Berlin, Nevada, is a treasure trove for paleontologists. Just down the street from now-abandoned gold and silver mines, a rocky collection of bones hints at an even richer past. The Berlin-Ichthyosaur State Park is teeming with dozens of ancient marine reptile fossils. That bone bed is so abundant and weird that researchers have been scratching their heads over it for decades.
“There are sites with much denser ichthyosaur skeletons, including sites in Chile and Germany,” said Nick Pyenson, curator of marine mammal fossils at the Smithsonian National Museum of Natural History. “But this place, Berlin-Ichthyosaur in eastern Nevada, has really eluded explanation for a long time.” In one quarry, at least seven individuals of the genus Shonisaurus — a bus-sized bloated dolphin with four limb-like fins — were essentially stacked on top of each other.
Previous hypotheses have largely focused on physical or environmental reasons for the cluster of fossils. One suggested that the animals stranded in shallow water and died as a group some 230 million years ago. Or maybe a volcanic eruption. Pyenson had another hunch, one his team tested using 3D visualizations of the location, as well as fossils and other clues in the geologic record.
In the journal Current Biology, Pyenson’s team writes today evidence that the shonisaurs came there to reproduce. The team concludes that the animals traveled long distances to give birth, as some whales do today. The discovery not only represents an example of “convergent evolution,” where the same traits evolve independently in different species, but is also the oldest example of migration in groups to a designated calving area.
“They make a pretty compelling case,” said Lene Liebe Delsett, a vertebrate paleontologist at the University of Oslo, Norway, who was not involved in the study. “Ichthyosaurs were the first large marine tetrapods. And throughout the Triassic they varied quite a bit, so there was great diversity. It’s just a really interesting period to learn more about.”
The shonisaur origin story begins with death – a lot of it.
Some 251 million years ago, between the Permian and Triassic periods, Earth’s largest extinction event wiped out about 95 percent of all marine species. This so-called “Great Dying” mowed down the ocean’s diverse landscape of creatures. Some of the animals that grew back in place turned out weirder and bigger than ever before.
The ensuing Triassic began an evolutionary arms race. Prey developed harder shells and better mobility, predators cracked through ammonite shells and hunted fish better than ever, and so on. Ichthyosaurs, which evolved from terrestrial reptiles into new species of varying sizes, partially pushed this pressure and quickly came to dominate the ocean. The genus Shonisaurus, in particular, grew to be one of the largest marine predators in existence. “They mostly reached whale sizes,” says Pyenson.
Pyenson is normally more of a whale man; he specializes in mammals, which split from reptiles some 325 million years ago. But ancient marine reptiles such as those under the order Ichthyosaur bear many similarities to extant marine mammals. Their ancestors came from land, they gave birth to live young, they had similar fins, and they are tetrapods, meaning they have four limbs. And Pyenson is well versed in these kinds of mysteries. About a decade ago, he and his South American collaborators in Atacama, Chile, used 3D mapping and chemical analysis to show that a tight-knit cluster of at least 40 fossilized whales must have died 7 to 9 million years ago from a toxic algal bloom.