Mammal tooth evolution toward complexity and then simplicity

Maybe the very last traits to learn in vertebrate paleontology,
are those ever-evolving mammal tooth cusps (Fig. 1), unless you are a mammal expert. Then you learn those first. Unfortunately, too often tooth cusps build false positive phylogenies when tested with more skeletal traits. That’s because they tend to devolve, (return to primitive shapes) especially in marine taxa (Fig. 1) and pre-toothless taxa.

Figure 3. Mammal tooth evolution alongside odontocete tooth evolution, reversing the earlier addition of cusps.

Figure 1. Mammal tooth evolution alongside odontocete tooth evolution, reversing the earlier addition of cusps. Colors added.

Hopson and Rougier 1993 were focusing
on the braincase of Vincelestes (Early Cretaceous, Fig. 1), which they considered “a therian mammal of pre-tribosphenic dental grade.” As we learned earlier with odontocetes (Fig. 1) and other mammals, sometimes mammal teeth loose their tribosphenic (= three cusped) morphology. The same can be said of Vincelestes (Fig. 2), which nests in the large reptile tree  LRT, 1366 taxa) as a derived marsupial (metathere) close to the much more derived sabertooth, Thylacosmilus. Hopson and Rougier, along with many later paleontologists, did not realize this phylogeny due to taxon exclusion.

Figure 2. Vincelestes drawing compared to Digimorph.org image. Colors applied here. The LRT indicates this is a sister taxon to the highly derived marsupial sabertooth, Thylacosmilus.

Figure 2. Vincelestes drawing compared to Digimorph.org image. Colors applied here. The LRT indicates this is a sister taxon to the highly derived marsupial sabertooth, Thylacosmilus.

The tribosphenic molar
Hopson and Rougier describe, “The molars of Vincelestes have the characteristic ‘reversed triangles’ pattern of therian mammals but differ from molars of tribosphenic therians in having a very small talonid without a true basin and in possessing a very small, low protocone. Therefore, Vincelestes may be the sister taxon of the Tribosphenida of McKenna (1975).”

Figure 5. Megazostrodon molars display a tribosphenic cusp arrangement. Therefore all mammals are derived from this cusp pattern.

Figure 3. Megazostrodon upper molars display a symmetrodont cusp arrangement. The lower molars have a tribosphenic cusp arrangement. This taxon is basal to all tested mammals in the LRT.

Don’t confuse
teeth evolving toward the tribosphenic morphology with those evolving away from the tribosphenic morphology.

In similar fashion, in traditional cladograms
Zhangheotherium (Early Cretaceous) nests with other so-called ‘symmetrodonts’ (Fig. 1) based on their simplified teeth, by only due to taxon exclusion. When you add in a few pangolins and pangolin ancestors, Zhangheotherium shifts to that clade as a basal member.

While it’s already January 2 in Australia,
it’s January 1, 2019 here in the USA. I wanted to take a moment to thank you for your readership. Essentially, this has been my diary, letting you know what I am learning as I learn it, while letting you know which taxa are added to the LRT as I add them. I’ve spent the last few days repairing mistakes I made in the clade Glires, guided by the low Bremer scores at certain nodes, alerting me to those errors. It’s a tricky clade.

References
Hopson JA and Rougier G 1993. Braincase structure in the oldest known skull of the therian mammal: Implications for mammalian systematics and cranial evolution. American Journal of Science 293-A-A:268–299.

Smith AJ website on dental morphology

wiki/molar

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Early mammal braincase bone labels

Usually the last bones any paleontologist learns
are the names for the carpals and tarsals. That these names change with mammals makes learning them… less easy.

And then, a little later,
one comes to the lateral braincase bones (Fig. 1), which also change with mammals. Braincase bones are typically obscured by the overlying dermal bones in non-cynodont tetrapods. Often they are fused in big brained birds and mammals.

Figure 1. Braincase bones of pre-mammals and mammals from Hopson and Rougier 1993, with some (Thylacosmilus, Tupaia and Kryptobaatar) added here. Colors added.

