Paleontologists studying a 236-million-year-old cynodont fossil from Argentina say they have found the earliest evidence yet of viviparity (live birth) in the evolutionary line that leads to mammals, pushing back the origin of this reproductive strategy by as much as 90 to 95 million years.
Life reconstruction of a group of Chiniquodon theotonicus individuals, a pregnant female (left) as well as a female (right) breastfeeding few, relatively large neonates. Image credit: María de los Ángeles Miceli Baro.
“Viviparity is an important acquisition in mammalian evolution, characteristic of modern therians and thought to have played a key role in their evolutionary success,” said Dr. Leandro Gaetano, a paleontologist at the National Scientific and Technical Research Council (CONICET), and his colleagues.
“This reproductive mode contrasts with oviparity, the plesiomorphic condition for vertebrates.”
“Nevertheless, viviparity has evolved independently more than 150 times among vertebrates.”
“Extant therians share a complex array of coadapted features — such as placentation, lactation, and its endocrine regulation — that suggest that viviparity is a synapomorphy of the clade.”
“Monotremes, despite presenting some sort of placentation (i.e., absorption of oviductal secretions as opposed to the yolk sac or chorioallantoic types of placenta present in therians), lay eggs.”
In their study, the researchers examined the fossilized remains of Chiniquodon theotonicus, a cat-sized cynodont that lived during the Triassic period in what is now northwestern Argentina.
They examined thin slices of the animal’s femur and ulna under a microscope and discovered a distinct transition in the bone tissue near the center of each bone — a ‘neonatal line’ marking the moment of birth, a feature well documented in living mammals but never before identified in such an ancient, non-mammalian relative.
By measuring the bone’s circumference at this line, they estimated the animal’s body mass at birth: roughly 1.68 kg, compared with nearly 12 kg as a full-grown adult.
This means the newborn weighed between 10 and 20% of its adult body mass, a proportion far closer to that of modern placental mammals than to egg-laying reptiles, birds, or even marsupials and monotremes, which produce comparatively tiny offspring.
Statistical analysis comparing the fossil to nearly 1,900 mammal, 2,600 reptile, and 780 bird species consistently classified the ancient creature alongside placental mammals.
“We show for the first time that live birth was present in at least one mammalian ancestor, Chiniquodon theotonicus, which lived approximately 236 million years ago,” Dr. Gaetano said.
“This implies that viviparity among early cynodonts originated in the mammalian lineage at least 95 to 90 million years earlier than previously thought.”
The findings challenge the long-held assumption that all non-mammalian cynodonts laid eggs, and that live birth arose only once, within the therian mammals that gave rise to modern placentals and marsupials.
The scientists propose two equally plausible evolutionary scenarios: either live birth evolved independently in Chiniquodon theotonicus’ lineage and again later in therian mammals, or it arose just once, early in mammalian ancestry, with egg-laying monotremes such as the platypus representing a later reversal back to the ancestral condition.
“It is very well possible that Chiniquodon theotonicus does not represent an isolated case of viviparity among cynodonts,” Dr. Gaetano said.
“It could be evidence of the general switch from laying eggs to giving birth to live young early on in the mammalian lineage.”
“But we need more evidence to test this hypothesis.”
“Still, it looks like some cynodonts were in fact very similar to present-day mammals.”
The study was published today in the journal Frontiers in Mammal Science.
_____
Leandro Carlos Gaetano et al. 2026. Early origin of viviparity in the mammalian lineage. Front. Mamm. Sci 5; doi: 10.3389/fmamm.2026.1845319
