"We show for the first time that live birth was present in at least one mammalian ancestor, Chiniquodon theotonicus, which lived approximately 236 million years ago," said lead author Dr. Leandro Gaetano, a paleontologist at the National Scientific and Technical Research Council (CONICET) in Argentina.

"This implies that viviparity among early cynodonts originated in the mammalian lineage at least 95 to 90 million years earlier than previously thought."

An inscrutable mystery

"Cynodonts thrived in the Triassic, a period of recovery and restructuring of ecosystems after one of the most devastating mass extinctions in life history," explained senior author Adriana Mancuso, a researcher at CONICET who focuses on the evolution of terrestrial ecosystems.

"This meant high competition for resources and strong predatory pressures. Combined with a trend toward aridity and strong seasonality, embryos of viviparous species would be better protected than those of egg-laying species."

The current study began during a postgraduate course when a growth mark was discovered in the bone microstructure of a fully grown C. theotonicus specimen found in northwestern Argentina. Data from living species suggested it was a neonatal line, a distinct growth ring that can be found in bones or teeth. It's the result of the strong acceleration in growth rate that occurs just after birth.