Ichthyosaurs have been strange animals. They looked like dolphins but were reptiles; her name means “fish lizards”. They didn’t lay eggs, instead they gave birth to their young alive. And when they reproduced, the ancient animals probably showed a behavior similar to that of whales. Scientists led by Neil Kelley from Vanderbilt University in Nashville, USA, have now found this out. As in the magazine Current Biology to report, the dinosaurs went to special places to bring their offspring into the world.
Kelley’s researchers studied seven ichthyosaur skeletons in the Luning Formation in Nevada’s West Union Canyon. When the dinosaurs were alive, around 250 to 90 million years ago, the region lay on a coast of the supercontinent Pangea and was covered with water. Ichthyosaurs is a collective term for dozens of species that could grow to be between 1 and 20 meters long. Depending on their own body size, they fed on everything from small fish to larger sea reptiles. In the Luning Formation, researchers found numerous bone fragments of the Shonisaurus, a subspecies of ichthyosaur. They only identified fossils of adult specimens and a few of embryos. However, the researchers did not find any remains of fish or marine reptiles on which the animals fed.
The animals had been returning here time and time again for hundreds of thousands of years
The scientists then looked for possible geological or biological reasons why there were so many skeletons at this point. However, they found no evidence that environmental disturbances triggered mass extinctions, such as changes in the carbon cycle, volcanic eruptions or a toxic alga. For example, they looked at the chemical composition of the rocks surrounding the fossils, said Randall Irmis, curator of paleontology at Utah’s National History Museum and co-author of the study, according to a University of Utah statement. In doing so, they would have looked for signs that environmental conditions had brought so many animals there. “When we determined that this was not the case, we focused on the possible biological reasons.”
The researchers created a colored, high-resolution model of the site. “A 3-D model would allow us to study the way these large fossils were arranged in relation to one another without losing the ability to examine bone by bone,” said Neil Kelley, lead author of the study. The model gave the researchers a clear overview. They also recognized that the groups of ichthyosaurs lay in different fossil layers. So the animals had come to this place over at least hundreds of thousands if not millions of years.
The researchers conclude that the reptiles had apparently visited these waters in large groups to give birth – and then shuttled back and forth between this place and a feeding site. Apparently there were few predators in this environment, so the newborns could be born safely. Group migration behavior is also typical for vertebrates in the sea, such as baleen whales. It is unclear why so many animals died at the birth site.