Caluromys and kin: Dromiciops + Marmosops AND placental tree shrews

Again,phylogenetic miniaturization
seems to be the guiding factor in the evolution of the smaller marsupials, Dromiciops and Marmosops, from larger Caluromys (Fig 1) in the large reptile tree (2340 taxa).

That’s because adult Dromiciops and Marmosops are smaller than
AND morphologically similar to a juvenile Caluromys (Fig 1).

All these marsupials are from Central and South America.

But wait! There’s more!
Look at the skull of the adult pen-tailed tree shrew, Ptilocercus (Fig 1). It is also smaller than and similar to a juvenile Caluromys (Fig 1). And Ptilocercus nests at the origin of the Placental 3 clade in the LRT. No taxa nest between Caluromys and Ptilocercus in the LRT so this phylogenetic split = dichotomy precedes several Jurassic multituberculates that follow. Ptilocercus today is restricted to the jungles of SE Asia.

Caluromys derbianus 
(Waterhouse 1841; Allen 1904; Fonseca and Astúa 2018; Snout to vent length = 30cm, total length = 70cm.) is the living ‘wooly opossum’, native to Central America. Juvenile skulls have only two molars. Adults increase this number to four. Five incisors are present on this omnivore. Litter sizes range from one to six. The tail is prehensile on this nocturnal marsupial. Five manual claws are present. Two pedal claws are present.

Marmosops ocellatus 
(Lund 1840, Voss 2004) is the extant spectacled slender opossum from South America, an insectivore and frugivore. It lacks a pouch.

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

Ptilocercus lowii 
(pen-tailed tree shrew, extant, Le Gros-Clark 1926) nests among other tree dwelling placentals (= Scandentia) with Tupaia at the base of Glires and directly derived from basal marsupials.

Tree shrews act like squirrels, but instead of eating nuts, they eat insects. Pouncing on prey while perched on a tree branch requires greater skills and a larger brain. To find prey tree shrews rely on their eyes and their hands – convergent with early primates, which have a more primitive dentition. Distinct from Glironia, the skull of Ptilocercus had a larger orbit completely surrounded by a postorbital ring – convergent with primates. Incisors are reduced from 4 to 2. The canine is absent. The radius is longer than the humerus. The ulna is reduced distally, to no more than one third the width of the radius (as in bats). The short fingers of Ptilocercus demonstrate the ability to spread so widely that digits 1 and 5 oppose one another by 180º, a character unheard of in mammals other than primates, including bats, by convergence.

Many problems have been recently resolved in the LRT.
The above is one of those issues now resolved. You won’t find this in any university textbooks…at present. Hopefully someone else will come along to test the LRT with a similar taxon list.

Don’t hold your breath.
Paleo is slow to accept the obvious. Ready and willing to accept the ridiculous.

References
Le Gros-Clark WE 1926. On the Anatomy of the Pen-tailed Tree-Shrew (Ptilocercus lowii.) Proceedings of the Zoological Society of London 96: 1179-1309. 
DOI – 10.1111/j.1096-3642.1926.tb02241.x
Lund PW 1840. [Preprint of] Blik paa Brasiliens Dyreverden för sidste Jordomvaeltning. Tredie Afhandling: Fortsaettelse af Pattedyrene. Kongelige Danske Videnskabernes Selskabs Naturvidenskabelige og Matematiske Afhandlinger 8: 219–272 + pls. xiv–xxiv. 
Marshall LG 1978. Mammalian species, Dromiciops australis, American Society of Mammalogists, pp. 1-5. pdf
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. 
Voss RS 2004. An Introduction to Marmosops (Marsupialia: Didelphidae), with the Description of a New Species from Bolivia and Notes on the Taxonomy and Distribution of Other Bolivian Forms. American Museum Novitates. 3466: 44 pp. PDF 

wiki/Marmosops
wiki/Dromiciops
wiki/woolly_opossum_Caluromys
wiki/Ptilocercus

Source: https://pterosaurheresies.wordpress.com/2026/08/09/caluromys-and-kin-dromiciops-marmosops-and-placental-tree-shrews/