Unhatched Dinosaur Egg Reveals the Surprising Face of a Baby Sauropod

Digital reconstruction of the embryonic sauropod skull, with eyes added to show forward-looking direction. (Image: Kundrat et al. /Current Biology)
Digital reconstruction of the embryonic sauropod skull, with eyes added to show forward-looking direction. (Image: Kundrat et al. /Current Biology)

A rare egg fossil containing an unhatched dinosaur suggests baby sauropods were not miniaturised versions of the four-legged, long-necked behemoths we all know and love, exhibiting a unique set of physical characteristics that included a little rhino-like horn.

Baby titanosaurs — the largest group of sauropods — featured a prominent facial horn and a pair of forward-facing eyes that enabled stereoscopic vision, according to new research published today in Current Biology. These facial characteristics eventually disappeared as the babies matured into adults, but not before serving a valuable purpose, speculate the authors, led by Martin Kundrat from Pavol Jozef Šafárik University in Slovakia.

“Titanosaur babies were on their own right after hatching,” explained Kundrat in an email. “They had to search for their food and defend themselves — two of the most important survival activities.”

Indeed, life during the Late Cretaceous couldn’t have been easy for these hatchlings. Accordingly, it would’ve been prudent for them to hide in a forest-like environment until they were big enough to move around in the open, said Kundrat. The baby titans, with their stereoscopic vision, were better able to source food and recognise danger. But their most striking feature — their beak-like premaxillary horn — could’ve been used to search for food and possibly even for defence, “at least for the period when they were most vulnerable,” Kundrat said.

The fossilized skull, with artist's reconstruction shown inset. (Image: Kundrat et al. /Current Biology)

These findings were made possible by analysing a rare egg fossil found in Patagonia, Argentina. Or at least, that’s where Kundrat and his colleagues believe the fossil came from. They can’t actually be certain, because the fossil was illegally exported from the country.

In 2001, an Argentinian dealer brought the egg to Terry Manning, a freelance paleontologist and a co-author of the new study. During a follow-up meeting in 2015 with study co-author John Nudds from the University of Manchester, the dealer said the fossil was sourced from the Allen Formation of Bajo de Santa Rosa in Rio Negro Province, Patagonia. The 80-million-year-old egg fossil has since been repatriated to a museum in Argentina, but there’s still uncertainty about its geographical origins.

The first fossilized sauropod eggs were found 25 years ago at Auca Mahuevo in Patagonia, the site of a former titanosaur nesting ground. Fossilized eggs containing embryonic remains are exceptionally rare, and there’s much we don’t know about the developmental stages of these animals. The newly analysed fossil is unique in that it’s the best three-dimensionally preserved embryonic skull belonging to a sauropod dinosaur, according to the paper. Sadly, other parts of the dinosaur were not recovered, and the exact species to which it belonged could not be determined.

To study the fossil, the researchers took it to the European Synchrotron Radiation Facility in France.

“The synchrotron is a particle accelerator that generates powerful X-rays called synchrotron light,” explained Kundrat. “This synchrotron light can penetrate highly dense objects, such as rocks or dinosaur eggs. Synchrotron microtomography is currently the leading imaging technology for working with fossils that are beyond the scanning capacity of industrial micro-CT systems.”

These scans revealed previously hidden features, such as the inner structure of the embryo’s bones, teeth, and even soft tissue. This data was subsequently used to reconstruct the skull, which measured 3 centimetres long, revealing the unexpected facial features.

“Part of the skull of these embryonic sauropods was extended into an elongated snout or horn, so that they possessed a peculiarly shaped face,” explained Nudds in a University of Manchester press release.

Interestingly, this rhino-like horn might’ve been a kind of “egg tooth,” which helped the baby break through its egg shell during hatching.

A fascinating study, no doubt, but there are some important limitations to point out.

As already mentioned, the researchers aren’t perfectly confident about where the fossil came from. This is more of an annoyance than a caveat, but it does make the dating of the fossil a bit precarious.

More serious limitations include the complete absence of the animal’s body and of supporting fossils, including hatched baby titanosaurs. Indeed, given “the fact that we do not know how long these sauropod embryos developed inside their eggs, it is difficult to assess how much these prenatal [characteristics] remained expressed in the cranial morphology at hatching,” wrote the authors in the study. It’s possible that this embryo was still subject to major developmental changes prior to hatching, so it may not be representative of baby titanosaurs in general.

Kundrat said more evidence will be needed to find out if the unusual face of this specimen also applies to other titanosaur embryos and hatchlings. Thankfully, more fossil eggs of sauropods have been found in Argentina, which could contain traces of embryos. Kundrat and his colleagues have made a very cool discovery, which has led to some very interesting, albeit speculative, conclusions. The good news is that this is all very testable stuff — we just need some more fossils.