Prehistoric Shorebirds Waded Alongside Their Dinosaur Cousins

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.

The spot where the bird tracks were discovered. From Lockley et al., 2014.
The spot where the bird tracks were discovered. From Lockley et al., 2014.

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.

Reference:

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

9 thoughts on “Prehistoric Shorebirds Waded Alongside Their Dinosaur Cousins

  1. If birds are dinosaurs, couldn’t it be said that I’m a fish? Or at least a monkey. Cladistically speaking, birds are dinosaurs, but their characteristics are quite different. Lack of teeth, extreme shortening of tail, extensive forelimb modification, bone fusion, etc. I mean, isn’t this all subjective?

  2. Just to note a subtlety in the story that I have trouble getting across to paleontologists lacking a geological background. While the bird track layer is in a layer mapped as Poison Strip Sandstone (122-121 Ma), that ledge represents a shoreline of the lake system the characterizes the underlying Yellow Cat Member of the Cedar Mountain Formation, so based on age data we have from those beds, a better age determination would be around 124 Ma or essentially the exact same age as the bird bearing lakes in Liaoning, China. These birds would be part of our Utahraptor-Gastonia fauna. The more typical Poison Strip Sandstone (a bit higher on cliff) represents gravelly river deposits that extended across the region and preserves a different, younger suite of dinosaurs that are still under study.

  3. Mr. O’Brien. Yes, you could say that you are a fish, cladistically speaking. However, you can’t say that all the dinosaurs died out, because birds are still around and they are dinosaurs. You can’t say that you are a monkey, using the commonly accepted idea of what a monkey is, because you did not descend from modern monkeys. You do have a common ancestor though. So you could say that you and monkeys are primates. If all the monkeys and apes died out, leaving only humans, you could not claim that all the primates died out. Essentially, that is what is going on when people talk about dinosaurs and birds as being different. They are confusing different levels in the hierarchy. Clearly, humans are different from other primates, yet we are still primates, just as all primates are mammals, which are all amniotes, which are all fish. The confusion here arises because people so rarely define which way they are using a term. The colloquial meaning for fish, meaning that subset of animals that looks to people like a fish, is not the same meaning as the cladistic definition of the group that includes all fish and all of their descendants. In the same way, what people think of as birds and dinosaurs are clearly different creatures, and as long as they understand how they are using the terms, everything is fine. It is when they then try to apply those terms in a more explicitly scientific meaning that the problems arise. If someone says that birds are not dinosaurs and is meaning that birds did not evolve from dinosaurs, they are incorrect. Sadly, most people not familiar with the science take the colloquial, categorical birds are not dinosaurs statement as a statement meaning there is no connection to them whatsoever.

  4. I’m pretty sure it’s incorrect to say that humans are fish cladistically because unlike dinosauria, fish are not a clade since they exclude tetrapods.

  5. We are sarcopterygians and tetrapods, just as we are primates, mammals, amniotes, and vertebrates — and just as birds are part of Dinosauria.

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