When Did Animals Start Giving Birth to Live Young? This 236-Million-Year-Old Fossil Might Rewrite Mammals’ Evolutionary History

A bone feature found in a mammal ancestor hints that the creature was relatively large as a newborn, more in line with the size of a creature birthed live than hatched from an egg. Still, it provides only indirect evidence

Margherita Bassi | Daily Correspondent

August 17, 2026 12:23 p.m.

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Most mammals today give birth to live offspring, rather than laying eggs. However, scientists are unsure of when exactly this characteristic emerged in the mammalian lineage. One theory suggests that mammals became viviparous, or birthed live babies, only in recent evolutionary history.

But a new study counters that notion. A growth mark found in a 236-million-year-old fossilized mammal ancestor hints that the creature was relatively large as a newborn, more in line with the size of a creature birthed live than hatched from an egg, researchers reported August 12 in the journal Frontiers in Mammal Science. The indirect evidence of live birth suggests that the reproductive process was taking place around 95 million years earlier than scientists previously thought.

The work is based on the remains of a Chiniquodon theotonicus, a dog-size member of a group of mammal-like reptiles called cynodonts that lived around 100 million to 260 million years ago. They were the ancestors of the first true mammals, and many researchers suspected that all non-mammalian cynodonts were oviparous, meaning they laid eggs.

But during a graduate school class, students noticed that an adult C. theotonicus fossil, which had been recovered from northwestern Argentina in 2011, had a bone with a strange line, reports New Scientist’s James Woodford. Further analyses revealed that the feature was a neonatal line—a growth mark akin to a tree ring—that can form in bones or teeth when newborns experience a growth spurt right after birth.

“When you are in an egg or in your mom’s belly, you have a small space and you grow very slowly,” study co-author Leandro Gaetano, a paleontologist at Argentina’s National Scientific and Technical Research Council, tells Live Science’s Chris Simms. “When you hatch or are born, you have a lot of space and you start growing very, very quickly.”

To investigate this line of thought, the team used the creature’s bone measurements and the neonatal line to deduce its weight at birth and death. It would have come into the world at about 3.7 pounds and died at around 26.4 pounds, translating to a 14 percent newborn-to-adult body mass ratio.

They then compared the ratio to that of living animals: 1869 mammals, 2619 reptiles and 782 birds. Today’s reptiles, such as turtles and snakes, are generally born relatively small. Adults that weigh between around 17.6 and 32 pounds have hatchlings that weigh between 0.3 and 1.9 ounces, resulting in a neonate-to-adult body mass ratio between 0.1 and 0.6 percent. Similarly, birds like certain cranes, pelicans and vultures with adult weights between 17.6 and 47.4 pounds have a neonate-to-adult body mass ratio of roughly 1.3 to 4.5 percent.

Living mammals, on the other hand, have relatively big babies. Those of comparable size to C. theotonicus, around 17.6 to 33.1 pounds, have a newborn-to-adult body mass ratio of up to nearly 19 percent—similar to the calculation for the extinct creature, the team found.

“It’s not the smoking gun, but it does suggest they are giving birth to live young,” Emily Rayfield, a paleobiologist at the University of Bristol in England who wasn’t involved in the study, tells Live Science.

Still, the indirect nature of the evidence makes evolutionary biologist Oliver Griffith instinctively skeptical of this conclusion, although he appreciates the researchers’ resolve.

“We may never get any of the kind of direct evidence that solves the mystery, for example a fossilized mammalian egg or a fossil mother during pregnancy,” Griffith, of Macquarie University in Australia, tells New Scientist. “So, it is kind of cool that they’re taking a different approach to trying to find an answer to that question.”

What’s more, University of Utah paleontologist Randall Irmis, who wasn’t involved in the research, says that the discovery raises another interesting conundrum: Did live birth enter the evolutionary stage just once in the mammalian lineage, or two or more times? “We’ll need more fossil evidence to test these two hypotheses,” he tells Science News’ Carolyn Gramling.

Cynodonts lived through a mass extinction event 252 million years ago and went on to flourish during the following Triassic period, when there were intense predatory pressures, strong resource competition and severe seasonal temperature swings, study co-author Adriana Mancuso, an evolutionary ecologist at Argentina’s National Scientific and Technical Research Council, says in the statement. Embryos in a mother’s womb likely would have been safer than those in eggs, she adds.

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.