Thermophysiologies of Jurassic marine crocodylomorphs inferred from the oxygen isotope composition of their tooth apatite

Abstract

Teleosauridae and Metriorhynchidae were thalattosuchian crocodylomorph clades that secondarily adapted to marine life and coexisted during the Middle to Late Jurassic. While teleosaurid diversity collapsed at the end of the Jurassic, most likely as a result of a global cooling of the oceans and associated marine regressions, metriorhynchid diversity was largely unaffected, although the fossil record of Thalattosuchia is poor in the Cretaceous. In order to investigate the possible differences in thermophysiologies between these two thalattosuchian lineages, we analysed stable oxygen isotope compositions (expressed as δ 18 O values) of tooth apatite from metriorhynchid and teleosaurid specimens. We then compared them with the δ 18 O values of coexisting endo-homeothermic ichthyosaurs and plesiosaurs, as well as ecto-poikilothermic chondrichthyans and osteichthyans. The distribution of δ 18 O values suggests that both teleosaurids and metriorhynchids had body temperatures intermediate between those of typical ecto-poikilothermic vertebrates and warm-blooded ichthyosaurs and plesiosaurs, metriorhynchids being slightly warmer than teleosaurids. We propose that metriorhynchids were able to raise their body temperature above that of the ambient environment by metabolic heat production, as endotherms do, but could not maintain a constant body temperature compared with fully homeothermic ichthyosaurs and plesiosaurs. Teleosaurids, on the other hand, may have raised their body temperature by mouth-gape basking, as modern crocodylians do, and benefited from the thermal inertia of their large body mass to maintain their body temperature above the ambient one. Endothermy in metriorhynchids might have been a by-product of their ecological adaptations to active pelagic hunting, and it probably allowed them to survive the global cooling of the Late Jurassic, thus explaining the selective extinction affecting Thalattosuchia at the Jurassic–Cretaceous boundary. This article is part of the theme issue ‘Vertebrate palaeophysiology'.

Vertebrate palaeophysiology

“…A previous analysis of the thermophysiology of marine reptiles showed that Plesiosauria, Ichthyosauria and (probably) Mosasauridae were endotherms [31]. Results obtained by Séon et al [8] for Thalattoshuchia (marine crocodylomorphs) are consistent with hydroxyapatite formed under temperatures intermediate between those of ectotherms and endotherms, and behavioural adjustments are speculated to have played a role in their thermoregulation.…”

“…'Vertebrate palaeophysiology' will promote a better understanding of how organism-environment interactions have evolved in terms of energy budgets, predator-prey relationships and sensitivity to environmental change. The research areas covered by this theme issue include: phospho-calcic metabolism [2], acid-base homeostasis [3,4], thermometabolism [4][5][6][7][8][9], respiratory physiology [10], skeletal growth [11], palaeopathophysiology [12,13], genome size and metabolic rate [14], and a concluding historical perspective [15]. Sometimes, the two components ( physiological mechanism and palaeobiological inference) are proposed in separate papers (for instance, three contributions devoted to mechanisms of thermogenesis mechanisms [5][6][7] and three papers dealing with the thermometabolic inferences in extinct taxa [4,8,9]).…”

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

“…The basal cetaceans that we consider most skeletally analogous to semiaquatic thalattosuchians-the nearshore remingtonocetids, which had teleosauroidlike elongate snouts, comparatively long necks, forelimbs and hindlimbs used in swimming, and robust pelves (1, 33)-had miniaturized pelagic-style labyrinths, and flourished for only a few million years. It may be that cetaceans were able to adapt more quickly to fully pelagic lifestyles, because they evolved from terrestrial ancestors that were already endothermic and gave live birth, whereas metriorhynchids had to develop these attributes after entering the water (34,35).…”

Cutting the Gordian knot: a historical and taxonomic revision of the Jurassic crocodylomorphMetriorhynchus

“…Fraas, 1902;von Arthaber, 1906;Andrews, 1913;Fernández & Gasparini, 2000Young et al, 2010;Herrera et al, 2013aHerrera et al, , 2013bSachs et al, 2019a;Schwab et al, 2020). It has also been suggested that metriorhynchids evolved a non-homeothermic mode of endothermy and were viviparous (see Herrera et al, 2015a;Séon et al, 2020).…”