The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour

Dinosaur behaviour has little legacy in the fossil record and the rarity of fossil soft tissues makes it difficult to evaluate. Indirect evidence from bonebeds, trackways, nesting traces and in-group comparisons with extant Archosauria suggests that the only substantive arguments to be made for dinosaur sociality concern cranial ornamentation and herding behaviour. There is currently no reliable method to determine gender from skeletal remains. Dinosaur reproductive anatomy was a unique combination of crocodilian and avian characters and extant models indicate that dinosaurs copulated using a reptilian ‘leg over back’ posture. Reliable evidence for post-hatching care in dinosaurs is lacking and extant archosaurs yield little insight. A hypothesis is proposed that for the majority of dinosaurs there was no post-hatching care provided which would have allowed adults energy acquisition that would otherwise have been required for defence and provisioning to be redirected towards growth and increased fecundity, both traits for which there is fossil evidence. Arguments suggesting that the more advanced aspects of extant avian care boasting an explicit coelurosaurian theropod origin are rejected as these behaviours appear unique to the Neornithes. Three ancestral care hypotheses are tested and none conform in a satisfactory manner with body fossil and ichnological evidence.