Thesis (Ph. D.)--University of Chicago, Dept. of Organismal Biology and Anatomy, August 1999. / Includes bibliographical references. Also available on the Internet.

With the transition from water to land in early tetrapods, and with transitions to secondarily aquatic habits in numerous tetrapod lineages, the functions of the vertebral column were transformed. Morphological changes in the vertebral column are a major mechanism by which vertebrates accommodate changes in locomotor forces. Although morphometric measurements from vertebrae have been correlated with axial mechanics and locomotor behaviour in numerous extant taxa, few studies have sought to test or apply these principles in non-mammalian tetrapods. In my thesis, I reconstructed the vertebral mechanics of fossil taxa that represent intermediate stages in water/land transitions of their lineages. Study taxa were the basal tetrapod Pederpes finneyae, which is one of the earliest known tetrapods to show indications of terrestrial adaptation, and three extinct crocodylomorphs, Terrestrisuchus, Protosuchus, and Pelagosaurus, which span the spectrum from fully terrestrial to primarily aquatic. I used a combination of morphometric measurements and 3D virtual models of bone morphology to estimate intervertebral joint stiffness and range of motion. For comparison, I also reconstructed the vertebral mechanics of four related extant taxa. Correlations between vertebral morphometrics and axial stiffness were statistically tested in (cadaveric) modem crocodylians, and I validated my methodology by comparing my results with data from extant taxa. My results reveal similarities and differences between the two lineages. Intervertebral joint compliance and range of motion tended to decrease with adaptation for terrestrial locomotion, as expected, but this trend seems to have reversed in later forms. Additionally, vertebral mechanics may have been largely controlled by different structural mechanisms in different lineages. The relationship between biomechanics of vertebrae and environment appears to be more complex than previously supposed. However, approaches that combine experimental measurements from extant animals, thorough analysis of fossil morphology, and explicit phylogenetic considerations have the potential to greatly improve locomotor reconstructions of extinct taxa.

No description available.

The Deadhorse Coulee Member of the Milk River Formation of southern Alberta preserves one of the oldest well-documented non-marine vertebrate assemblages in Canada. In this study, the taxonomic diversity of this member is updated, and vertebrate localities are placed in geographic and stratigraphic context. The stratigraphic provenance of specimens indicates all vertebrate material from this member is latest Santonian (83.5 Ma). A new species of turtle is described. Analyses of the rank and relative abundances of taxa support interpretations of this member as a prograding clastic wedge with localities approximately 40 km from the palaeoshoreline at time of deposition. Results support high local abundances of vertebrates in western North America, with faunal provincialism regulated by distance to the palaeoshoreline and mean annual temperatures. Morphologic changes in small theropod taxa through the latest Cretaceous of western North America act as a case study for evaluating species turnover of vertebrate microfossil material. / Systematics and Evolution

Jawed vertebrates (gnathostomes) account for over 99% of living vertebrate diversity, with origins that stretch back nearly half a billion years, and comprise two groups: Osteichthyes (fishes and land-dwelling vertebrates) and Chondrichthyes (sharks, rays and chimaeras). Osteichthyans are the dominant clade, with at least 60,000 species approximately evenly divided between two clades: the Actinopterygii and the Sarcopterygii. However, our understanding of early osteichthyan evolution is skewed in favour of sarcopterygians, leaving the origin of nearly half of all vertebrate diversity critically understudied. Furthermore, recent upheavals in the early gnathostome tree have destabilised relationships amongst fossil taxa and eroded our understanding of primitive anatomical conditions of key groups. Central to understanding early gnathostome evolution is the braincase, an anatomically complex structure that provides a wealth of morphological characters. However, braincases rarely fossilise, and their position inside the skull makes them difficult to attain. X-ray tomography allows a comprehensive description of the internal and external anatomy of fossils, including the braincase. This thesis sets out to target phylogenetically pivotal taxa and incorporate new anatomical data in building up a picture of character evolution in early jawed vertebrates. In particular, I target the gnathostome stem, describing a new taxon that helps bridge the morphological gap between placoderms and crown gnathostomes, allowing a more comprehensive understanding of both dermal and endoskeletal evolution. I also focus on early actinopterygians, describing the endoskeleton of the first members of the group in order to understand primitive anatomical conditions. I then investigate actinopterygian braincase anatomy in the context of a revised phylogenetic analysis, illuminating the early evolution of the actinopterygians. Finally, I present a synthetic review of braincase anatomy across the early gnathostome tree. These results provide a more accurate picture of braincase evolution across gnathostomes and actinopterygians, clarifying our understanding of their evolution while revealing new information about when key innovations arose in the brains of the very first ray-finned fishes.

A diverse assemblage of exquisitely-preserved fossil animals and plants were collected from Sawmill Sink blue hole in Abaco, Bahamas. More than 40 species have been identified including extinct tortoise, an extant crocodile, and birds that no longer live in The Bahamas. This study addresses the mechanisms that preserve fossils in blue holes and discusses paleoecological implications from faunal diversity and sediment composition. Blue holes are waterilled caves with surface openings that may trap animals and surface-driven vegetation. In Sawmill Sink the talus cone within the halocline acts as a substrate on which organics collect that drive microbe ecology. Their byproducts, hydrogen sulfide and anoxia, inhibit biological destruction and delay necrolysis. Low tidal flow and quiescent water conditions further enhance stability of the depositional environment. In addition, subaerial conditions during glacial lowstands allowed owls to roost; their deposits formed a rich assemblage of small fossil vertebrates.