236-Million-Year-Old Fossil Suggests Live Birth Arose Far Earlier in Mammal Ancestors

A study published in the journal Frontiers in Mammal Science provides the first compelling evidence that Chiniquodon theotonicus, a cynodont -- an ancestor group leading to mammals -- that lived about 236 million years ago, may have given birth to live young rather than laying eggs. "We show for the first time that live birth was present in at least one mammalian ancestor, Chiniquodon theotonicus," said lead author Dr. Leandro Gaetano, a paleontologist at Argentina's National Scientific and Technical Research Council (CONICET). "This implies that viviparity among early cynodonts originated in the mammalian lineage at least 90 to 95 million years earlier than previously thought."

The investigation began when researchers noticed an unusual growth mark inside the microscopic bone structure of a fully grown C. theotonicus fossil from northwestern Argentina. Comparisons with living species suggested the feature could be a neonatal line, a growth ring that forms when an animal's growth accelerates sharply soon after birth. To test that interpretation, the researchers estimated the fossil's weight at birth relative to its adult weight, then compared that ratio with measurements from several thousand living mammals, non-avian reptiles and birds.

The team calculated that C. theotonicus weighed about 1.7 kilograms at birth and around 12 kilograms at death -- a birth mass roughly 14% of its adult mass. Living reptiles of similar adult size produce hatchlings weighing only about 0.1% to 0.6% of adult mass, while similarly sized living mammals can give birth to young reaching up to about 18.77% of adult mass. "We were amazed to find that C. theotonicus grouped with extant placental mammals, being clearly distinct from other amniotes like reptiles or birds," Gaetano said.

"It is very well possible that C. theotonicus does not represent an isolated case of viviparity among cynodonts," 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." Senior author Adriana Mancuso, also of CONICET, noted that cynodonts lived through intense competition and predatory pressure following one of the most severe mass extinctions in life's history, a period when protected embryos would have had an advantage over eggs.