A 236-million-year-old fossil belonging to Chiniquodon theotonicus, a species of cynodont, suggests that mammal ancestors may have given live birth approximately 95 million years earlier than previously understood. The findings, published in the journal Frontiers in Mammal Science, could reshape the evolutionary history of mammalian reproduction.
For years, scientists have debated whether cynodonts, considered likely ancestors of modern mammals, reproduced by laying eggs like other reptiles or by giving birth to live young. Direct evidence of their reproductive methods remained elusive until the recent analysis of the C. theotonicus fossil. Researchers discovered embryonic tissue within the fossilized bones, which they identified as a neonatal line—a growth ring that forms after birth. This discovery provided a novel way to estimate the animal's body mass at birth relative to its adult size.
By comparing this ratio to data from thousands of modern bird, mammal, and reptile species, the researchers determined that the individual weighed approximately 3.7 pounds at birth and 26.5 pounds when fully grown. This 14% increase closely matches the birth-to-adult body mass ratio of species that give live birth, particularly placental mammals. In contrast, egg-laying species typically show a much larger relative increase in size from birth to adulthood.
Dr. Leandro Gaetano, lead author and paleontologist at the National Scientific and Technical Research Council in Argentina, stated that the results indicate some cynodonts were remarkably similar to present-day mammals. "It looks like some cynodonts were in fact very similar to present-day mammals," Gaetano said.
This adaptation to live birth may have provided an evolutionary advantage during the Triassic period, a time of significant ecosystem recovery and high competition following a major mass extinction. Senior author Adriana Mancuso, a researcher at CONICET specializing in terrestrial ecosystem evolution, believes that live birth offered greater protection to developing embryos in the increasingly harsh and seasonal environment of the Triassic, compared to eggs. "Due to the increasingly dry conditions and seasonality, embryos of live birth-giving animals would be better protected than those of egg-laying species," she explained.
Relatives of Chiniquodon theotonicus have also shown evidence of lactation, body hair, and brain sizes comparable to modern placental mammals, further supporting the idea that key mammalian traits, including live birth, may have emerged much earlier in evolutionary history. Researchers plan further studies to determine if this reproductive mechanism in C. theotonicus was an isolated case or representative of a broader trend among mammal ancestors.