Some other, smaller Stenopterygius individuals described in previous studies also have skin pigments, but they appear to be uniformly dark in color. This species may have changed its coloration as it grew up, going from dark juveniles to countershaded adults.
Incredibly, the branching structure of the pigment cells hints that ichthyosaurs may have been able to adjust their skin tone to lighter or darker shades.
Below the cutus is a thick layer of massive and fibrous tissue that Lindgren and colleagues interpreted as blubber. The researchers conducted experiments on porpoise skin to simulate the fossilization process, and found that the porpoise’s blubber looked almost identical to what they saw in Stenopterygius.
In living animals, blubber is only found in whales, dolphins, seals, manatees, dugongs, and leatherback turtles. Blubber helps streamline the body and provides buoyancy, but it’s only found in warm-blooded animals as its primary purpose is heat retention in frigid waters.
The presence of blubber in Stenopterygius adds to a growing list of evidence showing that ichthyosaurs, like dolphins, were also warm-blooded. This list also includes fast-growing vascularized bones, and oxygen isotope ratios in bones and teeth showing that the extremities were not significantly colder than the body’s core.
Ichthyosaurs evolved from terrestrial lizard-like ancestors 250 million years ago in the Early Triassic Period. They quickly became fully aquatic and dolphin-like, and filled a similar role in the oceans of the Mesozoic. Unable to return to land to lay eggs, ichthyosaurs gave birth to live young. They died out about 90 million years ago in the middle part of the Cretaceous Period, not long after a period of ocean anoxia, or low oxygen content.
This new specimen has allowed us to know much more about ichthyosaur appearance, behavior, and physiology, further reinforcing their similarities to dolphins. Their uncanny similarities are an example of convergent evolution, where different lineages end up looking alike because of similar selective pressures. New discoveries will only add to our understanding of these incredible marine predators of the Mesozoic.
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, Rodriguez-Meizoso I, Uvdal P, Ojika M, Schweitzer MH. 2018. Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur. Nature. https://doi.org/10.1038/s41586-018-0775-x