Skeletal pathologies track body plan evolution in ichthyosaurs

Judith M Pardo-Pérez,Benjamin P Kear,Erin E Maxwell

doi:10.1038/s41598-020-61070-7

Abstract

Changing predator-prey interactions during the Mesozoic Marine Revolution (MMR) profoundly altered the trajectory of marine tetrapod evolution. Here, we assess potential signatures of this landmark transition through the fossil record of skeletal pathologies in ichthyosaurs — iconic marine reptiles that developed increasingly ‘fish-like’ body plans over time. We surveyed a stratigraphically constrained sample of 200 Middle Triassic ichthyosaur specimens and compared the type, distribution and prevalence of pathologies with an approximately equivalent assemblage of Early Jurassic age. Overall, skeletal pathologies were equally prevalent in these groups, and most often manifested in species >4 m long. However, pathological bones were found to be concentrated in the hind limbs and tail of Triassic ichthyosaurs, whereas the jaws, forelimbs, and ribcage were preferentially affected in Jurassic taxa. We posit that the occurrence of ankylosed zygapophyses in the caudal peak of Triassic ichthyosaurs could represent a functional by-product of their primitive ‘eel-like’ swimming. Conversely, increased instances of broken ribs in Jurassic ichthyosaurs may infer ramming or tail strike behaviours that characterise morphologically ‘fish-like’ marine tetrapods, such as modern toothed whales. Different categories of skeletal pathologies thus evidently reflect structural modifications in the ichthyosaur body plan, and indirectly coincide with ecological turnover during the MMR.

Highlights

Changing predator-prey interactions during the Mesozoic Marine Revolution (MMR) profoundly altered the trajectory of marine tetrapod evolution
To test whether palaeoecological changes during the MMR exerted a measurable impact on the occurrence of skeletal pathologies in extinct marine tetrapods, we quantified a stratigraphically constrained sample of ichthyosaur fossils from the Middle Triassic Besano Formation of the Swiss-Italian Alps, and compared the type, distribution and prevalence of recognisable bone traumas and disease with an ecologically analogous assemblage recovered from the Lower Jurassic Posidonienschiefer Formation of southwestern Germany[10]
Our comprehensive survey of ichthyosaur skeletons from the Middle Triassic Besano Formation reveals a range and prevalence of diagnosable pathologies that is approximately equivalent to those reported from stratigraphically younger deposits[10]

Summary

Introduction

Changing predator-prey interactions during the Mesozoic Marine Revolution (MMR) profoundly altered the trajectory of marine tetrapod evolution. To test whether palaeoecological changes during the MMR exerted a measurable impact on the occurrence of skeletal pathologies in extinct marine tetrapods, we quantified a stratigraphically constrained sample of ichthyosaur fossils from the Middle Triassic Besano Formation of the Swiss-Italian Alps, and compared the type, distribution and prevalence of recognisable bone traumas and disease with an ecologically analogous assemblage recovered from the Lower Jurassic Posidonienschiefer Formation of southwestern Germany[10]. We treated all non-congenital skeletal damage incurred during the lifetime of the animal as pathological, including traumatic injuries, infection, post-natal articular ankyloses, and articular disease; this follows the classification scheme of Pardo-Pérez et al.[9] These approaches hypothesize that the ubiquitous constraints of an aquatic lifestyle will leave commensurate characteristics of pathological bone modification in fossil marine tetrapod assemblages given proportionate preservation, stratigraphical constraints and geographical representation. We aim to determine whether an analysis www.nature.com/scientificreports of skeletal pathologies in representative samples of Triassic versus Jurassic ichthyosaur skeletons can meaningfully track the large-scale changes in marine reptile ecosystems that occurred in conjunction with the MMR11

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