Montana lizard skeletons are among oldest, most complete iguanian fossils ever discovered
Nearly 40 years after Egg Mountain revealed fossils that would forever change the way people thought about dinosaurs, the site located on Montana’s Rocky Mountain Front continues to yield new wonders.
The latest finds to be described in scientific research belong to an iguana-like lizard, named Magnuviator ovimonsensis — “mighty traveler from Egg Mountain” — which lived about 75 million years ago. The two lizard skeletons are the oldest, most complete iguanian fossils ever discovered in the Americas.
“For North America in the late Cretaceous, most iguanians are known only from partial fragments,” said David DeMar, a postdoctoral research associate in the University of Washington biology department and the Burke Museum of Natural History and Culture, who studied the specimens.
“In the field, we knew we had some special specimens, as articulated lizards are exceptionally rare in the Cretaceous,” said David Varricchio, an associate professor of paleontology at Montana State University, who led the fossil dig. “But it wasn’t until Dave DeMar really examined the specimens that we knew it was a new species.”
DeMar analyzed CT scans of the fossils to identify the reptiles and in the process discovered that they were most closely related to Cretaceous-era lizards in East Asia, particularly fossils found in Mongolia, rather than others in the Americas.
“…Magnuviator belongs to the iguanomorphs, a large group of lizards that includes iguanas and horned lizards,” Varricchio said. “The specimen is about 20 million years older than the next oldest specimen from North America.”
Varricchio has led a dig at the famed site near Choteau since 2010, thanks to a National Science Foundation grant. It was one of Varricchio’s predecessors at MSU, paleontologist Jack Horner, who christened Egg Mountain after he and his crew discovered fossil eggs and clutches there in 1979.
“This was the first concentrated effort at Egg Mountain in 25 years,” Varricchio said.
75 million years ago the area along the eastern side of Montana’s Rocky Mountains was a flat coastal plain along the edge of the shallow Western Interior Seaway. The seaway split North America in half from the Arctic to the Gulf of Mexico. The eggs found at the site — up to 25 in clutches that measured 6 to 7 feet wide — belonged to Maiasaura, a large duck-billed, plant-eating dinosaurs.
The newly identified lizard — much smaller than a Maiasaura — likely lived in the nesting area, possibly eating wasps whose fossilized pupae are commonly found in the same rock. That seems like a puny diet for a lizard that measured about 14 inches long, but lizards that big are also known to eat plants.
“They are larger than 80 percent of modern-day lizards,” DeMar said. “The fact that they are a large-bodied reptile may have helped them move around between Asia and North America,” since a larger body makes it easier for the lizard to regulate its body temperature.
DeMar was the lead author of a paper about the lizards, “ A new Late Cretaceous iguanomorph from North America and the origin of New World Pleurodonta,” which was published on Jan. 25 in Proceedings of the Royal Society B.
It took four years for DeMar to research and write his paper.
“It was kind of a long haul going from straight out describing it, to the CT scans, to working with an artist,” he said. “But it was a pretty fun project and there’s more to come from Egg Mountain, other lizards and mammals. That’s pretty exciting as well.”