Evolution in biological radiations; insights from the Triassic archosaur radiation

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Virginia Tech


Adaptive radiations, or evolutionary diversifications, are the evolutionary divergence of a single lineage into many different adaptive forms. They play a critical role in the history of life as groups of organisms speciate and fill new ecological roles over geologically rapid time intervals. There is currently no agreed upon operational unit, timeframe, or amount of divergence for organisms to be considered to have undergone an adaptive radiation. Additionally, the paucity of both comparative and fossil studies has limited the utility of the adaptive radiation in framing macroevolutionary questions, such as, is ecological and morphological diversification simultaneous? An ideal fossil clade to test this question is the Archosauriformes (crocodylians, birds, and their closest relatives). Archosauriforms radiated following the end-Permian mass extinction and their lineage diversification through the Early to Late Triassic is well documented in the literature. Prior to the end-Permian mass extinction, these reptiles were both species poor and ecologically limited, but by the Late Triassic they dominated terrestrial ecosystems in both species abundance and ecological breadth. However, continued environmental instability following the end-Permian extinction has led to the hypothesis that ecological expansion of archosauriforms lagged behind the diversification of lineages. The first chapter of my dissertation uses a Middle Triassic archosauriform tooth assemblage from Tanzania to reconstruct dietary specialization, estimated by morphological disparity of teeth. In addition to comparing tooth disparity of isolated and in situ teeth, this also provides a lens for comparing the timing of dietary specialization and species diversification. I found the archosauriforms to be faunivorous with little morphological disparity amongst the teeth. The second chapter uses an Early Triassic reptile tooth assemblage from South Africa to reconstruct the dietary specialization of archosauriforms early in their radiation to compare the amount of morphological disparity and lineage diversity. I use methods from Chapter 1 and integrate 3D morphometrics to better capture shape. I described several tooth morphotypes including six new to the locality. The morphological and dietary differences were minimal, indicating a greater species diversity than ecological diversity. The third chapter is a description of a new pseudosuchian archosaur taxon from the Middle Triassic of Tanzania. As species descriptions form the basic data unit of macroevolutionary analyses, this assists future studies of the archosauriform radiation. I recover this new taxon as the oldest known aetosaur. This species provides insights into the evolution of an armored carapace in crocodylian-line archosaurs and shows morphology related to armor evolved prior to the evolution of an herbivorous diet.



macroevolution, reptiles, mass extinction, phylogeny, anatomy, teeth