Mammal ancestors were giving birth to live young 236 million years ago
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- Chiniquodon theotonicus fossil from Talampaya National Park (unearthed 2011) revealed a growth line in its leg bone recording a post-birth growth spurt, suggesting live birth 236 million years ago — about 90 million years earlier than previously assumed
- Leandro Gaetano and colleagues at Argentina's National Scientific and Technical Research Council calculated a newborn-to-adult mass ratio of 1.68 kg to roughly 12 kg, which matched placental mammals rather than egg-laying reptiles or birds
- The comparison drew on ratios from 1,869 mammals, 2,619 non-avian reptiles, and 782 birds; Chiniquodon grouped only with placental mammals, whose newborns can exceed 20% of adult body mass versus as little as 0.002% in some large birds
- Mike Benton (University of Bristol) endorsed the findings and pointed to the 'amniote egg gap' — the puzzling absence or rarity of eggs from the Permian and Triassic — as a long-standing mystery the result helps resolve
- Oliver Griffith (Macquarie University) said he was skeptical of the live-birth claim but called the indirect approach 'kind of cool,' noting that direct evidence — a fossilized cynodont egg or pregnant mother — may never surface
- The discovery originated from a chance observation during a postgraduate class at the University of Buenos Aires while students examined thin slices of the fossil's leg bones under a microscope
Why it matters: If the inference holds, live birth — one of the defining traits of most modern mammals — emerged 90 million years earlier in the mammalian lineage than previously thought, rewriting a core chapter of mammalian evolution. The catch: with no direct fossil evidence (a preserved pregnant cynodont or egg) likely ever to be found, the entire claim rests on indirect bone-growth proxies, which is why outside experts like Griffith remain openly skeptical even as Benton backs the team.
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