Figure 1. Braincase bones of pre-mammals and mammals from Hopson and Rougier 1993, with some (Thylacosmilus, Tupaia and Kryptobaatar) added here. Colors added. Is the large anterior lamina of Chulsanbaatar the result of fusion? It appears so, based on sister taxa. See figure 3.

Hopson and Rougier state, “The structure of the cranial wall [in Vincelestes] does distinguish monotremes and multituberculates form all other mammals in which the braincase is adequately known.”

Unfortunately,
this statement was made without a phylogenetic analysis testing a large suite of traits. By focusing on one or a few traits these authors are “Pulling a Larry Martin“. Moreover, by providing drawings alone, the authors did not permit the possibility of a misidentification.

Braincase bones and their alternate names

  1. Anterior lamina = prootic = lamina obturans
  2. Alisphenoid = epipterygoid
  3. Periotic = fused prootic + epiotic + opisthotic

Not sure why
the single lateral braincase bone in multituberculates, like Chulsanbaatar, was labeled the anterior lamina by Hopson and Rougier, while the same bone in Didelphis and Asioryctes was labeled the alisphenoid (Fig. 1). Do these bones fuse or does one shrink and disappear?

Note that Thylacosmilus
(Fig. 1) retains both an anterior lamina and alisphenoid, just like its sister in the LRT, Vincelestes. 

Figure 3. Daubentonia skull shares a long list of traits with multituberculate skulls.

Figure 3. Daubentonia skull shares a long list of traits with multituberculate skulls.

The braincase of the platypus,
Ornithorhynchus (Fig. 1), is greatly expanded, which explains its atypical appearance.

The origin of the braincase wall
Basal Tetrapoda have an ossified braincase buried beneath their dermal cranial bones. You can readily see braincase bones in therocephalians, like Lycosuchusas the lateral temporal fenestrae grow so large they nearly contact one another at the midline over the narrow parietal.

References
Hopson JA and Rougier G 1993. Braincase structure in the oldest known skull of the therian mammal: Implications for mammalian systematics and cranial evolution. American Journal of Science 293-A-A:268–299.

The walrus (genus: Odobenus) joins the LRT

No surprises here.
Odobenus, the walrus (Figs. 1, 2), nests with the seal, Phoca, in the large reptile tree (LRT, 1280 taxa). But I think you’ll see, the division between seals and walruses runs deep, perhaps with some parallel development of the flippers, fat, etc.

Figure 1. Walrus skeletons, swimming and walking, plus a view of the teeth, which barely erupt and cannot be seen in lateral view.

Figure 1. Walrus skeletons, swimming and walking, plus a view of the teeth, which barely erupt and cannot be seen in lateral view. Yes, that extra bone between the legs of the lower specimen resides in the penis.

Odobenus rasmanus (Linneaus 1758) is the extant walrus. The canines are much enlarged here. The other teeth are flat and barely erupt. The naris is elevated. The jaw joint is aligned with the bottom of the jaw and the retroarticular process is much reduced. The scapula is robust.

FIgure 2. Walrus skull with bones colorized.

FIgure 2. Walrus skull with bones colorized.

Walruses eat bivalve mollusk scraped from the sea floor bottom. 
According to Wikipedia, “The walrus’s body shape shares features with both sea lions (eared seals: Otariidae) and seals (true seals: Phocidae). As with otariids, it can turn its rear flippers forward and move on all fours; however, its swimming technique is more like that of true seals, relying less on flippers and more on sinuous whole body movements.[4] Also like phocids, it lacks external ears.” Earlier the LRT recovered separate terrestrial ancestors for seals and sea lions.

Figure 3. Ancestral walrus taxa from Robert Boessenecker. See references below.

Figure 3. Ancestral walrus taxa to scale from Boessenecker. 2014. Compare Neotherium to Puijila in figure 4. Neotherium nests closer to bears.

