250-million-year-old fossil proves mammal ancestors laid eggs
Laying eggs may have helped mammal ancestors thrive after Earth’s worst mass extinction
This reconstruction of a Lystrosaurus embryo shows the little creature in its partially preserved shell.
Advanced imaging of a 250-million-year-old fossil has provided the first definitive evidence that the ancestors of mammals laid eggs. This groundbreaking discovery addresses a long-standing mystery about the reproductive biology of our ancient predecessors and suggests how they thrived following Earth’s largest mass extinction.
Though scientists have long speculated that the ancestors of mammals, known as therapsids, reproduced by laying eggs similar to modern-day platypuses and echidnas, they had lacked concrete proof of synapsid eggs in the fossil record.
In a recent study, Julien Benoit from the University of the Witwatersrand in Johannesburg, South Africa, and his team examined three rock specimens containing fossils of a therapsid species called Lystrosaurus using x-ray microcomputed tomography (CT) and synchrotron radiation computed tomography (SRCT).
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The three specimens represent slightly varying stages of development around the time of birth. Evidence suggests the youngest Lystrosaurus was still inside its egg when it perished. For example, the rock nodule’s dimensions match those of an egg, and the embryo’s curled posture is similar to an ovoid shape. Additionally, the disjointed pelvis, vertebrae, and ribs indicate the animal was not ready to support its weight, a characteristic of an unhatched individual.
The newly identified egg, shown here, was large relative to its owner, Lystrosaurus.
Crucially, the imaging showed that the lower jaw halves of the youngest Lystrosaurus had not yet fused. In turtles and birds, this fusion happens before birth, enabling self-feeding post-hatching. The absence of this fusion indicates the animal was still in its egg when it died. The other two specimens appeared more developed; the largest was found in an outstretched position, suggesting it had moved after hatching.
Lystrosaurus, a pig-sized herbivore with tusks and a beak, was among the few tetrapods—four-limbed vertebrates—that survived the Permian mass extinction around 252 million years ago, which eliminated roughly 90% of Earth’s species. Following this extinction, amid dramatic climate shifts with intense heat and drought, Lystrosaurus thrived and became the most prevalent land vertebrate.
Egg-laying may have been a key to its survival. The reconstructed Lystrosaurus egg was relatively large and likely had a soft, leathery shell that didn’t fossilize well, explaining the absence of previous therapsid egg evidence. Larger eggs, with a lower surface-to-volume ratio, resist dehydration better, which is advantageous in droughts. Moreover, offspring of modern tetrapods from large eggs are typically more developed and independent at hatching compared to those from smaller eggs. In contrast, mammalian young, even from egg-laying species, require milk feeding post-birth.
These findings offer insights into how species might adapt to current environmental changes. “Understanding how past organisms survived global upheaval helps scientists better predict how species today might respond to ongoing environmental stress,” Benoit stated in a statement.
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