The marsupial–placental transition just changed in the LRT

Previously,
the marsupial genus without a pouch and with only one vagina, Monodelphis (Fig 2), nested basal to the clade Placentalia in the large reptile tree (LRT, 2322 taxa, subset Fig 1). That made so much sense because those are placental reproductive traits in a tiny, phylogenetically miniaturized marsupial.

Unfortunately, issues kept arising at this hypothetical transition.
It’s not over until it’s over.

With the present version of the LRT
the evolution of a single vagina and the loss of the pouch is no longer demonstrated at the placental transition. That is so because the recently extinct Tasmanian tiger, Thylacinus, had typical marsupial dual female reproductive organs and a posterior opening pouch.

That made Monodelphis (=’single womb’) convergent with placentals in this regard,
not homologous. That means phylogenetic miniaturization was not present at the precise genesis of the placental placenta.

Figure 1. Updated subset of the LRT focusing on the marsupial ancestors of the clade Placentalia. Blue taxa = Monodelphis. Gray are extinct taxa. Thylacinus is recently extinct. ” data-image-caption=”

Figure 1. Updated subset of the LRT focusing on the marsupial ancestors of the clade Placentalia. Blue taxa = Monodelphis. Gray are extinct taxa. Thylacinus is recently extinct.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/pre-placentals588-1.jpg?w=89″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/pre-placentals588-1.jpg?w=302″ tabindex=”0″ role=”button” class=”size-full wp-image-87664″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/pre-placentals588-1.jpg” alt=”Figure 1. Updated subset of the LRT focusing on the marsupial ancestors of the clade Placentalia. Blue taxa = Monodelphis. Gray are extinct taxa. Thylacinus is recently extinct.” width=”584″ height=”1979″ />

Figure 1. Updated subset of the LRT focusing on the marsupial ancestors of the clade Placentalia. Blue taxa = Monodelphis. Gray are extinct taxa. Thylacinus is recently extinct.

If the present subset of the LRT is correct
(Fig 1), the last common ancestor (LCA) of placentals, currently unknown as a single taxon, would have had placental reproductive methods and organs. It would have had a single vagina and no pouch as in Monodelphis.

See below for more instances of placental convergence.

The widely acknowledged similarity between members 
of the Creodonta and Carnivora has confused taxonomists for the last century. The LRT now indicates there is a good reason for that.

At least for the present in the LRT
Creodonta is the last marsupial clade prior to the placental transition. The Carnivora – Primates split is the first placental dichotomy. So Creodonta is phylogenetically close to Carnivora.

The present hypothesis must now be vigorously and independently tested and confirmed prior to acceptance. The present hypothesis is only the first step in the process. I’ll keep working on it. Hopefully others will do so at the same time.

Figure 2. Skulls of taxa at the current marsupial > placental transition. ” data-image-caption=”

Figure 2. Skulls of taxa at the current marsupial > placental transition.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/vulpavus_2mesocyon588-1.jpg?w=98″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/vulpavus_2mesocyon588-1.jpg?w=335″ tabindex=”0″ role=”button” class=”size-full wp-image-87667″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/vulpavus_2mesocyon588-1.jpg” alt=”Figure 2. Skulls of taxa at the current marsupial > placental transition.” width=”584″ height=”1784″ />

Figure 2. Skulls of taxa at the current marsupial > placental transition. No single taxon nests basal to Placentalia at present. A parasagittal crest develops on taxa that have them during ontogeny.

According to Wikipedia – Creodonta
“was coined by Edward Drinker Cope in 1875.^[1] Cope included the oxyaenids and the viverravid Didymictis but omitted the Hyaenodontidae. In 1880. he expanded the term to include families Miacidae (including Viverravidae), Arctocyonidae, Leptictidae (now Pseudorhyncocyonidae), Oxyaenidae, Ambloctonidae and Mesonychidae.Cope originally placed creodonts within the Insectivora. Hyaenodontidae was not included among the creodonts until 1909.^[William Diller Matthew regarded Creodonta as a suborder of order Carnivora, divided in three groups.”

I use the term Creodonta to indicate a clade of carnivorous marsupials.
Carnivora has always been and remains a clade of carnivorous placentals.

Figure 3. The oreodont, Merycoidodon, compared to scale with Oodectes. ” data-image-caption=”

Figure 3. The oreodont, Merycoidodon, compared to scale with Oodectes.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/merycoidodon.skull588.jpg?w=291″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/merycoidodon.skull588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87652″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/merycoidodon.skull588.jpg” alt=”Figure 3. The oreodont, Merycoidodon, compared to scale with Oodectes. ” width=”584″ height=”603″ />

Figure 3. The oreodont, Merycoidodon, compared to scale with Oodectes.

After nesting basal to placentals for several years in the LRT
Oodectes (Fig 3) now nests with oreodonts, like Merycoidodon,  Like several prior results in the LRT, the present hypothesis is fragile due to 1) allometry during ontogeny, 2) widespread convergence and 3) interpretation of traits from several disparate data sources  including photos, µCT scans and photographs.

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.”

In the three tiny species of Monodelphis (Fig 2) a placenta is absent
despite the presence of some placental-like reproductive organs,

References
wiki/Creodonta
wiki/Placentalia
wiki/Placenta