At least two and maybe four placenta origins
It all depends on tiny Monodelphis kunzi
(Fig 1) a finger-sized marsupial at the cusp of being a placental (it has only one vagina, hence ‘monodelphis‘) rather than the conventional two found in other marsupials.
Figure 2. Monodelphis kunsi now nests basal to primates + bats and Anagale (moes + tenrecs + odontocetes).
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/monodelphis-kunsi.invivo588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/monodelphis-kunsi.invivo588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88753″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/monodelphis-kunsi.invivo588.jpg” alt=”Figure 2. Monodelphis kunsi now nests basal to primates + bats and Anagale (moes + tenrecs + odontocetes).” width=”584″ height=”289″ />
With the proviso
that the mammal subset of the LRT (Fig 2) remains fragile and unfinished, Monodelphis kunsi currently nests basal to the large clade of large placental post-Cretaceous herbivores that ultimately produced sloths, elephants and artiodactyls.
Even though Monodelphis kunsi appears to be buried deep within Placentalia, you’ll note by tracing its lineage that Monodelphis kunsi maintains a direct ancestral track extending back to tiny Cretaceous Eomaia.
And this is why I think phylogenetic analysis can be so exciting and informative,
(when it isn’t so frustrating).
Earlier you might remember
Wortman 1902 was the first to hypothesize that placentas appeared several times by convergence within the Mammalia. That idea was generated long before the era of software-assisted analysis,
Housekeeping continues
on the mammal subset of the large reptile tree (LRT, 2324 taxa). That will continue as long as improvements can be made.
References
Wortman JL 1901-1903. Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum. Am. Jour. Sci. 11:333–348.
Source: https://pterosaurheresies.wordpress.com/2024/09/14/at-least-two-and-maybe-four-placenta-origins/
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