But the biggest surprise turned out to be what was inside skeletons of small fish that used the clams as cover from predators. In a seascape without rocks or reefs, the clams must have played a vital role in this ancient marine ecosystem.
"They provide important habitat for oysters growing on top and fishes hiding inside," said paleontologist Ken Carpenter, the museum's director.
Mancos Shale, mudstone deposited on a sea floor between 80 and 95 million years ago, has yielded smaller clams and ammonites in Utah, as well as giant clams in Colorado.
Logan and Carpenter were actually looking for the fossilized bones of mosasaurs, a predatory seagoing lizard, when they stumbled upon the clams on federal land about five miles north of Green River.
"The ground was littered with fragments of shell. What astounded me was some of the shell was an inch thick. The more we walked we saw more complete clams," Carpenter said.
They returned with volunteers to extricate two fragile fossils and encase them in plaster. Logan designed a display for a specimen that in life was 3 to 4 inches thick and weighed 50 pounds. That one, measuring 44 by 48 inches, is now on display in the Price museum. The other was shipped to USU's main campus in Logan.
During the late Cretaceous when these bivalves lived, eastern Utah was covered by a shallow sea, and shellfish inhabited the muddy floor and the mosasaur probably cruised these waters in search of prey.
"An ocean split North America in half. The western land mass was where a lot of the dinosaurs lived that have been excavated in the Grand Staircase. Off shore from that it was a fairly flat, muddy bottom," Carpenter said. "The only life that occurred was these clams that had such a large surface that they didn't sink into the mud. They acted as islands, giving other organisms a surface to grow on."
These clams are members of the genus Platyceramus (Latin for "flat clam"), the largest bivalves known to have ever lived. They are much larger than today's biggest species, which get about 2 feet in length and live among shallow coral reefs in the Indian and South Pacific oceans.
The Prehistoric Museum of Utah State University-Eastern is at 155 E. Main St. in Price. It is open 9 a.m. to 5 p.m. Monday through Saturday. Admission is free to USU students and children under 2, $5 for ages 13 and older, $4 for seniors 65 and older, $2 for ages 2 to 12.
O More information > http://eastern.usu.edu/museum or 435-613-5060