Fossil Evidence Pushes Live Birth Origin in Mammals Back 90 Million Years

Fossil evidence from a 236-million-year-old mammal ancestor suggests that live birth evolved much earlier in the mammalian lineage than previously believed, potentially reshaping the timeline of reproductive evolution. A study published on August 13 in the journal Frontiers in Mammal Science presents the first compelling indication that a cynodont, a group of animals bridging reptiles and mammals, gave birth to live young.

The research centers on a specimen of Chiniquodon theotonicus, a cynodont that lived during the Triassic Period in what is now northwestern Argentina. Researchers identified a microscopic growth mark in the animal's bone known as a neonatal line, which forms shortly after birth due to the metabolic stress of the transition from prenatal to postnatal life. This feature had never before been documented in a cynodont.

Using the neonatal line and other bone growth rings, the team estimated the animal weighed approximately 1.7 kilograms at birth and about 12 kilograms when it died. This yields a newborn-to-adult body mass ratio of roughly 14 percent. The researchers then compared this ratio to data from thousands of living mammals, reptiles, and birds. They found...