Stephanie E. Pierce

Patterns of morphospace occupation and mechanical performance in extant crocodilian skulls: A combined geometric morphometric and finite element modeling approach

Extant and fossil crocodilians have long been divided into taxonomic and/or ecological groups based on broad patterns of skull shape, particularly the relative length and width of the snout. However, these patterns have not been quantitatively analyzed in detail, and their biomechanical and functional implications are similarly understudied. Here, we use geometric morphometrics and finite element analysis to explore the patterns of variation in crocodilian skull morphology and the functional implications of those patterns. Our results indicate that skull shape variation in extant crocodiles is much more complex than previously recognized. Differences in snout length and width are the main components of shape variation, but these differences are correlated with changes in other regions of the skull. Additionally, there is considerable disparity within general classes such as longirostrine and brevirostrine forms. For example, Gavialis and Tomistoma occupy different parts of morphospace implying a significant difference in skull shape, despite the fact that both are traditionally considered longirostrine. Skull length and width also strongly influence the mechanical performance of the skull; long and narrow morphotypes (e.g., Tomistoma) experience the highest amount of stress during biting, whereas short and broad morphotypes (e.g., Caiman latirostris) experience the least amount of stress. Biomechanical stress and the hydrodynamic properties of the skull show a strong relationship with the distribution of crocodilians in skull morphospace, whereas phylogeny and biogeography show weak or no correlation. Therefore, ecological specializations related to feeding and foraging likely have the greatest influence on crocodilian skull shape. J. Morphol., 2008.

Three-dimensional limb joint mobility in the early tetrapod Ichthyostega

The origin of tetrapods and the transition from swimming to walking was a pivotal step in the evolution and diversification of terrestrial vertebrates. During this time, modifications of the limbs—particularly the specialization of joints and the structures that guide their motions—fundamentally changed the ways in which early tetrapods could move. Nonetheless, little is known about the functional consequences of limb anatomy in early tetrapods and how that anatomy influenced locomotion capabilities at this very critical stage in vertebrate evolution. Here we present a three-dimensional reconstruction of the iconic Devonian tetrapod Ichthyostega and a quantitative and comparative analysis of limb mobility in this early tetrapod. We show that Ichthyostega could not have employed typical tetrapod locomotory behaviours, such as lateral sequence walking. In particular, it lacked the necessary rotary motions in its limbs to push the body off the ground and move the limbs in an alternating sequence. Given that long-axis rotation was present in the fins of tetrapodomorph fishes, it seems that either early tetrapods evolved through an initial stage of restricted shoulder and hip joint mobility or that Ichthyostega was unique in this respect. We conclude that early tetrapods with the skeletal morphology and limb mobility of Ichthyostega were unlikely to have made some of the recently described Middle Devonian trackways.

Shape and mechanics in thalattosuchian (Crocodylomorpha) skulls: implications for feeding behaviour and niche partitioning

Variation in modern crocodilian and extinct thalattosuchian crocodylomorph skull morphology is only weakly correlated with phylogeny, implying that factors other than evolutionary proximity play important roles in determining crocodile skull shape. To further explore factors potentially influencing morphological differentiation within the Thalattosuchia, we examine teleosaurid and metriorhynchid skull shape variation within a mechanical and dietary context using a combination of finite element modelling and multivariate statistics. Patterns of stress distribution through the skull were found to be very similar in teleosaurid and metriorhynchid species, with stress peaking at the posterior constriction of the snout and around the enlarged supratemporal fenestrae. However, the magnitudes of stresses differ, with metriorhynchids having generally stronger skulls. As with modern crocodilians, a strong linear relationship between skull length and skull strength exists, with short‐snouted morphotypes experiencing less stress through the skull than long‐snouted morphotypes under equivalent loads. Selection on snout shape related to dietary preference was found to work in orthogonal directions in the two families: diet is associated with snout length in teleosaurids and with snout width in metriorhynchids, suggesting that teleosaurid skulls were adapted for speed of attack and metriorhynchid skulls for force production. Evidence also indicates that morphological and functional differentiation of the skull occurred as a result of dietary preference, allowing closely related sympatric species to exploit a limited environment. Comparisons of the mechanical performance of the thalattosuchian skull with extant crocodilians show that teleosaurids and long‐snouted metriorhynchids exhibit stress magnitudes similar to or greater than those of long‐snouted modern forms, whereas short‐snouted metriorhynchids display stress magnitudes converging on those found in short‐snouted modern species. As a result, teleosaurids and long‐snouted metriorhynchids were probably restricted to lateral attacks of the head and neck, but short‐snouted metriorhynchids may have been able to employ the grasp and shake and/or ‘death roll’ feeding and foraging behaviours.

