Mammal ancestors may have made the evolutionary leap from egg-laying to live birth at least 90 million years earlier than thought.
Growth rings within fossil bones from an adult Chiniquodon theotonicus , a species of cynodont that lived around 236 million years ago, reveal that the animal was relatively large when it was a baby, researchers report August 13 in Frontiers in Mammal Science . Such hefty newborns, relative to adult body size, are typical of modern mammals, but not of egg hatchlings, whether reptiles or birds. That, the team says, suggests that this cynodont fossil could be the earliest evidence of live birth.
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