A New Look Inside Dinosaur Eggs

by AMNH on

Research posts

A new study of fossilized dinosaur embryos suggests that the young of these prehistoric animals were slow to develop, with some spending up to sixth months inside their eggs before hatching. Detailed in the journal Proceedings of the National Academy of Sciences, this drawn-out development cycle not only surprised scientists—it may have contributed to the downfall of the dinosaurs.


Nearly complete hatchling protoceratops skeleton is revealed from it's earthen bed.
A hatchling Protoceratops andrewsi fossil from the Mongolia, examined in the study.
© AMNH/M. Ellison

“We know very little about dinosaur embryology, yet it relates to so many aspects of development, life history, and evolution,” said study co-author Mark Norell, Macaulay Curator of Paleontology at the American Museum of Natural History. “This work is a great example of how new technology and new ideas can be brought to old problems.”

Using a combination of computed tomography (CT) scanning and powerful microscopes, Norell and colleagues from the University of Calgary and Florida State University examined the teeth of fossilized dinosaur embryos in unprecedented detail, shining new light on specimens about which not much is known.

“Time within the egg is a crucial part of development with major biological ramifications,” said co-author Darla Zelenitsky, from the University of Calgary. “But it is poorly understood because dinosaur embryos are rare.”

Analysis and imaging of the fossil embryos of the sheep-sized Protoceratops and the towering duck-billed dinosaur Hypacrosaurus revealed “von Ebner” lines on the teeth of the fossils—telltale growth lines present in the teeth of every animal, dinosaurs (and, yes, humans) included. 


Microscopic view of the dentine growth lines, which visually resemble the rings of a tree.
This image shows the daily growth lines in the dentine of an embryonic tooth of Hypacrosaurus.
© G.M. Erickson

“They’re kind of like tree rings, but they’re put down daily,” said lead author and Florida State University professor Gregory Erickson. “So we could literally count them to see how long each dinosaur had been developing.”

The results of this high-resolution dental exam were revealing, not to mention unexpected. Scans showed that the embryonic Protoceratops had been in its shell for about three months when it died. The larger Hypacrosaurus took even longer to hatch—when that embryo died, it had already been in the shell for six months.


Chart compares the incubation times (y axis) to egg mass (x axis) of dinosaurs vs living birds and reptiles.
Dinosaur incubation periods compared to those of living reptiles and birds. (Click on the image to view larger.)
© G.M. Erickson

The long time spent developing inside the egg distinguishes dinosaurs from their modern ancestors—birds. Today’s avian embryos spend no more than 85 days inside the shell, and some hatch in under two weeks. Following the Cretaceous-Paleogene extinction event that occurred 65 million years ago, that extended incubation time may have put dinosaurs at a disadvantage against more quickly reproducing birds and mammals of the time.