Neotherium (Fig. 3)
shares a long list of traits with Puijila, which was originally hailed as a last common ancestor for seals, sea lions and walruses (Fig. 4). In the LRT Pujilia is not basal to sea lions. In the LRT Neotherium nests with Ursus, the bear, not with Odobenus, the walrus.

What are the giant canines used for?
According to Wikipedia, “Tusks are slightly longer and thicker among males, which use them for fighting, dominance and display; the strongest males with the largest tusks typically dominate social groups.  Tusks are also used to form and maintain holes in the ice and aid the walrus in climbing out of water onto ice. Analyses of abrasion patterns on the tusks indicate they are dragged through the sediment while the upper edge of the snout is used for digging.”

You can think of walruses
as aquatic bears or aquatic stylinodontids (Fig. 4). Ursus and Neotherium are sisters to the last common ancestor (LCA) of walruses and stylinodontids with Puijila the LCA of bears and walruses.

Figure 4. Ursus maritimus compared to ancestral and related taxa, Mustela, Puijila and Stylinodon. Seeing them together makes comparisons easier.

Figure 4. Ursus maritimus compared to ancestral and related taxa, Mustela, Puijila and Stylinodon. Seeing them together makes comparisons easier.

Figure 5. Puijila nests down the line from the walrus, a trait you can see it its profile and general morphology. Compare to Neotherium in figure 4.

Figure 5. Puijila nests down the line from the walrus, a trait you can see it its profile and general morphology. Compare to Neotherium in figure 4.

References
Boessenecker R 2014. The evolutionary history of walruses, parts1–5:

  1. http://coastalpaleo.blogspot.com/2014/08/the-evolutionary-history-of-walruses.html
  2. http://coastalpaleo.blogspot.com/2014/08/the-evolutionary-history-of-walruses_26.html
  3. http://coastalpaleo.blogspot.com/2014/09/the-evolutionary-history-of-walruses.html
  4. http://coastalpaleo.blogspot.com/2014/09/
  5. http://coastalpaleo.blogspot.com/2014/11/the-evolutionary-history-of-walruses.html

Linnaeus C von 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

wiki/Walrus

The diet of Thylacoleo, the giant sugar glider

The diet of Thylacoleo, the so-called ‘marsupial lion,’
has been a puzzle for decades. The jaws and teeth look dangerous and carnivorous, but Thylacoleo nests in the middle of an herbivorous clade of wombat-like marsupials.

That’s the problem.
Morphology and phylogeny provide the problem… and the answer to the diet of Thylacoleo. This answer could have been known decades earlier, but alas… the same taxon exclusion issue that pervades paleo was also present here.

Morphology
One look at the palate of Thylacoleo documents a very different sort of mammal palate:

  1. The jawline curves laterally near the premolars
  2. Several molars seem to have fused to become one giant tooth
  3. There is an asymmetry in the lineup of the posterior teeth
Figure 1. The palate of Thylacoleo is unusual in several respects. See text for details.

Figure 1. The palate of Thylacoleo is unusual in several respects. See text for details. Vertical arrows point to asymmetries. Horizontal arrow lines up with parasagittal plane.

 

Phylogeny
In the large reptile tree (LRT, 1250 taxa) the closest living sisters to Thylacoleo, the sugar gliders, like Petaurus, should provide some sort of natural guidance as to what the giant sugar glider ate. And they do.

Sugar glider diet
From the Wikipedia page on sugar gliders: Sugar gliders are seasonally adaptive omnivores with a wide variety of foods in their diet… In summer they are primarily insectivorous, and in the winter when insects (and other arthropods) are scarce, they are mostly exudativorous (feeding on acacia gum, eucalyptus sap, manna, honeydew or lerp). Sugar gliders have an enlarged caecum to assist in digestion of complex carbohydrates obtained from gum and sap.

