A newly published study suggests that one characteristic commonly associated with modern mammals—giving birth to live young—may have appeared in the mammalian lineage far earlier than palaeontologists previously realised.
Creationists prefer to imagine the major groups of organisms appearing fully formed, equipped from the outset with complete packages of supposedly defining characteristics. Evolution, however, produces no such conveniently bundled collections of features. Characteristics arise, change and sometimes disappear at different times, producing the mosaic of inherited and modified traits documented throughout the fossil record.
Creationists also rejoice at any suggestion that scientists might have to change their minds because, in their simplistic view of science, if something is not exactly and eternally true, it's wrong; there is no concept in creationism of incomplete truths or provisional conclusions contingent on future discoveries. Unlike creationism, though, changing one’s mind when the evidence changes is the hallmark of intellectual integrity, and what give science it's reliability and great strength as a tool for discovering the truth.
So here, to sow confusion in the minds of creationists, is an example of science changing its collective mind and still refuting creationists mythology, because it happened 236 million years before the mythical 'Creation Week'. Significantly too, the authors readily conceded that their conclusion is only provisional and they identify what additional information is needed to confirm or refute it. No such reasoning is ever advanced by creationist apologists because it must never be conceded that creationism could be falsified with evidence, lest that evidence be found.
Apart from the egg-laying monotremes, living mammals are viviparous: their embryos develop inside the mother and are born alive. Until now, this reproductive strategy was generally thought to have originated with the ancestral therian mammals—the lineage containing marsupials and placentals—about 160 million years ago.
However, an international team led by Leandro Carlos Gaetano of the University of Buenos Aires and Argentina’s National Scientific and Technical Research Council (CONICET) has found evidence that Chiniquodon theotonicus, a non-mammalian cynodont that lived about 236 million years ago, may also have given birth to live young. The researchers have published their findings in Frontiers in Mammal Science, accompanied by a Frontiers news release. The research has also been reported by Science News.
The evidence comes not from a fossilised embryo or a pregnant female, but from the microscopic structure of the femur and ulna of a partially articulated C. theotonicus skeleton recovered from the Late Triassic Chañares Formation of north-western Argentina. Inside both bones, the researchers identified an abrupt change between embryonic bone tissue and more rapidly deposited tissue formed later in life. They interpret this boundary as a neonatal line—a microscopic record of the physiological and mechanical changes accompanying birth.
Such a line, considered by itself, cannot reveal whether an animal was born alive or hatched from an egg. To distinguish between those possibilities, the team used the circumference of the femur at the line to estimate the animal’s size at that stage of its life. Their calculations suggest that it weighed about 1.68 kilograms at birth and about 12 kilograms when it died. In other words, the newborn was already approximately 14 per cent of the mass attained by the studied individual.
That is an extraordinarily large offspring-to-adult ratio compared with those of similarly sized egg-laying reptiles and birds, and even with those of monotremes and marsupials. When the researchers compared the estimates with data from thousands of living amniotes, C. theotonicus consistently grouped with placental mammals. Together, the apparent neonatal line and the inferred size of the newborn provide the first compelling, although not conclusive, evidence of live birth in a non-mammalian cynodont.
The conclusion must remain provisional. It is based on a single specimen, and its mass was calculated using equations derived from living carnivorous mammals, which are imperfect analogues for a Triassic cynodont. Even the authors acknowledge that additional specimens will be needed before anyone can determine whether viviparity was widespread among early cynodonts or peculiar to this lineage.
If the interpretation is confirmed, however, it would move the known occurrence of live birth in the mammalian stem lineage backwards by some 90–95 million years. It would leave at least two evolutionary possibilities: viviparity may have evolved independently in Chiniquodon and therian mammals, or it may have originated much earlier and subsequently been lost in the lineage leading to modern monotremes.
Either possibility is entirely evolutionary. The discovery does not overturn evolutionary biology, as sensational headlines about “rewriting” history might imply; it uses evolutionary relationships, comparative anatomy and bone histology to refine the history of one reproductive adaptation. It reveals a mammalian characteristic emerging among mammal-like ancestors roughly 230 million years before creationists imagine their mythical “Creation Week”—in an evolving lineage that their mythology says should never have existed.
How Can a Fossil Bone Reveal Live Birth? Bone is living tissue that records changes in an animal’s growth. Before birth or hatching, an embryo deposits a distinctive type of primary bone. The physiological changes and increased mechanical demands that accompany the beginning of independent life can then produce an abrupt change in the rate and structure of bone growth.The paper in Frontiers in Mammal Science was accompanied by a news release in Frontiers Science News:
Normally, this earliest tissue is destroyed as the marrow cavity expands. In the studied specimen of Chiniquodon theotonicus, however, unusually small marrow cavities and limited subsequent remodelling preserved embryonic tissue in both the femur and ulna. The boundary between this tissue and the more rapidly deposited later bone forms what the researchers interpret as a neonatal line.
A two-stage inference
Egg-laying reptiles and birds of comparable adult size generally produce much smaller hatchlings, whereas the estimated ratio for Chiniquodon falls within the range of many placental mammals. It is this combination of the probable neonatal line and the unusually large young—not the line alone—that supports the inference of live birth.
- The microscopic line: The change in bone structure records an important event around the beginning of independent life. Histology alone, however, cannot distinguish a live birth from hatching.
- The size of the young: The circumference of the femur at the line was used to estimate the animal’s mass at that time. The calculations suggest a mass of approximately 1.68 kilograms, compared with about 12 kilograms when the animal died—a ratio of roughly 14 per cent.
An important qualification: The result is based on one specimen, and the mass estimates rely on equations derived from living carnivorous mammals. The evidence is compelling but not yet conclusive.
