Two slabs of late-Jurassic frog fossils have returned to Dinosaur National Monument after more than a decade at the Carnegie Museum of Natural History in Pittsburgh, Penn.

These two rock slabs contain the fossilized skeletons of several small frogs, each about the size of a modern-day tree frog. Well-preserved frog skeletons such as these seldom occur in the fossil record because frogs are small, their skeletons are delicate, and they have cartilage in the skeleton, which does not fossilize well.

Although monument paleontologists have found many individual frog fossil bones, these entire skeletons tell a more complete story.

Amy Henrici, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, was studying the slabs. She determined that they are a previously unknown genus of frog.

The frogs share characteristics with a group of living frogs that burrow in mud, but lack the specific adaptations for burrowing. The fossilized frogs on the slab died when they were metamorphosing from a larval stage and consequently do not show all of their adult skeletal features. Therefore, it is possible that the fossilized specimens died before burrowing adaptation in their skeleton developed.

Although Dinosaur National Monument is best known for its dinosaur fossils, smaller fossils such as these are similarly important because they help paleontologists understand the ecosystem in which dinosaurs lived.

“We know that there was standing water at the site and that it was probably permanent,” said monument paleontologist Dan Chure. “Given the tremendous abundance of frog bones we have collected [at Dinosaur National Monument], it was frog heaven here back in the Jurassic.”