Not all seven ecotypes existed at once, although five existed simultaneously during the Early Jurassic period, when ichthyosaurs experienced a boom in diversity.
By the Middle Jurassic, the number of ichthyosaur ecotypes decreased. Specialized feeders, such as the swordfishlike Eurhinosaurus, and apex predators, including Temnodontosaurus, went extinct, leaving only two ecotypes, both of which lived in the open water.
These last two ecotypes included ichthyosaur genuses with large bodies and robust teeth for crushing bony fish or hard cephalopods, such as ammonites. The other ecotype was more dolphinlike; it had small teeth and likely ate soft prey, such as squid (also cephalopods), Dick said.
Ichthyosaurs eventually met their end during the Cenomanian-Turonian extinction event, in which spinosaurs (carnivorous swimming dinosaurs), plesiosaurs (long-necked marine reptiles) and roughly one-third of marine invertebrates (animals without a backbone) also went extinct, Dick said. [In Images: Digging Up a Swimming Dinosaur Called Spinosaurus]
With only two ecotypes of ichthyosaurs left, they would have been easily wiped out, Dick said.
"It's a slow ecological war of attrition, where they become more and more stranded on a single niche, and then the entire is depending on that niche remaining sustainable," he said. "And if that became unsustainable, then the entire group would become extinct."
It's unclear why ichthyosaurs lost their earlier niches, but they were likely "replaced, outcompeted by other species that adapted better," Dick said. For instance, plesiosaurs took over many of the near-shore niches, he said.
The study sheds light on ichthyosaurs' evolution and extinction, said Neil Kelley, a postdoctoral research fellow of paleobiology at the National Museum of Natural History in Washington, D.C., who was not involved in the new research.
According to the study, " get more and more confined to a specialized lifestyle," Kelley said. "Ultimately, they can never seem to re-evolve some of these more transitional lifestyles and body types that you see early on."
However, the study takes a broad view encompassing roughly 158 million years, so it loses some nuance in how these animals lived and why they went extinct, Kelley told Live Science. Furthermore, "just one weird fossil could totally rewrite that picture of what happened," by adding another ecotype, Kelley said.
The study was published online July 8 in the journal Biology Letters.
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