Mammalian Characteristics Evolved Piecemeal
Characteristic Evolutionary evidence Differentiated teeth and secondary palate These appeared among non-mammalian cynodonts, improving food processing and allowing breathing while chewing. Higher metabolism and more sustained activity Bone growth, posture and respiratory anatomy indicate that mammal-like physiology developed gradually among therapsids and cynodonts. Hair, whiskers and lactation Several lines of indirect anatomical evidence suggest that these originated among probainognathian cynodonts before the appearance of true mammals. Mammalian jaw and middle ear Fossils preserve transitional arrangements as former jaw bones became progressively reduced and incorporated into the hearing apparatus. Live birth The new evidence suggests that at least one non-mammalian cynodont may have been viviparous approximately 236 million years ago. Modern reproductive diversity Monotremes lay eggs, whereas marsupials and placentals give birth to live young but differ greatly in gestation and the developmental state of their newborns.
No single transition produced a completely modern mammal. Different mammalian characteristics appeared at different times, and some were subsequently modified or lost in particular descendant lineages.
One origin or two?
The present evidence permits two similarly economical evolutionary explanations. Live birth could have evolved independently in Chiniquodon and the ancestors of therian mammals, or it could have originated much earlier in probainognathians, followed by a later reversal to egg laying in the monotreme lineage. Fossils from additional cynodont species will be needed to distinguish between these possibilities.
New fossil evidence challenges the story of mammalian birth
For many years, cynodonts – animals that were a transitional link between early synapsids, a group of animals among which true mammals evolved, and the first true mammals – were believed to have laid eggs. Now, for the first time, a new study has found evidence to the contrary. Using a neonatal line, which is only rarely conserved in the fossil record, and comparisons of newborn-adult body weight ratios, a team in Argentina concluded that Chiniquodon theotonicus gave birth to live young. This pushes the timeline of this evolutionary adaptation back by up to 95 million years, said the team, and offers new avenues to reconstruct ecological aspects of the Triassic world.
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. This implies that viviparity among early cynodonts originated in the mammalian lineage at least 95 to 90 million years earlier than previously thought.
Leandro Gaetano, lead author
Instituto de Estudios Andinos “Don Pablo Groeber”
Universidad de Buenos Aires – Consejo Nacional de Investigaciones Científicas y Técnicas
Ciudad Autónoma de Buenos Aires, Argentina.
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. 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.
Adriana Mancuso, co-author.
Instituto Argentino de Nivología
Glaciología y Ciencias Ambientales
Centro Científico Tecnológico-Consejo Nacional de Investigaciones Científicas y Técnicas-Mendoza
Mendoza, Argentina.
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.
The neonatal line in Chiniquodon.Credit: Gaetano et al., 2026.
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. We came up with a somewhat ingenious set of methods to get at something very difficult to analyze in the fossil record.
Leandro Gaetano
Skull of the Chiniquodon theotonicus individual that was used for this study.Credit: Leandro Gaetano.
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%.
Representation of neonate and adult Chiniquodon theotonicus. Recovered (yellow) and histologically analyzed (orange) bones of specimen CRILAR PV109, the individual used in this study.Credit: Gaetano et al., 2026.
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.
Leandro Gaetano
Turning a theory on its head
In cynodonts, embryonic tissues were never observed before, let alone a neonatal line. Through its analysis, we found that a trait that is generally linked to evolutionary success was present in animals long before true mammals originated.
María de los Ángeles Miceli Baro, co-author.
Facultad de Ciencias Exactas y Naturales
Universidad de Buenos Aires
Ciudad Autónoma de Buenos Aires, Argentina.
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. Still, it looks like some cynodonts were in fact very similar to present-day mammals.
Leandro Gaetano
Publication:
Whichever interpretation further evidence eventually supports, neither offers creationism any comfort. If live birth evolved independently in Chiniquodon and therian mammals, it provides another example of convergent evolution producing similar solutions in separate lineages. If it evolved once in an early probainognathian ancestor and was later lost in the monotreme lineage, it illustrates the equally familiar evolutionary processes of inheritance, modification and reversal. No magic creation event is required in either case.
The discovery also adds to the evidence that mammals did not appear suddenly, equipped with a complete set of uniquely mammalian characteristics. Chiniquodon was not a mammal, yet it possessed a growing collection of mammal-like features and may have reproduced in a way previously associated only with much later members of the lineage. It is exactly the sort of transitional mosaic predicted by evolution and denied by creationist claims that organisms belong to separately created, sharply bounded “kinds”.
Then there is the unavoidable matter of time. This animal lived approximately 236 million years ago—more than 230 million years before creationists imagine their mythical “Creation Week”. Its bones were buried, fossilised and preserved within a well-dated sequence of Late Triassic rocks, retaining microscopic details of growth that allow scientists to reconstruct part of its life history. Neither the animal nor the geological and evolutionary history recorded in its bones can be reconciled honestly with a recently created Earth or a global flood a few thousand years ago.
The provisional nature of the conclusion provides no refuge for creationism. The authors openly acknowledge that the evidence comes from one specimen, that their mass estimates depend on comparisons with living animals and that more fossils are needed. That is how science works: conclusions remain proportional to the evidence and are revised when new evidence warrants it. Creationism, by contrast, begins with an immutable conclusion and must dismiss, distort or invent excuses for every discovery that contradicts it.
Here, the researchers used evolutionary relationships, comparative anatomy, statistics and bone histology to test competing explanations and potentially move the origin of live birth backwards by as much as 95 million years. At no point did they need to invoke supernatural intervention, nor did the unexpected result cause them to abandon evolutionary theory. It simply helped them refine another chapter in the long, branching history of mammalian evolution—a history written in fossil bone millions of years before creationists believe there was a world in which to write it.
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