World's Oldest Known Fossil Reptile Nests Discovered In Arizona's Petrified Forest

The fossil nests, dating to about 220 million years ago, are similar to modern day crocodile and turtle nests, said Stephen Hasiotis, the CU-Boulder research associate who discovered them. Hasiotis and colleague Anthony Martin of Emory University in Atlanta believe the nests extend the fossil record of reptile nests by roughly 110 million years.

The 62 bowl-like depressions found by the researchers in sandstone deposits appear to be trace fossils made by large, hole-nesting reptiles such as phytosaurs (primitive, crocodile-like animals), aetosaurs (armored reptiles from that period) or possibly ancient turtles, said Hasiotis. A paper by Hasiotis and Martin was presented at the annual meeting of the Geological Society of America held Oct. 25 to Oct. 29 in Toronto.

Located on the shoreline of an ancient river system, the nests -- which average about 12 inches wide and 18 inches deep -- are similar to modern day crocodile and turtle nests. All the nests were above the ancient water line of the river, the researchers said.

"The evidence now indicates this type of nesting behavior in reptiles has been continuous from at least the Triassic Period to the present," said Hasiotis, who received his doctorate from CU-Boulder in 1997. Hasiotis, who specializes in trace fossils, has previously been credited with discovering the world's oldest crayfish fossils as well as the oldest, bee, ant and termite nests.

Expansions below the narrow surface openings probably were used as egg chambers, which in some cases are overlain by broad, shallow and elliptical depressions that may represent "body pits" made by the egg-laying females, said the researchers. Several of the fossil chambers contain what appear to be scratch marks from digging activity and egg impressions, although no eggs or eggshells were found.

The fossil record for reptiles began in the Pennsylvanian Period beginning about 300 million years ago, said Hasiotis and Martin. The oldest previous evidence for reptile-nesting sites include 110 million-year-old sea turtle nests from the Front Range of Colorado and nests of a dinosaur group known as Hadrosaurus from Montana, estimated to be about 90 million years old.

Hasiotis first discovered the ancient reptile nests while conducting research in Arizona's Petrified Forest in 1996. He invited Martin to the site in 1997 and the two returned to conduct additional research in 1998. Surrounding some of the ancient nests are broad, shallow pits or irregular, trampled ground that appears to contain vertebrate tracks, Hasiotis said. The researchers also discovered distinct tracks of phytosaurs about one-half mile from the nesting sites.

The walls in the interior of the nests appears to have been compacted down by the inhabitants. Contemporary crocodile mothers tend to stay near their nests and guard them from predators, while turtles leave their nests immediately after the egg-laying process, said Hasiotis.

Some of the fossil nests are located adjacent to each other, said Hasiotis and Martin, who noted that modern crocodilians and sea turtles sometimes dig between five and 10 pits in an area before picking the appropriate place to lay their eggs. The research project was funded by the Petrified Forest Museum Association in Arizona.