Tracking a Cretaceous “Roadrunner”
1:12 am
Roadrunner Tracks
Shangdongornipes tracks
Photo credits: Martin Lockley
Roadrunner showing off his zygodactyl foot
From Wikipedia
After spending a decade in New Mexico, it is easy to develop an affinity towards roadrunners. Geococcyx californianus is our official State Bird, after all, and its legendary affinity to asphalt-based habitats ensure that most New Mexicans with a xeriscaped yard (or view overlooking a parking lot) can appreciate its cursorial, lizard-hunting habits. So it was with great interest that I read of a recent report of a fossil trackway that suggests a roadrunner-like bird lived alongside dinosaurs in China nearly 120,000,000 years ago.
Today’s roadrunners are specialized, ground-dwelling cuckoos. They inherited from their arboreal ancestors a condition known as zygodactyly, meaning their fourth toe has rotated clockwise to point more-or-less backwards, joining toe #1 in opposing the second and third pedal digits. (Unfortunately, the most famous representatives of the cuckoo family do not display this characteristic feature.) This foot type, which evolved independently in cuckoos, parrots, woodpeckers, and owls, gives roadrunners a distinct, X-shaped footprint, as seen in the roadrunner trackway to the right.
A team of researchers has identified a similar, zygodactyl trackway preserved in Early Cretaceous rocks from Shangdong, China. The tracks, named Shangdongornipes muxiai, appear to have been made by a roadrunner-sized bird as it ran across wet ground sometime between 110 and 120 million years ago. The rocks that preserve the Shandongornipes trackway also contain tracks made by ornithopod and theropod dinosaurs, including relatively rare tracks made by sickle-clawed dromaeosaur (“raptor”) dinosaurs.
Despite the overall similarities, the Shangdongornipes tracks were most certainly not made by roadrunners. Although roadrunners are the only modern birds capable of making similar tracks, roadrunners are only known from the American Southwest, and their fossil record only goes back to the Late Pleistocene—a couple of million years ago, at best. As previously mentioned, zygodactyly is known from a handful of different types of modern birds, but their oldest fossils show up after the Age of Dinosaurs, at most 65 million years ago. The Shangdongornipes tracks are almost twice as old, and show that some group of birds, so far unknown from fossil bones, had developed a zygodactyl foot long before any modern groups, and were experimenting with the roadrunner lifestyle over a hundred million years before today’s roadrunners first evolved.
Further reading:
Lockley, M. G., Li, R., Harris, J. D., Matsukawa, M. and Liu, M. 2007. Earliest zygodactyl bird feet: evidence from Early Cretaceous roadrunner-like tracks. Naturwissenschaften Published online: 27 March 2007. doi:10.1007/s00114-007-0239-x.
El PaleoFreak has the story with a “reconstruction” of the trackmaker.
Tip of the Hairy Museum toupee to Jerry Harris for bringing this story to my attention.
Tell me, did they find a Cretaceous Acme box nearby?
I can’t read the original paper because it’s behind a membership wall. Is there anything in it about why they’re sure this was a zygodactyl bird, and not a small theropod?
Is there anything in it about why they’re sure this was a zygodactyl bird, and not a small theropod?
Well, it’s a small theropod anyway — birds are theropod dinosaurs. But I’m sure that’s not what you meant — I think you meant more of “how did we know it was a zygodactyl bird and not a small non-avian theropod?” It’s a good question, and actually one that a reviewer of the submitted version of the paper asked, too (so we do address it in the paper itself). Basically, it boils down to the orientation of the first digit on the foot: it points backward in the tracks. Among bipedal animals that are known in the Mesozoic, only birds have this feature (and even then, only a subset of birds — even the famous Archaeopteryx doesn’t have this feature!). Sure, it’s possible that some non-avian theropod convergently evolved this feature as well, but in the absence of any evidence to show that, we have to go with the rule of parsimony, which essentially says that whatever is the most likely (or shortest) pathway — in other words, whatever requires the fewest number of steps — is the most likely explanation. Essentially, this means that in the absence of any actual skeletal fossils, given that birds are the only thing known to have a backward-pointing first digit, it makes more sense to accept these tracks as having been made by a bird than to posit that some non-avian theropod convergently evolved the same feature (the convergence would be a series of additional evolutionary events beyond the single evolution of the backward-pointing first toe within the avian lineage). Thus, an avian track maker is the most likely thing given what we presently know of theropod and bird evolution. Of course, someone could dig up a zygodactyl non-avian theropod tomorrow, which would throw this conclusion out the window!
…as to your question, John, there might be some answers in the paper cited here…
The followups are good reading, also.