Macroevolution of the Crocodylomorpha

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


The Crocodylomorpha are an ancient clade with origins in the Late Triassic, however their extant diversity is meagre in comparison with their fossil forebears. Fossil Crocodylomorpha includes hundreds of morphologically disparate species, including herbivorous, marine and semi-fossorial forms. How has the diversity and rate of crocodylomorph evolution changed through time? What are the driving factors of evolutionary rate and diversity? Do living crocodiles deserve status as living fossils?
This project presents a phylogenetic hypothesis of the crocodylomorpha based on meta-analysis of the literature. This hypothesis is utilised as a framework for comparative phylogenetic approaches to test for models of body size evolution, and for variable evolutionary rates. The phylogenetic hypothesis is applied as a phylogenetic correction to diversity through time and disparity through time data. Time-series datasets are analysed for relationships with environmental and preservation variables using univariate and multivariate linear modelling approaches.
The analyses presented here support a well-resolved phylogenetic hypothesis, supporting the monophyly of several established clades. Supertree approaches may be biased by the literature, but this architecture is also supported by supermatrix approaches. However, large supermatrices are limited by a build-up of inapplicable characters. The low evolutionary rate and diversity of extant Crocodylomorpha identifies them as living fossils. Climate is found to be a crucial factor in crocodylomorph evolution. Crocodylomorph body size and rates of body size evolution increase during periods of cooling, in accordance with Bergmann’s rule. Diversity is limited by temperature, decreasing during periods of cooling but recovering during periods of stability. Crocodylomorph morphological disparity is decoupled from diversity, resembling a ‘disparity- first’ model.The crocodylomorphs show strong support for a punctuated equilibrium model of evolution driven by environmental change, as defined by the court Jester hypothesis. The vulnerability of living crocodylomorphs to environmental change and their status as living fossils makes their conservation a priority.
Date of Award25 Sept 2018
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorMichael J Benton (Supervisor)

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