The great long-necked dinosaurs, or sauropods, of the Jurassic and Cretaceous periods were the largest animals ever to live on land. In a paper published in Current Biology, an international team of researchers including members from Uppsala University use cutting-edge imaging technology to give us a unique glimpse of the very beginning of a sauropod life: the face of an unhatched young inside an egg.
Sauropods like Brontosaurus and Argentinosaurus sometimes exceeded thirty metres in length and may have weighed as much as eighty tonnes, ten times as much as a really large African elephant.
And yet, these stupendous animals started life small: unlike modern-day giants such as elephants and whales, which are very large already at birth (a new-born blue whale is seven metres long), sauropods hatched out of eggs that were not much bigger than grapefruit. How were they able to grow so large from such small beginnings? And what did a baby sauropod look like?
80 million years old sauropod egg
Fossil eggs of sauropods are not terribly rare, but it is very uncommon to find anything inside them. Even when embryo bones are preserved they are extremely delicate and almost impossible to study. The fossil presented in the new paper, a Cretaceous sauropod egg from Patagonia in Argentina, about 80 million years old, was first treated with dilute acetic acid to expose some of the bones.
This has been done before with other sauropod embryos, but it only provides a partial view of the fossil because the bones are so fragile that they can never be removed from the rock and pieced together.
So the team took a radically different approach, taking the fossil to the European Synchrotron Radiation Facility (ESRF) in France, where extremely powerful x-rays produced by a giant accelerator ring were used to make a high-resolution three-dimensional image of the bones, including the parts still buried in the rocks.
The researchers could reconstruct the bones
The principle is the same as a hospital CT scanner, except that the x-ray beam from the synchrotron would kill you in seconds. Using special software the researchers could then reconstruct the bones, move them about in virtual space in the computer, and piece together the face of the tiny sauropod.
What a face it is! The eyes are huge and forward-facing, the snout short, the top of the head apparently round and domed although the bones of that region had not fully formed. Seen through human eyes it would have looked cute, as many baby animals do. The forward-facing eyes suggest that the hatchling would have been able to focus on objects in front of it with stereoscopic vision.
A more puzzling feature is a forward-pointing spike at the tip of the upper jaw. Modern birds and reptiles have a so-called egg tooth in approximately this position, which helps them break through the egg shell at hatching time. However, this is a horny structure on the skin that falls off soon after hatching, not a bony spike, and it points upwards not forwards.
The shape of the skull is different
Many puzzles remain, besides the function of the mystery spike. The shape of the skull is different from those of sauropod embryos previously described from Argentina, suggesting that they belong to different species, but at present it is not possible to assign any of them to known adult sauropods. All the sauropod embryos seem to show accelerated formation of the bones of the face compared to the rest of the skeleton, but the significance of this is also unclear.
What is clear, however, is that the application of synchrotron imaging to this ancient egg has given us an extraordinary and precious glimpse of a fleeting life that ended before it really began: a baby giant that never got to walk the earth.
Kundrát, M. et al. 2020. Specialized Craniofacial Anatomy of a Titanosaurian Embryo from Argentina. Current Biology. DOI: 10.1016/j.cub.2020.07.091