The moment of birth is written in bone. Paleontologists studying a 236-million-year-old cynodont from Argentina say they have found the earliest evidence yet of live birth — viviparity — in the evolutionary line that leads to mammals, pushing back the origin of this reproductive strategy by as much as 90 to 95 million years.

The fossil belongs to Chiniquodon theotonicus, a cat-sized cynodont that lived during the Triassic period in what is now northwestern Argentina. Cynodonts were the mammal-like reptiles that eventually gave rise to all modern mammals, and scientists had long assumed that the non-mammalian ones laid eggs.

The team, led by Dr. Leandro Gaetano of Argentina's National Scientific and Technical Research Council (CONICET), examined thin slices of the animal's femur and ulna under a microscope. In the bone tissue near the center of each bone they found a distinct transition: a 'neonatal line' marking the moment of birth — a feature well documented in living mammals but never before identified in such an ancient, non-mammalian relative.

Measuring the bone's circumference at that line let the team estimate the animal's body mass at birth: roughly 1.68 kilograms, against nearly 12 kilograms as a full-grown adult. The newborn therefore weighed between 10 and 20 percent of its adult body mass — a proportion far closer to modern placental mammals than to egg-laying reptiles, birds, or even marsupials and monotremes, which produce comparatively tiny offspring.

A statistical comparison against nearly 1,900 mammal species, 2,600 reptile species and 780 bird species consistently classified the ancient creature alongside placental mammals. "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," Gaetano said. "This implies that viviparity among early cynodonts originated in the mammalian lineage at least 95 to 90 million years earlier than previously thought."

The finding challenges the textbook assumption that live birth evolved only once, within the therian mammals that gave rise to modern placentals and marsupials. The researchers propose two equally plausible scenarios: either live birth evolved independently in Chiniquodon's lineage and again later in therian mammals, or it arose just once, very early in mammalian ancestry — with egg-laying monotremes such as the platypus representing a later reversal back to the ancestral condition.

"It is very well possible that Chiniquodon theotonicus does not represent an isolated case of viviparity among cynodonts," Gaetano said. "It could be evidence of the general switch from laying eggs to giving birth to live young early on in the mammalian lineage. But we need more evidence to test this hypothesis. Still, it looks like some cynodonts were in fact very similar to present-day mammals."

The study was published today in the journal Frontiers in Mammal Science (doi: 10.3389/fmamm.2026.1845319).

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