Overview
- A peer‑reviewed study published on Aug. 13, 2026 reports a neonatal growth line in a Chiniquodon theotonicus specimen (CRILAR PV109) from northwestern Argentina and interprets it as evidence the animal gave birth to live young.
- Researchers identified a neonatal line as a distinct microscopic growth marker that separates embryonic from postnatal bone tissue and used it to estimate the specimen’s size at birth.
- Using that histology, the team estimated the newborn weighed about 1.7 kg and the adult about 12 kg, giving a newborn‑to‑adult mass ratio near 14 percent that groups the fossil with living placental mammals rather than with reptiles or birds.
- The authors reached their conclusion by comparing mass ratios to thousands of living mammals, reptiles, and birds, but they stress the result rests on a single specimen and on indirect proxies and must be replicated with more fossils and analyses.
- If confirmed, the finding would push the origin of viviparity in the mammal lineage back roughly 90–95 million years and prompt reexamination of when other mammal‑like traits first appeared, while also likely driving targeted histological searches in museum collections.