Placental Macroscelides compares now to marsupial Dromiciops

Tuesday, August 18, 2026 4:22

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Not immediately obvious,
but helpful in increasing cladogram resolution, the extant terrestrial placental with the marsupial palate, Macroscelides (Figs 1, 2), now nests with the less derived South American arboreal Monito del Monte, Dromiciops in the large reptile tree (LRT, 2340 taxa).

I know, I know… Macroscelides has bounced around the LRT. (Welcome to my nightmare). That’s because this tiny mammal with an antorbital fenestra (Fig 5) is so different from all other mammals that the search was on for any relative, let alone a close relative.

Academics have been equally confused. See below.

Figure 1. Rhynchocyon (above) and Macroscelides (below) compared. Though both are considered elephant shrews, they nest in separate major mammal clades in the LRT.

The genus Macroscelides
(Figs 1, 2) from the relatively dry western Namibia and South Africa, commonly and mistakenly considered a relative of the other, ‘giant’ sengi (Rhynchocyon (Figs 1, 3) from the dense forests of eastern Africa. The two have different teeth, a different cranium, a different torso and different limbs.

In many ways Macroscelides stands alone in the mammal clade. That’s why it has been so difficult for academics and the LRT to classify.

Here’s a YouTube video on the sengi and its cute, round babies.

Figure 2. Terrestrial Macroscelides and arboreal Dromiciops in vivo.

Of the two ‘sengis’, which came first in the literature?
Macroscelides was published first. [A. Smith, 1829 (= Sorex proboscideus Shaw, 1800)]. Rhynchcyon [Peters 1847] (no relation, pure coincidence) was published later.

So let’s stop calling Rhynchocyon a sengi.
In the LRT it’s a little aardvark (genus Orycteropus (Fig 3), but smaller and with more teeth.

Figure 3. Above: Skeleton of the sengi, Rhynchoyon. Below: Skeleton of the aardvark, Orycteropus.

This new match is one of the last few puzzle pieces
in the mammal subset of the LRT. It resulted in greater resolution.

And a bit of relief and exultation = a eureka moment.

Figure 4. Skeleton of Macroscelides proboscides from Digimorph.org and used with permission. Note the high sacral spines, fused fibula-tibia and elongate metatarsals here.

Macroscelides stands alone
in many regards, but represents a fourth origin for the placenta in the LRT. No other placental taxa are in this lineage. In this regard, Macroscelides stands alone again.

All attempts at matching Macroscelides with other placental clades were not as parsimonious.

Figure 5. Left: The skull of the placental ‘sengi’ Macroscelides. Note the antorbital fenestra. Right: The skull of the marsupial without a pouch, Dromiciops. Note the similarities. Note the differences. DGS colors added here. Note the large palatine vacuities in both. This is a marsupial trait.

Once again, geography has something to say
(Fig 6). The current ranges for arboreal Dromiciops and terrestrial Macroscelides were once not separated by the Atlantic Ocean. Sometime during the Cretaceous environs changed from wet forests in the West to drier and drier areas in the East, giving Macroscelides reason to become terrestrial and increasingly adapted to the desert.

Is this a ‘just so story‘? Or does this model actual events?
Others are welcome to falsify this hypothesis with their own cladogram and timeline.

Figure 6. Late Cretaceous Gondwana. Fair usage claimed here for educational purposes. At this vague time, whatever time it was prior to the advent of the Atlantic Ocean, the current ranges for the extant Dromiciops and Macroscelides were much closer and traversable, with environs changing from wet forests to drier and drier areas as Macroscelides became terrestrial and adapted to the desert.

Dromiciops gliroides
(Thomas 1894, Marshall 1978) is the extant, monito del monte or colocolo opossum, a tiny living basal marsupial from the jungles of South Chile. The middle ear (endotympanic + alisphenoid (epiterygoid) is greatly inflated. Note the canine-like premolar 1. The canine is absent.

Macroscelides proboscideus
(Shaw 1800, 10-30cm) is the extant round-eared elephant shrew or sengi. It is not a sister to Scutisorex, the hero shrew, nor related to Rhynchocyon another traditional sengi. Macroscelides is not related to elephants either, contra Wikipedia. The cranium is expanded with acoustic space on this nocturnal desert dweller. Diet includes insects, shoots and roots. The limbs are long and slender, making it one of the fastest small mammals. In the LRT Dromiciops is an arboreal ancestor.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

References
Marshall LG 1978. Mammalian species, Dromiciops australis, American Society of Mammalogists, pp. 1-5. pdf
Shaw G 1800. General zoology or systematic natural history. Vol. I. Part 2. Mammalia. London: Printed for G. Kearsley.
Thomas O 1894. On Micoureus griseus, Desm., with the description of a new genus and species of Didelphyidae. Ann. Mag. Natural History Ser 9, 3:199-212.

wiki/Dromiciops
wiki/Macroscelides
wiki/Rhynchocyon

Source: https://pterosaurheresies.wordpress.com/2026/08/18/placental-macroscelides-compares-now-to-marsupial-dromiciops/