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The early Mesozoic rise of archosaurs: New insights into an exemplar evolutionary radiation

Final Report Summary - ARCHOSAUR RISE (The early Mesozoic rise of archosaurs: New insights into an exemplar evolutionary radiation)

A central goal of palaeontology and evolutionary biology is to understand the drivers and dynamics of the major evolutionary radiations that have shaped the history of life on Earth and governed the origins of the modern biota. On land, one of the most important evolutionary diversifications was the radiation of archosaurs, which began around 250 million years ago in the Triassic Period, following the greatest mass extinction in the history of life at the end of the preceding Permian Period. There is an astonishing species richness and morphological diversity of extinct archosaurs known from a rich fossil record, including well over 1000 Mesozoic species of dinosaurs, pterosaurs (flying reptiles), early birds and stem crocodylians. Following their origins in the Triassic, archosaurs subsequently dominated terrestrial ecosystems for around 170 million years of the Mesozoic, and reached gigantic body sizes unparalleled in the history of vertebrate life on land.

ARCHOSAUR RISE focused on the early archosaur radiation during the early Mesozoic (Triassic and Jurassic), between 250 and 150 million years ago, and aimed to constrain the diversity and biogeography of species involved in this radiation, and elucidate the radiation’s tempo and evolutionary patterns and mechanisms. The goal of the project was to provide new insights into how and why archosaurs became arguably the most successful group of vertebrates on land, the origin and rise to ecological dominance of dinosaurs, and more broadly into the nature of major evolutionary radiations, the recovery from mass extinction events, and the long-term interactions between biotic diversity and Earth system evolution.

ARCHOSAUR RISE was led by Professor Butler, following his move to a permanent, research-focused position as Birmingham Fellow at the University of Birmingham in 2013, and involved a team including a postdoctoral researcher, two PhD students, and international collaborations in the USA, Russia, South Africa, India, Argentina, Brazil and China (among others). The support provided by the CIG funding and the success of the ARCHOSAUR RISE project allowed Butler to consolidate his position in Birmingham, being promoted to Senior Research Fellow in 2015 and full Professor of Palaeobiology in 2017. Butler was also able to use the stability provided by the CIG funding to obtain additional grant support (from the European Research Council) and further expand his research group. The postdoctoral researcher and PhD students trained as part of ARCHOSAUR RISE have subsequently successfully obtained research positions in the UK, Germany and Argentina.

All of the major goals of the project were achieved. The project was divided into three objectives. The first objective was focused on revising the comparative anatomy, systematics and phylogenetics of key groups of early archosaurs and closely related taxa. This was the subject of the two PhD theses supported by the project, as well as >20 peer-reviewed papers in ISI-listed journals. These included: papers revising the taxonomy, diversity and distribution of important groups such as Erythrosuchidae, Proterosuchidae, Euparkeriidae and early phytosaurs; the description of highly important new archosaur taxa from the Middle Triassic of Tanzania; and the development of an explicit, phylogenetic framework encompassing all archosaurs and closely related taxa (archosauromorphs) from the first 30 million years of their evolutionary history (middle Permian–early Late Triassic).

The second objective of the project was to conduct fieldwork in South Africa and Argentina, with the goal of collecting new fossil material that would help illuminate the archosaur radiation. Two field seasons were conducted in South Africa in 2014 and 2017 in collaboration with the University of the Witwatersrand in Johannesburg, working in Middle Triassic and Late Triassic strata. These recovered several new taxa of cynodont and dicynodont synapsids from the Middle Triassic, and important new localities of early dinosaurs from the Late Triassic. The collected fossils are currently under study, and the newly discovered localities will be the subject of further fieldwork in 2018 and beyond. It was not possible to complete the planned fieldwork in Argentina, but this was substituted by fieldwork in the Early Triassic of India in collaboration with the Indian Statistical Institute (Kolkata) and by work in the Middle Jurassic of Scotland in collaboration with National Museums Scotland and the University of Oxford. Both of these field collaborations collected significant new fossil material that is the subject of current research and future publications.

The third objective of the project was to use the anatomical, systematic and palaeobiological data to conduct large-scale macroevolutionary analyses. This work has allowed us to characterize the archosauromorph radiation into the following sequence of events: (i) Permian origination and dispersal across the Pangaean supercontinent, but at low abundance and diversity; (ii) a morphologically conservative and globally distributed ‘disaster fauna’ immediately following the Permo-Triassic extinction event; (iii) a major but cryptic and poorly sampled phylogenetic diversification with significantly elevated evolutionary rates; and (iv) a marked increase in species richness, abundance, and disparity contemporaneous with global ecosystem stabilisation some 5 million years after the mass extinction. This multiphase event transformed global ecosystems, with far-reaching consequences for Mesozoic and modern faunas.