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Casey Holliday

University of Missouri Columbia, Integrative Anatomy, Faculty Member

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I am a vertebrate anatomist and paleontologist interested in the evolution of form and function. I received my B.S. in Zoology from the University of Florida and my Ph.D. in Ecology and Evolutionary Biology from Ohio University in 2006.

I employ a systems-based approach to understand the evolution of the feeding apparatus and locomotor system in lizards, crocodilians, birds, and their stem-group, fossil relatives including dinosaurs and croc ancestors. My research utilizes dissection, imaging, histology, and biomechanical modeling among other techniques. Ongoing projects include the evolution of jaw muscles an the temporal region in reptiles, biomechanics and evolution of cranial joints and sutures, and cartilage physiology.

I am significantly involved in medical education where I teach Clinical Anatomy to medical students and facilitate surgical anatomy training for senior medical students and residents. I train graduate students and undergradruates in topics in vertebrate anatomy and evolution.

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Appalachian State University

UNSL Universidad Nacional de San Luis

Maria Belen von Baczko

Consejo Nacional de Ciencia y Tecnología

Fundação Zoobotânica do RS

María Trotteyn

Ricardo N. Martínez

Universidad Nacional de San Juan

Sterling Nesbitt

Virginia Tech

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Papers by Casey Holliday

Morphology and diversity of the mandibular symphysis of archosauriforms

by Casey Holliday and Sterling Nesbitt

Geological Society, London, Special Publications, 2013

Trigeminal Nerve Morphology in Alligator mississippiensis and Its Significance for Crocodyliform Facial Sensation and Evolution

Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-inn... more Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-innervated dome pressure receptors speckle the face and mandible and sense mechanical stimuli. However, the morphological features of this system are not well known, and it remains unclear how the trigeminal system changes during ontogeny and how it scales with other cranial structures. Finally, when this system evolved within crocodyliforms remains a mystery. Thus, new morphological insights into the trigeminal system of extant crocodylians may offer new paleontological tools to investigate this evolutionary transformation. A cross-sectional study integrating histological, morphometric, and 3D imaging analyses was conducted to identify patterns in cranial nervous and bony structures of Alligator mississippiensis. Nine individuals from a broad size range were CT-scanned followed by histomorphometric sampling of mandibular and maxillary nerve divisions of the trigeminal nerve. Endocast volume, trigeminal fossa volume, and maxillomandibular foramen size were compared with axon counts from proximal and distal regions of the trigeminal nerves to identify scaling properties of the structures. The trigeminal fossa has a significant positive correlation with skull length and endocast volume. We also found that axon density is greater in smaller alligators and total axon count has a significant negative correlation with skull size. Six additional extant and fossil crocodyliforms were included in a supplementary scaling analysis, which found that size was not an accurate predictor of trigeminal anatomy. This suggests that phylogeny or somatosensory adaptations may be responsible for the variation in trigeminal ganglion and nerve size in crocodyliforms. Anat Rec, 296:670–680, 2013. © 2013 Wiley Periodicals, Inc.

Cranial biomechanics of Diplodocus (Dinosauria, Sauropoda): testing hypotheses of feeding behaviour in an extinct megaherbivore

Sauropod dinosaurs were the largest terrestrial herbivores and pushed at the limits of vertebrate... more Sauropod dinosaurs were the largest terrestrial herbivores and pushed at the limits of vertebrate biomechanics and physiology. Sauropods exhibit high craniodental diversity in ecosystems where numerous species coexisted, leading to the hypothesis that this biodiversity is linked to niche subdivision driven by ecological specialisation. Here, we quantitatively investigate feeding behaviour hypotheses for the iconic sauropod Diplodocus. Biomechanical modelling, using finite element analysis, was used to examine the performance of the Diplodocus skull. Three feeding behaviours were modelled: muscle-driven static biting, branch stripping and bark stripping. The skull was found to be 'over engineered' for static biting, overall experiencing low stress with only the dentition enduring high stress. When branch stripping, the skull, similarly, is under low stress, with little appreciable difference between those models. When simulated for bark stripping, the skull experiences far greater stresses, especially in the teeth and at the jaw joint. Therefore, we refute the bark-stripping hypothesis, while the hypotheses of branch stripping and/or precision biting are both consistent with our findings, showing that branch stripping is a biomechanically plausible feeding behaviour for diplodocids. Interestingly, in all simulations, peak stress is observed in the premaxillary-maxillary 'lateral plates', supporting the hypothesis that these structures evolved to dissipate stress induced while feeding. These results lead us to conclude that the aberrant craniodental form of Diplodocus was adapted for food procurement rather than resisting high bite forces.