To obtain sap and nectar from plants, sugar gliders will strip the bark off trees or open bore holes with their teeth to access stored liquid gum. Little time is spent foraging for insects, as it is an energetically expensive process, and sugar gliders will wait until insects fly into their habitat, or stop to feed on flowers. They are opportunistic feeders and can be carnivorous, preying mostly on lizards and small birds. They eat many other foods when available, such as nectar, acacia seeds, bird eggs, pollen, fungi and native fruits. Pollen can make up a large portion of their diet, therefore sugar gliders are likely to be important pollinators of Banksia species.”

Well, there you have it. 
Little sugar gliders can be carnivorous. They can also strip bark off trees to get at the gum inside. That’s a rare diet. As sister taxa, giant sugar gliders, like Thylacoleo, were therefore likely also carnivorous and/or stripped bark off trees to get at the gum. For the latter odd reason the odd skull of Thylacoleo was likely adapted, and predation, if you insist, but predators don’t have the odd palate and teeth that Thylacoleo has.

We don’t have to provide a narrow dietary answer for Thylacoleo
because the diet of living sugar gliders is diverse. AND sugar gliders provide the long-sought carnivorous exception to this herbivorous clade.

Petaurus breviceps (Waterhouse 1839; Early Miocene to present; up to 30cm) is the extant sugar glider, a nocturnal squirrel-like marsupial able to climb trees and glide with furry membranes between the fore and hind limbs. An opposable toe is present on each hind foot. Sharp claws tip every digit.

Thylacoleo carnifex (Owen 1859; Pliocene-Pleistocene; 1.14 m long) was a giant sugar glider like Petaurus. Thylacoleo had the strongest bite of any mammal with the largest, sharpest molars of any mammal. It had fewer but larger teeth than Petaurus. The manus included retractable claws. The pes had a very large heel bone (calcaneum). This supposedly carnivorous ‘marsupial lion’ nests with herbivores. Pedal digit 1 likely had a phalanx and claw, but it has not been shown. Sugar gliders strip bark off of trees and the very odd teeth of Thylacoleo could have done the same on a larger scale.

References
Owen R 1859. On the fossil mammals of Australia. Part II. Description of a mutilated skull of the large marsupial carnivore (Thylacoleo carnifex Owen), from a calcareous conglomerate stratum, eighty miles S. W. of Melbourne, Victoria. Philosophical Transactions of the Royal Society 149, 309-322.
Waterhouse GR 1838. Observations on certain modifications observed in the dentition of the Flying Opossums (the genus Petaurus of authors). Proceedings of the Zoological Society of London. 4: 149–153.

wiki/Petaurus
wiki/Thylacoleo
NOVA | Bone Diggers | Anatomy of Thylacoleo | PBS
https://en.wikipedia.org/wiki/Sugar_glider
https://www.wired.com/2009/06/thylacoleo-herbivore-or-carnivore/

Tooth replacement just prior to the origin of mammals

Everyone in paleo knows
that reptiles with teeth replace their teeth regularly. Dinosaurs and lizards shed teeth and replace them with new ones. This is known as polyphyodonty.

Everyone in paleo also knows
that therian mammals replace their front teeth (including the premolars) only once and their posterior molars appear only once. This is known as diphyodonty.

What happened at the transition to mammals
is the subject of today’s blogpost.

Most toothed reptiles
are born or are hatched with teeth. By contrast, as everyone knows, mammal babies are born toothless and depend on their mother’s milk and care during the early weeks of life. Milk teeth appear first, often at weaning. Milk teeth are typically lost and replaced with greater maturity. More molars appear as the mandible (dentary) grows in size to adulthood. Other mammals, especially toothless mammals, evolve other tooth replacement strategies.

Figure 1. Sinoconodon growth series including jaws and teeth, here colorized from Zhang et al. 1992.

Figure 1. Sinoconodon growth series including jaws and teeth, here colorized from Zhang et al. 1998. The huge canine in IVPP 8047 is worth noting.

Figure 2. Sinoconodon skull(s) showing some variation in the way they were drawn originally.

Figure 2. Sinoconodon skull(s) showing some variation in the way they were drawn originally.

