White et al 2023 report: marsupial birth no longer intermediate

Unfortunately
White et al based their phylogeny on genomics and skull morphology. Omitting fossil taxa is not a good method for determining phylogeny. Overlooking transitional taxa (Fig 1) is likewise not good. Trusting genes in deep time studies will also lead you astray.

In a paper devoted to marsupial reproduction you might find it ironic that White et al do not mention the words ‘pouch’ or ‘marsupium’ in their text.

From the White et al summary:
“Within mammals, different reproductive strategies (e.g., egg laying, live birth of extremely underdeveloped young, and live birth of well-developed young) have been linked to divergent evolutionary histories.”

This is oversimplified and misleading. Basal placentals in the trait-based large reptile tree (LRT, 2260 taxa) are also underdeveloped, while derived placentals (e.g. odontocetes, mysticetes, ungulates) are well-developed.

“How and when developmental variation across mammals arose is unclear.”

Perhaps because White et al eschew trait-based analysis and omit fossil taxa.

“While egg laying is unquestionably considered the ancestral state for all mammals, many long-standing biases treat the extreme underdeveloped state of marsupial young as the ancestral state for therian mammals (clade including both marsupials and placentals), with the well-developed young of placentals often considered the derived mode of development.

The LRT supports the traditional hypothesis, deriving basal placentals from taxa near Monodelphis, a marsupial without an enclosing pouch (Fig 1). White et al mention Monodelphis only once and then in regard to a skull bone, the supraoccipital.

White et al don’t name that last common ancestor, Megazostrodon (Fig 1), in the text.

“The mammalian crown group is defined by the divergence of monotremes from therians, which themselves diverged into the modern lineages of marsupials and placentals approximately 160 million years ago.”

That’s incorrect. A crown group is defined by a last common ancestor of all living clade members. Again, that would be the fossil, Megazostrodon. The Therian crown group would omit monotremes and thus mark the divergence of monotremes from therians.

Bottom line:
If White et al wanted to learn more about marsupial reproduction, the pouch and its absence in placentals is the place to start, not the skull shape.

I’ll say it again, White et al do not mention the words ‘pouch’ or ‘marsupium’ in their text.

References
White HE et al (8 co-authors) 2023. Paedomorphosis in the ancestry of marsupial mammals. Current Biology https://doi.org/10.1016/j.cub.2023.04.009

Publicity
phys.org/news/2023-05-marsupials-evolved-mammals.html

Lead author online bio
kcl.ac.uk/people/heather-white