Comparative axial morphology in pinnipeds and its correlation with aquatic locomotory behaviour

Regional variation in the axial skeleton of pinnipeds (seals and walruses) and its correlation with aquatic locomotory behaviour is examined using vertebral functional profiles. The results demonstrate clear morpho‐functional differences in the thoracolumbar region of modern pinnipeds (Phocidae, Otariidae, Odobenus) that can be strongly linked to swimming style. Phocid seals have a rigid thoracic region attached to a highly flexible lumbar region with long muscular lever arms providing the necessary mobility and leverage to perform pelvic oscillations. Conversely, otariid seals have extremely flexible inter‐vertebral joints along the length of the column which should enhance manoeuvrability and turning performance. They also have greater muscular leverage in the anterior thoracic region to support pectoral oscillations. Odobenus (walrus) shows vertebral characteristics most similar to phocids, but with some otariid qualities, consistent with an intermediate or mixed form of aquatic locomotion, with pelvic oscillation dominating over pectoral oscillation. Comparison of the vertebral functional profiles in the fossil taxon Allodesmus kernensis with those of modern pinniped clades reveals that this extinct pinniped may also have used a combination of pectoral and pelvic oscillatory movements during swimming, but in a manner opposite to that of Odobenus, with pectoral oscillatory movements dominating. This study raises questions about the evolution and diversification of pinniped locomotory behaviours, but also provides the necessary framework to begin to examine axial mechanics and locomotory stages in other fossil pinnipedimorphs and their relatives in more detail.

Morphological and biomechanical disparity of crocodile-line archosaurs following the end-Triassic extinction.

Mesozoic crurotarsans exhibited diverse morphologies and feeding modes, representing considerable ecological diversity, yet macroevolutionary patterns remain unexplored. Here, we use a unique combination of morphological and biomechanical disparity metrics to quantify the ecological diversity and trophic radiations of Mesozoic crurotarsans, using the mandible as a morpho-functional proxy. We recover three major trends. First, the diverse assemblage of Late Triassic crurotarsans was morphologically and biomechanically disparate, implying high levels of ecological variation; but, following the end-Triassic extinction, disparity declined. Second, the Jurassic radiation of marine thalattosuchians resulted in very low morphological disparity but moderate variation in jaw biomechanics, highlighting a hydrodynamic constraint on mandibular form. Third, during the Cretaceous terrestrial radiations of neosuchians and notosuchians, mandibular morphological variation increased considerably. By the Late Cretaceous, crocodylomorphs evolved a range of morphologies equalling Late Triassic crurotarsans. By contrast, biomechanical disparity in the Cretaceous did not increase, essentially decoupling from morphology. This enigmatic result could be attributed to biomechanical evolution in other anatomical regions (e.g. cranium, dentition or postcranium), possibly releasing the mandible from selective pressures. Overall, our analyses reveal a complex relationship between morphological and biomechanical disparity in Mesozoic crurotarsans that culminated in specialized feeding ecologies and associated lifestyles.

Historical Perspectives on the Evolution of Tetrapodomorph Movement

Over the past century, various modern analogs have been used to infer the evolution of locomotor performance in stem tetrapods and their fish ancestors, with varying success. Here, we conduct a phylogenetic review of these modern analogs, from chondrichthyans to mammals, highlighting the broad spectrum of vertebrate clades and locomotor behaviors. The pros and cons behind utilizing modern analogs for the early stages of the transition from water to land also are discussed. In particular, it is noted that any hypothesis about locomotion not only must be supported by evidence from living animals but must also be consistent with character transformations in the fossil record. A total-evidence approach that emphasizes what extinct taxa could not do, rather than focusing on the specifics of how they functioned, is thus recommended. An example of this approach, which investigates mobility of the limb joints in modern semi-aquatic animals and in the Devonian stem tetrapod Ichthyostega, is detailed. We propose that various locomotion behaviors of modern quadrupeds can be ruled out for Ichthyostega, but that forelimb crutching motions, as seen in living mudskippers, may have been possible. The potential for movement in other known Devonian stem tetrapods is assessed through an anatomical comparison of limb joint morphology-and associated mobility-with Ichthyostega, and deemed to have been quite similar.

Vertebral architecture in the earliest stem tetrapods

The construction of the vertebral column has been used as a key anatomical character in defining and diagnosing early tetrapod groups. Rhachitomous vertebrae—in which there is a dorsally placed neural arch and spine, an anteroventrally placed intercentrum and paired, posterodorsally placed pleurocentra—have long been considered the ancestral morphology for tetrapods. Nonetheless, very little is known about vertebral anatomy in the earliest stem tetrapods, because most specimens remain trapped in surrounding matrix, obscuring important anatomical features. Here we describe the three-dimensional vertebral architecture of the Late Devonian stem tetrapod Ichthyostega using propagation phase-contrast X-ray synchrotron microtomography. Our scans reveal a diverse array of new morphological, and associated developmental and functional, characteristics, including a possible posterior-to-anterior vertebral ossification sequence and the first evolutionary appearance of ossified sternal elements. One of the most intriguing features relates to the positional relationships between the vertebral elements, with the pleurocentra being unexpectedly sutured or fused to the intercentra that directly succeed them, indicating a ‘reverse’ rhachitomous design. Comparison of Ichthyostega with two other stem tetrapods, Acanthostega and Pederpes, shows that reverse rhachitomous vertebrae may be the ancestral condition for limbed vertebrates. This study fundamentally revises our current understanding of vertebral column evolution in the earliest tetrapods and raises questions about the presumed vertebral architecture of tetrapodomorph fish and later, more crownward, tetrapods.

