October 11, 2007—Odds are 50/50 that the dinosaur that left this mark in Montana was a Tyrannosaurus rex, says the scientist who announced the find yesterday.

Whatever it was, it was big.

"We have two large theropods to choose from [in this region], Nanotyrannus and T. rex," Phil Manning (pictured) told National Geographic News via email. "So either [dinosaur] (or an unknown) could have generated the track."

Referring to some breathless headlines of the past 24 hours, he added, "You can imagine which one the media wants!

"Unless you find an animal dead in its tracks, it is nigh impossible to identify the track maker," added the University of Manchester paleontologist, whose work is partially funded by the Expeditions Council of the National Geographic Society (which also owns National Geographic News).

If confirmed, it would be only the second known T. rex print. The first was found in 1983 in New Mexico.

Manning originally saw the three-foot-wide (meter-wide), 65-odd-million-year-old track in 2006 in the dinosaur-rich Hell Creek formation, and he is preparing a report for full scientific review.

In the meantime, he wrote, "we intend to keep the site secret until suitable measures can be taken to preserve the track."

—Ted Chamberlain

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