The renowned Berlin-Ichthyosaur State Park, part of Humboldt-Toiyabe National Forest in the Shoshone Mountains of west-central Nevada, the United States, has been studied for decades, but explanations for why so many individuals of Shonisaurus popularis, a species of ichthyosaur that lived during the Triassic period, died there some 230 million years ago have eluded paleontologists.
Ichthyosaurs are a group of dolphin-shaped marine reptiles that flourished in the world’s oceans for millions of years during the Triassic, Jurassic and Cretaceous periods.
These creatures had an elongated body, a relatively small head, a long snout, flipper shaped limbs, and dolphin-like tail flukes.
Shortly before most of them became extinct some 200 million years ago — only the familiar dolphin-like species survived until 90 million years ago — they evolved into gigantic forms.
Most hunted fish or other small prey, but some fed on other marine reptiles, much like killer whales that hunt other marine mammal species today.
Their fossils have been found on every continent but they are particularly well known from Europe, east Asia and North America.
In the new research, Smithsonian National Museum of Natural History curator Nicholas Pyenson and colleagues examined a rich fossil bed in the Berlin-Ichthyosaur State Park in Nevada, the United States.
“We present evidence that these ichthyosaurs died here in large numbers because they were migrating to this area to give birth for many generations across hundreds of thousands of years,” Dr. Pyenson said.
“That means this type of behavior we observe today in whales has been around for more than 200 million years.”
Over the years, some paleontologists have proposed that the ichthyosaurs in the Berlin-Ichthyosaur State Park died in a mass stranding event such as those that sometimes afflicts modern whales, or that the creatures were poisoned by toxins such as from a nearby harmful algal bloom.
To solve this prehistoric mystery, Dr. Pyenson and co-authors combined newer paleontological techniques such as 3D scanning and geochemistry with traditional paleontological perseverance by poring over archival materials, photographs, maps, field notes and drawer after drawer of museum collections for shreds of evidence that could be reanalyzed.
The researchers collected tiny samples of the rock surrounding the fossils and performed a series of geochemical tests to look for signs of environmental disturbance.
One test measured mercury, which often accompanies large-scale volcanic activity, and found no significantly increased levels.
Other tests examined different types of carbon and determined that there was no evidence of sudden increases in organic matter in the marine sediments that would result in a dearth of oxygen in the surrounding waters.
“There are so many large, adult skeletons from this one species at this site and almost nothing else,” Dr. Pyenson said.
“There are virtually no remains of things like fish or other marine reptiles for these ichthyosaurs to feed on, and there are also no juvenile Shonisaurus popularis skeletons.”
The scientists found a key piece of the puzzle when they discovered tiny ichthyosaur remains among new fossils collected at the Berlin-Ichthyosaur State Park and hiding within older museum collections.
Careful comparison of the bones and teeth using micro-CT X-ray scans revealed that these small bones were in fact embryonic and newborn Shonisaurus popularis.
Further analysis of the various strata in which the different clusters of ichthyosaur bones were found also revealed that the ages of the many fossil beds of the Berlin-Ichthyosaur State Park were separated by at least hundreds of thousands of years, if not millions.
“Finding these different spots with the same species spread across geologic time with the same demographic pattern tells us that this was a preferred habitat that these large oceangoing predators returned to for generations,” Dr. Pyenson said.
“This is a clear ecological signal, we argue, that this was a place that Shonisaurus popularis used to give birth, very similar to today’s whales.”
“Now we have evidence that this sort of behavior is 230 million years old.”
“One of the exciting things about this new work is that we discovered new specimens of Shonisaurus popularis that have really well-preserved skull material,” said Dr. Randall Irmis, chief curator and curator of paleontology at the Natural History Museum of Utah.
“Combined with some of the skeletons that were collected back in the 1950s and 1960s, it’s likely we’ll eventually have enough fossil material to finally accurately reconstruct what a Shonisaurus popularis skeleton looked like.”
The findings appear today in the journal Current Biology.
Neil P. Kelley et al. 2022. Grouping behavior in a Triassic marine apex predator. Current Biology 32 (24): P5398-5405; doi: 10.1016/j.cub.2022.11.005
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