Mammal ancestors were giving birth to live young 236 million years ago

Fossilized bones with distinctive growth rings suggest that mammalian ancestors were giving live birth as far back as 236 million years ago, potentially pushing back the timeline for when this reproductive strategy emerged by up to 90 million years. The discovery reexamines assumptions about the evolutionary history of viviparity in mammals, based on evidence from an ancestral species in the mammalian lineage.

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