Glironia and Ptilocercus: the marsupial to placental 2 transition in the LRT

Glironia is a mouse-sized pouchless marsupial
with a thick, hairy tail (Figs 1, 2). Ptilocercus (Figs 1, 2) is similar, but reproduces with a placenta and has a hairless tail – except for the feathery tip. Both are extant, but represent a split that occurred during the Early Jurassic.

Glironia remained nocturnal. Ptilocercus became diurnal.

Here
in the large reptile tree (LRT, 2337 taxa) these two taxa document the transition from akynodont marsupial to akynodont placental 2, the clade that includes hedgehogs, moonrats, tree shrews, tenrecs, pakicetids and odontocetes.

in this transition
from Glironia to Ptilocercus (Figs 1, 2) premaxilla teeth wee reduced in number from four to two, but increased in size and sharpness. The mandible became more robust in the latter. The palates are similar. The molars are larger and deeper in the latter. So is the postorbital bar, which encircles the orbit in Ptilocercus.

Glironia venusta
(Thomas 1912; snout-vent length 20cm) is the extant bushy-tailed opossum. It is nocturnal, arboreal, small and omnivorous. Glironia lacks a marsupium (= pouch), by convergence with several marsupials, but similar to Placental 2 taxa like Ptilocercus. The upper canine is a vestige (Fig 2). The first premolar has the appearance of a canine. Dromiciops is another akynodont relative. Following tree shrews, placental moon rats, shrews, tenrecs, archaeocetes and odontocetes are descendant taxa in the LRT.

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 derived from pouchless marsupials, like Glironia. Tree shrews must have originated in the Early Jurassic, prior to the earliest multituberculates, but their fossils go back only 3 million years.

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.

Distinct from Glironia, the skull of Ptilocercus had a larger orbit completely surrounded by a postorbital ring. Incisors are reduced from 4 to 2. The canine is a vestige.

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 bats, by convergence.

A tree shrew cannot retract its eyeballs back into their sockets. Nor is a nictating membrane (clear third eyelid) present. Color vision makes a return, as does better binocular vision. A pair of feathery extra tongues are found beneath the main tongue. These are also found in lemurs by convergence.

The pen-tailed tree shrew lives in trees full time. The first digit is somewhat thumb-like, able to grasp objects. Pen tails retain a scaly tail up to the last inch which is tipped with long feathery hairs.

Tree shrews are active during daylight, which sets them apart from more primitive relatives, like Glironia. Because of this their internal temperatures are set higher, closer to the 98.6 degrees that is normal for humans. To maintain their temperature and their active lifestyle a tree shrew must eat its weight in food each day.

Tree shrews produce a few sounds, from a snarling hiss to a piercing squeal. They also chatter when alarmed. Most have cartilage that shapes the outer ear. Like a dog, tree shrews can draw back their upper lip to expose their teeth, which is their only facial expression.

Males and females are almost identical. Females give birth to between one and three young after 50-day pregnancy. Babies are born naked with ears and eyes closed. Species that give birth to a single baby have a single pair of breasts. Those that give birth to three at a time have three pairs of breasts. Newborns are nestbound. They receive maternal visits every other day for no more than 10 minutes at a time. After 6 weeks they are weaned. They mature in 4 months.

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

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
Thomas O 1912. A new Genus of Opossums and a new Tuco-tuco. The Annals and Magazine of Natural History. Ser. 8. 9 (50): 239–241. doi:10.1080/00222931208693127

wiki/Glironia
wiki/Ptilocercus

Source: https://pterosaurheresies.wordpress.com/2025/10/29/glironia-and-ptilocercus-the-marsupial-to-placental-2-transition-in-the-lrt/