You heard it here first: Multituberculates had placental reproduction

…because multituberculates nest within Placentalia
in the large reptile tree (LRT, 2206 taxa). That nesting happened in 2012.

Ten years later, Weaver et al 2022 reported,
“Multituberculate bone histology closely resembles that of placentals, suggesting that they had similar life history strategies. A stem-therian clade exhibiting evidence of placental-like life histories supports the hypothesis that intense maternal-fetal contact characteristic of placentals is ancestral for therians. Alternatively, multituberculates and placentals may have independently evolved prolonged gestation and abbreviated lactation periods.”

Consider a third more parsimonious (= fewer steps) alternative: According to the LRT multituberculates were placental close to rodents and the aye-aye. Here’s an otherwise excellent paper on the histology of mammals undercut by taxon exclusion. This happens too often out there.

Figure 1. Animation of the mandible of the multituberculate Kryptobaatar showing the sliding of the jaw joint producing separate biting and grinding actions, just like rodents, their closest relatives in the LRT. ” data-image-caption=”

Figure 1. Animation of the mandible of the multituberculate Kryptobaatar showing the sliding of the jaw joint producing separate biting and grinding actions, just like rodents, their closest relatives in the LRT.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2018/10/kryptobaatar_moving-jaw588.gif?w=300″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2018/10/kryptobaatar_moving-jaw588.gif?w=584″ src=”https://pterosaurheresies.files.wordpress.com/2018/10/kryptobaatar_moving-jaw588.gif?w=584″ alt=”Figure 1. Animation of the mandible of the multituberculate Kryptobaatar showing the sliding of the jaw joint producing separate biting and grinding actions, just like rodents, their closest relatives in the LRT.” class=”wp-image-33332″ />

Publicity:
According to the U of Washington online:
“In a paper published July 18 in The American Naturalist, a team led by researchers at the University of Washington and its Burke Museum of Natural History and Culture present evidence that another group of mammals — the extinct multituberculates — likely reproduced in a placental-like manner. Since multituberculates split off from the rest of the mammalian lineage before placentals and marsupials evolved, these findings question the view that marsupials were “less advanced” than their placental cousins.”

By contrast, in the LRT multituberculates (Figs 1, 2) nest with aye-ayes and rodents as derived members of the clade Glires (gnawers) within Placentalia.

“This study challenges the prevalent idea that the placental reproductive strategy is ‘advanced’ relative to a more ‘primitive’ marsupial strategy,” said lead author Lucas Weaver, a postdoctoral researcher at the University of Michigan who conducted this study as a UW doctoral student. “Our findings suggest that placental-like reproduction either is the ancestral reproductive route for all mammals that give birth to live young, or that placental-like reproduction evolved independently in both multituberculates and placentals.”

No. Their traditional cladogram is wrong. Add taxa to figure this out for yourself.

“Weaver and his colleagues obtained cross sections of 18 fossilized femurs — the thigh bone — from multituberculates that lived approximately 66 million years ago in Montana. he researchers then examined femoral cross sections taken from 35 small-bodied mammalian species that are living today — 28 placentals and seven marsupials, all from Burke Museum collections. Nearly all of the placental femurs showed the same “sandwich” organization as the multituberculates. But all of the marsupial femurs consisted almost entirely of organized bone, with only a sliver of disorganized bone. “This is compelling evidence that multituberculates had a long gestation and a short lactation period similar to placental mammals, but very different from marsupials,” said Weaver.”

“Based on this correlation, the researchers estimate that multituberculates had a lactation period of approximately 30 days — similar to today’s rodents.”

The LRT recovered a close interrelationship between rodents and multis back in 2012 (time-stamped link below). That link has not wavered in the eleven years since.

Figure 2. Multituberculates to scale. Carpolestes is the proximal outgroup taxon. ” data-image-caption=”

Figure 2. Multituberculates to scale. Carpolestes is the proximal outgroup taxon.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2021/03/multituberculates2scale588.jpg?w=106″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2021/03/multituberculates2scale588.jpg?w=363″ src=”https://pterosaurheresies.files.wordpress.com/2021/03/multituberculates2scale588.jpg” alt=”Figure 2. Multituberculates to scale. Carpolestes is the proximal outgroup taxon.” class=”wp-image-53605″ />

The Weaver team wrote,
“Multituberculata, an extinct mammalian clade that is phylogenetically stemward of Theria.”

That hypothesis is not supported here. The post-dentary bones that suggest a more stemward node are the result of neotony causing a reversal, likely due to the large propalinal movement of the jaws (Fig 1) interfering with the inner ear bones normal development. Members of Placentalia recapitulate their phylogeny during their development as embryos. Multis simple stopped developing inner ear bones in this hypothesis. Reversals, like odontocete molar shapes, also reverse to earlier morphologies.

References
Weaver LN et al (6 co-authors) 2022. Multituberculate Mammals Show Evidence of a Life History Strategy Similar to That of Placentals, Not Marsupials. The American Naturalist 220(3) 000-000. https://www.journals.uchicago.edu/doi/epdf/10.1086/720410