title: Near-Stasis in the Long-Term Diversification of Mesozoic Tetrapods.
abstract: How did evolution generate the extraordinary diversity of vertebrates on land? Zero species are known prior to ~380 million years ago, and more than 30,000 are present today. An expansionist model suggests this was achieved by large and unbounded increases, leading to substantially greater diversity in the present than at any time in the geological past. This model contrasts starkly with empirical support for constrained diversification in marine animals, suggesting different macroevolutionary processes on land and in the sea. We quantify patterns of vertebrate standing diversity on land during the Mesozoic-early Paleogene interval, applying sample-standardization to a global fossil dataset containing 27,260 occurrences of 4,898 non-marine tetrapod species. Our results show a highly stable pattern of Mesozoic tetrapod diversity at regional and local levels, underpinned by a weakly positive, but near-zero, long-term net diversification rate over 190 million years. Species diversity of non-flying terrestrial tetrapods less than doubled over this interval, despite the origins of exceptionally diverse extant groups within mammals, squamates, amphibians, and dinosaurs. Therefore, although speciose groups of modern tetrapods have Mesozoic origins, rates of Mesozoic diversification inferred from the fossil record are slow compared to those inferred from molecular phylogenies. If high speciation rates did occur in the Mesozoic, then they seem to have been balanced by extinctions among older clades. An apparent 4-fold expansion of species richness after the Cretaceous/Paleogene (K/Pg) boundary deserves further examination in light of potential taxonomic biases, but is consistent with the hypothesis that global environmental disturbances such as mass extinction events can rapidly adjust limits to diversity by restructuring ecosystems, and suggests that the gradualistic evolutionary diversification of tetrapods was punctuated by brief but dramatic episodes of radiation.
authors.affiliation: Department of Earth Sciences, University of Oxford, Oxford, United Kingdom.
authors.forename: Roger B J
authors.initials: RB
authors.lastname: Benson
authors.affiliation: School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, United Kingdom.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Butler
authors.affiliation: Department of Biological Sciences, Macquarie University, New South Wales, Australia.
authors.forename: John
authors.initials: J
authors.lastname: Alroy
authors.affiliation: Department of Earth Science and Engineering, Imperial College London, London, United Kingdom.
authors.forename: Philip D
authors.initials: PD
authors.lastname: Mannion
authors.affiliation: Department of Paleobiology, Smithsonian Institution, Washington, D.C., United States of America.
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Carrano
authors.affiliation: Department of Biological Sciences, Macquarie University, New South Wales, Australia.
authors.forename: Graeme T
authors.initials: GT
authors.lastname: Lloyd
mesh_terms: D000818:Animals; D044822:Biodiversity; D005075:Biological Evolution; D005580:Fossils; D008962:Models, Theoretical; D014714:Vertebrates