this end, the morphology of the specimens is being described to provide a comparison to known chondrichthyan egg cases. The ultrastructure and composition are being analyzed using mi-croXRD using TEM to determine the nature of phosphatization, thin sectioning for both light microscopy and SEM analysis, and energy dispersive spectroscopy (EDS) to evaluate the elemental variations within individual laminae.
Six enigmatic fossils from the Famennian (Devonian) Cleveland Shale in Ohio, U.S.A., are interpre... more Six enigmatic fossils from the Famennian (Devonian) Cleveland Shale in Ohio, U.S.A., are interpreted here as arthrodiran (Placodermi) egg cases. Recognition as egg cases is confirmed based on the observation of layered collagen fibers. The presence of a tuberculated bone fragment preserved within one case confirms a vertebrate source. The nature of the tubercles and the unique morphology of the egg cases supports the interpretation of an arthrodiran source. Reports of Devonian egg cases are limited to either assumed chondrichthyan producers or a putative ‘egg sac’ with a morphology atypical for any vertebrate. The Cleveland Shale egg cases thus represent the first record for a non-chondrichthyan producer. Among placoderms, behaviors of a pelagic life style with obligate nesting sites, reef fishes with live birth, and estuarine and fluvial nurseries, along with eggcase oviparity testifies to the diversity of reproductive strategies. As with modern fishes these strategies may be ecol...
The Cleveland Shale fauna represents a unique view of the time after a major Devonian extinction ... more The Cleveland Shale fauna represents a unique view of the time after a major Devonian extinction event (Frasnian–Famenian) with the recovery of arthrodires (Placodermi) best represented by this most specious North American fauna. This time was followed by an additional event (Hangenberg Biocrisis) leading to the extinction of arthrodires (and all other placoderms). An understanding of the diversity and interrelationships of North American arthrodires can aid our understanding of this critical time in vertebrate evolution. A new aspinothoracid arthrodire Hlavinichthys jacksoni gen. et sp. nov. is described from the Late Devonian of northern Ohio, U.S.A., which adds to our knowledge of this group. It provides a point of comparison to other members of the fauna whose interrelationships are poorly known. A phylogenetic analysis supports an assignment of Hlavinichthys jacksoni gen. et sp. nov. among the aspinothoracid arthrodires. This work has drawn attention to the continued need for ...
The Famennian fish fauna from the Cleveland Shale Member (Ohio Shale Formation) represents a 160-... more The Famennian fish fauna from the Cleveland Shale Member (Ohio Shale Formation) represents a 160-year collection effort starting in the mid-19th Century and continuing today. Three periods of increased research include 1857-1909 with such notable names as Newberry, Claypole, Dean, and Hussakof. This was followed in the 1930-1947 period with the work of Dunkle and Bungart. Since 1964 there has been a steadily increasing interest in the Cleveland Shale fauna. Hlavin (1976) completed the first comprehensive review of the fauna and noted 40 vertebrate taxa. This number has grown to 66 taxa (Carr & Jackson, 2008), more than double the next-most speciose Famennian fauna (32 species, Tafilalt, Morocco; Lelièvre, 2003), and our knowledge of the Cleveland Shale fauna continues to grow. This growth includes a new genus and species of selenosteid arthrodire, other new species in the shoreward laterally-equivalent Chagrin Shale Member (Ohio Shale Formation), and the preparation of new material for taxa that are only know from limited or incomplete specimens. The discovery of placoderm egg cases in the Cleveland Shale (Carr, 2010a) demonstrates the equal antiquity of oviparity and viviparity among the basal gnathostomes. The pelagic nature of the large arthrodires (Carr, 2010b) associated with these egg cases suggests the possibility of arthrodiran hatcheries in the eastward shallower parts of the Appalachian Basin. An interesting note regarding the diversity of the Cleveland Shale fauna is the lack of any analysis of microvertebrates, although bone beds and lag deposits have been identified in the field. The Cleveland Shale was considered at one time to represent the latest Devonian with the overlying