Researchers examining a 236-million-year-old fossil have found compelling evidence that early mammalian ancestors gave live birth rather than laying eggs, according to a study published in Frontiers in Mammal Science.
Fossil Discovery Rewrites Early Mammal Reproduction History
The discovery centers on Chiniquodon theotonicus, a cynodont species that lived during the late Triassic period, according to reporting by Gizmodo. Scientists previously believed that cynodonts—a major group of mammal-like reptiles that gave rise to all modern mammals—laid eggs, similar to modern monotremes like platypuses and echidnas. The new findings push the timeline of viviparity, or the development of an embryo inside the mother’s body, back by up to 95 million years.
Uncovering Microscopic Evidence in Northwestern Argentina
The investigation began during a postgraduate course when researchers analyzed the bone microstructure of a fully grown C. theotonicus fossil discovered in northwestern Argentina. Co-author Maria Miceli Baro, a graduate student at the University of Buenos Aires, noticed unusual embryonic tissue while examining the specimen.
She came with her samples, and we were looking at them, and we said, ‘Hey, look, you have embryonic tissue. This is very weird, I have never seen this,’
lead author Leandro Gaetano, a paleontologist at Argentina’s National Scientific and Technical Research Council, told Gizmodo.
By comparing the bone features with data from living animals, the team identified a neonatal line. This distinct growth ring forms in bones or teeth due to the sharp acceleration in growth that occurs shortly after birth.
Body Mass Comparisons Mirror Modern Mammals
To confirm their findings, the researchers estimated the body mass of the C. theotonicus individual at birth and at death using measurements from the fossilized bones. Their calculations indicate that the animal weighed about 1.7 kilograms (3.7 pounds) when it was born and roughly 12 kilograms (26.5 pounds) when it died.
When comparing these values with data from thousands of living mammals, non-avian reptiles, and birds, the researchers found that reptiles and birds did not match the fossil’s proportions. Instead, placental mammals—which comprise about 95% of all living mammal species—show newborn-adult body mass ratios that closely align with C. theotonicus.
For comparison, the African bay duiker antelope gives birth to offspring that weigh about as much as the estimated newborn cynodont.
Ecosystem Pressures and Future Research
During the Triassic period, cynodonts thrived amidst ecosystem recovery and restructuring following a massive extinction event. Senior author Adriana Mancuso, a researcher at CONICET focusing on the evolution of terrestrial ecosystems, noted that high competition for resources, strong predatory pressures, a trend toward aridity, and strong seasonality meant that embryos of viviparous species would be better protected than those of egg-laying species.
It is very probable that there was a general shift from egg laying to live birth in these cynodonts,
Gaetano said.
The research team plans to search for embryonic tissues in other specimens of these animals to replicate their findings and further clarify the origins of mammalian reproduction.
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