The disappearance of the soft-bodied Ediacara biota at the Ediacaran–Cambrian boundary potentiall... more The disappearance of the soft-bodied Ediacara biota at the Ediacaran–Cambrian boundary potentially represents the earliest mass extinction of complex life, although the precise driver(s) of this extinction remain unresolved. The ‘biotic replacement’ model proposes that an evolutionary radiation of metazoan ecosystem engineers in the latest Ediacaran profoundly altered marine palaeoenvironments, resulting in the extinction of Ediacara biota and setting the stage for the subsequent Cambrian Explosion. However, metazoan ecosystem engineering across the Ediacaran–Cambrian transition has yet to be quantified. Here, we test this key tenet of the biotic replacement model by characterizing the intensity of metazoan bioturbation and ecosystem engineering in trace fossil assemblages throughout the latest Ediacaran Nama Group in southern Namibia. The results illustrate a dramatic increase in both bioturbation and ecosystem engineering intensity in the latest Ediacaran, prior to the Cambrian bo...
Reconstructing Precambrian eukaryotic paleoecology is pivotal to understanding the origins of the... more Reconstructing Precambrian eukaryotic paleoecology is pivotal to understanding the origins of the modern, animal-dominated biosphere. Here, we combine new fossil data from southern Namibia with computational fluid dynamics (CFD) to test between competing feeding models for the Ediacaran taxon Ernietta. In addition, we perform simulations for multiple individuals, allowing us to analyze hydrodynamics of living communities. We show that Ernietta lived gregariously, forming shallow marine aggregations in the latest Ediacaran, 548 to 541 million years (Ma) ago. We demonstrate enhanced vertical mixing of the water column above aggregations and preferential redirection of current into body cavities of downstream individuals. These results support the reconstruction of Ernietta as a macroscopic suspension feeder and also provide a convincing paleoecological advantage to feeding in aggregations analogous to those recognized in many extant marine metazoans. These results provide some of the ...
Toothed whales (Cetacea: Odontoceti) are the most diverse group of modern cetaceans, originating ... more Toothed whales (Cetacea: Odontoceti) are the most diverse group of modern cetaceans, originating during the Eocene/Oligocene transition approximately 38 Ma. All extant odontocetes echolocate; a single origin for this behaviour is supported by a unique facial source for ultrasonic vocalizations and a cochlea adapted for hearing the corresponding echoes. The craniofacial and inner ear morphology of Oligocene odontocetes support a rapid (less than 5 Myr) early evolution of echolocation. Although some cranial features in the stem odontocetes Simocetus and Olympicetus suggest an ability to generate ultrasonic sound, until now, the bony labyrinths of taxa of this grade have not been investigated. Here, we use µCT to examine a petrosal of a taxon with clear similarities to Olympicetus avitus . Measurements of the bony labyrinth, when added to an extensive dataset of cetartiodactyls, resulted in this specimen sharing a morphospace with stem whales, suggesting a transitional inner ear. This ...
Narwhals (Monodon monoceros) and belugas (Delphinapterus leucas) are the only extant members of t... more Narwhals (Monodon monoceros) and belugas (Delphinapterus leucas) are the only extant members of the Monodontidae, and are charismatic Arctic-endemic cetaceans that are at risk from global change. Investigating the anatomy and sensory apparatuses of these animals is essential to understanding their ecology and evolution, and informs efforts for their conservation. Here, we use X-ray CT scans to compare aspects of the endocranial and inner ear labyrinth anatomy of extant monodontids and use the overall morphology to draw larger inferences about the relationship between morphology and ecology. We show that differences in the shape of the brain, vasculature, and neural canals of both species may relate to differences in diving and other behaviors. The cochleae are similar in morphology in the two species, signifying similar hearing ranges and a close evolutionary relationship. Lastly, we compare two different methods for calculating 90var – a calculation independent of body size that is increasingly being used as a proxy for habitat preference. We show that a ‘direct’ angular measurement method shows significant differences between Arctic and other habitat preferences, but angle measurements based on planes through the semicircular canals do not, emphasizing the need for more detailed study and standardization of this measurement. This work represents the first comparative internal anatomical study of the endocranium and inner ear labyrinths of this small clade of toothed whales.
