Stenopterygius was a Jurassic marine reptile that according to a research was warm-blooded

An article published in the journal “Nature” shows a molecular and microstructural analysis of a fossil of Stenopterygius, a marine reptile that belonged to the order of ichthyosaurs and lived about 180 million years ago. A team of researchers examined a fossil found in the Holzmaden quarry in Germany of which various soft tissues also got fossilized. This allowed to find chemical traces consistent with blubber, fatty tissue similar to whale fat and other aquatic or semi-aquatic mammals’ fat, all warm-blooded species. The conclusion is that Stenopterygius was warm-blooded as well.

Stenopterygius is a genus of ichthyosaurs that lived in the Early and Mid-Jurassic period in today’s Europe. It was similar to the reptiles of the genus Ichthyosaurus that give the name to the whole order but because of some different characteristics it was classified in a separate genus. Some fossils such as those of the German Holzmaden quarry have an extraordinary level of conservation that over the years allowed to discover some specific characteristics of Stenopterygius.

In particular, it’s the soft tissues that got fossilized in many Stenopterygius specimens, an event out of the ordinary that allowed to see a number of similarities between their physical characteristics and those of today’s animals such as dolphins and tunas. The skeleton of a female who was giving birth shows how these ichthyosaurs were ovoviviparous, which means that their eggs matured inside the mother’s body, and that the newborns came out tail-first, just like the modern ceteceans, an adaptation that prevented them from suffocating during birth.

Johan Lindgren, an associate professor at Lund University in Sweden and the lead author of this article, assembled a team to analyze one of the best Stenopterygius fossils, cataloged as MH 432 and part of the collection of a Holzmaden museum. The image (courtesy Johan Lindgren, all rights reserved) shows a picture of the fossil and a diagrammatic representation for a total of about 85 cm in length, about half of the original length of this animal.

The preservation of parts of the skin of this Stenopterygius specimen makes it possible to observe individual cell layers within it. Modern technologies, specifically some forms of spectrometry, together with microscopic analysis, help to understand the characteristics of the skin of this ichthyosaur because they indicate which proteins or products of their decay are present within it.

Among the findings there’s the evidence of the presence of subcutaneous adipose tissue similar to whale fat and other warm-blooded animals called blubber. The various characteristics of the skin are indeed more similar to those of mammals than to those of reptiles, also because the study shows that it lost its scales, a characteristic useful to improve the maneuverability under water and lower its drag resistance.

Paleontologists have long been suspected that Stenopterygius possessed that type of fatty tissue, but the simple observation of fossils wasn’t evidence good enough like this new chemical analysis. This is the most recent confirmation that ichthyosaurs were warm-blooded and weren’t the only water reptiles: an article published in May 2016 in the journal “Palaeontology” described a research whose conclusion is that mosasaurs were warm-blooded.

In the case of the Stenopterygius specimen, various types of spectrometric examinations were performed but there’s still the risk of false positives caused by a contamination that occurred after its death. Examining other specimens whose skin is in a sufficient state of preservation would offer a verification at Johan Lindgren’s team’s conclusion. This type of exam is still relatively new in the field of paleontology so further studies are absolutely needed.

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