Can social and sexual selection explain the bizarre snout of proterosuchid archosauriforms?

Abstract

Proterosuchids are a clade of quadrupedal, carnivorous Permo-Triassic diapsids crucial to understand the successful evolutionary radiation of archosaurs during the Mesozoic. The importance and good fossil record of proterosuchids nourished a renewed interest in recent years, but no function has been proposed for their bizarre snouts. An oversized and downturned premaxilla with up to nine teeth with continuous replacement is present in all proterosuchid species and seems to have represented a physiologically costly phenotype that increased towards adulthood. A non-functional or a species recognition hypothesis are not supported as evolutionary mechanisms that drove this phenotype because features expected for these explanations tend to have a very low or zero physiological cost. There is no evidence favouring – but neither rejecting − that this morphology can be explained by non-sexual and non-social natural selection alone. Mutual social and/or sexual selection is favoured here as the most unambiguously supported explanation for the function and origin of the bizarre snout of proterosuchids based on several lines of evidence, including costliness, positive allometry, positive changes in growth rates and modern analogues. Social and/or sexual selection may have been important evolutionary mechanism in the dawn of the lineage that gave rise to crocodiles and dinosaurs.

Keywords:

• Archosauromorpha
• Proterosuchidae
• Proterosuchus
• Triassic
• mass extinction
• ontogeny

Acknowledgements

We thank the following curators, researchers and collection managers who provided access to specimens under their care for the purpose of this research: Bernhard Zipfel, Bruce Rubidge, Jonah Choiniere and Fernando Abdala (BP); Markus Moser and Oliver Rauhut (BSPG); Ellen de Kock (GHG); Liu Jun and Corwin Sullivan (IVPP); Elize Butler and Jennifer Botha-Brink (NMQR); Andrey Sennikov (PIN); Sheena Kaal and Roger Smith (SAM-PK); and Heidi Fourie (TM). I appreciate the discussions and comments on an early version of the manuscript with Federico Agnolín, Richard Butler, Leandro Canessa, Julia Desojo, Lucas Fiorelli, Agustín Martinelli and Damián Perez. I thank Ernest Keeley for allowing the usage of a photograph of a coho salmon taken by him. I appreciate the comments and suggestions of two anonymous reviewers that helped to improve the manuscript.