Bedford Shale and Berea Sandstone Formations representing the earliest Mississippian (e.g., Lewis, 1988). This entire sequence, found in the Cleveland, Ohio area and NW Pennsylvania, is now considered to represent the latest Devonian (Brezinski et al., 2010; Baird et al., 2011; Baird, 2013) and is equivalent to the Hangenberg shales and sandstones of the European Hangenberg Biocrisis (Kaiser et al., 2016). Associated with the biocrisis is an episode of end-Devonian glaciation. This is documented in the eastern Appalachian Basin by a succession of glacigenic tillites (Brezinski et al., 2010). In Kentucky, a 3-ton granitic dropstone is embedded within the Cleveland Shale (Lierman et al., 2009) suggesting ice rafting from eastern glaciated regions. The correlation between the Ohio and NW Pennsylvania sequences with those in the glaciated regions of the eastern Appalachian Basin remains unclear. However, the presence of a series of unit-boundary bone beds and bone-containing lag deposits within the Cleveland Shale offer the opportunity to correlate these units, to tie the Appalachian events to those of Europe, and to document the pattern of extinction for the vertebrates in Cleveland Shale fauna. Sallan & Coates (2010) report both a loss of diversity and a complete restructuring of the vertebrate worldwide communities associated with the Hangenberg Biocrisis; however, they note a lack of
The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occur... more The synarcual is a structure incorporating the anterior vertebrae of the axial skeleton and occurs in vertebrate taxa such as the fossil group Placodermi and the Chondrichthyes (Holocephali, Batoidea). Although the synarcual varies morphologically in these groups, it represents the first indication, phylogenetically, of a differentiation of the vertebral column into separate regions. Among the placoderms, the synarcual of Cowralepis mclachlani Ritchie, 2005 (Arthrodira) shows substantial changes during ontogeny to produce an elongate, spoolshaped structure with a well-developed dorsal keel. Because the placoderm synarcual is covered in perichondral bone, the ontogenetic history of this Cowralepis specimen is preserved as it developed anteroposteriorly, dorsally and ventrally. As well, in the placoderm Materpiscis attenboroughi Long et al., 2008 (Ptyctodontida), incomplete fusion at the posterior synarcual margin indicates that both neural and haemal arch vertebral elements are added to the synarcual. A survey of placoderm synarcuals shows that taxa such as Materpiscis and Cowralepis are particularly informative because perichondral ossification occurs prior to synarcual fusion such that individual vertebral elements can be identified. In other placoderm synarcuals (e.g. Nefudina qalibahensis Lelièvre et al., 1995; Rhenanida), cartilaginous vertebral elements fuse prior to perichondral ossification so that individual elements are more difficult to recognize. This ontogenetic development in placoderms can be compared to synarcual development in Recent chondrichthyans; the incorporation of neural and haemal elements is more similar to the holocephalans, but differs from the batoid chondrichthyans. Keywords Vertebral fusion Á Synarcual Á Placodermi Á Chondrichthyes Á Holocephali Á Batoidea Á Vertebral column Communicated by A. Schmidt-Rhaesa.
A new species of the actinolepidoid arthrodire Bryantolepis Camp, Welles, and Green, 1949, is des... more A new species of the actinolepidoid arthrodire Bryantolepis Camp, Welles, and Green, 1949, is described from the Water Canyon Formation of northern Utah and southern Idaho, where it is characteristic of the uppermost part of the Lower Devonian in the Grassy Flat Member. Bryantolepis williamsi n. sp. is about 60% larger than the only other previously recognized species, B. brachycephala (Bryant, 1932), from the Early Devonian Beartooth Butte Formation of Wyoming, but is otherwise very similar to it. The new material provides information on the endocranial morphology showing that it is very similar to that of Kujdanowiaspis Stensiö , 1942 and Lehmanosteus Goujet, 1984, the only other actinolepidoids for which the endocranium is known. The parasphenoid is described for the first time in this genus, as is the anterior superognathal. The superognathal is only the second to be definitely attributed to an actinolepidoid and is shown to be a crushing rather than a shearing element.
