Multituberculate Reproduction: where assumptions rest
Mesodma and her brood by Andrey Atuchin.
Previously, it was assumed that placental mammals were the only mammals capable of gestating long term pregnancies. While there is the adage that “monotremes lay eggs, marsupials have pouches and placentals get pregnant”, in practise the former two have the same means of reproduction: producing tiny fetus like young (with en egg lasting a few days in the former). It was thus no wonder that it was assumed that the same applied to all non-placental mammals in the fossil reccord, including early eutherians.
Well, a study on multituberculates laid that assumption to rest. Multituberculates, or at least the derived cimolodonts, had bones that indicated short periods of lactation, much like most placentals (aside from bats and primates, which have unusually long parental care) but unlike marsupials and monotremes.
Thus, it seems that some assumptions need to be re-examined.
Why they existed in the first place?
Placentals are the only mammals that lack epipubic bones: projections that jut out of the front part of the pelvis in virtually all other mammals, fossil or alive (though many marsupials and close relatives like sparassodonts have vestigial ones). Epipubic bones offer many benefits — from torso stighthening to better lung ventilation — at the alleged expense of preventing abdominal expansion, thus preventing long term pregnancies and forcing mammal mothers to produce fetal young.
Clearly, this is not the case, as multituberculates have epipubic bones. Even the derived cimolodonts in the study; complete skeletons are known for Ptilodus and various Asian djadochtatherioideans, after all. In fact, previous studies such as Zofia Kielan-Jaworowska‘s masterpiece drew comparisions to marsupial pelvises, emphasising their thinness and how impossible large young would have been. Even that didn’t stop them, however.
Obviously, this assumption relied on somewhat faulty logic in the first place. Correlation =/= causation, so the absence of mammals with a placental mode of reproduction and epipubics at the same time =/= evidence that such a thing is impossible. At the same time, the fact that both surviving groups with epipubics have fetal young is too convenient to ignore. We cannot fault this reasoning, but it was bound to be broken eventually.
Is there truth to it, though?
Kayentatherium by Maija Karala.
Previously, the mystery of tritylodontid reproduction had been solved: these close relatives of mammals produced dozens of small, but fairly well developed young. And indeed, they had epipubic bones, considered a trait acquired by their last common ancestor with true mammals.
Key here is the young being small. Perhaps there could be some truth to the epipubics restraining the torso, but nothing says small young can’t come out well developed…
Is altriciality even reversible?
Ultimately, this new finding offers a more organic view to how mammalian evolution evolved. While evolution zig-zags all the time, a reversal from altricial (extremely altricial in this case, given how fetus-like joeys and puggles are) to well-gestated is rare if non-existent; animals with altricial young tend to specialise towards them, and rarely reverse this condition.
The cursorial, able-to-run-at-birth young of ungulates and some lagomorphs and rodents clearly evolved from ancestors with less developed young, but clearly not a condition as extreme as in monotremes and marsupials. Likewise, no bird lineages seem to have reversed from altricial young to precocial, although megapodes did revert to a superprecocial state from ancestors with loose parental care.
This new model, that mammals started with parental care and latter independently specialised to more extreme dependence instead of jumping to losing it, offers an intuitive progression, so to speak. Of course, it is possible that future finds will prove extreme reversals of altriciality, but for now this seems to be a specialised condition hard to reverse.
A commission by Diego Anatol Ortega of a Gobiconodon pushing an enantiornithean bird in its pouch. This relied on the assumption eutriconodonts reproduced like marsupials and monotremes, which can now be put into question.
Extinct organisms may resemble extant ones, but close analogues are oftentimes regrettably lost. Hopefully, as our knowledge of the past improves, we will have a clearer picture of the inner workings of these fascinating creatures sadly no longer with us.