Morphology and diversity of the mandibular symphysis of archosauriforms

Trigeminal Nerve Morphology in Alligator mississippiensis and Its Significance for Crocodyliform Facial Sensation and Evolution

Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-inn... more Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-innervated dome pressure receptors speckle the face and mandible and sense mechanical stimuli. However, the morphological features of this system are not well known, and it remains unclear how the trigeminal system changes during ontogeny and how it scales with other cranial structures. Finally, when this system evolved within crocodyliforms remains a mystery. Thus, new morphological insights into the trigeminal system of extant crocodylians may offer new paleontological tools to investigate this evolutionary transformation. A cross-sectional study integrating histological, morphometric, and 3D imaging analyses was conducted to identify patterns in cranial nervous and bony structures of Alligator mississippiensis. Nine individuals from a broad size range were CT-scanned followed by histomorphometric sampling of mandibular and maxillary nerve divisions of the trigeminal nerve. Endocast volume, trigeminal fossa volume, and maxillomandibular foramen size were compared with axon counts from proximal and distal regions of the trigeminal nerves to identify scaling properties of the structures. The trigeminal fossa has a significant positive correlation with skull length and endocast volume. We also found that axon density is greater in smaller alligators and total axon count has a significant negative correlation with skull size. Six additional extant and fossil crocodyliforms were included in a supplementary scaling analysis, which found that size was not an accurate predictor of trigeminal anatomy. This suggests that phylogeny or somatosensory adaptations may be responsible for the variation in trigeminal ganglion and nerve size in crocodyliforms. Anat Rec, 296:670–680, 2013. © 2013 Wiley Periodicals, Inc.

Ontogeny of the Alligator Cartilago Transiliens and Its Significance for Sauropsid Jaw Muscle Evolution

PLoS ONE, Jan 1, 2011

The cartilago transiliens is a fibrocartilaginous structure within the jaw muscles of crocodylian... more The cartilago transiliens is a fibrocartilaginous structure within the jaw muscles of crocodylians. The cartilago transiliens slides between the pterygoid buttress and coronoid region of the lower jaw and connects two muscles historically identified as m. pseudotemporalis superficialis and m. intramandibularis. However, the position of cartilago transiliens, and its anatomical similarities to tendon organs suggest the structure may be a sesamoid linking a single muscle. Incompressible sesamoids often form inside tendons that wrap around bone. However, such structures rarely ossify in reptiles and have thus far received scant attention. We tested the hypothesis that the cartilago transiliens is a sesamoid developed within in one muscle by investigating its structure in an ontogenetic series of Alligator mississippiensis using dissection, 3D imaging, and polarizing and standard light microscopy. In all animals studied, the cartilago transiliens receives collagen fibers and tendon insertions from its two main muscular attachments. However, whereas collagen fibers were continuous within the cartilaginous nodule of younger animals, such continuity decreased in older animals, where the fibrocartilaginous core grew to displace the fibrous region. Whereas several neighboring muscles attached to the fibrous capsule in older individuals, only two muscles had significant contributions to the structure in young animals. Our results indicate that the cartilago transiliens is likely a sesamoid formed within a single muscle (i.e., m. pseudotemporalis superficialis) as it wraps around the pterygoid buttress. This tendon organ is ubiquitous among fossil crocodyliforms indicating it is a relatively ancient, conserved structure associated with the development of the large pterygoid flanges in this clade. Finally, these findings indicate that similar tendon organs exist among potentially homologous muscle groups in birds and turtles, thus impacting inferences of jaw muscle homology and evolution in sauropsids in general.

