Three origins for placental reproduction in mammals
According to Wikipedia – Placenta
“The placenta has evolved independently multiple times, probably starting in fish, where it originated multiple times, including the genus Poeciliopsis. Placentation has also evolved in some reptiles.
Now the LRT is recovering three independent origins
for placental reproduction.
1. The extant marsupial Dromiciops
(Figs 1, 3) now scores close to the extant placental, Ptilocercus (Figs 2, 3) in the large reptile tree (LRT, 2323 taxa). The premaxillary teeth are different. So is the postorbital bar. So is the canine. Note the large, circular auditory bulla in both, derived from the angular bone of basal synapsid mandibles.
The auditory bulla frames the eardrum and protects the middle ear bones, also derived from posterior jaw elements.
Figure 1. Dromiciiops skull compared to Ptilocercus.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ptilocercus_skull588-1.jpg?w=131″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ptilocercus_skull588-1.jpg?w=447″ tabindex=”0″ role=”button” class=”size-full wp-image-88219″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ptilocercus_skull588-1.jpg” alt=”Figure 1. Dromiciiops skull compared to Ptilocercus. ” width=”584″ height=”1337″ />
Figure 1. Dromiciiops skull compared to Ptilocercus. Note the loss of the canine (orange) as in all members of Glires, the gnawing clade.
In vivo images of
Dromiciops and Ptilocercus (Fig 2) are also similar. Might be hard to tell them apart, other than that feathery tail on one and a pouch on the other.
Figure 1. Ptilocercus and Dromiciops in vivo.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ptilocercus.dromiciops.invivo588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ptilocercus.dromiciops.invivo588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88197″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ptilocercus.dromiciops.invivo588.jpg” alt=”Figure 1. Ptilocercus and Dromiciops in vivo.” width=”584″ height=”379″ />
Figure 2. Ptilocercus and Dromiciops in vivo.
2. Close by, but morphologically different,
the arboreal marsupial, Dactyopsia now scores close to the Paleocene Ignacius (Figs 3, 4), Carpolestes and Plesiadapis (Fig 3), all basal to multituberculates and rodents.
Figure 3. The extant marsupial, Dacylopsia compared to Paleocene Ignacius. Carpolestes, and Plesiadapis. Note the broader phylogenetic difference between taxa these taxa and those above.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/dactylopsila-588-4.jpg?w=111″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/dactylopsila-588-4.jpg?w=380″ tabindex=”0″ role=”button” class=”size-full wp-image-88223″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/dactylopsila-588-4.jpg” alt=”Figure 3. The extant marsupial, Dacylopsia compared to Paleocene Ignacius. Carpolestes, and Plesiadapis. Note the broader phylogenetic difference between taxa these taxa and those above.” width=”584″ height=”1573″ />
Figure 3. The extant marsupial, Dacylopsia compared to Paleocene Ignacius. Carpolestes, and Plesiadapis. Note the broader phylogenetic difference between taxa these taxa and those above. All are arboreal. Note the tube-like auditory bulla opening, convergent with primates.
Ignacius
is a traditional plesiadapiform, an arboreal clade that includes Plesiadapis (Fig 3) and the extant aye-aye, Daubentonia in the LRT. These taxa are close to extinct, squirrel-like rodents, like Paramys as we learned earlier here.
According to Wikipedia,
“Ignacius, along with the other plesiadapiforms, possess large, procumbent lower incisors convergent with those found in rodents, however, unlike rodents, the incisors of plesiadapiforms were not continuously growing.”
That’s a derived trait in rodents. This novel hypothesis provides the first clue as to the transition from on type of incisor to another came about.
Figure 5. Dactylopsia and Ignacius skeletons, not to scale.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/dactylopsila_skeleton588.jpg?w=221″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/dactylopsila_skeleton588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88203″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/dactylopsila_skeleton588.jpg” alt=”Figure 5. Dactylopsia and Ignacius skeletons, not to scale. ” width=”584″ height=”794″ />
Figure 5. Dactylopsia and Ignacius skeletons, not to scale.
3. Previously in the LRT,
the marsupial genus without a pouch and with only one vagina, Monodelphis, nested basal to the clade Placentalia. That made so much sense because those are placental reproductive traits in a tiny, phylogenetically miniaturized marsupial.
Now two more transitions to placental reproduction appear after phylogenetic analysis in the LRT.
Monodelphis remains basal to all other placentals in the LRT.
Figure 6. Monodelphls and pups exposed as no pouch is present in this basal placental taxon. Note the tail is not bushy.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-24463″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2016/10/monodelphis_and_pups588.jpg” alt=”Figure 6. Monodelphls and pups exposed as no pouch is present in this basal placental taxon. Note the tail is not bushy.” width=”584″ height=”554″ />
Figure 6. Monodelphls and pups exposed as no pouch is present in this marsupial taxon. Only one vagina = monodelphis is present in this taxon, as in placentals, distinct from other marsupials.
Traditionally and previously here
these taxa (Figs 1–4) were considered examples of convergence. The reason for the long time in getting to this new hypothesis is best explained by the power of the paradigm.
Remember, all science passes through a human filter. This one did, too.
Don’t expect the mammal subset
of the LRT to be up-to-date for awhile.
This new hypothesis shakes things up a bit. Hopefully it will settle all remaining issues.
Source: https://pterosaurheresies.wordpress.com/2024/08/14/three-origins-for-placental-reproduction-in-mammals/
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