How did the prehistoric relatives of mammals reproduce? A 250-million-year-old fossil provides the first direct evidence that they laid eggs. Researchers have identified an egg that contained the embryo of a Lystrosaurus. This was an early mammalian ancestor that survived the mass extinction at the Permian-Triassic boundary. Analysis of the find suggests that the Lystrosaurus young animals were not yet nursing, but were able to fend for themselves quickly after hatching. This could have been crucial for the species to thrive under extremely unstable environmental conditions.
At the end of the Permian period around 252 million years ago, the most devastating mass extinction in Earth’s history occurred. Within a few millennia, it is estimated that around three quarters of all land creatures and 95 percent of species native to the oceans disappeared. The cause was probably extreme volcanic activity that seriously changed the Earth’s climate. One of the few species that not only survived the mass extinction but actually expanded during this period of unstable, hot-dry climate was Lystrosaurus. It was a resilient herbivore that is considered one of the earliest ancestors of mammals.
Unhatched embryo
But how did Lystrosaurus reproduce? This question was a mystery for a long time, because even though many fossils of older animals had already been found, there was no trace of eggs. Did the ancestors of mammals even lay eggs? Researchers led by Julien Benoit from the University of the Witwatersrand in Johannesburg, South Africa, can now answer this question with yes. Back in 2008, a team led by Bennoit’s colleague Jennifer Botha found a fossil the size of a fist during excavations in South Africa’s Karoo Basin, which turned out to be a curled-up Lystrosaurus juvenile. “I suspected even then that it had died in the egg, but at the time we simply didn’t have the technology to confirm this,” says Botha.
Using modern synchrotron X-ray computed tomography, the researchers were now able to take a detailed look inside the fossil. In fact, they found numerous evidence that it was an embryo that had not yet hatched: On the one hand, the individual, with a skull length of just 34 millimeters, is the smallest Lystrosaurus specimen ever found. The curled up posture suggests that it was once surrounded by an eggshell. On the other hand, the CT scans show that the limbs and pelvis were only slightly ossified and the lower jaw had not yet grown together. This feature is only found in today’s birds and turtles before hatching. Since no calcified eggshell has been preserved, the researchers assume that it was soft and leathery and therefore did not survive millions of years. This would also explain why no eggs from these early mammalian ancestors have yet been found.
Survival advantage in mass extinctions
According to the researchers’ calculations, the volume of the egg was probably around 115 cubic centimeters – about twice as much as a chicken egg. In relation to its body size, Lystrosaurus laid relatively large eggs. Together with the analyzes of the embryo’s skeleton, this suggests that the young animal was able to develop so much in the egg that it quickly became independent after hatching and did not need to be fed by its parents. Unlike later mammals, Lystrosaurus probably did not produce milk for its offspring.
The size of the egg also offered the advantage of being more resistant to drying out. This could have been crucial for the survival of the species during the mass extinction period, when there were repeated droughts. “By producing large, yolk-rich eggs and precocious young, Lystrosaurus was able to thrive in the harsh, unpredictable conditions following the end-Permian mass extinction,” the researchers explain. The results could also be important in light of current climate change: “Understanding how organisms have survived global upheavals in the past helps scientists better predict how species might respond to ongoing environmental stress today,” says Benoit.
Source: Julien Benoit (University of the Witwatersrand, Johannesburg, South Africa) et al., PLOS One, doi: 10.1371/journal.pone.0345016