The effects of the organopollutant PCB 126 on bone density in juvenile Diamondback terrapins (< i> Malaclemys terrapin</i>)

Aquatic Toxicology, Jan 1, 2011

Bone is a dynamic tissue with diverse functions including growth, structural support, pH balance ... more Bone is a dynamic tissue with diverse functions including growth, structural support, pH balance and reproduction. These functions may be compromised in the presence of organopollutants that can alter bone properties. We exposed juvenile diamondback terrapins (Malaclemys terrapin) to 3,3 ,4,4 ,5pentachlorobiphenyl (PCB 126), a ubiquitous anthropogenic organochlorine, and measured organic content, apparent bone mineral density (aBMD) using radiography and computed tomography, and quantified bone microstructure using histological preparations of femora. PCB-exposed terrapins were smaller in total size. Skulls of exposed animals had a higher organic content and a skeletal phenotype more typical of younger animals. The femora of exposed individuals had significantly reduced aBMD and significantly more cortical area occupied by non-bone. Because bone is an integral component of physiology, the observed skeletal changes can have far-reaching impacts on feeding and locomotor performance, calcium reserves and ultimately life history traits and reproductive success. Additionally, we caution that measurements of bone morphology, density, and composition from field-collected animals need to account not only for relatedness and age, but also environmental pollutants.

Archosaur adductor chamber evolution: integration of musculoskeletal and topological criteria in jaw muscle homology

Journal of Morphology, Jan 1, 2007

Cranial kinesis in dinosaurs: intracranial joints, protractor muscles, and their significance for cranial evolution and function in diapsids

Journal of Vertebrate Paleontology, Jan 1, 2008

New insights into dinosaur jaw muscle anatomy

… Record: Advances in Integrative Anatomy and …, Jan 1, 2009

Hydrodynamic performance of the minke whale (Balaenoptera acutorostrata) flipper

Journal of …, Jan 1, 2008

Evolution and function of the jaw musculature and adductor chamber of archosaurs (crocodilians, dinosaurs, and birds)

UMI, ProQuest ® Dissertations &amp; Theses. The world&#x27;s most comprehensive collectio... more

Cephalic vascular anatomy in flamingos (Phoenicopterus ruber) based on novel vascular injection and computed tomographic imaging analyses

… Record Part A: …, Jan 1, 2006

A three-dimensional animation model of Edmontosaurus (Hadrosauridae) for testing chewing hypotheses

Palaeontologia …, Jan 1, 2008

The epipterygoid of crocodyliforms and its significance for the evolution of the orbitotemporal region of eusuchians

Journal of Vertebrate Paleontology, Jan 1, 2009

Cartilaginous epiphyses in extant archosaurs and their implications for reconstructing limb function in dinosaurs

PloS one, Jan 1, 2010

Microanatomy of the Mandibular Symphysis in Lizards: Patterns in Fiber Orientation and Meckel's Cartilage and Their Significance in Cranial Evolution

The Anatomical …, Jan 1, 2010

An Osteological and Histological Investigation of Cranial Joints in Geckos

The Anatomical Record: …

The impact of bone and suture material properties on mandibular function in Alligator mississippiensis: testing theoretical phenotypes with finite element analysis

Journal of …

Free body analysis, beam mechanics, and finite element modeling of the mandible of Alligator mississippiensis

Journal of …

Morphology and diversity of the mandibular symphysis of archosauriforms

by Casey Holliday and Sterling Nesbitt

Geological Society, London, Special Publications, 2013

Trigeminal Nerve Morphology in Alligator mississippiensis and Its Significance for Crocodyliform Facial Sensation and Evolution

Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-inn... more Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-innervated dome pressure receptors speckle the face and mandible and sense mechanical stimuli. However, the morphological features of this system are not well known, and it remains unclear how the trigeminal system changes during ontogeny and how it scales with other cranial structures. Finally, when this system evolved within crocodyliforms remains a mystery. Thus, new morphological insights into the trigeminal system of extant crocodylians may offer new paleontological tools to investigate this evolutionary transformation. A cross-sectional study integrating histological, morphometric, and 3D imaging analyses was conducted to identify patterns in cranial nervous and bony structures of Alligator mississippiensis. Nine individuals from a broad size range were CT-scanned followed by histomorphometric sampling of mandibular and maxillary nerve divisions of the trigeminal nerve. Endocast volume, trigeminal fossa volume, and maxillomandibular foramen size were compared with axon counts from proximal and distal regions of the trigeminal nerves to identify scaling properties of the structures. The trigeminal fossa has a significant positive correlation with skull length and endocast volume. We also found that axon density is greater in smaller alligators and total axon count has a significant negative correlation with skull size. Six additional extant and fossil crocodyliforms were included in a supplementary scaling analysis, which found that size was not an accurate predictor of trigeminal anatomy. This suggests that phylogeny or somatosensory adaptations may be responsible for the variation in trigeminal ganglion and nerve size in crocodyliforms. Anat Rec, 296:670–680, 2013. © 2013 Wiley Periodicals, Inc.

