Marmosa, Caluromys and Chironectes: the living, breathing origin of the Eutheria

These are the mouse, wooly and water opossums:
Marmosa (Fig. 1), Caluromys (Fig. 4) and Chironectes (Figs. 8, 9). As traditional didelphids, they’ve received too little attention. In a world in love with DNA phylogenetic analysis, they’ve received too little attention. In the large reptile tree (LRT, 1252 taxa, subset Fig. 3) these are the much sought after transitional taxa between Metatheria (marsupials) and Eutheria (placentals).

As simple and logical as this sounds
the present hypothesis of interrelationships (Fig. 3) is heretical. From Novacek 1989, 1992 to Tarver et al. 2016 other workers have placed armadillos, pangolins and elephants at the base of the Eutheria using gene analyses. As mentioned earlier, it is discouraging to see serious paleontologists (references below, including a certain science blogger) among the ‘believers’ as they embrace and put their faith in a method (gene analysis) that fails to deliver a gradual accumulation of derived traits at every node in large phylogenetic analyses, hoping for eventual redemption. They just accept the results without questioning. And that is surprising, because as a professor, you can’t really explain to students how these results gradually evolve. Rather these studies mix up and confuse the placental clades as others have mixed up the bird clades using DNA. We’ll take a look at these influential placental DNA papers and list their problems in detail in a few days. It’ll be horrible, untenable and illogical, so prepare yourself.

FIgure 1. Marmosa murina in vivo.

FIgure 1. Marmosa murina in vivo. Yes, this pouch less marsupial is carrying babies in front of its thigh. This is what basal placentals, like bats and flying lemurs do.


Marmosa murina
(Gray 1821, Voss and Jansa 2009) 
is one of 19 species of ‘mouse opossums’ native from Mexico to Argentina. In the large reptile tree (LRT, 1252 taxa, Fig. 3) Marmosa nests at the base of the last metatherian clade prior to the origin of eutherians (placentals), the clade that includes Monodelphis, and Chironectes (a swimmer). Like other mouse opossums (Fig. 5) Marmosa lacks a marsupium (= pouch) like its sisters (Fig. 5).

Marmosa waterhousi (Gray 1821) skull is shown below (Fig. 2).

Figure. 2. Marmosa waterhousi skull.

Figure. 2. Marmosa waterhousi skull.


Caluromys derbianus
(Allen 1904; Fonseca and Astúa 2018; Fig. 4) is the living ‘wooly opossum’, native to Central America. Sometimes it feeds inverted as seen in bats and hypothesized for pre-bats. It is an omnivore, like related placental carnivore, Nandinia.

Caluromys nests just inside of the first placental clade, Carnivora, alongside Vulpavus (Fig. 6), a taxon omitted from all prior papers on didelphids. Basal Carnivora are larger than other basal shrew- and mouse-sized placentals. In like fashion, Caluromys is the largest of these opossums, similar in size and shape to Vulpavus.

Figure 4. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

Figure 3. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

As we discussed earlier
here regarding Mondelphis (a genus including 22 species of short-tailed opossum) and the origin of bats and dermopterans, the transition from metatherians to eutherians was a gradual one that took place at this phylogenetic transition. So there is no great revelation here, just more evidence piling on.

Figure 1. Caluromys skull and mandible (sized to fit).

Figure 4 Caluromys skull and mandible (sized to fit).

Voss and Jansa found a ‘pouch’ in Caluromys,
but no pouch in the slightly more primitive and perhaps more plesiomorphic, Marmosa and Monodelphis. They report, “The marsupium of Caluromys philander uniquely consists of deep lateral skin folds that enclose the nursing young and open in the midline.” 

But wait!
In this regard the marsupium of Caluromys more closely resembles that of placental dermopterans and bats, taxa that expand these deep lateral skin folds to create newborn nurseries and ultimately, gliding membranes. Voss and Jansa do not mention the term ‘Eutheria’ and do not mention placentals as descendants of mouse opossums in their paper. This was an opportunity missed, but resolved here.

