Ichnofossil Green River Formation Tracks

December is notorious for the snow that it brings—especially to Buffalo, New York. A beautiful side of this frozen precipitation is finding numerous different animal tracks in it, providing insight to their way of living, diets and homes. This month’s Fossil of the Month also exhibits tracks from various animals, however, these tracks are from prehistoric animals that walked through mud around 50 million years ago. 

This extremely rare and unbelievably well-preserved limestone trace fossil plate is only 27 x 20 inches, yet shows evidence of no less than four types of animals. Known as an “ichnofossil,” this fossil record of activity is from the Green River Formation along the Utah-Wyoming border. The Green River Formation consisted of three large lakes—Fossil Lake, Lake Uinta and Lake Gosiute—that persisted for almost 12 million years over what is now Colorado, Utah and Wyoming. It is one of the most prominent sources for spectacularly preserved Eocene-aged fossils, most notably fossil fish and flora. 

Trace fossils like this are just as important as fossils of skeletons and exoskeletons because trace fossils show behavior and patterns of organisms. For example, there are three sets of footprints from an Eohippus, which was a 14-inch tall horse with three-hoofed toes on its back legs and four on the front. The close placement of these 2-inch long prints indicate these mammals must have browsed and traveled in herds along the lush and muddy banks of the ancient lakes. 

Mixed in between the set of horse prints is the 2.5-inch long tridactyl tracks from a species of shorebird, possibly the Jindongornipes falkbuckleyi. This prehistoric bird might have resembled a present-day plover or sandpiper. Not only did birds and mammals leave an eternal impression in the mud, but organisms as tiny as insects, insect larvae or small arthropods left their own miniature trackways. On the lower right side of the plate, there is the beginning of a “wave-like” trail that could belong to a marine nematode, or roundworm.

A spectacular fossil like this shows the importance that the Green River Lake System played in the survival of species that could not be more different from each other. 



Fossil Tracks: Horses, wading bird, invertebrate (insect? arthropod?), nematode
Eocene
Green River Formation
Wyoming/Utah Border