Macroevolution of early tetrapods

  • Armin Elsler

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

About 390 million years ago the transition from water to land paved the way for the success story of the most iconic clade of animals, the four-limbed tetrapods. The first half of the evolutionary history of Tetrapoda was characterised by major radiations, massive biotic crises, restructuring of vertebrate ecosystems, and ultimately the appearance of most modern tetrapod subclades. Previous macroevolutionary analyses of early tetrapods focused on smaller subclades, relied on imprecise stratigraphic data, did not incorporate phylogenetic relationships, and ignored the potential for heterogeneity in evolutionary rates. Here I present the only comprehensive database (2,142 valid species) and associated supertrees spanning the first 200 million years of tetrapod evolution, with state-of-the-art alpha taxonomy and substage level stratigraphic resolution. Using this dataset and appropriate phylogenetic comparative methods I am able to show that rates of body size evolution were highly heterogeneous in terrestrial early tetrapods following the Simpsonian concept of quantum evolution (chapter 2). Unlike previously suggested, high rates of body size evolution were not responsible for the success of early tetrapod clades and were potentially a signal for stress. Similar to the rates of body size evolution, rates of the mandibular functional complex were also heterogeneous in early archosauromorphs (chapter 3). Despite the rate heterogeneity I find no evidence for positive phenotypic selection acting on the lower jaw of early archosauromorphs, consistent with a non-competitive model of replacement of faunas in the aftermath of the Permo-Triassic mass extinction event (PTME). Finally, I am able to show the complexity of speciation dynamics in early tetrapods prior to the PTME (chapter 4). ‘Amphibians’ were in decline tens of millions of years before the mass extinction, but amniotes were thriving up to the last moment indicating that without the PTME Palaeozoic terrestrial ecosystems probably would have persisted much longer into the Mesozoic.
Date of Award7 May 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMichael J Benton (Supervisor), Marcello Ruta (Supervisor) & Alexander Dunhill (Supervisor)

Keywords

  • Macroevolution
  • Tetrapoda
  • Permo-Triassic mass extinction event
  • Evolutionary rates
  • Archosauromorpha
  • Dinosauria
  • Pseudosuchia
  • Synapsida
  • Therapsida
  • Temnospondyli
  • Basal tetrapods
  • Body size
  • Disparity
  • Speciation rates

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