Why giant marine reptiles gathered on the territory of modern Nevada is still unknown
About 230 million years ago, huge dolphin-like reptiles called ichthyosaurs gathered to breed in safe waters, like many modern whales.
Researchers came to this conclusion after studying a mysterious cemetery of ichthyosaurs in the Berlin Ichthyosaur Park in Nevada. The park is home to the world’s richest collection of fossils Shonisaurus popularis one of the largest ichthyosaurs which have ever been discovered.
“That’s what we see in modern marine vertebrates — gray whales make the trip to Baja California every year” to breed, says Randall Irmis, a paleontologist at the National Museum of Utah History in Salt Lake City. Protected warm water provides safety for whales.
Nova find described on December 19 in Current Biology , shows that this behavior “goes back at least 230 million years,” Irmis says. “It really ties the past to the present in a big way.”
The idea of ichthyosaur birth zones has been proposed before, and is even known enough to be often included in artists’ depictions of the creatures, says Erin Maxwell, a paleontologist at the State Museum of Natural History in Stuttgart, Germany, who was not involved in the new study. But, she said, this study “is the first to support these assumptions with data.”
Ichthyosaur fossils in Nevada have puzzled paleontologists for decades. One of the points of interest is the many ichthyosaur fossils clustered in what is now a park, but was part of a tropical sea about 230 million years ago. Another oddity is that this place looks as if it is almost entirely inhabited by giant 14-meter-tall adults S. popularis . And here there is an exciting question about what caused the death.
Scientists had previously speculated that the reptiles, which as adults could be about the length of a school bus, clustered together for some unknown reason before something caused their mass death.
Several pockets, or quarries, of specimens are scattered throughout the park. In total, Irmis and his colleagues identified at least 112 ichthyosaurs in these quarries, including one site where park officials left previously discovered bones half-encased in rock for public viewing.
According to Neil Kelly, a paleontologist at Vanderbilt University in Nashville, this snapshot of death meant scientists could examine how the fossils were positioned relative to each other, possibly offering insight into the reptile’s behavior.
Kelly, Irmis and their colleagues used digital cameras and a laser scanner to collect hundreds of measurements of the bone bed of half-buried reptiles, combining the data into a 3-D model of the site. The team also studied the sizes and shapes of bones from around the park, including some now in museum collections. The researchers analyzed the chemical composition of the surrounding rocks and delved into older photographs and field records.
These bits of evidence helped researchers understand what they were looking at — and perhaps solve at least one long-standing mystery: what brought these creatures together.
Despite the fact that almost all skeletons Shonosaurus in the park are adults, scientists have discovered that there are several very tiny remains of ichthyosaurs at the site. Using micro-computed tomography, a three-dimensional imaging technique that uses X-rays to see inside fossils, the researchers discovered that some of the tiny bones belonged to an embryo and a newborn Shonosaurus .
The find led the team to conclude that this place was the birthplace. Researchers say this could explain why there were so many of the same creatures in one place alongside the newborns.
The place also seems to have been a birthplace Shonisaurus for a long time. Instead of all the quarries dating to roughly the same time, the different quarries are separated by at least hundreds of thousands of years, the researchers found.
As for what killed the reptile, “we don’t know,” Irmis says.
Among the hypotheses of mass mortality were harmful algal blooms or large-scale volcanic activity. But new data on the rock’s chemistry ruled out these events as the culprits.
Some of the animals in each quarry could still die en masse. Grouping the creatures together to breed could have made the reptiles vulnerable to a sudden disaster that buried them in sediments, such as an underwater landslide.
But the fossils could also indicate “just normal mortality over time,” Irmis says, given that the creatures seem to have visited the site again and again.
