A microscopic growth line preserved in the bones of a cynodont from Argentina, along with an estimated unusually high birth weight, suggests that early relatives of mammals were already giving birth live births during the Triassic period. However, the conclusion is based on a single fossil and does not prove that all cynodonts reproduced this way.
A thin line preserved within two fossilized bones may change the timeline of one of the most important developments in mammalian evolution: the transition from egg-laying to live birth. Researchers in Argentina report that Chiniquodon theotonicus, a cainodont that lived about 236 million years ago, bore a mark in its bones corresponding to a line formed around the time of birth.
The cynodonts were an ancient group of synapsids, from which the mammal-like and later mammals themselves evolved. They already displayed several mammalian features, including different types of teeth, a secondary palate, and changes in jaw structure. Chiniquodon itself is not recognized as a direct ancestor of mammals, but rather as an ancient relative of the evolutionary branch from which they emerged.
The study, published in a peer-reviewed journal Frontiers in Mammal Science, suggests that the birth of life appeared in this lineage 90–95 million years earlier than previously assumed.
A line formed near birth – or hatching
The fossil, cataloged as CRILAR-PV109, was discovered in the Chenieres Formation in northwestern Argentina. It is a partially articulated skeleton that includes a skull and several body bones. The size of the skull, the fusion of the bones, the eruption of teeth, and the complete development of the skeleton indicate, according to the researchers, that this is an adult individual.
The researchers prepared thin sections of the femur and tibia and examined them under a polarizing microscope. Within both bones, a sharp change in tissue structure was found: a transition from embryonic bone with relatively small blood vessels to tissue that formed at a faster rate later.
In modern animals, such a change may mark the transition from embryonic to postnatal life. It is called a "birth line," and is formed by the acceleration of bone growth and the change in the loads placed on them as the offspring begins to bear its own weight.
But tissue alone cannot distinguish with certainty between a line formed following birth and a line hatching from an egg. To examine both possibilities, the researchers turned to another measure: the ratio between the size of the offspring at the moment the line is formed and its size as an adult.
Offspring weighing 1.7 kilograms
Using the circumference of the femur inside and outside the birthmark, the researchers estimated that the individual weighed about 1.68 kilograms at birth and about 12 kilograms at maturity. According to this calculation, the offspring weighed about 14% of the adult individual's weight.
The researchers compared the ratio to data on 1,869 mammal species, 2,619 non-avian reptile species, and 782 bird species living today. Adult reptiles that weigh about the same as a Chiniquodon They usually hatch weighing only 9–53 grams. In birds of similar weight, hatching weight ranges from about 110 to 357 grams.
In contrast, placental mammals with an adult weight of 8–15 kilograms may give birth to offspring weighing up to 1.87 kilograms. The ratio calculated for Chiniquodon It is therefore within the range of placental mammals and far from the relationships typical of reptiles and birds.
"We show for the first time that live birth existed in at least one ancient relative of mammals," said lead researcher Dr. Leandro Gaetano of the National Council for Scientific and Technical Research in Argentina.
It is unclear whether live birth evolved once or more than once.
If the interpretation is correct, it leaves two evolutionary possibilities. According to the first, live birth evolved in Chiniquodon Apart from its later appearance in marsupials and placental mammals, live birth has evolved independently more than 150 times among vertebrates, so such a scenario is possible.
According to the second possibility, live birth appeared already in ancient cynodonts and was inherited by mammals. In this case, modern-day duck-billed mammals – the platypus and the porcupine – returned to egg-laying at some point. The researchers state that both options are equally economical from an evolutionary perspective, and the current finding does not allow us to choose between them.
The emergence of live birth during the Triassic period may have provided an advantage in an environment characterized by seasonality, aridity, competition for resources, and pressure from predators. Embryos developing inside the mother's body are better protected from temperature fluctuations, desiccation, and egg predation. However, the study does not directly test whether these conditions drove the evolution of the trait.
Intriguing evidence, but not yet definitive proof
The conclusion is based on a single fossil specimen, and the weight estimates also include uncertainty. The equations used by the researchers were developed based on modern carnivorous mammals, whose body structure and growth patterns are not necessarily identical to those of a Triassic caenodont. The researchers themselves note that the equations sometimes tend to overestimate weight in very small specimens.
The evidence from other fossils also does not present a uniform picture. Kayentatherium wellesi, an ancient relative that lived about 196 million years ago, an adult individual was found alongside 38 tiny offspring – an arrangement that resembles more an egg-laying than a modern mammal's brood.
Paleontologist Eva Hoffman of the American Museum of Natural History, who was not involved in the study, She made a reservation in a conversation with Science News. From the assertion that live birth has been proven in a non-mammal caenodont. She says such a claim requires particularly strong evidence. Paleontologist Randall Irmis also noted that more fossils will be needed to decide whether live birth evolved once early or in several separate branches.
Thus, the finding does not prove that all cynodonts gave birth to live offspring. It presents for the first time a combination of a microscopic sign associated with birth and a relatively large offspring, more consistent with the reproductive pattern of placental mammals than with hatching from an egg. Finding similar growth lines in other fossils, alongside embryos, preserved pelvises, or adjacent offspring, could either support or refute the hypothesis.
Questions and Answers
what is Chiniquodon theotonicus?
A carnivorous caenodont that lived in South America during the Late Triassic period, about 236 million years ago. It was not a true mammal, but belonged to a group close to the branch from which mammals evolved.
How can one deduce from a fossil whether an animal was born or hatched from an egg? The researchers identified a change in bone structure that occurs around the onset of independent life. Since the change can occur at both birth and hatching, they compared the estimated size of the offspring to the adult size.
Why is the weight of the offspring important? Offspring hatching from eggs are usually very small compared to their parents. The estimated weight of Chiniquodon The young were about 14% of their adult weight – a ratio closer to placental mammals.
Does the study prove that animal birth evolved only once? No. The trait may have evolved in Chiniquodon independently, and it may have appeared early and passed to mammals. The existing findings do not allow for a conclusion.
More on the subject on the science website
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- The duck genome unfolds the story of mammalian evolution
- Things that donors know: why is there lactose in milk?
- The oldest placental mammals lived just after the end of the dinosaur era
- Dinosaur ancestors walked 16 kilometers through Earth's "hell"
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