The origin of Pterosaurs

Abstract

Our understanding of the pterosaurs' place within the reptilian lineage has had a long and complex history. The unusual morphology of pterosaurs, which is inextricably linked to their habit of powered flight, has generated numerous proposals over the years regarding their exact origin and systematic position. Though it was concluded early on in pterosaur research history that these animals represented a group of derived flying reptiles, their exact origination remained mysterious for a long time and is still somewhat controversial. A rough consensus has now been reached that pterosaurs are derived archosaurs and are likely close relatives of the dinosaurs, united with them in the clade Ornithodira, though some still challenge this view. The anatomical evidence in support of this position close to Dinosauria is also admittedly fairly limited at present, largely owing to a lack of any clear-cut transitional ‘proto-pterosaur’ taxa (albeit that some fragmentary specimens have been suggested to represent exactly this). Differing hypotheses have also recently been put forward as to the exact interrelationships between the pterosaurs and other non-dinosaurian and dinosaurian ornithodirans. Here the previous hypotheses of where pterosaurs fit into the reptilian lineage and the anatomical evidence in support of the current hypotheses are reviewed. Results of new analyses are included that looked to test the origin and systematic position of the Pterosauria using an expanded version of a large anatomical dataset of archosaurs, within which several previously unconsidered early pterosaur taxa and a suit of new anatomical characters were considered. The analyses in this study support the close affinities between pterosaurs and dinosauriforms within Ornithodira; Pterosauria is recovered as the sister-taxon to Lagerpetidae. Such a result suggests that the clade Pterosauria belongs with Lagerpetidae as part of a broader Pterosauromorpha that then, with Dinosauriformes, falls within Ornithodira. The anatomical evidence in support of this position within Ornithodira is also discussed in detail.

Introduction

Pterosaurs were a long-lived and highly diverse clade of flying reptiles that first appeared in the fossil record in the early-middle Late Triassic (Benton, 1985; Bennett, 1997; Barrett et al., 2008; Upchurch et al., 2015). The oldest definitive specimens are Norian in age (227–208.5 million years old) (e.g., Wild, 1978), and come from a range of localities that are exclusively within the Northern Hemisphere (Barrett et al., 2008). A single Carnian-Norian (237–208.5 million years old) specimen that could represent a ‘basal’ pterosaur, named Faxinalipterus minima, is also known from the Caturrita Formation in Brazil, but this specimen is currently of uncertain affinity (Bonaparte et al., 2010).

Recent time-calibrated phylogenetic analyses (based upon the hypothesis that pterosaurs are archosaurs closely related to dinosaurs) have put the origination of the Pterosauria near to the Olenekian-Anisian boundary. This equates to the end of the Early Triassic and start of the Middle Triassic, roughly 250 million years ago (Nesbitt et al., 2017). This would also suggest that there is a substantial ghost lineage for the clade spanning almost 40 million years. This gap in the fossil record has yet to be filled, despite that fact that an abundance of other terrestrial archosaurs, including members of Avemetatarsalia (currently believed to be the clade that pterosaurs are nested in within Archosauria), are continuously being discovered in Middle and early Late Triassic localities (Ferigolo and Langer, 2006; Kammerer et al., 2012, Kammerer et al., 2020; Nesbitt et al., 2010, Nesbitt et al., 2017; Martz and Small, 2019; Pacheco et al., 2019). There are also currently no known ‘transitional’ pterosaur taxa that display a combination of plesiomorphic and derived anatomical characters suggestive of a lack of powered flight capability. As yet we do not know of any ‘missing-link’ pterosaurs that appear more as if they could glide or perhaps adopt other types of behaviour that differ from all currently known pterosaurs. This has made it difficult to understand precisely how the pterosaurs acquired their strange suite of unique anatomical characteristics. However, regardless of how and when pterosaurs originated, not long after their first appearance the clade rapidly diversified, radiating and spreading through the Late Triassic and Jurassic, lasting until the end-Cretaceous mass extinction, and achieving in this time a great diversity and disparity, as well as a global distribution (Unwin, 2003a; Barrett et al., 2008; Kellner et al., 2019; Bestwick et al., 2020).