Mammal ancestors laid eggs, and this 250-million-year-old fossil finally proves it

A 250-million-year-old fossil egg containing a Lystrosaurus embryo provides definitive proof that the ancestors of mammals reproduced by laying eggs. | Contesto: cronaca

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• Mammal ancestors laid eggs, and this 250-million-year-old fossil finally proves it

Contesto

A 250-million-year-old fossil, discovered in the Karoo Basin of South Africa, has delivered conclusive evidence that the ancestors of all mammals laid eggs. The fossil, an exceptionally preserved egg containing the curled-up skeleton of an embryonic Lystrosaurus, ends a decades-long scientific debate. The find, detailed in a study published in the journal Nature, confirms that these hardy, tusked creatures that dominated the planet after the Permian mass extinction reproduced by oviparity, a pivotal discovery for understanding the evolution of mammalian life. The embryo was identified through advanced synchrotron scanning, a non-destructive imaging technology that allowed researchers to peer inside the mineralized egg. The scans revealed the delicate bones of a tiny Lystrosaurus, a dog-sized, herbivorous animal belonging to a group called therapsids, which are the direct evolutionary forerunners of mammals. The egg itself is relatively large and, critically, shows a soft, leathery shell structure, lacking the crystalline calcite layers of hard-shelled reptile and bird eggs. This suggests the eggs were buried in moist soil or vegetation, similar to those of modern monotremes like the platypus and echidna, the only egg-laying mammals alive today. The significance of this discovery is magnified by the era from which it comes. Lystrosaurus thrived in the immediate aftermath of the Permian-Triassic extinction event approximately 252 million years ago, a catastrophe that wiped out more than 70% of terrestrial vertebrate life. Its ability to survive and then proliferate across the supercontinent of Pangaea has long fascinated paleontologists. The confirmation of its reproductive strategy provides a crucial piece of the puzzle for its success. Producing large, nutrient-rich, soft-shelled eggs would have been a significant advantage in the unstable post-apocalyptic environments, potentially allowing for faster incubation and higher survival rates for offspring compared to live-bearing species of the time. For years, the reproductive biology of early mammal ancestors was inferred from skeletal anatomy and phylogenetic bracketing, but direct fossil evidence was absent....

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Categoria: cronaca