Chiniquodon reproduction

Gaetano et al 2026
proposed viviparity in a cynodont based on the relatively large size of the neonate.

Abstract
“Viviparity is a reproductive mode in which the embryo completes their development inside the female body.

Yes… and no. Pterosaurs, and several other lepidosaurs (like snakes) hold eggs within the gravid female until the egg is about to hatch. Then the egg is laid. Shortly thereafter the fully developed hatchling = flapling opens the egg ready to fly when dry. This is called ovoviviparity. This mode may be present in Chiniquodon (Fig 1).

The term ‘ovoviviparity’ is absent from the authors’ text. This possibility was overlooked.

Figure 1. Top images from Gaetano et al 2026. Egg shapes, birth and pregnancy images added to scale with neonate image provided. This assumes a single neonate pregnancy, not twins or more, based on the neonate image provided. ” data-image-caption=”

Figure 1. Top images from Gaetano et al 2026. Egg shapes, birth and pregnancy images added to scale with neonate image provided. This assumes a single neonate pregnancy, not twins or more, based on the neonate image provided.

” data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2026/08/chiniquodon.neonate588-1.jpg?w=584″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2026/08/chiniquodon.neonate588-1.jpg?w=584″ alt=”Figure 1. Top images from Gaetano et al 2026. Egg shapes, birth and pregnancy images added to scale with neonate image provided. This assumes a single neonate pregnancy, not twins or more, based on the neonate image provided. ” class=”wp-image-97319″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2026/08/chiniquodon.neonate588-1.jpg?w=584 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2026/08/chiniquodon.neonate588-1.jpg?w=150 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2026/08/chiniquodon.neonate588-1.jpg?w=300 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2026/08/chiniquodon.neonate588-1.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Abstract continues:
“Present-day mammals, with the exception of monotremes, are viviparous, as opposed to the egg-laying reproductive mode of most vertebrates. Non-therian cynodonts are regarded as oviparous although direct evidence on the reproductive modes in extinct taxa is lacking. We identified, in a palaeohistological study, a neonatal line in two bones of the early probainognathian Chiniquodon theotonicus from the Late Triassic of South America. The body mass at birth is comparable to that observed in living placentals, and notably higher than the one registered for monotremes, marsupials, lepidosaurs, crocodiles, and birds.

Pterosaur hatchlings are consistently 1/8 the size of the adult and otherwise nearly identical with the exception that crested adults have crestless hatchlings. The Chiniquodon neonate graphic is about 1/3 the size of the adult (Fig 1). A neonate small enough to pass through the deep pelvic opening would have been about 1/4 the size of the adult.

Our results reveal that viviparity is not exclusive of therians among synapsids, pushing back its recognition in the clade ~90–95Myr.”

That’s jumping to conclusions. Viviparity is not the same as ovoviviparity. Vivipariy in basal mammals produces premature offspring = crawling toothless embryos capable of suckling.

The artwork in some of the publicity
(Fig 2) shows a mammal-like Chiniquodon suckling twins. At that size she should be weaning them. They are ready to face the world!

That’s jumping to conclusions. If Chiniquodon was suckling infants by definition it was a mammal – but it appears in the large reptile tree (LRT, 2340 taxa) and ALL other similar gamut synapsid cladograms – several nodes more primitive than the last common ancestor of all mammals.

Among monotremes = basal mammals.
Echidnas lay a single small egg. Platypuses lay one to three small eggs. Another basal mammal, the basal marsupial, Myremecobius, gives birth to four tiny, premature, embryonic offspring that crawl to the teats in the pouch. So Chiniquodon, as envisioned here (Fig 2) skipped = leapfrogged several reproductive steps.

According to the publicity for this paper
“Scientists have long thought cynodonts, the ancestors of all modern mammals, laid eggs. For the first time, researchers have found evidence to the contrary.”

Not so.

“This discovery pushes the timeline of this evolutionary adaptation back by up to 95 million years, according to the researchers.”

Not so.

“Co-author Maria 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, and we were looking at them, and we said, ‘Hey, look, you have embryonic tissue. This is very weird, I have never seen this,’” lead author Leandro Gaetano, a paleontologist at Argentina’s National Scientific and Technical Research Council who is based at the University of Buenos Aires, told Gizmodo. 

“Data from living species helped Gaetano and his colleagues identify a neonatal line, a distinct growth ring that can be found in bones or teeth. It results from the strong growth-rate acceleration that occurs just after birth. The researchers realized this discovery could 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 this idea about using body size as a proxy,” Gaetano explained.”

