Dinosaur-Era Nursery

An ancient field of death?

Actually, quite the opposite.

The mystery has long haunted paleontologists : What could explain a mass death event recorded by fossils of large marine reptiles from the dinosaur era in the West Union Canyon in Nevada?

The area, once part of a shallow tropical ocean, was a habitat for whale size ichthyosaurs : Shonisaurus popularis.

One theory was that this was the site of an ancient mass stranding event, like those by whales today. But that hypothesis did not match the sedimentology : There was no sign of tidal flats or sandbars to trap the animals.

To rule out poisoning or asphyxiation from, say, volcanic eruptions, a team from the Natural History Museum of Utah assessed levels of mercury and oxygen in the rock and found nothing suspicious.

Researchers finally found their answer in the dusty cabinets of museum collections : tiny Shonisaurus bones from the canyon that couldn't have belonged to adults.

Further field work turned up more signs of embryonic and newborn ichthyosaurs. Rather than a scene of mass death, the canyon seems to have been a wellspring of mass life : a birthing ground. Some animals died there, because so many of them gathered there.

''I think this was a place where giant ichthyosaurs came to give birth,'' said Nicholas Pyenson, a paleontologist at the Smithsonian and an author of the study.

That behavior would be similar to the way today's whales and sharks routinely migrate from one place where they feed to another where they give birth. [ Darren Incorvaia ]


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