A 236-million-year-old fossil indicates that a very ancient relative of mammals may already have given birth to live young.
The specimen belongs to Chiniquodon theotonicus, a cynodont from the Triassic. These animals were not yet mammals, but they belonged to the evolutionary branch that would give rise to them. Researchers reexamined this fossil with a specific question: did the animal come from a laid egg, or was it born directly from its mother’s body?
Fossil of Chiniquodon - Photo taken at the Tübingen Museum of Paleontology
Image Wikimedia
To understand their reasoning, we need to examine bone growth. In many vertebrates, hard tissues record physiological changes. The researchers identified a mark in the fossil interpreted as a neonatal line. This corresponds to a growth alteration occurring around birth.
This trace is particularly significant because it is found on an extremely young individual. Its development would therefore have continued after this event. The authors see this as evidence supporting live birth, known as viviparity, rather than a traditional hatching outside the mother’s body.
The team also compared the estimated size of the young individual with that of adults of the same species. This relationship more closely resembles that observed in mammals that give birth to live young. Several independent clues thus converge on the same scenario, even though a fossil cannot directly reveal reproduction.
The dating substantially changes the proposed chronology. Chiniquodon lived approximately 236 million years ago, during the Triassic. If the interpretation is correct, live birth would have appeared in the lineage leading to mammals about 95 million years earlier than some previous estimates indicated.
In other words, several characteristics associated with mammals today may have begun developing long before mammals appeared. Cynodonts already possessed a combination of traits heralding this transition. Reproduction may now join that list, with implications for how researchers reconstruct their way of life.
Viviparity has evolved independently many times among vertebrates. It exists in various fish, reptiles, and mammals. Keeping the embryo inside the mother’s body changes its protection and the exchanges with the mother. However, this strategy does not automatically imply the existence of a placenta comparable to that of humans.
It remains to be determined whether Chiniquodon represented an isolated case or a more widespread characteristic among its close relatives. Other cynodont fossils, particularly very young individuals, may be sought to find the same growth marks and clarify when this reproductive transformation became established.