Cranial biomechanics of Diplodocus (Dinosauria, Sauropoda): testing hypotheses of feeding behaviour in an extinct megaherbivore

Sauropod dinosaurs were the largest terrestrial herbivores and pushed at the limits of vertebrate... more Sauropod dinosaurs were the largest terrestrial herbivores and pushed at the limits of vertebrate biomechanics and physiology. Sauropods exhibit high craniodental diversity in ecosystems where numerous species coexisted, leading to the hypothesis that this biodiversity is linked to niche subdivision driven by ecological specialisation. Here, we quantitatively investigate feeding behaviour hypotheses for the iconic sauropod Diplodocus. Biomechanical modelling, using finite element analysis, was used to examine the performance of the Diplodocus skull. Three feeding behaviours were modelled: muscle-driven static biting, branch stripping and bark stripping. The skull was found to be 'over engineered' for static biting, overall experiencing low stress with only the dentition enduring high stress. When branch stripping, the skull, similarly, is under low stress, with little appreciable difference between those models. When simulated for bark stripping, the skull experiences far greater stresses, especially in the teeth and at the jaw joint. Therefore, we refute the bark-stripping hypothesis, while the hypotheses of branch stripping and/or precision biting are both consistent with our findings, showing that branch stripping is a biomechanically plausible feeding behaviour for diplodocids. Interestingly, in all simulations, peak stress is observed in the premaxillary-maxillary 'lateral plates', supporting the hypothesis that these structures evolved to dissipate stress induced while feeding. These results lead us to conclude that the aberrant craniodental form of Diplodocus was adapted for food procurement rather than resisting high bite forces.

Morphology and diversity of the mandibular symphysis of archosauriforms

Trigeminal Nerve Morphology in Alligator mississippiensis and Its Significance for Crocodyliform Facial Sensation and Evolution

Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-inn... more Modern crocodylians possess a derived sense of face touch, in which numerous trigeminal nerve-innervated dome pressure receptors speckle the face and mandible and sense mechanical stimuli. However, the morphological features of this system are not well known, and it remains unclear how the trigeminal system changes during ontogeny and how it scales with other cranial structures. Finally, when this system evolved within crocodyliforms remains a mystery. Thus, new morphological insights into the trigeminal system of extant crocodylians may offer new paleontological tools to investigate this evolutionary transformation. A cross-sectional study integrating histological, morphometric, and 3D imaging analyses was conducted to identify patterns in cranial nervous and bony structures of Alligator mississippiensis. Nine individuals from a broad size range were CT-scanned followed by histomorphometric sampling of mandibular and maxillary nerve divisions of the trigeminal nerve. Endocast volume, trigeminal fossa volume, and maxillomandibular foramen size were compared with axon counts from proximal and distal regions of the trigeminal nerves to identify scaling properties of the structures. The trigeminal fossa has a significant positive correlation with skull length and endocast volume. We also found that axon density is greater in smaller alligators and total axon count has a significant negative correlation with skull size. Six additional extant and fossil crocodyliforms were included in a supplementary scaling analysis, which found that size was not an accurate predictor of trigeminal anatomy. This suggests that phylogeny or somatosensory adaptations may be responsible for the variation in trigeminal ganglion and nerve size in crocodyliforms. Anat Rec, 296:670–680, 2013. © 2013 Wiley Periodicals, Inc.

Ontogeny of the Alligator Cartilago Transiliens and Its Significance for Sauropsid Jaw Muscle Evolution

