Float, explode or sink: postmortem fate of lung-breathing marine vertebrates

Abstract

What happens after the death of a marine tetrapod in seawater? Palaeontologists and neontologists have claimed that large lung-breathing marine tetrapods such as ichthyosaurs had a lower density than seawater, implying that their carcasses floated at the surface after death and sank subsequently after leakage of putrefaction gases (or ''carcass explosions''). Such explosions would thus account for the skeletal disarticulation observed frequently in the fossil record. We examined the taphonomy and sedimentary environment of numerous ichthyosaur skeletons and compared them to living marine tetrapods, principally cetaceans, and measured abdominal pressures in human carcasses. Our data and a review of the literature demonstrate that carcasses sink and do not explode (and spread skeletal elements). We argue that the normally slightly negatively buoyant carcasses of ichthyosaurs would have sunk to the sea floor and risen to the surface only when they remained in shallow water above a certain temperature and at a low scavenging rate. Once surfaced, prolonged floating may have occurred and a carcass have decomposed gradually. Our conclusions are of significance to the understanding of the inclusion of carcasses of lung-breathing vertebrates in marine nutrient recycling. The postmortem fate has essential implications for the interpretation of vertebrate fossil preservation (the existence of complete, disarticulated fossil skeletons is not explained by previous hypotheses), palaeobathymetry, the physiology of modern marine lung-breathing tetrapods and their conservation, and the recovery of human bodies from seawater.

Ichthyosaur embryos outside the mother body: not due to carcass explosion but to carcass implosion

“…The bottom, which was 50-150 m deep (important for the attack by Reisdorf et al (2012) on the putrefaction gases hypothesis), and the topmost decimetres of the bottom sediments would, according to some authors, have been 'soupy' during some intervals, probably nearly fluid (Hofmann 1958;Martill 1993). This would have enabled ichthyosaurs and other vertebrates that had sank to the bottom to become embedded in the bottom sediment (Schimmelmann et al 1994).…”

“…In a contribution by Reisdorf et al (2012) to the special issue of this journal devoted to taphonomic processes in terrestrial and marine environments (Wuttke and Reisdorf 2012), the postmortem fate of large lung-breathing marine vertebrates was dealt with. One of the aspects in this work that are of special interest for palaeontologists is the analysis of the event(s) that has led to the relatively common occurrence of (almost) intact skeletons of female ichthyosaurs, with bones of foetuses scattered nearby but outside the mother skeleton.…”

“…One of the aspects in this work that are of special interest for palaeontologists is the analysis of the event(s) that has led to the relatively common occurrence of (almost) intact skeletons of female ichthyosaurs, with bones of foetuses scattered nearby but outside the mother skeleton. As an example, Reisdorf et al (2012) mention a skeleton of a Stenopterygius specimen from the Posidonienschiefer (= Posidonia Shale) Formation at Holzmaden (Germany), stored at the Staatliches Museum für Naturkunde Stuttgart as number SMNS 50007 (Fig. 1).…”

“…Böttger (1990) detailed this further by ascribing the foetus bone positions to the displacement of the already disarticulated foetus embryos during the expulsion of putrefaction gases through the ruptured body wall of the mother. As Reisdorf et al (2012) state: "In spite of the lack of (direct) evidence for these processes, these ideas have never been questioned". Reisdorf et al (2012) attack the above putrefaction gases hypothesis, and come to the conclusion that this hypothesis is not tenable, at least not for the fossils preserved in the Toarcian (Early Jurassic) Posidonienschiefer Formation.…”

“…As Reisdorf et al (2012) state: "In spite of the lack of (direct) evidence for these processes, these ideas have never been questioned". Reisdorf et al (2012) attack the above putrefaction gases hypothesis, and come to the conclusion that this hypothesis is not tenable, at least not for the fossils preserved in the Toarcian (Early Jurassic) Posidonienschiefer Formation. Unfortunately, they do not explicitly state what other process(es) might be responsible for the foetus bones outside the mother skeleton, but they seem implicitly to adhere to the original bottom-current hypothesis of Osborn (1905).…”