is being tested at the hominin rich site of Malapa, South Africa. There are several advantages to... more is being tested at the hominin rich site of Malapa, South Africa. There are several advantages to incorporating a virtual preparation technique. First, this considerably shortens overall preparation time, making more efficient use of a given number of preparation hours. Exact positions of fossils within the hard matrix can be communicated to preparators before extraction begins. Second, this method provides an opportunity to finish particularly delicate preparations virtually, which saves specimens from damage during risky extractions. Copies of specimens can be produced using 3D printing technology. Third, the method removes an additional element of chance from discovery by allowing more systematic searching of matrix for fossils (i.e., fossils buried completely in blocks of matrix are no longer invisible). This procedure also reduces the potential for damaging unknown specimens that are encountered as a surprise during preparation of known specimens. Results are scanner-specific and the choice of scan parameters will produce varying results. Thus, there appears to be no single 'best practice', although similar protocols may be applicable for blocks from similar sedimentary units. Formalizing the process by creating reporting forms to aid in the prioritization of preparation, as well as involving technicians in the process by showing the location, direction and position of fossils within the matrix have produced excellent results. For example, the methods employed have resulted in the discovery and identification of early hominid fossils that were not visible on the surface of individual blocks, thereby enhancing our search capabilities for typically rare fossils, and by further minimizing destructive extractive and preparation techniques.
The Cleveland Member (Late Famennian) fish fauna represents one of the most speciose and well-col... more The Cleveland Member (Late Famennian) fish fauna represents one of the most speciose and well-collected faunas from the Late Devonian; however, our understanding of the fauna's paleoecology is limited. Published interpretations of placoderm paleoecology typically suggest that most species are obligate bottom-living forms or are tied closely to life on the bottom. Dunkleosteus terrelli (Newberry, 1873) (Placodermi, Arthrodira), however, was a pelagic organism. This interpretation is based on an analysis of the nature of the distal Appalachian Basin depositional habitat and the distribution of this species' remains within the basin. The species presence as fossils represents the remains of organisms that lived within the water column in the basin rather than an allochthonous accumulation of floating carcasses. Thus, the disarticulation seen in specimens of Dunkleosteus terrelli is a result of local post-mortem flotation and possible scavenging within the water column. This hyp...
Remarkably preserved specimens of Cowralepis mclachlani Ritchie, 2005 (Proc Linn Soc NSW 126:215-... more Remarkably preserved specimens of Cowralepis mclachlani Ritchie, 2005 (Proc Linn Soc NSW 126:215-259) (Phyllolepida, Placodermi) represent a unique ontogenetic sequence adding to our understanding of anatomy, function, and phylogeny among basal jawed vertebrates (gnathostomes). A systematic review demonstrates that the Phyllolepida are a subgroup of the Arthrodira. Consideration of visceral and neurocranial characters supports the hypothesis that placoderms are the sister group to remaining gnathostomes. Placoderms possess, as adult plesiomorphic features, a number of characters that are only seen in the development of extant gnathostomes-a peramorphic shift relative to placoderms. Developmental evidence in vertebrates leads to a revised polarity of character transitions. These include 1) hyomandibula-neurocranium and ventral parachordal-palatoquadrate articulations (vertebrate synapomorphies); 2) jointed pharynx, paired basibranchials, anterior ethmoidal-palatoquadrate articulation, short trabeculae cranii, and anterior and posterior neurocranial fissures (gnathostome synapomorphies); and 3) fused basibranchials, dorsal palatoquadrate-neurocranium articulation, loss of the anterior neurocranial fissure, elongated trabeculae cranii, and transfer of the ventral parachordal-palatoquadrate articulation to the trabeculae (crown group gnathostomes). The level of preservation in C. mclachlani provides the basis for a reinterpretation of phyllolepid anatomy and function. Cowralepis mclachlani possesses paired basibranchials allowing the reinterpretation of the visceral skeleton in other placoderms. Mandible depression in C. mclachlani follows an osteichthyan pattern and the ventral visceral skeleton acts as a functional unit. Evidence for hypobranchial musculature demonstrates the neural crest origin of the basibranchials and that Cowralepis was a suction feeder. Finally, the position of the visceral skeleton relative to the neurocranium in placoderms parallels the condition in selachians and osteichthyans, but differs in the elongation of the occiput. The cucullaris fossa of placoderms (interpreted as a site of muscle attachment) is shown to represent, in part, the parabranchial chamber.
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