X-ray computed tomography (CT) provides a nondestructive means of studying the inside and outside... more X-ray computed tomography (CT) provides a nondestructive means of studying the inside and outside of objects. It allows accurate visualization and measurement of internal features, that are otherwise impossible to obtain nondestructively, and is a lasting digital record that can be made available to future researchers, museums, and the general public. Here, an overview of CT scanning methodologies and protocol is provided, as well as some recent examples of how this technology is allowing paleontologists to make new inroads into understanding the ecology, evolution, and development of both extant and extinct organisms. Lastly, some frontiers and outstanding questions in the acquisition, processing, and storage of digital 3-D morphological data are highlighted.
The Pliocene fossil porpoise SDSNH 65276 has extremely elongate mandibular morphology, unlike tha... more The Pliocene fossil porpoise SDSNH 65276 has extremely elongate mandibular morphology, unlike that of any marine amniote, and is superficially most similar to the living bird species known as skimmers (Rynchops sp.). Endocasts of the pterygoid sinuses and endocranial cavity were digitally segmented from high-resolution X-ray CT scans of the specimen to explore internal anatomy of functionally and phylogenetically important anatomical features of this specimen and odontocetes in general. The sinuses are similar in volume and shape to extant porpoise species, but the dorsal extension of the preorbital lobes are particularly elongate as in the harbor porpoise (Phocoena phocoena). The cranial endocast also shows similarities with extant porpoises, but has much deeper interhemispheric fissures, which are filled by ossified meninges, particularly a deep falx cerebri and shallower tentorium cerebelli. Ossifications of these parts of the meninges may reflect faster angular accelerations of the head, deeper diving ability, or both. Penetrations of the endocranial cavity for cranial nerves and blood vessels are like those of extant porpoises. The internal skull morphology of this unique delphinoid sheds additional light both on its phylogenetic affinities and novel odontocete adaptations.
The evolution of hearing in cetaceans is a matter of current interest given that odontocetes (too... more The evolution of hearing in cetaceans is a matter of current interest given that odontocetes (toothed whales) are sensitive to high frequency sounds and mysticetes (baleen whales) are sensitive to low and potentially infrasonic noises. Earlier diverging stem cetaceans (archaeocetes) were hypothesized to have had either low or high frequency sensitivity. Through CT scanning, the morphology of the bony labyrinth of the basilosaurid archaeocete Zygorhiza kochii is described and compared to novel information from the inner ears of mysticetes, which are less known than the inner ears of odontocetes. Further comparisons are made with published information for other cetaceans. The anatomy of the cochlea of Zygorhiza is in line with mysticetes and supports the hypothesis that Zygorhiza was sensitive to low frequency noises. Morphological features that support the low frequency hypothesis and are shared by Zygorhiza and mysticetes include a long cochlear canal with a high number of turns, st...
Establishing how Ediacaran organisms moved and fed is critical to deciphering their ecological an... more Establishing how Ediacaran organisms moved and fed is critical to deciphering their ecological and evolutionary significance, but has long been confounded by their non-analogue body plans. Here, we use computational fluid dynamics to quantitatively analyse water flow around the Ediacaran taxon Parvancorina, thereby testing between competing models for feeding mode and mobility. The results show that flow was not distributed evenly across the organism, but was directed towards localized areas; this allows us to reject osmotrophy, and instead supports either suspension feeding or detritivory. Moreover, the patterns of recirculating flow differ substantially with orientation to the current, suggesting that if Parvancorina was a suspension feeder, it would have been most efficient if it was able to re-orient itself with respect to current direction, and thus ensure flow was directed towards feeding structures. Our simulations also demonstrate that the amount of drag varied with orientat...
