Argentine Cynodont Fossil Rewrites Live Birth Timeline

The thigh bone of an animal that died about 236 million years ago in what is now northwestern Argentina carries a mark never before documented in mammal ancestors. A team from Argentina’s National Scientific and Technical Research Council (CONICET) published an analysis of that bone on August 13 in the journal Frontiers in Mammal Science and concluded that the species Chiniquodon theotonicus gave birth to live young. If the interpretation holds, live birth in mammal ancestors is 90 to 95 million years older than previously thought.

The lead author of the study is Leandro Gaetano, a paleontologist at the same institution in Buenos Aires.

For the first time, we show that live birth existed in at least one mammal ancestor, the species Chiniquodon theotonicus.

That means viviparity in early cynodonts arose in the mammalian lineage at least 95 to 90 million years earlier than thought.

Viviparity is the technical term for giving birth to live young, as opposed to laying eggs. Cynodonts are a group that appeared near the end of the Permian, about 260 million years ago, and mammals are the only lineage within it that has survived to the present. Live birth had until now been tied to the emergence of therian mammals about 160 million years ago, a much later point in that story. Platypuses and echidnas, the only egg laying mammals alive today, remain the exception that picture allows for.

What Is a Neonatal Line?

It is a thin, dark ring in bone that records a sudden jump in growth rate right after birth, separating tissue formed before birth from tissue formed after it. In living species it shows up in bones and teeth, but it had never been documented in cynodonts before. In the thigh bone of the adult specimen examined, the line appears as a darker boundary, and the tissue above it carries a denser network of blood vessels. The find turned up during a graduate course, in a specimen recovered in northwestern Argentina.

The Mass Ratio That Places It Among Placental Mammals

From the bone measurements, the team estimated two figures: a birth mass of about 1.7 kilograms and a mass of about 12 kilograms at the time of death. The newborn thus carried close to 14 percent of the adult’s mass. That ratio was then compared against values from several thousand living animals, including reptiles, birds, platypuses and echidnas, marsupials and placental mammals.

Among living species of similar size, the comparison looks like this:

• reptiles such as turtles, snakes and crocodiles: roughly 0.1 to 0.6 percent
• birds such as cranes, pelicans and vultures: roughly 1.3 to 4.5 percent
• mammals weighing between 8 and 15 kilograms: up to 18.77 percent

In that analysis, Chiniquodon theotonicus fell among today’s placental mammals rather than among egg laying animals. Its value is nearly identical to that of a small African antelope called the bay duiker.

We were stunned to find that Chiniquodon theotonicus groups with living placental mammals.

Why would such a reproductive strategy have made sense specifically in the Triassic? Adriana Mancuso, a paleontologist at the same institution and one of the study’s authors, explains: the Triassic was a period of ecosystem recovery after the mass extinction at the end of the Permian, marked by fierce competition for resources and severe drought.

Embryos of viviparous species would have been better protected than embryos of egg laying species.

What This Finding Does Not Prove

The conclusion rests on a single specimen and on an indirect indicator, the ratio between newborn and adult mass, rather than on an embryo preserved inside the mother’s body. Embryonic tissue has not yet been found in cynodonts, so the most direct kind of confirmation is still missing.

Paleontologists not involved in the research are looking for exactly that kind of evidence. Randal Irmis of the University of Utah says the finding raises the question of whether live birth in the mammalian lineage arose once, but much earlier, or at least twice.

We will need more fossil evidence to test these two hypotheses.

Eva Hofman of the American Museum of Natural History in New York, who studies cynodont evolution, sets a stricter bar.

Strong evidence would be needed to prove viviparity.

There are also findings that point the other way. In the dicynodont Lystrosaurus, which lived about 250 million years ago, an embryo was found preserved inside an eggshell. In the cynodont Kayentatherium wellesi, about 196 million years old, 38 young were found alongside an adult, a number that fits a clutch of eggs better than a live litter. The finding from Argentina, then, speaks to one species, not to the whole group.