The pre-mammal, Sinoconodon,
(Fig.1) is transitional in that the incisors and canines are replaced ‘multiple times’ according to Zhang, Crompton, Luo and Schaff 1998. They found juvenile specimens in which premolars were replaced only once before being permanently lost in the oldest specimens.  (That’s an autapomorphic twist seen in some basal mammals, but not others). In Sinoconodon molars 1 and 2 were lost after one replacement. Molars 3–5 were were replaced once. Thus, there were no permanent teeth in this pre-mammal (as there are in the basal mammals Megazostrodon and Hadrocodium). Based on this data, the authors determined  that Sinoconodon lacked lactation and determinate growth.

The large reptile tree (LRT, 1245 taxa) agrees with the authors that Sinoconodon is close to ancestor of mammals. Therioherpeton (Fig. 3) and the clade of Spinolestes are closer.

Figure 4. Therioherpeton nests at the base of the Mammaliaformes with Brasilodon, between Yanaconodon and Sinoconodon, not far from Megazostrodon.

Figure 3. Therioherpeton nests between Pachygenelus and the tritylodontids, not far from Megazostrodon.

The take-away:
Phylogenetic miniaturization in the Late Triassic, including a shorter maturation and faster life cycle, with fewer tooth replacements along with lactation, were key to mammal survival—but only after the Cretaceous. A wide variety of tritylodonts, like Repenomamus, lived alongside mammals throughout the Mesozoic, but not beyond.

Figure 1. Basal mammals and their ancestors to scale. At 72 dpi the image is about actual size.

Figure 4. Basal mammals and their ancestors to scale. At 72 dpi the image is about actual size. The blue teeth in the Sinoconodon skull are not present in the skull, as I learned after reading the Zhang et al. paper. By comparison, you can that is odd and atypical. 

References
Zhang F, Crompton AW, Luo Z and Schaff CR 1998. Pattern of dental replacement of Sinoconodon and its implication for evolution of mammals. Vertebrata PalAsiatica 36(3):197–217.

wiki/Sinoconodon

Chaoyangodens: a transitional monotreme with big canines

Hou and Meng 2014
described a new Jehol eutriconodont mammal, Chaoyangodens lii, (Fig. 1) from the Yixian formation, Early Cretaceous. “The new species has a tooth formula I5-C1-P1-M3/i4-c1-p1-m4, unique among eutriconodonts in having only one premolar in lower and upper jaws, respectively, and a distinctive diastema between the canine and the premolar. Its simple incisors and reduced premolars show a mosaic combination of primitive and derived features.” In other words, this is a transitional taxon, as most are.

Later, Meng and Hou 2016
described ‘the earliest known mammalian stapes’ from the same specimen. “The stapes of Chaoyangodens is reduced in size compared to those of non-mammalian cynodonts and is within the size range of extant mammals.”

Figure 1. Chaoyangodens lii in situ and restored skull in lateral view. This taxon is a monotreme basal to both the echidna and platypus.

Figure 1. Chaoyangodens lii in situ and restored skull in lateral view. At a screen resolution of 72 dpi, this image is twice life size. This mouse-sized taxon is a monotreme basal to both the echidna and platypus.

Figure 2. Subset of the LRT focusing on monotremes and Chaoyangodens.

Figure 2. Subset of the LRT focusing on monotremes and Chaoyangodens.

Here in the large reptile tree (LRT, 1137 taxa, Fig. 2) Chaoyangodens nests between Kuehneotherium and Akidolestes, basal  to the living monotremes, Ornithorhynchus and Tachyglossus.

The top of the Chaoyangodens skull is buried in the matrix. The shape of the skull in lateral view, or at least parts of it, like the position of the orbit (Fig. 1), can be surmised by phylogenetic bracketing.

Based on the nesting of Chaoyangodens
and relatives, like Brasilitherium and Kuehneotherium (Late Triassic), these taxa are all crown mammals, not stem mammals (contra traditional thinking).

Luo, Kielan-Jaworowska and Cifelli (2002)
also nested eutriconodonts within crown mammals and this was confirmed by many later workers. The LRT nests many traditional triconodonts and eutriconodonts elsewhere, both more primitive and more derived.

