Deep in the stone at the Berlin-Ichthyosaur State Park (BISP) in Nevada’s Humboldt-Toiyabe National Forest, many 50-foot-long ichthyosaur (Shonisaurus popularis) specimens lay petrified and frozen in time. This order of extinct marine reptiles (and Nevada’s state fossil) looked like a chunky dolphin and lived during the late Triassic age, roughly 237-227 million years ago.
New research also suggests that the predator may have performed similar migrations to modern whales. Today’s blue and humpback whales make annual migrations thousands of miles across oceans to breed and give birth in regions where predators are scarce. Many of these whales gather together year after year along the same stretches of coastline.
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Shonisaurus may have done something very similar. An international team of researchers published their findings Monday in the journal Current Biology, explaining how at least 37 of these marine reptiles died in the same location—a question that has stumped paleontologists for more than 50 years.
“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,” said co-author and Smithsonian National Museum of Natural History curator Nicholas Pyenson, in a statement. “That means this type of behavior we observe today in whales has been around for more than 200 million years.”
Some paleontologists have proposed that BISP’s ichthyosaurs died in a mass stranding event similar to the ones seen in whales today, or that a harmful algal bloom may have poisoned the animals. But these hypotheses do not have strong scientific evidence supporting them.
To try to solve this prehistoric puzzle, the team combined 3D scanning and geochemistry and combed through archival materials, photographs, maps, and field notes, for shreds of evidence.
Within BISP is a barn-like building that researchers call Quarry 2, which houses partial skeletons from an estimated seven individual ichthyosaurs that all appear to have died around the same time.
“When I first visited the site in 2014, my first thought was that the best way to study it would be to create a full-color, high-resolution 3D model,” lead author Neil Kelley, an assistant professor of geology at Vanderbilt University, said in a statement. “A 3D model would allow us to study the way these large fossils were arranged in relation to one another without losing the ability to go bone by bone.”
The team then collaborated with Jon Blundell, a Smithsonian Digitization Program Office’s 3D Program team member, and Holly Little, informatics manager in the museum’s Department of Paleobiology. Little and Blundell used digital cameras and a spherical laser scanner to take hundreds of photographs and millions of point measurements. These were then stitched together using specialized software to create a 3D model of the fossil bed while the paleontologists on the team physically measured the bones.
“Our study combines both the geological and biological facets of paleontology to solve this mystery,” co-author Randall Irmis, a paleontology professor at the University of Utah and the chief curator of the Natural History Museum of Utah’s Department of Geology & Geophysics, said in a statement. “For example, we examined the chemical make-up of the rocks surrounding the fossils to determine whether environmental conditions resulted in so many Shonisaurus in one setting. Once we determined it did not, we were able to focus on the possible biological reasons.”
Geochemical tests in the rock didn’t reveal any signs that these ichthyosaurs died due to a major environmental event like a harmful algal bloom that would have also disturbed the ecosystem. They expanded their search beyond Quarry 2 to the surrounding geology and fossils that scientists had previously excavated from the area.
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The geologic evidence showed that when the ichthyosaurs died, their bones sank to the bottom of the sea over time instead of collecting along the shoreline, which would have suggested stranding. The area’s mudstone and limestone were also full of large adult Shonisaurus specimens but not as many specimens of other marine vertebrates.
“There are so many large, adult skeletons from this one species at this site and almost nothing else,” said Pyenson. “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 skeletons.”
After ruling out the algae and stranding hypotheses, the team found a key clue in tiny ichthyosaur remains among some of the new fossils collected at the park and hiding within older museum collections. Micro-CT x-ray scans and a comparison of the bones and teeth showed that the small bones were embryonic and newborn Shonisaurus.
“Once it became clear that there was nothing for them to eat here, and there were large adult Shonisaurus along with embryos and newborns but no juveniles, we started to seriously consider whether this might have been a birthing ground,” said Kelley.
Additional analysis revealed that the ages of the many fossil beds of BISP were actually separated by at least hundreds of thousands of years, if not millions of years.
“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,” said Pyenson. “This is a clear ecological signal, we argue, that this was a place that Shonisaurus used to give birth, very similar to today’s whales. Now we have evidence that this sort of behavior is 230 million years old.”
The next step for this research is to look into other ichthyosaur and Shonisaurus sites in North America with these new findings in mind. It will help scientists recreate this ancient world by looking for other breeding sites or places with greater diversity of other species that could have provided rich feeding grounds for this extinct apex predator.
“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,” Irmis said. “Combined with some of the skeletons that were collected back in the 1950s and 1960s that are at the Nevada State Museum in Las Vegas, it’s likely we’ll eventually have enough fossil material to finally accurately reconstruct what a Shonisaurus skeleton looked like.”