One Polyprotodonty transition to Diprotodonty finds a transitional taxon

Some marsupials
have five premaxillary incisors and four dentary incisors – all about the same size. Dromiciops (Fig 1 top) is an example of that morphology = polyprotodonty = ‘many front teeth’.

This is the plesiomorphic condition for all tetrapods and their finned ancestors.

Other marsupials
greatly enlarge the central incisors of both the premaxilla and dentary. Petaurus (Fig 1 right) is an example of that morphology = diprotodonty = ‘two front teeth’. The dentary is much shorter than the rostrum which accommodates the enlarged, elongated and forward-facing lower incisors

The placental 2 clade that includes multituberculates and rodents (yes, including rabbits and manatees) are derived from marsupials with diprotodonty. .

Paleontologists know this transition happened.
Unfortunately, paleontologists have not yet identified transitional taxa sharing traits, gaining traits and losing traits that model actual evolutionary events.

Genes have not been helpful here despite all the extant taxa (Fig 1) with deep roots.

The extremely rare Leadbeater’s possum,Gymnobelideus
(Fig 1 left) from a geographic pinpoint in South Australia is similar in size, shape and morphology to the South American Monito del monte, Dromiciops (Fig 1 top), and the flying phalanger, Petaurus (Fig 1 right). These three share a long list of traits – but the teeth are different – evolving from the primitive to the derived condition with Gymnobelideus as the only tested transitional taxon.

In this evolutionary sequence
(Fg 1) the number of premaxillary teeth disappear from back to front while the central = anterior tooth gets bigger. Meanwhile, as the mandible shortens, the first two dentary teeth enlarge, elongate and laminate while incisors 3 and 4 become leftovers. The lower canine (orange) is disappears. There are three dentary premolars in all three taxa, which helps identify what teeth remain despite their reduction and shape change.

Note the lower incisors of Dromiciops (Fig 1 top) are already oriented anteriorly on a slightly shortened mandible. The canine (orange) and no longer sharp is still large and present.

According to WikipediaLeadbeaters_possum
“Their diet is omnivorous: feeding on a range of wattle saps and exudates, lerps, and a high proportion of arthropods which they find under the loose bark of eucalypts, including spiders, crickets, termites and beetles. Plant exudates make up 80% of their energy intake, but the protein provided by the arthropods is essential for successful breeding.”

Diet drives tooth evolution.

These are all extant taxa. Their phylogenetic split preceded the geologic breakup of Gondwanaland around 180 million years ago. So these genera saw dinosaurs.

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

References

wiki/Diprotodontia

wiki/Leadbeaters_possum

Source: https://pterosaurheresies.wordpress.com/2026/10/29/one-polyprotodonty-transition-to-diprotodonty-finds-a-transitional-taxon/