Now, paleontologists understand another quirk of the many far-flung species that once made up the genus Lystrosaurus: These proto-mammals laid eggs — and the sheer size of those eggs was likely crucial to their survival.
In fact, Lystrosaurus reproduced via eggs so large compared to its own body weight, that researchers now believe that infant Lystrosaurus were born ready to roll – far advanced in their development to escape predators, feed themselves, and even create more tiny Lystrosaurus. According to a new study of the first confirmed eggs containing Lystrosaurus embryos, their larger eggs may also have had another major advantage during this extremely hot post-apocalypse: a lower “surface area to volume ratio” that would have made them less likely to fatally dry out, or “desiccation.”
“This fossil was discovered during a field trip led by me in 2008,” paleontologist Jennifer Botha, a professor at the University of the Witwatersrand in South Africa and co-author of the new study, said in a statement. “I still suspected that it had died inside the egg, but at the time, we didn’t have the technology to confirm it.”
Ghost in the Shell
At the field site, Botha was confident that his team had uncovered a “fully curled Lystrosaurus hatchling”, but getting a clear picture of this fossil would require new and more sophisticated X-ray scanning via an intense beam within a particle accelerator – known as a synchrotron. Notably, such an instrument would take more than a decade to come online, and would be a massive construction project at the European Synchrotron Radiation Facility (ESFR). In 2022, ESFR’s BM18 beamline synchrotron X-ray CT imager became operational.
That’s where Botha’s coauthor, Vincent Fernandez, a permanent beamline scientist at the ESFR in Grenoble, France, came in. Fernandez placed the fossil under both X-ray micro-computed tomography (CT) and the new BM18 beamline imager. The high-energy X-rays employed by BM18 gave the team a resolution of just less than 18 micrometres inside the fossil.
“It was essential that we scan the fossil correctly to capture the level of detail required to resolve such small, delicate bones,” Fernandez said in a statement.
The scan uncovered a hidden detail in the skeletal structure of the Lystrosaurus hatchling, which confirmed that it was more likely that the specimen had fossilized as an embryo rather than as a newborn.
“When I saw the incomplete mandibular symphysis, I got really excited,” paleontologist and co-author Julien Benoit said in a statement. “The mandible, the lower jaw, is composed of two parts that must fuse before the animal can feed. The fact that this fusion has not yet occurred indicates that the individual must have been unable to feed itself.”
In other words, the hatchling had not yet hatched.
Soft-shell, alternative architecture
Three researchers speculate that Lystrosaurus hatchlings likely hatched from “soft and leathery” eggs, which would solve the complex issue of why no pre-eggs for these creatures have been discovered anywhere in the existing fossil record.
Dinosaur eggs, which typically have very hard shells, would easily calcify and become fossilized – as the wet mineral deposits essentially weathered them, turning the eggs literally to stone. The more soft-shelled and more mammalian eggs of Lystrosaurus, on the other hand, would likely decay into dust anonymously.
“Understanding reproduction in mammalian ancestors has been a long-standing puzzle,” Fernandez said, “and this fossil provides an important piece of the puzzle.”
According to the official statement from the ESRF, the team’s discovery is not only “the first direct evidence of egg-laying in mammalian ancestors”, but also “a powerful explanation of how Lystrosaurus came to dominate the ecosystem after its extinction.”
Despite their comfortable reputation as sedentary, leaf-chewing burrowers – happy to escape a torturous, nearly two-million-year-long extinction event – Lystrosaurus survived by growing rapidly.
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