A 236-million-year-old fossil could rewrite the story of mammalian birth

"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," said lead author Dr. Leandro Gaetano, a paleontologist at the National Scientific and Technical Research Council (CONICET) in Argentina. "This implies that viviparity among early cynodonts originated in the mammalian lineage at least 95 to 90 million years earlier than previously thought."

A Longstanding Mystery About Mammal Reproduction

"Cynodonts thrived in the Triassic, a period of recovery and restructuring of ecosystems after one of the most devastating mass extinctions in life history," explained senior author Adriana Mancuso, a researcher at CONICET who focuses on the evolution of terrestrial ecosystems. "This meant high competition for resources and strong predatory pressures. Combined with a trend toward aridity and strong seasonality, embryos of viviparous species would be better protected than those of egg-laying species."

The investigation began during a postgraduate course, when researchers noticed an unusual growth mark in the microscopic structure of a fully grown C. theotonicus fossil discovered in northwestern Argentina. Comparisons with living animals suggested that the feature was a neonatal line, a distinct growth ring that can occur in bones or teeth. Such lines form in response to the sharp acceleration in growth that takes place shortly after birth.

To investigate whether this interpretation was correct, the researchers estimated the animal's body mass as a newborn and compared it with its adult body mass. They then compared those values with data from several thousand living mammals, non-avian reptiles, and birds.

"If mammalian ancestors were egg-laying or viviparous has been considered an inscrutable mystery," said Gaetano. "We came up with a somewhat ingenious set of methods to get at something very difficult to analyze in the fossil record."

An Unusually Large Newborn

The researchers used measurements from the fossilized bones to estimate how large and heavy the C. theotonicus individual was at birth and at death. The neonatal line provided information about its size at birth, while the outer surface of the bone reflected its final size.

Their calculations indicate that the animal weighed about 1.7kg when it was born and roughly 12kg when it died. That means the newborn C. theotonicus was about 14% of the body mass it eventually reached.

That ratio is striking when compared with living reptiles. Some turtles, snakes, and crocodilians weighing between 8kg and 14.5kg produce hatchlings that weigh only nine to 53g. Their neonate-adult body mass ratios are therefore extremely low, at roughly 0.1% to 0.6%.

Birds also tend to produce relatively small offspring. Some cranes, pelicans, and vultures weighing between 8kg and 21.5kg have hatchlings ranging from 110g to around 357g, corresponding to neonate-adult body mass ratios of about 1.3% to 4.5%.

A Birth Pattern More Like Modern Mammals

Mammals of a similar adult size (weighing between 8kg and 15kg) can give birth to much heavier young, ranging from 35.5g to 1.87kg. Their neonate-adult body mass ratios can reach as high as 18.77%.

The bay duiker antelope, for example, gives birth to offspring that weigh about as much as the estimated newborn C. theotonicus. The researchers excluded non-placental mammals that lay eggs or harbor their newborns in belly-pouches and produce extremely small younglings from these calculations.

"We were amazed to find that C. theotonicus grouped with extant placental mammals, being clearly distinct from other amniotes like reptiles or birds," said Gaetano.

Rethinking When Live Birth Evolved

"In cynodonts, embryonic tissues were never observed before, let alone a neonatal line," said co-author María Miceli Baro, a graduate student at the University of Buenos Aires. "Through its analysis, we found that a trait that is generally linked to evolutionary success was present in animals long before true mammals originated."

Live birth has generally been viewed as a relatively recent evolutionary development within the mammalian lineage. These findings suggest it may have appeared much earlier, raising the possibility that other traits thought to have evolved later were already present in cynodonts.

"It is very well possible that C. theotonicus does not represent an isolated case of viviparity among cynodonts. 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," concluded Gaetano. "Still, it looks like some cynodonts were in fact very similar to present-day mammals."