A 236-million-year-old fossil challenges the story of mammalian live birth

Key Points:

• A new study provides the first strong evidence that cynodonts, mammalian ancestors from around 236 million years ago, may have given birth to live young (viviparity) much earlier than previously thought—by 90 to 95 million years.
• Researchers discovered a neonatal growth line in the bones of Chiniquodon theotonicus, indicating live birth, and estimated its newborn weighed about 14% of its adult mass, a ratio similar to modern placental mammals but much higher than reptiles or birds.
• The findings suggest that viviparity may have evolved early in the mammalian lineage during the Triassic period, possibly as an adaptation to harsh environmental conditions and predation pressures.
• This challenges the previous assumption that live birth was a relatively recent evolutionary development in mammals and raises questions about other advanced traits that cynodonts might have possessed.
• While C. theotonicus may not be an isolated case, further evidence is needed to confirm if viviparity was widespread among early cynodonts, indicating they were more similar to modern mammals than once believed.