Millions of years after the reptilian animals we know as dinosaurs disappeared, some of their traits persisted in surprising ways among their descendants — birds — a new fossil shows.
The fossil, discovered in the mudstone of northwestern Nebraska, is a bird’s egg that was laid about 35 million years ago — some 30 million years after dinosaurs like T. rex and velociraptor became extinct.
But the new-found egg is more like that of a dinosaur than a bird, says Dr. Frankie Jackson, paleontologist at Montana State University.
“If it had been discovered in the Cretaceous [layer, dating from 66 million to 145 million years ago], you would likely think it was that of a theropod dinosaur rather than a bird; it has more characteristics of the former than the latter,” she said in an interview.
The fossil is so unusual that it appears to be a type of egg that’s entirely new to science, she said.
Indeed, Jackson and her colleagues say the egg is likely evidence of an ancient bird with some unexpectedly dinosaur-like traits — one whose reproductive features, at least, reveal the piecemeal transition by which two-legged dinosaurs evolved into modern birds.
The team describes the find in the new issue of the Journal of Vertebrate Paleontology.
Jackson specializes in the study of fossil eggs, teasing out the evolutionary links between extinct dinosaurs and birds, which are so closely related that birds are in fact considered living, avian dinosaurs.
Scientists can often tell a lot about an ancient animal from the fossils of its eggs, Jackson explained, even if only the shells remain, or if the embryo inside can’t be analyzed.
“The eggshell structure of both modern and fossil eggs is distinct, providing information about the animal that laid the egg in the absence of identifiable embryonic remains,” Jackson said.
“For example, the crystalline structure of bird eggs is easily distinguished from crocodile or turtle.”
But because non-avian dinosaurs and birds are so closely related, and because they coexisted for millions of years, it can often be difficult to differentiate between their eggs, as their traits often overlap.[Learn about a similar discovery made nearby: "Extinct Tropical Bird Discovered in Wyoming"]
In the case of the Nebraska egg, one of its most salient and confusing characteristics was right on its surface.
“Unlike the smooth surface of most modern eggs, dinosaur eggs typically exhibited surface ornamentation — raised bumps or ridges,” Jackson said.
And despite its relatively young age, the Nebraska fossil egg, too, had an unusually textured surface, with tiny lumps and waves that she and her team describe as an “undulating ornamentation.”
The egg was also more spherical than what we consider today — thanks to modern birds — to be egg-shaped.
And the shell was significantly thicker than a modern bird’s egg of its size would have been, the researchers note.
Together these traits not only reflect how eggs gradually changed as birds evolved, they also yield clues into how other physical features, and probably some behaviors like nesting, transformed over time, Jackson said.
“We’re very interested in egg and reproductive characteristics in the non-avain theropod to avian theropod transition, especially when these characteristics first arose,” she said.
“These features include eggshell structural aspects, as well as things like brooding, incubation methods, feathers and other features.”[Learn more about feathered dinosaurs: "New Feathered Dinosaur Discovered in Texas’ Big Bend National Park"]
The interval after non-avian dinosaurs disappeared “was a time when bird lineages were diversifying rapidly,” Jackson noted.
“[It], therefore, is an interesting time, and the fossil record of avian eggs during this interval is fairly limited.”
Frankie D. Jackson, David J. Varricchio, & Joseph A. Corsini (2013). Avian eggs from the Eocene Willwood and Chadron formations of Wyoming and Nebraska Journal of Vertebrate Paleontology DOI: 10.1080/02724634.2013.769445