The disappearance of the soft-bodied Ediacara biota at the Ediacaran–Cambrian boundary potentiall... more The disappearance of the soft-bodied Ediacara biota at the Ediacaran–Cambrian boundary potentially represents the earliest mass extinction of complex life, although the precise driver(s) of this extinction remain unresolved. The ‘biotic replacement’ model proposes that an evolutionary radiation of metazoan ecosystem engineers in the latest Ediacaran profoundly altered marine palaeoenvironments, resulting in the extinction of Ediacara biota and setting the stage for the subsequent Cambrian Explosion. However, metazoan ecosystem engineering across the Ediacaran–Cambrian transition has yet to be quantified. Here, we test this key tenet of the biotic replacement model by characterizing the intensity of metazoan bioturbation and ecosystem engineering in trace fossil assemblages throughout the latest Ediacaran Nama Group in southern Namibia. The results illustrate a dramatic increase in both bioturbation and ecosystem engineering intensity in the latest Ediacaran, prior to the Cambrian bo...
Reconstructing Precambrian eukaryotic paleoecology is pivotal to understanding the origins of the... more Reconstructing Precambrian eukaryotic paleoecology is pivotal to understanding the origins of the modern, animal-dominated biosphere. Here, we combine new fossil data from southern Namibia with computational fluid dynamics (CFD) to test between competing feeding models for the Ediacaran taxon Ernietta. In addition, we perform simulations for multiple individuals, allowing us to analyze hydrodynamics of living communities. We show that Ernietta lived gregariously, forming shallow marine aggregations in the latest Ediacaran, 548 to 541 million years (Ma) ago. We demonstrate enhanced vertical mixing of the water column above aggregations and preferential redirection of current into body cavities of downstream individuals. These results support the reconstruction of Ernietta as a macroscopic suspension feeder and also provide a convincing paleoecological advantage to feeding in aggregations analogous to those recognized in many extant marine metazoans. These results provide some of the ...
Toothed whales (Cetacea: Odontoceti) are the most diverse group of modern cetaceans, originating ... more Toothed whales (Cetacea: Odontoceti) are the most diverse group of modern cetaceans, originating during the Eocene/Oligocene transition approximately 38 Ma. All extant odontocetes echolocate; a single origin for this behaviour is supported by a unique facial source for ultrasonic vocalizations and a cochlea adapted for hearing the corresponding echoes. The craniofacial and inner ear morphology of Oligocene odontocetes support a rapid (less than 5 Myr) early evolution of echolocation. Although some cranial features in the stem odontocetes Simocetus and Olympicetus suggest an ability to generate ultrasonic sound, until now, the bony labyrinths of taxa of this grade have not been investigated. Here, we use µCT to examine a petrosal of a taxon with clear similarities to Olympicetus avitus . Measurements of the bony labyrinth, when added to an extensive dataset of cetartiodactyls, resulted in this specimen sharing a morphospace with stem whales, suggesting a transitional inner ear. This ...
Narwhals (Monodon monoceros) and belugas (Delphinapterus leucas) are the only extant members of t... more Narwhals (Monodon monoceros) and belugas (Delphinapterus leucas) are the only extant members of the Monodontidae, and are charismatic Arctic-endemic cetaceans that are at risk from global change. Investigating the anatomy and sensory apparatuses of these animals is essential to understanding their ecology and evolution, and informs efforts for their conservation. Here, we use X-ray CT scans to compare aspects of the endocranial and inner ear labyrinth anatomy of extant monodontids and use the overall morphology to draw larger inferences about the relationship between morphology and ecology. We show that differences in the shape of the brain, vasculature, and neural canals of both species may relate to differences in diving and other behaviors. The cochleae are similar in morphology in the two species, signifying similar hearing ranges and a close evolutionary relationship. Lastly, we compare two different methods for calculating 90var – a calculation independent of body size that is increasingly being used as a proxy for habitat preference. We show that a ‘direct’ angular measurement method shows significant differences between Arctic and other habitat preferences, but angle measurements based on planes through the semicircular canals do not, emphasizing the need for more detailed study and standardization of this measurement. This work represents the first comparative internal anatomical study of the endocranium and inner ear labyrinths of this small clade of toothed whales.
