Paleontologists May Have Solved the Mystery Behind a Prehistoric Reptile Graveyard

Ichthyosaur mothers likely migrated to the site to give birth

Shonisaurus Illustration
A reconstruction of adult and newly born Triassic ichthyosaurs Shonisaurus Gabriel Ugueto

No one knew why so many immense reptiles were bunched together. In the middle of the Nevada desert, in rocks dating back over 225 million years, paleontologists uncovered the remains of at least seven immense marine reptiles called ichthyosaurs in one place. Explanations for the creatures’ death have run the gamut from a mass stranding on Triassic shores to a toxic algal bloom, but a new analysis published Monday in Current Biology offers a compelling theory of why they were together in the first place. The ichthyosaur pileup occurred in waters where the great saurians were gathering to give birth.

Many bones and skeletons of the ichthyosaur Shonisaurus have been found in Nevada’s Berlin-Ichthyosaur State Park over the course of more than a century, but most famous of all is what 20th-century paleontologist Charles Camp simply called “quarry 2.” Paleontologists have been stumped to why the marine reptiles—which could reach sizes comparable to today’s humpback whales—have been found in such startling abundance. But by combing over the quarry as well as the fossils in the broader rock layers around it, Smithsonian National Museum of Natural History paleontologist Nicholas Pyenson and colleagues have proposed a new interpretation of this site. Hundreds of millions of years ago, Shonisaurus traveled to this place as part of their life cycle.

The key to unraveling the mystery is in the fossiliferous demographics of the Shonisaurus preserved in quarry 2. Almost all of the specimens documented inside the quarry are adults. Almost. But fossils of embryonic Shonisaurus as well as those that had only just begun to swim on their own were also in the area. “Embryonic material was mentioned from the locality in a 1980 monograph but wasn’t figured, and no details were provided in the publication,” says Erin Maxwell, a paleontologist at the State Museum of Natural History in Stuttgart, Germany, who was not involved in the new study.

The new paper finally documents these fossils, interpreting them as evidence that Shonisaurus were coming to this place to give birth. “We found a dominance of adult-sized Shonisaurus and then a smaller bump of embryonic to neonatal specimens,” Pyenson says. No fossils of juvenile Shonisaurus were found by the team, which experts would expect if the deposit represented an ecosystem struck by unexpected disaster like volcanic activity or toxic plankton.

Ichthyosaur Tooth
A hand holds an ichthyosaur tooth. Natural History Museum of Utah

The new research is an extension of what Pyenson and other researchers have studied at places like Cerro Ballena in Chile’s Atacama Desert. There, paleontologists have found dozens of skeletons of prehistoric whales and other marine mammals that appear to have died during toxic algal blooms and washed onto a tidal flat. Pyenson and colleagues wanted to see if something similar had happened at Berlin-Ichthyosaur and applied some of the same research techniques. “We applied laser scanning, photogrammetry and computer vision workflows in the same fashion,” Pyenson says. The multiyear effort created a digital data set of the site that allowed for a broader analysis than simply looking at museum specimens.

“It’s a really fascinating site, and it’s exciting to see new research being focused on this important ichthyosaur graveyard,” says University of Manchester paleontologist Dean Lomax, who was not involved in the new study.

Previous studies proposed that Shonisaurus lacked teeth and was a gentle giant, filter feeding or slurping up ancient cephalopods. But this interpretation overlooked some of Camp’s finds from the 1950s. Shonisaurus had large, sharp teeth, indicating that this ichthyosaur was a member of the “cut guild,” Pyenson and colleagues report, and was more like an immense killer whale.

Shonisaurus seems almost out of place where the majority of the fossils have been found. The problem is that there doesn’t seem to be any prey large enough for such a carnivore in the same rocks other than Shonisaurus itself. “I am surprised by the lack of non-Shonisaurus remains at the locality, especially the apparent dearth of bony fishes,” Maxwell says. Lacking evidence of cannibalism, therefore, the paleontologists propose that Shonisaurus did their hunting and feeding elsewhere and deposited their babies in the warm, relatively predator-free waters of what eventually became Berlin-Ichthyosaur. “We infer Shonisaurus congregated close to what was then the Triassic coastline, even though it was fairly deep water,” Pyenson says. The pattern is similar to that of whales that travel to give birth in places like Monterey Bay, just in a different oceanic setting.

If the paleontologists’ migratory hypothesis is correct, then Shonisaurus returned to the same area time and again to give birth over during a span of more than 100,000 years.

Precisely why so many Shonisaurus perished and were buried in this relatively small geographic area is unclear. “It would be fair to say that Berlin-Ichthyosaur represents two mysteries,” Lomax says, “why so many Shonisaurus were found together, and what killed them.” The new study addresses the first question, but the second remains open, a puzzle whose clues still reside in the Triassic rock. Experts don’t know why several marine reptiles might have died at once, but we now have a better idea of why they gathered in the first place—to leave their offspring in the spot that they themselves likely started their lives.