Birds are dinosaurs. This fact is easily understood by looking at the scaly feet of a chickadee or by comparing a chicken wing to a Velociraptor arm. But given that birds are the only “terrible lizards” around today, it’s easy to forget that they also thrived alongside their non-avian kin for 84 million years. The first birds evolved in the Late Jurassic, roundabout 150 million years ago, and they became a widespread and successful branch of the dinosaur family tree.
The trouble is finding those birds. They were often so small and so delicate that their bones didn’t make it into the fossil record in the same abundance as their larger, more robust relatives. And that’s where a different sort of fossil comes to the rescue. Thanks to tracks, paleontologists have been able to detect the presence of Mesozoic birds in strata where their bones have remained elusive. Among the latest to be uncovered are dozens of birds tracks found in eastern Utah.
Discovered in 2005 by fossil reconstruction expert Rob Gaston and paleontologist John Foster, the tracks were distributed across eight blocks that had tumbled out of their original position as “float” upon the older rocks below. With a little geological sleuthing, though, Foster, Martin Lockley, Lisa Buckley, Jim Kirkland, and Don Deblieux were able to trace the slabs back to the 122-119 million year old Poison Strip Member of the Cedar Mountain Formation. This formation has been producing a fantastic array of discoveries over the past few years, including a group of Utahraptor mired in quicksand that was discovered near the bird tracksite.
All told, Lockley and colleagues counted over 130 tracks representing 43 different trackways. Most were made by the same species of bird but two were left by non-avian theropod dinosaurs. Exactly what the birds looked like is unknown. No one has found the bones to reconstruct their skeletons and see, if like other Cretaceous birds, they still had teeth. But the thin-toed, spread anatomy of their feet was very similar to that of modern shorebirds. Along with the fact that the tracks were imprinted in the sand along a Cretaceous lake, the collection of tracks likely represents a single species of bird that inhabited ancient shores.
The Utah traces are among the oldest bird tracks in North America. (Shorebird tracks found in South Dakota are the only ones known from the continent that may be a little older.) This helps fill in the pattern of how avian dinosaurs evolved alongside their non-avian relations.
As is the case with tracks of about the same age in South Dakota and Canada, the Utah fossils represent a single species of shorebird. But bird tracks from later in the Cretaceous show multiple foot shapes and, therefore, multiple species. This pattern could be upset by future finds of Early Cretaceous tracksites preserving the footprints of multiple bird species, but, as it stands now, North America’s fossils suggest increasing diversity through time. If you were to walk along a lake shore in 122 million year old Utah, you’d likely see a gaggle of similar shorebirds skittering along the sand – an early look at an evolutionary bloom that birds are carrying on to this day.
Lockley, M., Buckley, L., Foster, J., Kirkland, J., Deblieux, D. 2014. First report of birds tracks (Aquatilavipes) from the Cedar Mountain Formation (Lower Cretaceous), eastern Utah. Palaeogeography, Palaeoclimatology, Palaeoecology. doi: 10.1016/j.palaeo.2014.12.014