Maybe the wrong proxy. Better to test pelvic opening size, phylogeny and the possibility of ovoviviparity or just oviparity. Need to see the skull to see it had teeth.

“The researchers measured the neonatal line and the specimen’s outer surface to estimate its size and weight at birth and at death. They determined it weighed roughly 3.7 pounds (1.7 kilograms) at birth and 26.5 pounds (12 kg) when it died. That means the newborn C. theotonicus weighed 14% of the mass it reached by the time of its death.”

That 14 percent of the adult mass
comes to about 1/7 the mass of the adult. Mass represents the cube of the length. So their graphic of the Chiniquodon neonate is a little too large It should be revised to be more the size of the revised smaller size capable of passing through the cloaca (Fig 1). Of course, mammals and other archosauromorphs (crocs + dinos) mature allometrically, not isometrically as illustrated (Fig 1). The skull and orbits were likely relatively larger in the neonate, as in living mammals generally.

“The team then compared this newborn-adult body mass ratio to other groups of living animals to see which group C. theotonicus most resembled. Present-day reptiles such as turtles, snakes, and crocodiles that weigh between 17.6 pounds (8 kg) and 32 pounds (14.5 kg) tend to produce hatchlings that weigh between 9 and 53 grams. This translates to very low newborn-adult body mass ratios of roughly 0.1% and 0.6%. Larger birds like some cranes, pelicans, and vultures also exhibit low newborn-adult body mass ratios of around 1.3% to 4.5%.”

“Those groups clearly weren’t a good fit, but placental mammals—which make up about 95% of all living mammal species—have newborn-adult body mass ratios that closely align with that of C. theotonicus. Those that weigh between 17.6 pounds (8 kg) and 33.1 pounds (15 kg) can have newborns that weigh between 35.5 grams and 4.1 pounds (1.87 kg), which translates to newborn-adult body mass ratios as high as 18.77%.”

Phylogeny indicates oviparity and no lactation. The authors conclude lactation developed prior to Chiniquodon – then monotremes re-developed egg-laying and a very primitive, breast-less form of lactation = a reversal.

“In cynodonts, embryonic tissues were never observed before, let alone a neonatal line,” Miceli Baro said in a statement. “Through its analysis, we found that a trait that is generally linked to evolutionary success was present in animals long before true mammals originated.”

“While the researchers only studied a single cynodont species, Gaetano said his team’s findings align with previous studies showing that some features of present-day placental mammals—such as lactation, body hair, and brain size—were already appearing in relatives of C. theotonicus. Such traits are also linked to the genetic basis that underlies live birth, suggesting these changes may have emerged together.”

Academic paleontologists, as a rule, are prone to produce headline stories from non-headline discoveries, usually by doubling down on odd and untenable hypotheses.

“It is very probable that there was a general shift from egg laying to live birth in these cynodonts,” Gaetano said.

And then a shift back to egg-laying, followed by another shifteither recapitulating viviparity or developed anew by convergence, according to their hypothesis.

Gaetano et al showed no knowledge of Wortman 1902
who determined a triple origin for placental reproduction from marsupia ancestors. The LRT confirmed that hypothesis.

References
Gaetano LC et al (3 co-authors) 2026. Early origin of viviparity in the mammalian lineage. ORIGINAL RESEARCH article. Front. Mamm. Sci., Sec. Evolution, Anatomy and the Paleosciences. Volume 5 – 2026 | https://doi.org/10.3389/fmamm.2026.1845319
Matthew WD 1906. The Osteology of Sinopa, a Creodont Mammal of the Middle Eocene.
Proc. U. S. Nat. Mus., Vol. XXX, pp. 203-233, pl. XVI.
Wortman JL 1901-1903. Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum. Am. Jour. Sci. 11:333–348.

wiki/Ovoviviparity
sciencenews.org/article/mammal-ancestors-birthed-live-earlier

Publicity
gizmodo.com/this-236-million-year-old-fossil-just-rewrote-the-story-of-live-birth

sciencenews.org/article/mammal-ancestors-birthed-live-earlier

Science News
“Growth rings within fossil bones from an adult Chiniquodon theotonicus, a species of cynodont that lived around 236 million years ago, reveal that the animal was relatively large when it was a baby, researchers report August 13 in Frontiers in Mammal Science. Such hefty newborns, relative to adult body size, are typical of modern mammals, but not of egg hatchlings, whether reptiles or birds. That, the team says, suggests that this cynodont fossil could be the earliest evidence of live birth.”

Or it could be an error. That’s OK. The scientific method permits errors.