Cracking a puzzle

How reptilian were dinosaur eggs?

DID dinosaur eggs hatch quickly, like those of birds (which are dinosaurs’ direct descendants), or slowly, like those of modern reptiles (which are dinosaurs’ collateral cousins)? That is the question addressed by Gregory Erickson of Florida State University and his colleagues in a paper just published in the Proceedings of the National Academy of Sciences. It is pertinent because it touches on the wider matter of just how “reptilian” the dinosaurs actually were. Researchers already know that many were warm-blooded, and that some had insulation in the form of feathers, even though they could not fly. Fast-developing embryos would drive a further wedge between them and their truly reptilian kin.

To investigate, Dr Erickson looked at two sets of fossilised dinosaur eggs. The first, from a Mongolian nest , was laid by Protoceratops andrewsi, a sheep-sized creature that lived 70m years ago. The second, from Canada, was laid by Hypacrosaurus stebingeri—a species contemporary with P. andrewsi that grew to something between the weights of a rhinoceros and an elephant.

In each case the researchers used an Xray scanner to examine the teeth of embryos found inside the eggs. In cross-section, dinosaur teeth display growth rings, called von Ebner lines, that are reminiscent of the annual growth rings of a tree trunk. In all living species which have von Ebner lines those lines represent a day’s growth. It therefore seems reasonable to believe that this was true for dinosaurs as well.

Assuming also, as Dr Erickson and his colleagues did, that dinosaurs’ teeth began to grow about halfway through embryonic development (which is when a crocodile’s embryonic teeth first appear), they conclude that the P. andrewsi eggs they looked at were about 83 days old, making that the lower

Researchers already know that many dinosaurs were warm-blooded, and that some had insulation in the form of feathers, even though they could not fly. Fast-developing embryos would drive a further wedge between them and their truly reptilian kin.

bound of their incubation period. This compares with the 42 days an ostrich egg takes to incubate and the 200-plus days required by a Komodo dragon egg—both of these animals being, when adult, of comparable size to P. andrewsi.

The bigger eggs of H. stebingeri needed, according to Dr Erickson’s calculations, a minimum of 171 days incubation. Sadly, no egg-laying animal of its size is around today for comparison. But projections based on size and incubation-period data from modern birds and reptiles suggest 171 days is substantially more than would be expected if the eggs of H. stebingeri were developing in a birdlike way.

The truth, then, is that in this as in other matters, dinosaurs are less reptilian than was once thought, but not as avian as some revisionists would like to believe. A messy answer, perhaps. But, in nature, things are not always clear-cut.

AP The study, examining fossilsed dinosaur eggs, is pertinent because it touches on the wider matter of just how ‘reptilian’ the dinosaurs actually were.