A tiny growth ring preserved in the bones of a Triassic creature could rewrite textbook knowledge on mammal evolution. The fossil, found in the Chañares Formation of La Rioja province in Argentina, belongs to the species Chiniquodon theotonicus, a cynodont (a group of advanced reptiles closely related to mammals) that lived approximately 236 million years ago.
The Discovery That Surprises Paleontology
The study, published on August 13, 2026, in the journal Frontiers in Mammal Science and led by paleontologist Leandro Gaetano from CONICET (the National Scientific and Technical Research Council of Argentina), analyzed the bone microstructure of a specimen identified as CRILAR-PV109. The researchers discovered a neonatal line, a non-cyclical growth mark that in modern species is associated with the physiological change following birth.
“We found a very peculiar growth ring. It is a mark formed by a sudden change, an acceleration in the growth rate immediately after birth,” Gaetano explained.
This line represents the circumference of the bone at the moment of birth, allowing scientists to estimate the animal's body mass at birth for the first time.
How Much Did It Weigh at Birth? The Game-Changing Data
The team's calculations determined that the specimen weighed approximately 1.7 kilograms at birth and around 12 kilograms in adulthood. That means the newborn was equivalent to about 14% of the adult weight, a surprisingly high proportion.
To put this value into context, the scientists compared the data with a database comprising 1,869 mammals, 2,619 reptiles, and 782 birds alive today. The results were conclusive: in reptiles of comparable mass, offspring represent between 0.1% and 0.6% of adult weight; in similar birds, 1.3% to 4.5%. Only placental mammals reach values near 18.77%.
Birth Weight vs. Adult Weight Comparison
| Group | Offspring/Adult Ratio |
|---|---|
| Reptiles | 0.1% – 0.6% |
| Birds | 1.3% – 4.5% |
| Chiniquodon theotonicus | ≈ 14% |
| Placental Mammals | Up to 18.77% |
A Paradigm Shift in Evolution
Until now, the scientific community maintained that synapsids prior to modern mammals, including cynodonts, laid eggs. This new analysis does not offer direct observation of a live birth, but it does provide solid histological evidence pointing in another direction.
“We demonstrated for the first time that the birth of live young was present in at least one of the ancestors of mammals,” Gaetano stated. The researcher added that viviparity “originated in the mammalian lineage at least 95 to 90 million years earlier than previously believed.”
The finding raises two possible scenarios: either viviparity appeared independently in probainognathian cynodonts and, much later, in modern therians; or it emerged once in a common ancestor and current monotremes (like the platypus) retained or regained the egg-laying condition.
An Environmental Context That Explains It
The authors link this reproductive strategy to the harsh conditions of the Triassic period, marked by the reorganization of ecosystems after one of the great mass extinctions. Aridity, strong seasonality, and predator pressure could have favored retaining embryos inside the mother's body.
“In this context, the embryos of viviparous species would have been protected from desiccation and could have been kept at temperatures more suitable for their development than those of egg-laying species,” Gaetano noted.
An International Team and an Exceptional Fossil
The research involved specialists from the Institute of Andean Studies Don Pablo Groeber (UBA-CONICET), the National University of Río Negro, the Carlos Ameghino Museum, the Argentine Institute of Snow, Glaciology and Environmental Sciences, and the Evolutionary Studies Institute at the University of Witwatersrand in South Africa.
Fossil CRILAR-PV109 is one of over 50 known specimens of the species and the largest recorded to date. Its exceptional preservation allowed researchers to study the fibula and femur, where bone remodeling did not destroy early embryonic tissues—a rare occurrence in adult vertebrates.
If future discoveries confirm this pattern in other cynodonts, the timeline of one of the most characteristic reproductive adaptations of mammals will have to be pushed back to the Late Triassic, long before its previously assumed appearance between the Late Jurassic and Early Cretaceous.
Sources: Infobae, LA NACION, Diario Jornada, El Eco de Tandil, and Muy Interesante. Image: Leandro Gaetano / María de los Ángeles Miceli Baro.