An experimental and morphometric test of the relationship between vertebral morphology and joint stiffness in Nile crocodiles (Crocodylus niloticus)

Despite their semi-aquatic mode of life, modern crocodylians use a wide range of terrestrial locomotor behaviours, including asymmetrical gaits otherwise only found in mammals. The key to these diverse abilities may lie in the axial skeleton. Correlations between vertebral morphology and both intervertebral joint stiffness and locomotor behaviour have been found in other animals, but the vertebral mechanics of crocodylians have not yet been experimentally and quantitatively tested. We measured the passive mechanics and morphology of the thoracolumbar vertebral column in Crocodylus niloticus in order to validate a method to infer intervertebral joint stiffness based on morphology. Passive stiffness of eight thoracic and lumbar joints was tested in dorsal extension, ventral flexion and mediolateral flexion using cadaveric specimens. Fifteen measurements that we deemed to be potential correlates of stiffness were taken from each vertebra and statistically tested for correlation with joint stiffness. We found that the vertebral column of C. niloticus is stiffer in dorsoventral flexion than in lateral flexion and, in contrast to that of many mammals, shows an increase in joint stiffness in the lumbar region. Our findings suggest that the role of the axial column in crocodylian locomotion may be functionally different from that in mammals, even during analogous gaits. A moderate proportion of variation in joint stiffness (R2=0.279–0.520) was predicted by centrum width and height, neural spine angle and lamina width. These results support the possible utility of some vertebral morphometrics in predicting mechanical properties of the vertebral column in crocodiles, which also should be useful for forming functional hypotheses of axial motion during locomotion in extinct archosaurs.

The Importance of Cultivating a Preference for Complexity in Veterinarians for Effective Lifelong Learning

Much attention has been paid to the link between students approaches to study and the quality of their learning. Less attention has been paid to the lifelong learner. We conceptualized a tripartite relationship between three measures of learning preference: conceptions of knowledge (construction and use vs. intake), need for cognition (high vs. low), and approach to study (deep vs. surface) and hypothesized that an individuals profile on these three measures-reconceptualized as a preference for complexity versus simplicity-would affect their attitude toward continuing professional development (CPD). A questionnaire was mailed to 2,000 randomly selected, home-practicing UK veterinarians to quantify their learning preferences, motivation to engage in CPD, and perception of barriers to participation and to assess the relationships between these constructs. Analysis of 775 responses (a 38.8% response rate) confirmed our tripartite model of learning and showed that a preference for complexity was negatively correlated with barriers and positively correlated with intrinsic, social, and extrinsic motivating factors, suggesting that all play a role in the continuing education of this group of professionals. A preference for simplicity was negatively correlated with social motivation and positively correlated with barriers. This study demonstrates that approach not only affects the quality of learning but crucially affects motivation to engage in CPD and perception of barriers to lifelong learning. This should emphasize to veterinary educators the importance of fostering a preference for complexity from an early age, both in terms of its immediate benefits (better understanding) and longer-term benefits (continued engagement with learning).

Fostering Integrated Learning and Clinical Professionalism Using Contextualized Simulation in a Small-Group Role-Play

Teaching and learning in a clinical setting is important in veterinary and medical training but presents many challenges, including providing enough hands-on experience while not putting patients (animal or human) at risk. Some of the issues have been addressed with the introduction of clinical skills laboratories and communication skills training using role play. However, in both instances skills are learned in isolation, whereas the real task requires the integration of many skills including technical competencies, effective communication, decision making, and professionalism. In our study, we trialed contextualized simulation by combining role play with a simulator, the haptic cow, in a small-group tutorial, the Simulated Fertility Visit. Students took turns as the veterinarian; they had to establish the cows history from the farmer (a role player), palpate the simulation, make a diagnosis, and decide on treatment, if appropriate. We included scenarios varying from common cases to challenging situations. The tutorial was introduced in the farm-animal clinical rotation, and feedback was gathered from students by means of a questionnaire. The tutorial was attended by 178 students (98% of that years students), and 151 questionnaires were returned (85% response rate). Students reported that the tutorial was a positive learning experience and recognized that it presented an opportunity to integrate the skills needed for clinical work. Student feedback suggests that contextualized simulation provides a valuable complement to clinical cases, and we recommend extending this teaching method to other clinical scenarios and species, particularly because it provides a safe environment in which to experience, and learn from, mistakes.