Take a look at the nesting of Didelphis in the LRT (subset in Fig. 3) and you’ll see that this is the primitive clade from which all other metatherians evolved. Most large carnivores and herbivores split off on a separate clade, leaving the mouse-sized didelphids (the Proeutherians) a more direct route to the Eutherian grade. This hypothesis of interrelationships has not been noticed or published before.

Pouch-less marsupials?
Why not just call them what they are? Transitional taxa. This is exactly how the Eutheria evolved from the Metatheria. Is this a heretical hypothesis? Or is it just another overlooked hypothesis that should have been proposed a century ago.

Figure 6. Mammary glands in pouchless marsupials. These taxa have not been tested in the LRT.

Figure 5. Mammary glands in pouch-less marsupials (mouse opossums). Pouch-less marsupials? Why not just call them what they are? Transitional taxa.

Other hypotheses
In the pre-cladistic era, Lillegraven et al. 1987 described the origin of Eutherian mammals “with high intensity food habits, small body masses and adaptations to very cold climates.” The authors focused on soft tissue traits the involve reproduction and metabolismn and put forth a hypotheses as to how nonspecific eutherians could have arisen from nonspecific metatherians… when they could have just studied mouse, wooly and water opossums and removed the guesswork. As mentioned above, modern authors delved far astray in their search for taxa at this transition.

In a very real sense
when you look at these images of mouse, wooly and water opossums you’re looking at an excellent example of the last common ancestors of all placental mammals, probably originating in the Early Jurassic (based on the first appearance of placental multituberculate Megaconus in the Middle Jurassic, in the LRT). These small didelphids are not terminal taxa. They are living breathing late-surviving representatives of an Early Jurassic split between pouch-less metatherians and pouch-less eutherians.

Figure 8. Caluromys, the largest of the mouse opossums, to scale with its LRT sister, Vulpavus, a basal member of Carnivora.

Figure 6. Caluromys, the wooly opossum, to scale with its LRT sister, Vulpavus, a basal member of Carnivora.

Here’s an unexpected finding:
Caluromys, the woolly opossum, nests as the basalmost member of the Carnivora (Fig. 3), but it retains a pouch. Time in the pouch is not particularly short. Size at birth is not particularly large. Sister taxa, including Vulpavus and Deltatherium, are both extinct, so we don’t know whether they had a pouch, but we know that on the main branch of carnivores, starting with Nandinia, the pouch was gone, convergent with mouse opossums (Fig. 5). Caluromys also has more molars than other carnivores and a longer nasal bone.

But remember,
in phylogeny it’s not the particular cherry-picked traits that determine what clade a taxon is a member of, its the nesting within a clade based on a suite of traits that is paramount.

So, similar to mammal-like reptiles,
amphibian-like reptiles, walking whales and dinosaur-like birds, Caluromys was a very basal metathere-like carnivore. And that’s how evolution really works in trait analysis.

Figure 8. Chironectes minimus skull.

Figure 8. Chironectes minimus, the water opossum, skull.

We didn’t spend much time with the water opossum, Chironectes.
It’s important to note that it, too, has a pouch. This sole aquatic marsupial has a water-proof pouch with a unique sphincter for access. And it nests in the LRT as the proximal outgroup taxa to the Eutheria, although the aquatic niche and webbed feet are autapomorphies not retained in descendant taxa among the placental mammals. These traits have had the entire Cretaceous and Cenozoic to develop after that phylogenetic split.

When you’re looking for transitional taxa,
keep looking for the little, plain, brown taxa and you will often find them.

Figure 9. Chironectes minimus, the water opossum, in vivo.

Figure 9. Chironectes minimus, the water opossum, in vivo. This sole aquatic marsupial has a water-proof pouch with a unique sphincter for access.

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wiki/Caluromys derbianus

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