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Session Schedule & Abstracts

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Friday 1st July, 2016

Paleontology 5

Room: Salon F   2:30 pm–4:00 pm

Moderator(s): Lambertz M, Tsai HP
PAL5-1  2:30 pm  The cartilage cone of archosauromorphs: biomechanical implications for hip joint loading and femoral ossification. Tsai HP*, Brown University; Middleton KM, University of Missouri; Holliday CM, University of Missouri
Abstract: The cartilage cone is a convex extension of the epiphyseal hyaline cartilage that inserts into the metaphyseal growth plate of long bones. Among extant archosaurs, the cartilage cone results from delayed endochondral ossification relative to perichondral ossification during embryological development, and is absent in neonates. In contrast, the proximal femora of many post-neonatal fossil archosauromorphs possess evidence of uncalcified cartilage cones. This study investigated the evolutionary transitions, functional roles, and ontogenetic significance of the cartilage cone in archosauromorphs. Femora of 140 taxa were studied and digitized. Key phylogenetic transitions in cartilage morphology were estimated using likelihood ancestral state reconstruction on the osteological correlates, and analyzed using phylogenetically corrected correlation to reveal trends in body size evolution. The cartilage cone arose independently in multiple lineages, including dinosauriforms and paracrocodylomorphs, but was secondarily reduced in sauropods, theropods, crocodylomorphs, and phytosaurs. Although adult body size does not predict the presence of the cartilage cone, it is often absent in large adults but persist in locomotor patent conspecific juveniles. The cartilage cone likely provided mechanical support to the thick epiphyseal hyaline cartilage against tensile and shear strain by increasing metaphyseal surface contact. In sauropods and phytosaurs, reduction of the cone coincides with highly rugose growth plates; whereas reduction of the cone in theropods coincides with smooth growth plates. These divergent adaptations are hypothesized to associate with transitions in cartilage thickness and locomotor-induced loading regimes. Overall this study indicates that multiple lineages of basal archosauromorphs used uncalcified hyaline cartilage as load bearing tissues on par with subchondral bones, illustrating a key innovation in locomotor tissues.

PAL5-2  2:45 pm  Developmental trajectories of convergence, recapitulation, and evolutionary novelty in the crocodylian skull. Morris Z. S.*, Harvard University; Abzhanov A. , Imperial College London; Pierce S. E., Harvard University
Abstract: The crocodylian skull is a unique and complex structure that has been extensively used in phylogenetic and functional studies, and it has served as a model system for interpreting the paleobiology of extinct species. Despite this great interest, we still know very little about how the crocodylian skull evolved or how it is shaped through development. Here we present a geometric morphometric (GMM) analysis that integrates and explores the ontogeny of craniofacial development in extant crocodylian species. Our dataset includes embryonic series for nine species that represent all major adult cranial shapes found in extant crocodylians (i.e., generalized, slender-snouted, and blunt-snouted), as well as post-hatchling ontogenetic series for all extant crocodylian species. Our shape analysis finds that all embryonic specimens cluster together in a unique region of morphospace characterized by blunt rostra, even though adult alligatorids, crocodylids, and Gavialis occupy distinct regions. Slender-snouted crocodylids and Tomistoma have similar ontogenetic trajectories, suggesting a shared developmental pattern. However, the ontogenetic trajectory for Gavialis proceeds in the opposite direction. These results suggest the long, slender snout of Gavialis may have developed via a novel developmental pathway. In addition, analysis of the palate appears to “recapitulate” the evolutionary modification of the internal nares, with the opening moving posteriorly through development, and only later exiting through the pterygoid. Overall, our data suggest that ontogenetic changes in the crocodylian palate better reflect crocodylomorph evolution, while snout shape captures later, crown group, evolutionary divergence. We intend to test this further by incorporating fossil taxa, from across the crocodylomorph clade, into our shape morphospace.

PAL5-3  3:00 pm  Evolution of the flexible avian neck: insights from 3D cervical joint kinematics in wild turkeys. Kambic RE*, Harvard University; Biewener AA, Harvard University; Pierce SE, Harvard University
Abstract: Birds have extremely flexible necks, as demonstrated by the variety of elaborate poses they adopt during preening, feeding, and sleeping. Evolution of this flexible musculoskeletal system from non-avian theropod dinosaurs is not well-understood – particularly the acquired capacity to achieve complicated neck configurations. Historically, measuring inter-vertebral joint motion in the neck has been challenging, due to overlying skin, abundance of soft tissue, and the numerous serially repeating, inter-articulating bones. To overcome these obstacles, we used X-ray Reconstruction of Moving Morphology (XROMM) to measure 3D joint kinematics during simple (e.g. dorsoventral/lateral bending) and complex (e.g. multiple-axis twisting, looking over shoulder) neck maneuvers in cadaveric wild turkeys (Meleagris gallopavo), with soft tissues intact (e.g. skin, muscle, ligament). Our results reveal that inter-vertebral joint movements are not strongly regionalized, although the middle portion of the neck (C5-C6, C7-C8) appears to be somewhat less flexible than other regions. Furthermore, joints in the middle portion appear to work together, rotating similarly to achieve poses, while cranial and caudal joints behave more independently. We recorded torsion in joints caudal to the atlas-axis complex (C3-C4, C4-C5), indicating that heterocoelous centra do not always restrict axial rotations. We also discovered that zygapophyses maintain very little overlap during certain poses (e.g. dorsiflexion). Our use of XROMM to record 3D cervical joint kinematics is thus revealing a hidden world of complexity that has not been fully appreciated using previous methods. We aim to further our investigations of the evolution of the avian neck by integrating experimental and modeling approaches, including in vivo measurements of cervical joint function in living birds and musculoskeletal simulations of cervical joint motion in extinct non-avian theropod dinosaurs.

PAL5-4  3:15 pm  Paleobiology of caseids (Synapsida: Caseidae) and the functional morphology of their respiratory apparatus: implications for the evolutionary origin of the mammalian diaphragm. Lambertz M*, Sektion Herpetologie, Zoologisches Forschungsmuseum Alexander Koenig; Institut für Zoologie, Rheinische Friedrich-Wilhelms-Universität Bonn; Shelton CD, Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn; Spindler F, Institut für Geologie, Technische Universität Bergakademie Freiberg; Perry SF, Institut für Zoologie, Rheinische Friedrich-Wilhelms-Universität Bonn
Abstract: The origin of the diaphragm remains a poorly understood yet crucial step in the evolution of terrestrial vertebrates as this unique structure serves as the main respiratory motor for mammals. We analyzed the paleobiology and the respiratory apparatus of one of the oldest lineages of mammal-like reptiles: the Caseidae. Combining quantitative bone histology and functional morphological modeling approaches we conclude that an auxiliary ventilatory structure must have been present. Histology of the long bones revealed an extremely osteoporotic-like microstructure in the phylogenetically advanced caseids. This biomechanical impediment renders the traditional hypothesis that these taxa were primarily terrestrial functionally implausible. Putting the barrel-shaped, short-necked and tiny-headed animals into water furthermore solves the problems that were associated with their limited feeding envelope and their difficulty to drink. An aquatic lifestyle, on the other hand, resulted in severe constraints on the caseian ventilatory system, which consequently had to cope with diving-related problems. Our modeling of breathing parameters revealed that these caseids were capable of only limited costal breathing and must have employed some auxiliary ventilatory mechanism to quickly meet their oxygen demand upon surfacing. The bucket-handle articulation of the ribs allowed for a maximal vital capacity of only about 40% of total lung capacity, which is far from being physiologically plausible. Given caseids' phylogenetic position at the base of Synapsida, it is most parsimonious to assume that a homologue of the mammalian diaphragm was already present about 50 Ma earlier than previously assumed. This early origin is interpreted as an exaptation that allowed the large caseids to become aquatic and also paved the way for the evolution of the bronchioalveolar lung towards crown synapsids through the formation of a negative-pressure intracoelomic compartment: the pleural cavity.

PAL5-5  3:30 pm  Functional morphology of the pectoral girdle and forelimbs of a new burrowing cistecephalid dicynodont (Therapsida: Anomodontia). Lungmus J K*, University of Chicago; Angielczyk K D, Field Museum of Natural History
Abstract: Dicynodonts were one of the most abundant and successful clades of Permo-Triassic synapsids, and provide early examples of the evolution of new ecomorphologies. Cistecephalid dicynodonts were specialized scratch diggers, with mole-like forelimbs and highly modified box-like skulls. Fieldwork conducted in the Zambezi Basin of Zambia uncovered a new partially articulated Permian cistecephalid (NHCC LB366). Despite its likely earlier stratigraphic occurrence than other cistecephalids, the specimen shows markedly derived characters, including reduced orbits and a flared snout. Micro-CT scan data reveal that the specimen includes the whole pectoral girdle, complete humeri, and the proximal ends of both ulnae. Here we present a description of this new specimen along with biomechanical analyses, emphasizing the forelimbs and associated soft tissue anatomy as it pertains to fossoriality. Although the forelimbs differ in some details from later cistecephalids it is clear that they were still optimized for very powerful movements. Specifically, the elbow joint possesses an enlarged olecranon, and the humerus shows a unique combination of primitive articulation surfaces with robust muscle attachment sites. An analysis of Index of Fossorial Ability (IFA), an established functional index informative of digging ability, across dicynodonts and extant burrowers confirms the strong potential for digging habits in NHCC LB366. To further validate the inferred fossorial ecology, we conducted comparative geometric morphometric analysis on extant burrowing mammals and reptiles, and used both physical 3D-printed models and computerized models, to quantify joint surface area and range of motion in the humerus (55.56° anterior-posterior) and the ulna (54.39° anterior-posterior; 88° laterally). Together, these analyses demonstrate that cistecephalids evolved sophisticated specializations for digging earlier in dicynodont history than previously recognized.

PAL5-6  3:45 pm  Triassic wheelbarrow race: revisiting cynodont forelimb posture with a musculoskeletal model. Lai PH*, Harvard University; Biewener AA, Harvard University; Pierce SE, Harvard University
Abstract: Highly adducted limbs operating in a parasagittal plane are a key mammalian innovation, decoupling pectoral and pelvic limb function and enabling the selection of dynamic, asymmetrical gaits by decreasing the lateral component of ground reaction forces. By contrast, tetrapods with more abducted stances typically employ stable, symmetrical gaits where mediolateral bending of the axial skeleton contributes substantially to stride length. As the immediate evolutionary precursors to mammals, cynodonts are reconstructed with sprawling forelimbs and parasagittal hindlimbs. However, no extant tetrapod has reliably been observed to employ this combination of limb stances—in the absence of a clear modern analogue, new methods and a broader, comparative perspective are necessary for inferring the locomotory ability and kinematics of these animals. It has been hypothesized that full hindlimb parasagittalism in the synapsid line was preceded by a period of facultative adduction, as seen in modern crocodilians. Here we consider the possibility of the cynodont forelimb following a similar dual-stance path to parasagittalism, and explore the gait implications of dissimilar fore- and hind-limbs. We present preliminary data from a musculoskeletal model of Massetognathus pascuali, a cynodont from the middle Triassic, combining µCT-data with muscle anatomy based on an extant phylogenetic bracket reconstruction of forelimb and pectoral girdle myology. By looking at how skeletal geometry and simulated muscle function interact across a series of hypothesized poses, we are able to evaluate the potential postural space available to this organism and shed light on the acquisition of mammalian parasagittalism.

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