Fossoriality In Snakes: Perspectives From Morphology And The Fossil Record

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Howard, Alexandra 

Snakes are a speciose clade of squamates that have diversified into almost all habitats, all with the same elongate limbless body plan. This thesis aims to improve scientific understanding of snake evolution by focusing on novel perspectives using both extant snakes and the fossil record.

  1. Alternate phylogenetic positions of fossils affects body size estimates in snakes Body size is a readily available metric for both extant and extinct snakes. The competing hypothesis of ecological snake origins also predict different outcomes for the evolution of body size in snakes. I used a combination of ancestral state reconstructions, body size estimates of fossils from regression models, and evolutionary rate modelling to examine the evolution of body size in snakes, with a particular focus on early snake evolution. This project showed that snakes achieve a wide range of diversity of body size during the Cretaceous, trends that are not observed when using data from extant taxa.
  2. Cranial osteology of Typhlopidae (Serpentes: Scolecophidia) Scolecophidia are a distinctive group of snakes that occupy the basal most diverging branches of the snake phylogeny. However, due to the small overall size of many taxa, they have been relatively understudied in regards to comparative anatomy. In this project I examined segmented CT scans of 10 different species of Typhlopoidea, the largest clade of Scolecophidia. Using these comparisons I identified variability in several morphological characters not previously thought to be variable in Scolecophidia, as well as the identification of features in large typhlopids such as the lateral wings of the basisphenoid, which were previously thought to be limited only to Alethinophidia.
  3. Morphometrics provide evidence for the fossorial origin of snakes The problem of homoplasy, particularly in relation to adaptation to fossoriality, is potentially responsible for the incongruences between phylogenetic hypothesis of snakes that use either morphological or molecular data. In this project I used CT scans of 73 species of alethinophidian snake and geometric morphometrics to examine morphological correlates to fossoriality. I found that there is a clear morphological signal between the parietal of alethinophidian snakes and ecology. In general, the parietal of fossorial snakes is more elongate with a deep u or v shaped fronto-parietal suture, with an elongated posterior shelf that overlaps the braincase. These projects show how integrative research using both extant and palaeontological data can inform questions about the early evolution of clades, particularly the heavily debated ecological origin of snakes.

Head, Jason
Cranial, Cranial Osteology, Evolution, Fossil, Fossoriality, Geometric Morphometrics, Herpetology, Morphology, Morphometrics, Osteology, Palaeontology, Reptiles, Snake
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge