Hairy Museum of Natural History

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.

—Matt Celeskey.

File under: Birds, Cretaceous.

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…this edition inspired by mangoes. Start your Wednesday with the fruits of two week’s worth of science and nature blogging, collected and online at Balancing Life.

—Matt Celeskey.

File under: Miscellany.

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A 95,000,000-year-old fossil shows the earliest example of vestigial limbs in lizards and may provide insight into the way snakes lost their legs, according to a new paper in the Journal of Vertebrate Paleontology.

The fossil was quarried over a century ago in what is now Slovenia, but recent preparation has shown it to be a new species of the extinct aquatic lizard Adriosaurus. Adriosaurus was well-adapted for cruising the Cretaceous seas. It had heavy, thickened (pachyostotic) ribs to keep it from floating to the surface of the water. It added several extra vertebrae to its spinal column, giving it a long, flexible body. And it had shortened both its front and back legs, allowing it to tuck its limbs along its body and swim with graceful, snakelike undulations.

Detail of the holotype of Adriosaurus microbrachis
Credit: University of Alberta. From the EurekAlert press release.

This newly-described fossil appears to have taken this limb-shortening to another level. Other Adriosaurus specimens possess short yet fully-developed arms, complete with five fingers on a small hand. But this latest specimen shows no sign of hands or forearms, and its humeri (upper-arm bones) are reduced to mere slivers. Drs. Alessandro Palci and Michael Caldwell have named the new species A. microbrachis—the “small-armed” Adriosaurus.

Because of the slender shape and excellent preservation of the existing arm bones, Palci and Caldwell believe that the missing forearms reflect the actual skeleton of A. microbrachis, and were not lost in fossilization. “There was a moment when I said, ‘I think we stumbled on a new fossil illustrating some portion of the aquatic process of losing limbs,’” said Caldwell. “There are lots of living lizards that love to lose their forelimbs and then their rearlimbs, but we didn’t know it was being done 100 million years ago and we didn’t know that it was happening among groups of marine lizards.”

The forelimbs of A. microbrachis were so severely reduced that they could not have played any role in locomotion, either in water or on land. In both environments, A. microbrachis must have slithered like a snake, perhaps using its short, yet fully-developed, hindlimbs to assist in pushing its body forward.

A. microbrachis might provide a snapshot of the morphological changes that occurred during the transition from lizards to snakes. The origin and early evolution of snakes is, at present, riddled with controversy, but one prominent hypothesis suggests that the limblessness and elongation of snakes first evolved as adaptations to an aquatic lifestyle.

Some of the best evidence supporting this hypothesis comes from the fossil record, which preserves an easy-to-follow continuum from terrestrial varanid lizards to the aquatic aigialosaurs and mosasaurs, to the elongated aquatic dolichosaurs, through elongated aquatic forms with reduced forelimbs (including Adriosaurus), finally reaching primitive snakes like Pachyrhachis, which lost their forelimbs but retained vestigial hindlimbs and are widely believed to have been aquatic in habit. Evidence from other fossils, and from myological, developmental, and molecular studies of modern-day lizards and snakes throw their fair share of kinks into this transitional series. But the discovery of Adriosaurus microbrachis adds an interesting data point to future discussions on the origins of snakes.

Further reading:

Palci, Alessandro and Caldwell, Michael W. 2007. Vestigial forelimbs and axial elongation in a 95 million-year-old non-snake squamate. Journal of Vertebrate Paleontology 27 (1): 1–7.

The EurekAlert press release, and a similar story at Science Daily.

Dr. Michael Caldwell’s faculty page, with more info on his work on snake origins. (And I’d be remiss if I didn’t mention his enviable membership in the Luxuriant Flowing Hair Club for Scientists.)

And a tip of the toupee to Afarensis, who brought the story to my attention.

—Matt Celeskey.

File under: Cretaceous, Recent Discoveries, Reptiles.

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Well, here’s just the sort of story to shake me out of my posting hiatus. A team of researchers led by Dr. David Varricchio have just announced the discovery of a hypsilophodont dinosaur that dug burrows and cared for its young in dens.

In a new paper, Varricchio et al. describe Oryctodromeus cubicularis, “digging runner of the lair,” from the remains of three individuals, and adult and two juveniles. The authors note that the fossils were found within a sinuous sandstone body that crosscuts three layers of a completely different type of mudstone. This indicates that the three Oryctodromeus were preserved deep within a sediment-filled burrow, and the mixed age of the specimens suggests that the burrow was a den where Oryctodromeus cared for its young.

There is every reason to believe that Oryctodromeus dug these burrows itself. The relative dimensions of the burrow match what an animal the size of an adult Oryctodromeus would make. And while the overall proportions of its skeleton suggest it was a better sprinter than tunneler, there are key details that suggest Oryctodromeus was quite capable of excavating a cozy little home. Its beak was solidly fused and expanded in the front, perhaps an adaptation to loosen or scoop out soil. Its shoulder girdle was remarkably robust compared to other hypsilophodonts, providing leverage for powerful forelimb muscles used in digging. The hips and base of the tail were also stoutly reinforced, allowing it to brace itself while tunneling.

Some of these features occur in other Montanan hypsilophodonts, such as Orodromeus and Zephyrosaurus, which appear to be the closest relatives to Oryctodromeus. Varricchio et al.’s careful study and description of the Oryctodromeus fossils and their surrounding sediments has given us insight into a newly discovered dinosaurian behavior, and suggests that Montana may have been home to a mid-Cretaceous radiation of burrowing hypsilophodonts.

Further reading:

Varricchio, D. J., Martin, A. J., and Katsura, Y. 2007. First trace and body fossil evidence of a burrowing, denning dinosaur. Proceedings of the Royal Society B: Biological Sciences. FirstCite early online publishing.

Padian, K. 2007. Dinosaurs digging deeper. Proceedings of the Royal Society B: Biological Sciences. FirstCite early online publishing.

The press release (with images) from Montana State University.

New Scientist has a good article, as does News in Science.

—Matt Celeskey.

File under: Cretaceous, Dinosaurs, Recent Discoveries.

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