The enigmatic evolutionary relationships of Palaeocene mammals and their relevance for the Tertiary radiation of placental mammals

• Main Author: Halliday, T. J. D.
• Published: University College London (University of London) 2015
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.668500

Understanding the general pattern of how a clade evolves over time is a central aim of palaeontology and evolutionary biology. The observation that the tree of life is asymmetric in species distribution necessitates that rates of evolution, speciation, and extinction vary through time and across phylogeny. The way this variation is distributed can help to inform on historic events, selection pressures, and relationships. Often, at the origination of a clade, it is supposed that there is an ‘early burst’ of diversification, before rates of speciation and morphological evolution slow down as the clade ages. One example of a supposed ‘early burst’ is that of placental mammals, but the internal relationships of the earliest members of this group have prevented further study of macroevolutionary parameters. In this thesis, by building the largest cladistic data matrix to date, I test the relationships of mammals from the earliest Cenozoic, and from the resulting phylogenies, test the hypotheses that the end-Cretaceous mass extinction resulted in an adaptive radiation of placental mammals. I show that Phenacodontidae are most parsimoniously ancestral to Perissodactyla, that a division between Boreoeutheria and Atlantogenata is better supported than one between Xenarthra and Epitheria or Afrotheria and Exafroplacentalia at the root of Placentalia, and that all “condylarths” can be placed, with varying degrees of confidence, as stem members of laurasiatherian orders. I show that there was an increase in rate of morphological evolution immediately after the end-Cretaceous mass extinction, that Placentalia is extremely likely to have originated less than 70 million years ago, and that the rise of Placentalia was associated with an increase in morphospace occupation, and, with a lag, mean pairwise dissimilarity of taxa. These conclusions support the contention that the end-Cretaceous mass extinction was not just an important time in Earth’s ecological history, but crucial to the diversification of mammals to the level observed today.