Mass Extinction and Adaptive Radiation in Lizards and Snakes: Evidence from Fossils and Molecular Data

Abstract

Squamates (snakes, lizards and amphisbaenians) are represented by over 10,000 living species, with a rich diversity of ecologies and life history traits. They split from their closest relatives some 250 million years ago, and as a group have survived several large environmental upheavals, notably the Jurassic-Cretaceous extinction event, the Cretaceous-Palaeogene (K-Pg) mass extinction, and the Eocene-Oligocene (E-O) cooling event. In order to understand their modern diversity, biogeography and ecology more fully, it is important that we have a more complete picture of how squamates diversified through time and responded at times when other fauna experienced elevated rates of extinction.

In particular, I use molecular clock analyses to show that the Triassic-Jurassic mass extinction (201 Mya) probably had a limited effect on squamates, since their initial low diversity and abundance were maintained until the late Jurassic (\~{}160 Mya), when modern groups of squamates first appeared. Diverse snake faunas existed by the Cenomanian, (\~{}100 Mya), as evidenced by new fossils described from Morocco. Using a study of fossil vertebrae, I demonstrate that the disparity (morphological diversity) of snakes was overall higher after the K-Pg mass extinction (66 Mya) than before it: an increase that was largely driven by the diversification of marine palaeophiid snakes. An overarching pattern of post K-Pg success after the mass extinction is corroborated by molecular clock analyses and biogeographical reconstructions, which support both the diversification of snakes in the extinction’s aftermath, and the successful colonisation of Asia by the group at this time. Following this rapid post-extinction recovery, the E-O cooling event (\~{}33 Mya) caused further faunal turnover. We show that modern colubrids and viperids diversified rapidly at this time in the southern continents. Contrarily, aquatic snakes went extinct at the E-O boundary, as a result of falling temperatures and initiation of ice sheets at the poles. This was also reflected in a decline in disparity. Despite rapid diversification in the Oligocene, disparity remained low for 13 My. These macroevolutionary patterns suggest a complex interaction between climate and the evolution of modern squamates, with survival through environmental upheaval hinging on a multitude of factors - including the ability to find shelter, food, and adapt to new opportunities - rather than a singular mechanism.

Date of Award	22 Jul 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Matthew Wills (Supervisor), Daniel J. Field (Supervisor), Davide Pisani (Supervisor), Mark Puttick (Supervisor) & Jakob Vinther (Supervisor)