The vast majority of mammals alive today (including us) give birth to live young, but this reproductive mode is neither unique to mammals nor universal among them. In fact, it evolved independently many times in other vertebrate groups, and some mammal species even lay eggs. For years, scientists believed that cynodonts (a major group of mammal-like reptiles that gave rise to all modern mammals) probably also laid eggs. After all, egg-laying is common among reptiles, as well as the oldest lineage of living mammals: the monotremes, which include platypuses and echidnas. However, efforts to determine whether cynodonts laid eggs or gave birth have been inconclusive so far. A study published today in the journal Frontiers in Mammal Science presents the first fossil evidence to suggest that a species of cynodont gave birth. Chiniquodon theotonicus lived approximately 236 million years ago, during the Late Triassic period. According to the researchers, this discovery pushes the timeline of this evolutionary adaptation back up to 95 million years. Stumbling upon an important discovery Co-author María Miceli Baro, a graduate student at the University of Buenos Aires in Argentina, was examining the bone microstructure of a fully grown C. theotonicus specimen when something surprising appeared. “She came with her samples, we were looking at them and we said, ‘Hey, look, you have embryonic tissue. This is very strange, I’ve never seen this,'” lead author Leandro Gaetano, a paleontologist at Argentina’s National Scientific and Technical Research Council based at the University of Buenos Aires, told Gizmodo. Data from living species helped Gaetano and his colleagues identify a neonatal lineage, a distinctive growth ring that can be found on bones or teeth. It results from the strong acceleration of the growth rate that occurs just after birth. The researchers realized that this discovery might finally offer a way to test the universally accepted hypothesis that cynodonts laid eggs. “We started trying to figure out how we could use this new information to test the working hypothesis, and we came up with the idea of using body size as an indicator,” Gaetano explained. Putting together a reproductive puzzle The researchers measured the neonatal line and the outer surface of the specimen to estimate its size and weight at birth and death. They determined that he weighed approximately 1.7 kilograms (3.7 lb) at birth and 12 kg (26.5 lb) when he died. That means that the newborn C. theotonicus weighed 14% of the mass it reached at the time of its death. The team then compared this newborn-to-adult body mass ratio to other groups of living animals to see which group C. theotonicus most resembled. Today’s reptiles, such as turtles, snakes, and crocodiles, which weigh between 17.6 pounds (8 kg) and 32 pounds (14.5 kg), tend to produce offspring weighing between 9 and 53 grams. This translates into very low newborn-to-adult body mass ratios of approximately 0.1% and 0.6%. Larger birds, such as some cranes, pelicans, and vultures, also exhibit low newborn-to-adult body mass ratios, of around 1.3% to 4.5%. Those groups clearly didn’t fit well, but placental mammals, which make up about 95% of all living mammal species, have newborn-to-adult body mass ratios that closely align with those of C. theotonicus. Those weighing between 17.6 pounds (8 kg) and 33.1 pounds (15 kg) can have newborns weighing between 35.5 grams and 4.1 pounds (1.87 kg), which translates to newborn-to-adult body mass ratios as high as 18.77%. Take the bay duiker antelope as an example. This African species gives birth to young that weigh about the same as a newborn C. theotonicus. “Embryonic tissues have never been observed before in cynodonts, much less a neonatal line,” Miceli Baro said in a statement. “Through their analysis, we discovered that a trait that is generally linked to evolutionary success was present in animals long before true mammals originated.” While the researchers only studied one species of cynodont, Gaetano said his team’s findings align with previous studies showing that some characteristics of extant placental mammals, such as lactation, body hair and brain size, were already appearing in relatives of C. theotonicus. These traits are also related to the genetic basis underlying live births, suggesting that these changes may have arisen together. “It is very likely that there was a general shift from egg laying to live births in these cynodonts,” Gaetano said. He and his colleagues hope to look for embryonic tissues in other specimens of these animals to try to replicate their findings. Their future work could finally reveal the emergence of mammalian reproduction as we know it today.