Recently
we looked at the echidna sister/ancestor, Cifelliodon, here. It also had fewer and bigger teeth in the jaws, though none of those erupted beyond the gum line.

References
Hou S-L and Meng J 2014. A new eutriconodont mammal from the Early Cretaceous Jehol Biota of Liaoning, China. Chinese Science Bulletin 59, 546–553.
Luo Z-X, Kielan-Jaworowska  z and Cifelli RL 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica. 47 (1): 1–78.
Meng J and Hou S-L 2016. Earliest known mammalian stapes from an early Cretaceous eutriconodontan mammal and implications for evolution of mammalian middle ear. Palaeontologica Polonica 67:181–196.

wiki/Eutriconodonta

Mystery solved: Thylacoleo is a giant sugar glider…

no doubt, a little too big to glide…
and Thylacoleo (Fig. 2) is looking even less carnivorous in phylogenetic bracketing.

Sugar gliders
(Fig. 1) are phalangers (Fig. 6), a marsupial clade nesting between kangaroos and wombats (Fig. 5).

Figure 1. Petaurus breviceps skeleton in two views, plus a skull with mandible, lacking in the skeleton.

Figure 1. Sugar glider, Petaurus breviceps, skeleton in two views, plus a skull with mandible, lacking in the skeleton.

Adding the marsupial sugar glider,
Petaurus (Figs. 1, 3), and the cuscus, Phalanger (Fig. 6), to the large reptile tree (LRT, 1231 taxa) resolves a decades-old phylogenetic problem because Petaurus, the sugar glider, nests as a sister to Thylacoleo, the marsupial lion (Figs. 2, 4). Phalanger, the cuscus, nests as their last common ancestor, which has been suggested earlier.

According to the AustraliaMuseum website
“Most palaeontologists think that the ancestors of thylacoleonids were herbivores, an unusual occurrence since most carnivores evolved from other carnivorous lineages. One proposal suggests that thylacoleonids evolved from a possum ancestor (Phalangeroidea) based on dental formula, the skull of the cuscus Phalanger, and on a phalangerid-like musculature. Alternatively, evidence from certain skull features may show that thylacoleonids branched off the vombatiform line, the lineage that includes wombats and koalas.”

In the LRT,
wombats and koalas are now sister taxa to the cuscus clade. Without the sugar glider and the cuscus, the marsupial lion earlier nested with the wombat, Vombatus.

Just to be clear,
Phalanger is not an ancestor to Didelphis, the Virginia opossum, in the LRT, even though the Australian Museum called it a ‘possum ancestor.’

Figure 2. Thylacoleo skeleton compared to Petaurus skeleton to scale.

Figure 2. Thylacoleo skeleton compared to Petaurus skeleton to scale.

Long thought to be a super predator, 
in the midst of a clade of gentle wombat-like herbivores, Thylacoleo had, for its size, the strongest bite of any mammal, living or extinct, despite having tiny upper canines. This linking with sugar gliders further erodes the carnivorous hypothesis. 

Figure 3. Skulls of the genus Petaurus with many more teeth than in Thylacoleo, but in the same general pattern. Note the lower third premolar and its similarity to the same tooth in Thylacoleo.

Figure 3. Skulls of the genus Petaurus with many more teeth than in Thylacoleo, but in the same general pattern. Note the lower third premolar and its similarity to the same tooth in Thylacoleo. The big organe tooth at the tip of the dentary is the canine. The lower incisors are absent.

Arboreal or not?
Wikipedia reports, “The claws [of Thylacoleo] were well-suited to securing prey and for climbing trees.” And now we know how that came to be. Petaurus, despite its arboreal abilities, does not have a divergent thumb, like the one found in Thylacoleo.

