# 236-Million-Year-Old Fossil Pushes Back Origin of Live Birth by 90 Million Years

*2026-08-14 18:01 UTC · Science · 1 min read*

> A 236-million-year-old cynodont fossil from Argentina shows live birth evolved in mammal ancestors up to 95 million years earlier than scientists thought.

A fossilized cynodont from northwestern Argentina has given scientists the first compelling evidence that live birth evolved in mammal ancestors around 236 million years ago — roughly 90 to 95 million years earlier than previously thought.

The specimen, Chiniquodon theotonicus, carried a "neonatal line" in its bone microstructure: a growth ring marking the burst of bone growth that follows birth. Researchers led by Dr. Leandro Gaetano at Argentina's CONICET estimated the newborn weighed about 1.7 kg against an adult mass of roughly 12 kg, a ratio close to 14%. That matches placental mammals of similar size, whose newborns run 8-15% of adult mass, and is far above egg-laying reptiles (0.1-0.6%) and birds (1.3-4.5%) of comparable size.

"We show for the first time that live birth was present in at least one mammalian ancestor," the team wrote, challenging the long-held assumption that all non-mammalian cynodonts laid eggs and that live birth arose only once, later, within the therian mammals that gave rise to today's placentals and marsupials. The study appears in Frontiers in Mammal Science.

## Sources

- [phys.org](https://phys.org/news/2026-08-million-year-fossil-story-mammalian.html)
- [sciencedaily.com](https://www.sciencedaily.com/releases/2026/08/260813045534.htm)

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TechTools News · https://techtools.cz/tools/news/article/2188-236-million-year-old-fossil-pushes-back-origin-of-live-birth-by-90-million-years
Česká verze: https://techtools.cz/tools/news/clanek/2261-236-milionu-let-stary-fosilni-nalez-posouva-pocatek-zivorodosti-o-90-milionu