Of Sirens and Ichthyosaurs

There have been some very interesting studies published this week regarding aquatic creatures: a living amphibian, and an extinct reptile, both very peculiar in their own way.

First, a new species of salamander has been described from a very bizarre and little-known family called the Sirenidae. The sirens are aquatic salamanders, known for possessing serpentine bodies, reduced forelimbs, no hindlimbs at all, and external gills even in their adult stage, like axolotls. Before this study, only four living species of siren were recognised, limited to the southern United States and northern Mexico. However, a type of siren found in wetland habitat in Alabama and Florida has been determined to be distinct from any currently recognised species. Nicknamed the “leopard eel” due to its distinctive spotted pattern, it has been given the scientific name Siren reticulata. Not only is it one of the largest amphibians alive today – with the biggest of the known specimens being over two feet long from nose to tail – but it is one of the largest new species of animal to be described in the United States in the past century!

Second, a fossil of an ichthyosaur – fish-like marine reptiles which lived at the same time as the dinosaurs – has been described which is so well preserved that it actually features the remains of soft tissues. The animal, which is named Stenopterygius and swam over what is now Germany about 180 million years ago, exhibits traces of pigment cells which indicate it was countershaded – darker on top and lighter on the belly. This colour pattern, found in many living marine vertebrates such as sharks, helps in camouflaging a sea-going animal, whether viewing it from above against darker water, or from below against the surface light.

Not only that, but Stenopterygius also possessed a layer of fatty blubber, like modern whales and seals, but unlike any living reptile with the exception of the leatherback turtleStenopterygius‘s blubber would have served as an insulator, allowing it to maintain a body temperature higher than its surroundings, as the leatherback turtle can – again, this sets them apart from most other reptiles, whose body temperature is dependent upon the environment. I love it when we discover new details like this that expand our image of extinct animals.

References:

Graham SP, Kline R, Steen DA, Kelehear C (2018), Description of an extant salamander from the Gulf Coastal Plain of North America: The Reticulated Siren, Siren reticulata. PLoS ONE 13 (12): https://doi.org/10.1371/journal.pone.0207460

Lindgren J, Sjövall P, Thiel V, Zheng W, Ito S, Wakamatsu K, Hauff R, Kear BP, Engdahl A, Alwmark C, Eriksson ME, Jarenmark M, Sachs S, Ahlberg PE, Marone F, Kuriyama T, Gustafsson O, Malmberg P, Thomen A, Rodríguez-Meizoso I, Uvdal P, Ojika M, Schweitzer MH (2018), Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur. Nature (2018); https://doi.org/10.1038/s41586-018-0775-x