Dentary canines
traditionally considered large, rodent-like incisors due to their placement, the anterior-most (medial-most) dentary teeth are actually canines. The incisors and their alveoli have disappeared. This can only be traced via phylogeny (see Arctocyon and Didelphis). The ancestrally small lower incisors are gone, replaced with ancestrally large large lower canines that meet medially like typical incisors. Notably, the lower canines maintain their traditional placement relationship to the upper canines (Fig. 6).

Even more interesting,
some marsupial taxa that experience a phylogenetic miniaturization, like Eurygenium (basal to Toxodon) the incisors reappear and the canines are not much larger than the incisors. That’s called a reversal or an atavism.

Figure 4. Thylacoleo skull. Many times larger than Petaurus, with fewer larger teeth, this is a giant sugar glider.

Figure 4. Thylacoleo skull. Many times larger than Petaurus, with fewer larger teeth, this is a giant sugar glider. The large orange tooth is the lower canine. The upper canine is a vestige. 

Size
Thylacoleo was 71 cm tall at the shoulder, about 114-150cm long from head to tail tip, about the size of a jaguar.

Petaurus is 40cm long to the tail tip, about the size of a ‘flying’ squirrel. Loose folds of skin spanning the fore and hind limbs to the wrists and ankles are used to extend glides from tree to tree, or up to 140m. The diet includes sweet fruits and vegetables.

The sugar glider in vivo.

Figure 5. The sugar glider, Petaurus, in vivo. Note the wrinkled fur between the fore and hind limb. That’s the gliding membrane.

Petaurus species
According to Wikipedia, “There are six species, sugar glidersquirrel glidermahogany glidernorthern glideryellow-bellied glider and Biak glider, and are native to Australia or New Guinea.” Whichever one is closest to Thylacoleo has not been tested or determined.

Figure 2. Thylacoleo skeleton compared to Petaurus skeleton to scale.

Figure 5. Subset of the LRT focusing on Marsupialia, Metatheria and then nesting of Thylacoleo.

Petaurus breviceps (Waterhouse 1839; Early Miocene to present; up to 30cm) is the extant sugar glider, a nocturnal squirrel-like marsupial able to climb trees and glide with furry membranes between the fore and hind limbs. An opposable toe is present on each hind foot. Sharp claws tip every digit.

Phalanger orientalis (Pallas 1766; 34 cm in length) is a nocturnal arboreal folivore marsupial known as thte Northern common cuscus. Commonly considered a ‘possum’ the cuscus nests between wombats and kangaroos, basal to sugar gliders and marsupial lions.

Figure 6. The cuscus (genus: Phalanger orientalis) nests with Petaurus and Thylacoleo in the LRT.

Figure 6. The cuscus (genus: Phalanger orientalis) nests with Petaurus and Thylacoleo in the LRT. Those anterior dentary teeth look like incisors, but phylogenetically are actually canines.

Thylacoleo carnifex (Owen 1859; Pliocene-Pleistocene; 1.14 m long) was a giant sugar glider like Petaurus. Thylacoleo had the strongest bite of any mammal with the largest, sharpest molars of any mammal. It had fewer but larger teeth than Petaurus. The manus included retractable claws. The pes had a very large heel bone (calcaneum). This supposedly carnivorous ‘marsupial lion’ nests with herbivores. Pedal digit 1 likely had a phalanx and claw, but it has not been shown.

References
Goldingay RL 1989. The behavioral ecology of the gliding marsupial, Petaurus australis. Research Online. University of Wollongong Thesis Collection. PDF
Owen R 1859. On the fossil mammals of Australia. Part II. Description of a mutilated skull of the large marsupial carnivore (Thylacoleo carnifex Owen), from a calcareous conglomerate stratum, eighty miles S. W. of Melbourne, Victoria. Philosophical Transactions of the Royal Society 149, 309-322. 
Waterhouse GR 1838. Observations on certain modifications observed in the dentition of the Flying Opossums (the genus Petaurus of authors). Proceedings of the Zoological Society of London. 4: 149–153.

wiki/Petaurus
wiki/Thylacoleo
https://australianmuseum.net.au/thylacoleo-carnifex