In a small area in the US state of Nevada, researchers find an unusual number of ichthyosaurs – remains of dozens of ichthyosaurs, each about the size of a bus. There has long been speculation about the reason for the accumulation. Now there are crucial clues to solving the puzzle.
For years, paleontologists have been puzzling over the finds of dozens of fossilized fish dinosaurs in a small area in the US state of Nevada. Now, an international research team has come up with an explanation as to why remains of at least 37 school bus-sized ichthyosaurs were discovered in Berlin-Ichthyosaur State Park (BISP). These marine reptiles, which lived between 250 and 66 million years ago, migrated seasonally to areas where they gave birth to their young around 230 million years ago.
This is similar to the migrations of modern-day whales, writes the team led by Neil Kelley from the National Museum of Natural History in Washington. “We present evidence that these ichthyosaurs died out here in large numbers because generations of them migrated to this area over hundreds of thousands of years,” says co-author Nicholas Pyenson, curator at the natural history museum. “This means that the behavior we see in whales today has been around for more than 200 million years.”
At that time, the area in Nevada was on the edge of the primordial ocean Panthalassa, which surrounded what was then the continent of Pangea. The approximately two square kilometer find area contains remains of dozens of individuals of the species Shonisaurus popularis. These animals, whose shape resembled today’s dolphins, were up to 16 meters tall – their skull alone was as long as a human being. The Quarry 2 site received particular attention, where at least seven Shonisaurus individuals were found close together – they presumably died at the same time.
Researchers speculated early on about the cause of the accumulation: assumptions ranged from mass strandings to mass poisoning, for example due to algal blooms or natural disasters. By analyzing the rock strata, the team initially ruled out to a large extent that environmental changes cost the animals their lives at the time. They found no evidence of volcanism, such as traces of mercury. There was also no indication of a generally increased mortality of sea creatures there, triggered, for example, by changes in the oxygen content in the water. The team also ruled out stranding as a cause of death.
“There is no direct geological evidence of any major environmental disturbance either concurrent with, immediately preceding or subsequent to the deposition of the bones at Quarry 2 to explain a cause of the die-off,” the team writes in the journal Current Biology.
Instead, it proposes a different explanation – backed up with concrete evidence. The researchers first noticed that the rocks contained hardly any other vertebrates apart from the Shonisaurus specimens, but rather only small invertebrates such as ammonites, extinct cephalopods. “There were virtually no remains of animals such as fish or other marine reptiles that these ichthyosaurs fed on,” explains Pyenson. The animals, he concluded, probably fed in a different region.
At the same time, only remains of adult shonisaurs were found in the area, measuring 11 to 16 meters in length, but no evidence of juveniles. The decisive clues were then provided by a targeted search in fossils that came from the find area and are now stored in museums. Here the team found tiny bones and teeth that came from three either newborn or unborn Shonisaurs.
“When it became clear that there was nothing for them to eat here and that these were large, adult shonisaurs with embryos and newborns, we began to seriously consider whether this might not have been a region for the birth of the offspring,” says Kelly.
The deposits across different layers of rock show that the ichthyosaurs visited this area for at least hundreds of thousands, maybe even millions of years. One reason for this could be that the newborn offspring were less threatened by predators here.
The researchers suspect that the animals move back and forth seasonally between feeding and breeding waters – the pronounced body size also indicates long migrations. This is a parallel to the migration of modern whales such as blue or humpback whales. This behavior could therefore be at least 230 million years old.