PLoS ONE, Jan 1, 2011

The cartilago transiliens is a fibrocartilaginous structure within the jaw muscles of crocodylian... more The cartilago transiliens is a fibrocartilaginous structure within the jaw muscles of crocodylians. The cartilago transiliens slides between the pterygoid buttress and coronoid region of the lower jaw and connects two muscles historically identified as m. pseudotemporalis superficialis and m. intramandibularis. However, the position of cartilago transiliens, and its anatomical similarities to tendon organs suggest the structure may be a sesamoid linking a single muscle. Incompressible sesamoids often form inside tendons that wrap around bone. However, such structures rarely ossify in reptiles and have thus far received scant attention. We tested the hypothesis that the cartilago transiliens is a sesamoid developed within in one muscle by investigating its structure in an ontogenetic series of Alligator mississippiensis using dissection, 3D imaging, and polarizing and standard light microscopy. In all animals studied, the cartilago transiliens receives collagen fibers and tendon insertions from its two main muscular attachments. However, whereas collagen fibers were continuous within the cartilaginous nodule of younger animals, such continuity decreased in older animals, where the fibrocartilaginous core grew to displace the fibrous region. Whereas several neighboring muscles attached to the fibrous capsule in older individuals, only two muscles had significant contributions to the structure in young animals. Our results indicate that the cartilago transiliens is likely a sesamoid formed within a single muscle (i.e., m. pseudotemporalis superficialis) as it wraps around the pterygoid buttress. This tendon organ is ubiquitous among fossil crocodyliforms indicating it is a relatively ancient, conserved structure associated with the development of the large pterygoid flanges in this clade. Finally, these findings indicate that similar tendon organs exist among potentially homologous muscle groups in birds and turtles, thus impacting inferences of jaw muscle homology and evolution in sauropsids in general.

The effects of the organopollutant PCB 126 on bone density in juvenile Diamondback terrapins (< i> Malaclemys terrapin</i>)

Aquatic Toxicology, Jan 1, 2011

Bone is a dynamic tissue with diverse functions including growth, structural support, pH balance ... more Bone is a dynamic tissue with diverse functions including growth, structural support, pH balance and reproduction. These functions may be compromised in the presence of organopollutants that can alter bone properties. We exposed juvenile diamondback terrapins (Malaclemys terrapin) to 3,3 ,4,4 ,5pentachlorobiphenyl (PCB 126), a ubiquitous anthropogenic organochlorine, and measured organic content, apparent bone mineral density (aBMD) using radiography and computed tomography, and quantified bone microstructure using histological preparations of femora. PCB-exposed terrapins were smaller in total size. Skulls of exposed animals had a higher organic content and a skeletal phenotype more typical of younger animals. The femora of exposed individuals had significantly reduced aBMD and significantly more cortical area occupied by non-bone. Because bone is an integral component of physiology, the observed skeletal changes can have far-reaching impacts on feeding and locomotor performance, calcium reserves and ultimately life history traits and reproductive success. Additionally, we caution that measurements of bone morphology, density, and composition from field-collected animals need to account not only for relatedness and age, but also environmental pollutants.

Archosaur adductor chamber evolution: integration of musculoskeletal and topological criteria in jaw muscle homology

Journal of Morphology, Jan 1, 2007

Cranial kinesis in dinosaurs: intracranial joints, protractor muscles, and their significance for cranial evolution and function in diapsids

Journal of Vertebrate Paleontology, Jan 1, 2008

New insights into dinosaur jaw muscle anatomy

… Record: Advances in Integrative Anatomy and …, Jan 1, 2009

Hydrodynamic performance of the minke whale (Balaenoptera acutorostrata) flipper

Journal of …, Jan 1, 2008

Evolution and function of the jaw musculature and adductor chamber of archosaurs (crocodilians, dinosaurs, and birds)

UMI, ProQuest ® Dissertations &amp; Theses. The world&#x27;s most comprehensive collectio... more

Cephalic vascular anatomy in flamingos (Phoenicopterus ruber) based on novel vascular injection and computed tomographic imaging analyses

… Record Part A: …, Jan 1, 2006

A three-dimensional animation model of Edmontosaurus (Hadrosauridae) for testing chewing hypotheses

Palaeontologia …, Jan 1, 2008

The epipterygoid of crocodyliforms and its significance for the evolution of the orbitotemporal region of eusuchians

Journal of Vertebrate Paleontology, Jan 1, 2009

Cartilaginous epiphyses in extant archosaurs and their implications for reconstructing limb function in dinosaurs

PloS one, Jan 1, 2010

Microanatomy of the Mandibular Symphysis in Lizards: Patterns in Fiber Orientation and Meckel's Cartilage and Their Significance in Cranial Evolution

The Anatomical …, Jan 1, 2010

An Osteological and Histological Investigation of Cranial Joints in Geckos

The Anatomical Record: …

The impact of bone and suture material properties on mandibular function in Alligator mississippiensis: testing theoretical phenotypes with finite element analysis

Journal of …

Free body analysis, beam mechanics, and finite element modeling of the mandible of Alligator mississippiensis

Journal of …