Hoffman and Rowe 2018
bring us a large field jacket dotted with 38 tiny hatchlings of Kayentatherium, a tritylodontid synapsid the size of a cat (Figs. 1,2). In this wonderful and unique discovery the authors report, “Here we present what is, to our knowledge, the first fossil record of pre- or near-hatching young of any non-mammalian synapsid. The single clutch comprises at least 38 individuals, well outside the range of litter sizes documented in extant mammals. This discovery confirms that production of high numbers of offspring represents the ancestral condition for amniotes, and also constrains the timing of a reduction in clutch size along the mammalian stem.”
That last statement needs to be taken as conjecture
because we don’t have data for a long list of predecessor taxa going back to Devonian tetrapods. The authors’ statement could be true. On the other hand, the tritylodontids, being derived herbivores, might have created lots of babies, while their omnivore and carnivore ancestors, more in the line of mammal ancestry, laid smaller numbers of larger eggs. We just don’t know. The authors provided one puzzle piece. That’s not enough to make a conclusive statement.
Then Hoffman and Rowe double down:
“The discovery of a large clutch in a stem mammal provides material evidence that producing high numbers of offspring is the ancestral condition for amniotes, and that small litters represent a derived mammalian trait.” Wait a minute… lobe-fin coelacanths embryos hatch within the female and only a few are produced at a time. What happened between coelacanths and tritylodontids? We just don’t have the data for a long list of taxa between these two. Best not to guess and make it sound like scientific canon.
Note the narrow braincase in Kayentatherium,
slightly narrower than in ancestors, like Sinoconodon (Fig. 3) and basal mammals, like Sinodelphys. A U of Texas article (ref. below) reports, “The 3D visualizations Hoffman produced allowed her to conduct an in-depth analysis of the fossil that verified that the tiny bones belonged to babies and were the same species as the adult. Her analysis also revealed that the skulls of the babies were like scaled-down replicas of the adult, with skulls a tenth the size but otherwise proportional. This finding is in contrast to mammals, which have babies that are born with shortened faces and bulbous heads to account for big brains.”
“The discovery that Kayentatherium had a tiny brain and many babies, despite otherwise having much in common with mammals, suggests that a critical step in the evolution of mammals was trading big litters for big brains, and that this step happened later in mammalian evolution. ‘Just a few million years later, in mammals, they unquestionably had big brains, and they unquestionably had a small litter size,’ Rowe said.”
Actually brains stayed relatively small
until we get to more recent prototheres, more recent metatheres (by convergence) and more recent placentals (again, by convergence). Check out the following basal mammal taxa for cranium ‘narrowness’
To show that cranium width can narrow
or become relatively smaller in highly derived placental mammals check out the following taxa:
So the skull can balloon, or narrow, depending on the situation over millions of years.
According to the authors, the skull length of a hatchling
was 1/20 that of an adult with an isometric rostrum and a smaller, allometric, temporal fenestra. Is that correct? See for yourself (Fig. 2). It looks like the orbit was larger, while the rostrum and temple were both smaller. Hate to nit-pick, but there you are…
Again, this was a wonderful find and a great presentation.
We just don’t want to get ahead of ourselves after one discovery, when other hypotheses are currently possible and now on the table.
Hoffman EA and Rowe TB 2018. Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth.