The Origins of Mammal Live Birth: Chiniquodon theotonicus

Publication Info:

Transcript:

Leandro Carlos Gaetano et al. 2026. Early origin of viviparity in the mammalian lineage. Front. Mamm. Sci 5; doi: 10.3389/fmamm.2026.1845319

Most of us think of mammals as giving live birth, but for sometime most people have thought that our earliest non-mammal cynodont ancestors probably laid eggs. This brand new research might be changing that! There's new evidence that 236 million years ago, Chiniquodon theotonicus gave birth to live young!

Episode 327

Notes:

In case you didn't know, live birth isn't just a mammal thing, and its origins just got pushed back another 90 million years. So, I know a lot of us think of live birth or viviparity as being a mammal trait, or at least a mammal thing. I mean, ignoring some specific weirdos, but it's actually evolved independently in multiple groups of animals, something like 150 times. When I say independently, I do mean it's evolved independently. These are groups that laid eggs, and then over time evolved live birth. Anyway, 236 million years ago, millions of years before the first true mammal mammals, and right about the time the earliest dinosaurs were starting to show up, one of our non-mammal relatives was starting to do something weird. This is Chiniquodon theotonicus, and like true mammals, it was a cynodont. And even though it looks very mammal-like, it's not, or at least it's not a true mammal. And in the past, it's been thought that these non-mammal cynodonts laid eggs, but that's changing. By analyzing thin slices of Chiniquodon's limb bones, researchers were able to find a distinct line, or a neonatal line. And by comparing the size of its first bone when it was born, and its adult limb bones, and they determined that a newborn Chiniquodon already weighed nearly 4 lb. And this is an animal that as an adult was only right around 25. Researchers then compared that to the growth of 1,900 mammals, 2,600 reptiles, and 780 species of birds, and determined that that like 10 to 20% body size range as a newborn is consistent with live birth, and not ovaparity or egg-laying. Meaning that most likely Chiniquodon was not laying eggs. Now, like I said, viviparity has evolved independently over 150 times. So, just because Chiniquodon gave live birth doesn't mean that's the same lineage that led to true mammals having live birth, but it's just going to take more research to find out. As usual, if you want more information, here's the source.