Quantifying variation in teeth of Late Triassic vertebrates: implications for identification, palaeoecology, and biostratigraphy

Abstract

The Triassic Period (~252–201.5 Ma) represents one of the most dynamic intervals in Earth's history, characterised by major evolutionary radiations following the end-Permian mass extinction, the emergence and diversification of key vertebrate lineages, and the establishment of modern terrestrial ecosystems. Whereas macrofossils tend to capture greater attention, small fossils are equally as important to reconstructing vertebrate communities of ancient ecosystems. This is especially true of fossils found from microvertebrate localities (deposits where 75% of remains are <50mm) as these record both small vertebrates and small parts of larger organisms. Tooth fossils are among the most readily preserved in these sites thanks to their relative hardness and resistance to chemical and physical weathering. However, when teeth are found isolated, it can be difficult to ascribe them to a species or even clade as many teeth come from currently unknown animals, and there is a high degree of convergence with little variation among many archosaur teeth in the Triassic Period. Different methods have been used to quantify variation and identify isolated teeth, particularly within Dinosauria, but 3D methods have been underutilised, despite the increasing availability of scanned fossils and increased information on dimensionality that can be captured by using 3D models. My dissertation quantified variation in tooth morphologies at three different taxonomic levels: within a species, within a clade, and among clades with similar inferred diet, and employs quantitative and qualitative analyses on a broad range of taxa. The goal of this work was to test whether the variation in tooth morphology, once quantified, allows for the identification of isolated teeth of unknown taxa in microvertebrate deposits, and as an aid to determine the biostratigraphic utility of the specimens tested. In my first chapter, I looked at variation in tooth shape and discrete characters within a clade, using aetosaurs (quadrupedal terrestrial pseudosuchians with extensive armour across most of the body) and their close outgroups as an example. Interestingly, isolated aetosaur teeth are exceptionally rare in deposits where other aetosaur fossils, such as osteoderms, are common; however, it is unknown whether this is an artifact of aetosaurs generally having fewer teeth than other archosaurs or the result of the difficulty of identifying isolated aetosaur teeth. To determine if aetosaur teeth are diagnostic and can be identified apart from other Triassic Period archosaurs, I created matrices that use 3D geometric morphometrics (3DGM) and non-metric multidimensional scaling (NMDS) to both examine variation and evaluate whether these techniques could be used to help identify isolated aetosaur teeth, particularly in microvertebrate deposits. I found that, broadly, aetosaurs can be distinguished from the outgroups tested. When isolated teeth suspected to be from aetosaurs were added into the matrix, they plotted within the space of the majority of aetosaurs in the 3DGM. In the NMDS morphospace, they plotted more disparately, but still away from the outgroups, suggesting that these methods are useful in identifying isolated aetosaur teeth. My second chapter utilised the methods from my first chapter and applied them to a wider group of animals – the carnivorous archosaurs of the Triassic Period (e.g. Coelophysis, Batrachotomus, Smilosuchus) to test whether distantly related carnivorous taxa retain a similar tooth form, and whether it is possible to identify teeth to major clade despite these taxa sharing the ancestral tooth shape for this clade (i.e., recurved, serrated, and laterally compressed). I found that even though some taxa (e.g. Diandongosuchus, Ornithosuchus, Riojasuchus) overlap greatly in dental morphospace using 3D morphometrics and within the NMDS analysis morphospace plots, dinosaurs tend to plot away from pseudosuchians in the 3DGM, implying that 3DGM may be useful in separating these two clades. My third and final chapter further addressed intraspecific tooth variation and described a new species of hybodontiform shark based on teeth from a highly productive microvertebrate locality called the 'Green Layer' near the Petrified Forest National Park, Arizona, USA. Chondrichthyan (shark) teeth can be useful for biostratigraphy, but differences in tooth shape throughout the mouth can make disentangling species from one another difficult. Additionally, specimens are also known from a second site within Petrified Forest National Park, and I examined variation between the two assemblages using NMDS and conducted a 3DGM analysis on the 'Green Layer' teeth revealing that there is limited morphological variation in this new species. When I conducted a NMDS analysis, the two assemblages strongly overlapped suggesting that these assemblages are indeed the same species and as tooth shape is highly conserved, they are a useful index taxon for the early Revueltian. My dissertation found that the two methods (3DGM and NMDS) used in each chapter are useful on a broad range of vertebrate taxa in combination, from pseudosuchians to chondrichthyans and highlights the importance of small and isolated fossils in reconstructing vertebrate communities.