The Evolution of Parental Care in Archosaurs

Abstract

With up to 18,000 extant representatives and many more extinct, archosaurs are hugely species rich, have enormous ecological diversity and re-radiated spectacularly in the wake of the P-Tr, T-J and K-Pg extinctions. The success of the archosaurs has been attributed to a variety of factors, not least their unique locomotory adaptations, varied diet, hardiness to environmental change, unique dentition and, in some lineages, endothermy and derived metabolic adaptations. A factor that is often overlooked in comparing the radiation of the archosaurs with that of other amniote clades (eg. mammals and squamates) is their unique parental care behaviours. Some of these behaviours count among the greatest parental investments within the tetrapods, and the story of the evolution of parental care in the archosaurs is a fascinating field of inquiry.

Here, I review current understanding of parental care in archosaurs, its origin and
evolution. This collated information is presented as an introductory chapter. Following this are five pieces of novel research, presented in the form of scientific publications: (1) An analysis of the evolution of egg mass in extant birds, and the biological implications of previously masked allometric relationships within Orders. (2) An investigation into the parental sex roles in extant birds, with emphasis on the relationship between female biological investment and male behavioural investment. (3) An overview and novel analysis of the reproductive traits of extant crocodylians (4) An ancestral state reconstruction of the reproductive traits of extinct dinosaurs, with focus predominantly
on the morphological changes to the egg in the theropod lineage ancestral to modern birds. (5) A description of the most complete juvenile spinosaur material recovered from the Kem Kem of Morocco.

I conclude that: (1) Previous examinations of the relationship between egg and body mass in extant birds have masked underlying between-Order trends that offer significant evolutionary insight into the adaptive conditions and underlying genetic mechanisms present in the evolution of birds. (2) I report the first evidence of a significant negative relationship between male incubation and proportional female pre-laying investment in birds, with no such relationship recovered for other aspects of male care (feeding and brooding). (3) I report the first evidence of a latitudinal body mass trend in extant Crocodylia, as well as significant relationships between important reproductive traits (female mass, egg mass, clutch size, hatchling mass). No significant relationship was identified between breeding latitude and hatchling mass, suggesting a different type
of selection from that in testudines. (4) I report strong evidence of a morphological change in the egg of theropod dinosaurs along the lineage that led to birds, consistent with previous revelations of changes to the adult morphology, nest and egg structure. (5) I describe seven new cranial and postcranial specimens of juvenile spinosaur from the Moroccan Kem Kem, suggesting a full individual size of 3-5m. I conclude that these juveniles, recovered in the same geological deposits as the adults, likely shared a common habitat, though this is less likely to have been the case for hatchlings.

Date of Award	22 Jun 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Matthew Wills (Supervisor) & Tamas Szekely (Supervisor)