T. magnoliana dates to a time before sea levels had fallen enough for primates to cross over to North America from Europe, Beard said.
"We know the sea level was high when our fossil primates lived in Mississippi, because the actual bed that yielded our fossils is a marine bed," he noted.
Most of the fossils at the site were shark teeth and similar objects. The primate's skeleton, he said, likely washed into the shallow estuary from the coastline.
A second fossil bed in a higher layer was laid down by a river or stream, not an ocean, indicating that sea level continued to fall after the primate fossils were deposited, Beard added.
The same sea-level imprints are found in the rock section where Teilhardina was found in Europe. This indicates that Teilhardina arrived there after the sea level had dropped.
"So we know the Teilhardina fossils we're finding in Mississippi are older than the ones that have been found in Europe and along with that they are anatomically more primitive," Beard said.
Philip Gingerich is a paleontologist at the University of Michigan in Ann Arbor. He argued in a 2006 PNAS study that ancient primates migrated to North America via Europe.
Gingerich's study was based largely on comparison of carbon isotope signatures from the fossil beds where the ancient primates were found in Europe, Asia, and Wyoming. Scientists use the signatures to identify and date events within the PETM.
Beard acknowledges that his study lacks an appropriate carbon isotope signature.
Gingerich said the missing data makes Beard's claims impossible to validate.
Sea level rose and fell several times during the PETM, making geologic markings difficult to interpret, Gingerich added.
"We're talking about events here that could be separated by as little as 10 or 15 or 20 thousand years," he said.
But though Beard has been unable to find the carbon isotope record at his site in Mississippi, he said he has found an aquatic microorganism that is associated with the record. That, combined with the other geologic evidence, makes him confident in his result.
T. magnoliana is best suited to live in hot, muggy climates, Beard added.
So scientists can infer that even the northern reaches of North America were forested, warm, and wet when these creatures migrated across the Bering land bridge, Beard said.
If T. magnoliana indeed predates the Teilhardina find in Wyoming, it would also indicate the primates stuck to the coasts for tens of thousands of years before the climate changed enough for them to migrate inland.
"It took time for the North American ecosystems, especially in the interior part of the continent, to kind of adapt to this big warming event," Beard said.
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