A single line in a 236-million-year-old bone changed the history of mammals
A barely visible line inside a fossilised bone has prompted palaeontologists to rethink the reproductive history of mammalian ancestors. In the animal Chiniquodon theotonicus, which lived around 236 million years ago, researchers discovered a microscopic mark which they interpreted as the boundary between pre- and post-natal development. Calculations have shown that its offspring may have been born unusually large — similar to modern live-bearing mammals. This suggests that live birth may have emerged 90–95 million years earlier than previously thought.
Chiniquodon theotonicus lived during the Late Triassic in what is now Argentina. It belonged to the cynodonts — a group that included the ancestors of mammals, but was not yet a true mammal itself.
Researchers studied the microscopic structure of the bones of an adult specimen and discovered a distinct growth line in the femur and ulna.
In modern animals, such marks are known as neonatal lines. They may arise due to a sudden change in metabolism and growth rate during the transition from foetal development to life after birth.
Crucially, below this line, the researchers discovered bone tissue that had formed at an early stage of development. Such features had not previously been described in non-mammalian cynodonts.
Details
A single line was not sufficient to draw a conclusion about live birth. The team therefore sought to determine how large Chiniquodon was at the time of birth.
Based on the position of the line within the bone, the researchers estimated its birth weight at approximately 1.7 kg. By the time of its death, the specimen weighed around 12 kg.
This means that the newborn accounted for approximately 14 per cent of the animal’s body mass at the time of death.
This turned out to be an unusually high proportion.
In modern reptiles of comparable size, hatchlings usually account for only a small fraction of an adult’s body mass. In birds, the figure is also significantly lower. Chiniquodon’s result, however, was much closer to that observed in modern placental mammals.
To test the hypothesis, the scientists compared the ancient animal’s measurements with data on several thousand modern mammals, birds and reptiles.
In terms of the ratio of the size of the newborn to that of the adult, Chiniquodon fell within the range typical of placental mammals, rather than that of most egg-laying animals.
The authors regard this as the first convincing evidence that at least some ancient cynodonts were capable of giving birth to live young.
If this interpretation is correct, live birth existed within the mammalian lineage as early as around 236 million years ago — long before the emergence of true mammals.
The study has a fundamental limitation.
Palaeontologists did not find an embryo inside the animal’s body, nor did they discover a pregnant female. The conclusion is based on the microscopic structure of the bones, an estimate of the newborn’s size, and a comparison with modern animals.
It is therefore more accurate to speak of strong evidence for live birth rather than definitive proof.
Furthermore, the analysis is based on a single specimen of Chiniquodon. This does not yet allow us to conclude that all members of this group reproduced in this way. Independent palaeontologist Randall Irmis also noted that further fossil evidence will be required to choose between possible evolutionary scenarios.
The discovery presents an interesting evolutionary puzzle.
Modern platypuses and echidnas lay eggs, whereas marsupials and placental mammals give birth to live young.
If Chiniquodon was indeed viviparous, there are various possible explanations. Viviparity could have arisen independently in it and later again in other mammalian ancestors. Alternatively, this ability may have emerged very early on but was lost in some subsequent lineages.
It is not yet possible to determine which scenario is correct. Other ancient relatives of mammals are also known, and the evidence for these suggests that they laid eggs. For example, the Kayentatherium fossils, dating back around 196 million years, include an adult specimen with 38 young — a number more reminiscent of a clutch of eggs than a small litter of modern mammals.
Why this is important
The transition from egg-laying to live birth was one of the most significant changes in the evolution of mammalian reproduction. However, pinpointing exactly when this occurred is extremely difficult: eggs, embryos and soft tissues are very rarely preserved in the fossil record.
New research shows that a clue may lie even within the bone itself.
If other findings confirm these results, live birth will have to be regarded as a much more ancient evolutionary experiment than previously thought: it may have emerged even when modern mammals were still tens of millions of years away.
Source
Study: Leandro Carlos Gaetano, María de los Ángeles Miceli Baro, Ignacio Alejandro Cerda, Adriana Cecilia Mancuso. Early Origin of Viviparity in the Mammalian Lineage.
Journal: Frontiers in Mammal Science, 2026.