Mammal ancestors may have made the evolutionary leap from egg-laying to live birth at least 90 million years earlier than thought.
Growth rings within fossil bones from an adult Chiniquodon theotonicus, a species of cynodont that lived around 236 million years ago, reveal that the animal was relatively large when it was a baby, researchers report August 13 in Frontiers in Mammal Science. Such hefty newborns, relative to adult body size, are typical of modern mammals, but not of egg hatchlings, whether reptiles or birds. That, the team says, suggests that this cynodont fossil could be the earliest evidence of live birth.
It’s long been a mystery when mammalian ancestors became viviparous, giving birth to live young, says Leandro Gaetano, a paleontologist at the National Scientific and Technical Research Council in Buenos Aires. Mammals are the only surviving lineage of the cynodonts (meaning “dog teeth”), a group of creatures that appeared around 260 million years ago, toward the end of the Permian Period.
Cynodonts had some powerful evolutionary advances relative to their own ancestors, the therapsids. They had differentiated teeth, sporting canines, incisors and molars, which made it possible to process a variety of foods through cutting, tearing and grinding. Their jaws were adapted for chewing, including a secondary palate that allowed them to chew and breathe at the same time, speeding up digestion and making it possible to fuel a higher metabolic rate.
As a result, cynodonts were at least partially warm-blooded. That, along with their burrowing abilities, may have helped them survive the mass extinction at the end of the Permian, around 252 million years ago.
Live birth is a characteristic of most modern mammals, with the exception of egg-laying monotremes like platypuses. But viviparity wasn’t thought to appear until much later in the cynodont lineage, with the emergence of a group called the therian mammals around 160 million years ago.
The new timeline comes from analyses of a C. theotonicus specimen found in northwestern Argentina. The team examined growth rings in the creature’s bones and identified a neonatal line, a microscopic dark ring that marks the intense metabolic stress of being born, separating prenatal tissue from postnatal tissue.
Using the neonatal line and other bone-growth rings, the team estimated that the animal weighed about 1.7 kilograms at birth and about 12 kilograms when it died. That’s a newborn-to-adult body mass ratio of about 14 percent.
The researchers then plotted C. theotonicus’ newborn-to-adult body mass ratio alongside that of thousands of modern creatures. That list included reptiles such as turtles, snakes and crocodiles; birds such as cranes and vultures; nonplacental mammals that lay eggs such as echidnas; pouch-rearing marsupials such as kangaroos; and placental mammals that give birth to fully developed live young, including humans.
“We were amazed to find that C. theotonicus grouped with extant [living] placental mammals,” Gaetano says. Its body mass ratio was nearly identical to that of a small African antelope called the bay duiker. By contrast, the newborn-to-adult body mass ratio for monotremes and marsupials can be around 0.15 percent. For reptiles and birds, it can be 0.1 percent to 4.5 percent. That comparison strongly suggests that C. theotonicus gave live birth, Gaetano and his colleagues say.
One implication of this suggestion is that it opens the question of whether mammalian live birth evolved once, but much earlier in mammal ancestors, or whether it actually evolved at least twice, says Randall Irmis, a paleontologist at the University of Utah in Salt Lake City who wasn’t involved in the study. “We’ll need more fossil evidence to test these two hypotheses,” Irmis says.
But that’s challenging, because researchers haven’t yet found other fossilized embryonic tissues in cynodonts that would help make the case that this reproductive style had evolved earlier.
And there is strong evidence that certain other early mammal ancestors and relatives were still laying eggs. Lystrosaurus, a dicynodont that lived around 250 million years ago and also survived the Permian extinction event, certainly laid eggs: Scientists have found a tightly curled Lystrosaurus embryo preserved inside a shell. Then there’s Kayentatherium wellesi, a cynodont that lived around 196 million years ago; fossils of this creature, found in northern Arizona, revealed an adult with 38 individual young, a number that’s more suggestive of a clutch of eggs than a litter of babies.
“It would require strong evidence to demonstrate viviparity,” says Eva Hoffman, a paleontologist at the American Museum of Natural History in New York City who studies cynodont evolution. “And I haven’t seen that for any non-therian cynodont.”
