An Ichthyosaur is a cross between a fish and a dinosaur, that looks like a dolphin.
The word comes from the ancient Greek for “Fish Lizard.” One wonders what they were smoking.
But seriously, this category of creature is an entire order unto itself, part of the Reptile class. They first appear in the fossil record around 250 million years ago, and dwindled into extinction about 90 million years ago. There origin occurs at about the same time as the beginning of the Triassic, becoming diversified and abundant by the end of the triassica nd early Jurassic.
Very little is known about how the Ichthyosaur first evolved from a land-based reptilian stock. A separate group, called the Hupehsuchia, date to the very early Ichthyosaur times, and resemble them, but with distinct differences. Hupehsuchia did not last very long. It is likely that Hupehsuchia represent a link between an unknown pre-aquatic reptile and actual Ichthyosaurs, in indirectly, and arose from near the base of the Ichthyosaur linage. This is debated.
But never mind, for now, the early beginnings of the enigmatic fish lizard. Consider instead what it ended up becoming. Even early in their evolutionary history, Ichthyosaurs really do resemble modern day toothed whales, especially the dolphins and porpoises. Here are the skeletons of an Ichthyosaur and a Dolphin compared.
The differences are not small, but they relate more to the underlying skeletal patterns in reptiles vs mammals. For example, reptiles have a lot of vertebra and thus ribs, and the number seems to vary evolutionary across genera as needed, while mammal vertebrae and rib patterns are much more constrained. Also, those weird sclerotic plates in the eyes … these are eye bones found across vertebrates but not mammals (or crocodilians).
So, Ichthyosaurs are basically reptilian dolphins in overall appearance. It turns out they are dolphinesque in another important way as well, according to just published research.
In Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur by Yohan Lindgren, Peter Sjövall, Volker Thiel, Wenxia Zheng, Shosuke Ito, Kazumasa Wakamatsu, Rolf Hauff, Benjamin P. Kear, Anders Engdahl, Carl Alwmark, Mats E. Eriksson, Martin Jarenmark, Sven Sachs, Per E. Ahlberg, Federica Marone, Takeo Kuriyama, Ola Gustafsson, Per Malmberg, Aurélien Thomen, Irene Rodríguez-Meizoso, Per Uvdal, Makoto Ojika & Mary H. Schweitzer, we learn:
Here we show that this resemblance [to Dolphins] is more than skin deep. We apply a multidisciplinary experimental approach to characterize the cellular and molecular composition of integumental tissues in an exceptionally preserved specimen of the Early Jurassic ichthyosaur Stenopterygius. Our analyses recovered still-flexible remnants of the original scaleless skin, which comprises morphologically distinct epidermal and dermal layers. These are underlain by insulating blubber that would have augmented streamlining, buoyancy and homeothermy. Additionally, we identify endogenous proteinaceous and lipid constituents, together with keratinocytes and branched melanophores that contain eumelanin pigment. Distributional variation of melanophores across the body suggests countershading, possibly enhanced by physiological adjustments of colour to enable photoprotection, concealment and/or thermoregulation. Convergence of ichthyosaurs with extant marine amniotes thus extends to the ultrastructural and molecular levels, reflecting the omnipresent constraints of their shared adaptation to pelagic life.
So they may have been homeothermic (warm blooded, more or less). They may also have had countershading seen in today’s fishy and other swimmy things, with a lighter belly and darker flanks. This may have served as both camouflage and protection from UV light.