Some cynodonts may have been giving birth to live young much sooner in evolutionary history than previously assumed. A new Frontiers in Mammal Science study has offered the first compelling evidence that cynodonts may have been viviparous – a reproductive mode characterized by live 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."
An inscrutable mystery
"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 current study began during a postgraduate course when a growth mark was discovered in the bone microstructure of a fully grown C. theotonicus specimen found in northwestern Argentina. Data from alive species suggested it was a neonatal line, which is a distinct growth ring that can be found in bones or teeth. It's the result of the strong acceleration of the growth rate that occurs just after birth.
To test their forming hypothesis, the team evaluated the neonate body mass relative to the adult body mass and compared the values of C. theotonicus to those of several thousand mammals, non-avian reptiles, and birds alive today.
"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."
Big babies
The size and weight of the C. theotonicus specimen at the time of its birth and death were estimated from measurements of its bones. This was done by measuring the neonatal line and the external surface, respectively. The team determined it weighed around 1.7kg at birth and around 12kg when it died. This means the newly born C. theotonicus weighed around 14% of the mass it would have reached by the time of its death.
Living reptiles, like some turtles, snakes, and crocodilians that weigh between 8kg and 14.5kg, produce hatchlings weighing between nine and 53g, which translates to very low neonate-adult body mass ratios of roughly 0.1% and 0.6%. Birds like some cranes, pelicans, or vultures that weigh between 8kg and 21.5kg produce hatchlings weighing between 110g and around 357g; a neonate-adult body mass ratio of around 1.3% to 4.5%.
When it comes to mammals of comparable size (weighing between 8kg and 15kg), newborns can be much heavier at between 35.5g and 1.87kg, which results in neonate-adult body mass ratios as high as 18.77%. The bay duiker antelope, for example, gives birth to young that weigh about as much as the newborn C. theotonicus. These calculations exclude non-placental mammals that lay eggs or harbour their newborns in belly-pouches and produce extremely small younglings.
"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.
Turning a theory on its head
"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."
Until now, birthing live young was considered a relatively modern evolutionary acquisition in the mammalian lineage. The finding raises questions regarding which other traits believed to have appeared much later were already present among cynodonts, said the team.
"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."
