Ichthyosaurs were one of the most successful lineages ever. These dolphin-looking marine reptiles of the Mesozoic Era evolved to occupy a vast range of niches: Some were massive apex predators hunting in the open ocean, others were small suction feeders swimming in shallow seas. All ichthyosaurs went extinct tens of millions of years before the dinosaurs were wiped out during the famous Cretaceous-Paleogene extinction event. But we still don’t know why. According to new “ecospace” modeling results published in Biology Letters this month, ichthyosaur species became so specialized that they were restricted to a very specific way of life. When one went extinct, another wasn't able to replace it.
To see what sorts of narrow niches they occupied, Daniel Dick and Erin Maxwell from Staatliches Museum für Naturkunde Stuttgart examined fossils from 45 different ichthyosaur genera, paying particular attention to body sizes, teeth shapes, and feeding strategies. They grouped these ichthyosaurs into seven genetically and geographically distinct categories called ecotypes. These include shallow-water suction, soft-prey specialist, apex pelagic (or open water), slash pelagic, and pelagic generalist. A lot of known ichthyosaurs belonged to what they called circalittoral generalist: They were small, had robust and blunt posterior teeth, and they lacked open-water swimming adaptations like a long body.
D.G. Dick & E. E. Maxwell / Biology Letters 2015.
Not all seven ecotypes occurred at once, Live Science explains, though five of them existed simultaneously during the Early Jurassic. But by the Middle Jurassic, their ecotypes decreased. Some specialized feeders, such as the swordfish-like Eurhinosaurus (slash pelagic), as well as hypercarnivorous predators like the massive Temnodontosaurus (apex pelagic), went extinct. It’s unclear why this happened exactly, though it’s possible that plesiosaurs, for example, took over many of their niches.
By the Late Cretaceous, only two ecotypes were left – a generalist and a soft-prey specialist – and both lived in the open water. With only two ecotypes left, they would have been easily wiped out, Dick says. Sure enough, we lost all the ichthyosaurs during the Cenomanian–Turonian extinction event around 90 million years ago.
“It's a slow ecological war of attrition, where they become more and more stranded on a single niche, and then the entire [group] is depending on that niche remaining sustainable,” Dick tells Live Science. “And if that became unsustainable, then the entire group would become extinct.” The ichthyosaurs simply weren’t able to “re-evolve” their more ancient, more transitional body types.