Ichthyosaurs are an interesting challenge for evolutionary biologists. They are intrinsically interesting to anybody trying to build a coherent narrative of the development of life on Earth, since as a class of underwater lizard species, they are animals out of place. Whether it’s a squirrel in flight, a bird confined to the ground, of a bacterium surviving indefinitely under kilometers of arctic ice, determining how and why life moves into odd new environments is an obvious points of attack for anyone studying evolution.
However, the history of ichthyosaurs is poorly understood, and in the fossil record the group simply springs into existence a bit over 200 million years ago, already furnished with marine physiology and quite a bit of species diversity. The group’s development is also poorly understood, but now a new fossil from Scotland is shedding light on an abrupt and unexplained shift in their physiology.
The fossil in question was found in the Isle of Skye, and shows paleontologists a new species from about 170 million years ago, which they’ve termed Dearcmhara shawcrossi. Its age puts it near the beginning of a large-scale switch from smaller ichthyosaurs to larger ones, the cause of which is still unknown. It’s assumed that by studying the ichthyosaur species that bookended this period of change, we might be able to figure out why the newer, larger species came to dominate. Believe it or not, at a mere 14-foot length Dearcmhara was actually one of the smaller, earlier species. This puny paleo-shrimp would have been dwarfed by the 30-foot-plus monsters that largely replaced it.
As mentioned, it’s assumed that ichthyosaurs evolved from land-based lizards (a fair assumption), even though there are no fossils showing transitional forms. This is roughly like what would later happen in marine mammals, with land-adapted life finding new opportunities back in the water. Tetrapods, the classical “missing link” between sea- and land-based life, were probably less of a stepping stone in a bacterium-to-human evolutionary straight shot than they were a branching point. Ichthyosaurs were a line that saw incredible success, likely dominated the oceans as local apex predators, until they went abruptly extinct a little less than 100 million years ago. The predominant explanation is climate change, which makes sense since a wide swathe of species were all wiped out over a relatively short period of time.
Incredibly well-preserved specimens like these are not representative of average fossil quality, to say the least.
Whatever killed them, they had lifestyles rather like modern whales. Likely warm-blooded and air-breathing, they would have had to surface to breathe and hunt in generally similar fashions. Low, poorly adapted nostrils meant that they probably had to rear far out of the water to get access to air, unlike other air-breathing swimmers whose breathing is located more helpfully on their dorsal side. Ichthyosaur bone structure indicates that they were warm blooded, but there is still some controversy over whether all ichthyosaurs were indeed, in biological parlance, endothermic.
Dearcmhara would be one of the older pre-shift ichthyosaurs, and it’s rather incredible that any evidence of its existence is still visible at all. The fossils found here are very incomplete, as we’d expect given that it has been aging through multiple ice ages. Bear in mind that these are not robust elephant bones but something much closer to fish bones, or a pre-pre-pre-historic version of them. The existence of holes in the fossil record is an unavoidable side effect of history. If paleontologists had their druthers, they might actually opt for a 5,000-year-old Earth; it’d be a whole lot easier to study than what we’ve actually got.