A new study published in Frontiers in Mammal Science presents the earliest known indication that some cynodonts gave birth to live young. Researchers examined a specimen of Chiniquodon theotonicus from northwestern Argentina dated to about 236 million years ago. Analysis of bone microstructure revealed a neonatal line, a growth mark formed shortly after birth. The newborn weighed roughly 1.7 kilograms, or about 14 percent of the adult mass of 12 kilograms. This ratio aligns closely with modern placental mammals and differs markedly from egg-laying reptiles and birds of similar size. The findings indicate that viviparity in the mammalian lineage may have begun at least 90 million years earlier than previously estimated. The discovery occurred during a postgraduate course when the growth mark was identified in the fossil. Comparisons with thousands of living species supported the interpretation that the animal was viviparous. The Triassic setting, marked by resource competition and seasonal aridity, would have favored internal development of offspring.