X-ray computed tomography (CT) provides a nondestructive means of studying the inside and outside... more X-ray computed tomography (CT) provides a nondestructive means of studying the inside and outside of objects. It allows accurate visualization and measurement of internal features, that are otherwise impossible to obtain nondestructively, and is a lasting digital record that can be made available to future researchers, museums, and the general public. Here, an overview of CT scanning methodologies and protocol is provided, as well as some recent examples of how this technology is allowing paleontologists to make new inroads into understanding the ecology, evolution, and development of both extant and extinct organisms. Lastly, some frontiers and outstanding questions in the acquisition, processing, and storage of digital 3-D morphological data are highlighted.
The Pliocene fossil porpoise SDSNH 65276 has extremely elongate mandibular morphology, unlike tha... more The Pliocene fossil porpoise SDSNH 65276 has extremely elongate mandibular morphology, unlike that of any marine amniote, and is superficially most similar to the living bird species known as skimmers (Rynchops sp.). Endocasts of the pterygoid sinuses and endocranial cavity were digitally segmented from high-resolution X-ray CT scans of the specimen to explore internal anatomy of functionally and phylogenetically important anatomical features of this specimen and odontocetes in general. The sinuses are similar in volume and shape to extant porpoise species, but the dorsal extension of the preorbital lobes are particularly elongate as in the harbor porpoise (Phocoena phocoena). The cranial endocast also shows similarities with extant porpoises, but has much deeper interhemispheric fissures, which are filled by ossified meninges, particularly a deep falx cerebri and shallower tentorium cerebelli. Ossifications of these parts of the meninges may reflect faster angular accelerations of the head, deeper diving ability, or both. Penetrations of the endocranial cavity for cranial nerves and blood vessels are like those of extant porpoises. The internal skull morphology of this unique delphinoid sheds additional light both on its phylogenetic affinities and novel odontocete adaptations.
The evolution of hearing in cetaceans is a matter of current interest given that odontocetes (too... more The evolution of hearing in cetaceans is a matter of current interest given that odontocetes (toothed whales) are sensitive to high frequency sounds and mysticetes (baleen whales) are sensitive to low and potentially infrasonic noises. Earlier diverging stem cetaceans (archaeocetes) were hypothesized to have had either low or high frequency sensitivity. Through CT scanning, the morphology of the bony labyrinth of the basilosaurid archaeocete Zygorhiza kochii is described and compared to novel information from the inner ears of mysticetes, which are less known than the inner ears of odontocetes. Further comparisons are made with published information for other cetaceans. The anatomy of the cochlea of Zygorhiza is in line with mysticetes and supports the hypothesis that Zygorhiza was sensitive to low frequency noises. Morphological features that support the low frequency hypothesis and are shared by Zygorhiza and mysticetes include a long cochlear canal with a high number of turns, st...
Establishing how Ediacaran organisms moved and fed is critical to deciphering their ecological an... more Establishing how Ediacaran organisms moved and fed is critical to deciphering their ecological and evolutionary significance, but has long been confounded by their non-analogue body plans. Here, we use computational fluid dynamics to quantitatively analyse water flow around the Ediacaran taxon Parvancorina, thereby testing between competing models for feeding mode and mobility. The results show that flow was not distributed evenly across the organism, but was directed towards localized areas; this allows us to reject osmotrophy, and instead supports either suspension feeding or detritivory. Moreover, the patterns of recirculating flow differ substantially with orientation to the current, suggesting that if Parvancorina was a suspension feeder, it would have been most efficient if it was able to re-orient itself with respect to current direction, and thus ensure flow was directed towards feeding structures. Our simulations also demonstrate that the amount of drag varied with orientat...
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Papers by Rachel Racicot