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Of koalas and marsupial lions: the vombatiform radiation, part I


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That recent article on tree-kangaroos really brought home to me just how little marsupial-themed information I’ve published here on Tet Zoo. This marsupial drought really isn’t deliberate, since I find marsupials among the most fascinating of mammals. It’s just that I’ve never found the time to write about them much. Here’s an effort to rectify that, in part.

Skeletal reconstructions of (mostly extinct) vombatiforms, all by Peter Murray, from Murray (1991).

ResearchBlogging.org

Kangaroos (aka macropods) belong to a large, mostly herbivorous Australasian marsupial clade termed Diprotodontia. Shared characters that unite diprotodontians include diprotodonty (where there are just two lower incisors), a special epitympanic wing of the squamosal bone in the braincase, and the presence of an extra band of fibres (termed the fasciculus aberrans) that connect the two hemispheres of the brain. The monophyly of Diprotodontia is also well supported by molecular characters.

In addition to macropods, ‘possums’ (petauroids and phalangeroids) are diprotodontians, as are the members of the koala-wombat clade, collectively termed Vombatiformes. Various different topologies have been suggested for Diprotodontia but most studies find vombatiforms to be the sister-group to a macropod + ‘possum’ clade (e.g., Amrine-Madsen et al. 2003, Horovitz & Sánchez-Villagra 2003, Asher et al. 2004). We’ll deal with macropods and possums some other time: the aim of this article (and the following one) is to review, as briefly and succinctly as possible, the vombatiform radiation. Here we go.

So far, very few published phylogenies incorporate data from fossil diprotodontians. There’s basically Munson (1992), with Archer et al. (1999), Weisbecker & Archer (2008) and some other studies depicting cladograms based on Munson’s work. These trees agree in showing koalas and marsupial lions to be outside Vombatoidea, a clade that includes diprotodontoids (Diprotodon and kin) and wombats. The term Vombatomorphia has been used (e.g., Black 2007) for the group that includes everything except koalas, in which case marsupial lions are non-vombatoid vombatomorphian vombatiforms (the adjacent, highly simplified cladogram should help). Molecular phylogenies dated with a representative sample of fossils indicate that the earliest divergences within Vombatiformes (e.g., that between koalas and vombatoids) occurred in the Eocene (Beck 2008).

Koalas past and present

Captive male Koala, photographed by Quartl, from wikipedia.

Phylogenetic studies that sample fossil species have generally found the Koala Phascolarctos cinereus to be the sister-group to the rest of Vombatiformes. If you only sample extant marsupial species, you of course find the Koala to be the sister-taxon to wombats (Vombatidae), and this has created all kinds of amusing and erroneous assumptions about koala and wombat evolution (more on this in another article).

The ecomorphological and behavioural peculiarities of the living Koala are well known. Sedentary, solitary, arboreal and tailless, it has a proportionally small brain (0.2% of body mass) and enormous guts (proportionally, the largest of any mammal), is specialised for a diet of eucalyptus, is virtually able to go without drinking, and possesses a zygodactyl* grip (manual digits I and II oppose III-V) that it uses in hand-over-hand vertical climbing and clinging. It’s quite large for an arboreal folivore, with males exceptionally exceeding 20 kg. Koalas have selenodont molars (meaning that the cusps are lunate when seen in occusal view, with the concave side of the curve facing outwards) and hence are unlike most macropods and diprotodontoids, which are lophodont (that is, with transversely aligned ridges, or lophs). Koalas famously possess human-like fingerprints and I’m sure I’ve heard it said that a Koala fingerprint left at a human crime scene would seriously and unquestionably be assumed to be that of a human.

* ‘Zygodactyl’ has been used by some authors but is arguably inaccurate (see comments). The term schizodactyl and forcipate have also been used to describe Koala hand anatomy.

Modern Koala skeleton, by Sklmsta, from wikipedia.

A descended larynx allows male Koalas to make especially loud roaring calls and sexual dimorphism is prominent: males are about 50% heavier than females and have broader faces, proportionally smaller ears, and a large, odiferous chest gland. Despite common perception, the Koala is not fragile or over-specialised but actually able to tolerate an impressive diversity of wooded habitats. These range from moist, montane forests in the south-east of Australia to tropical vine thickets and rainforests in the far north, and to semi-arid woods in the drier parts of its range.

Koalas were commercially exploited at massive scale during the early decades of the 20th century, with more than 2 million animals killed for their skins in 1924 alone. Extensive local extinction and a range-wide population crash eventually led people to seek conservation measures for the species, though the large populations present historically are definitely a thing of the past. The prevalence of Chlamydia in wild Koala populations is well known and seems to be causing increasing mortality and declines in fertility in wild populations. The Chlamydia strains involved are the same as those found in domestic sheep and cattle, suggesting that Koalas have been infected by cross-species transmission.

Numerous fossil koalas (about 18 species) are known (the named genera are Madakoala, Perikoala, Nimiokoala, Litokoala and Invictokoala) [skull of Nimiokoala greystanei shown here; from Louys et al. (2009)]. None of these are much different from Phascolarctos and indicate that this group has been rather conservative during its history, apparently persisting since the Oligocene (or Eocene) at low diversity. Some are about half as big as the living species while one (P. yorkensis, originally given its own genus: Cundokoala) was about twice the size of it.

Comparison of these fossil forms to the living species has led authors to propose several interesting ideas about trends in koala evolution. Similar ear regions in fossil and living koalas suggest that a sedentary lifestyle and loud calling habit may have evolved early in the group, but differences in palate and tooth structure between Phascolarctos and other koalas suggest that the former is more strongly adapted for a tough, nutrient-poor diet (Louys et al. 2009). It has also been proposed that P. cinereas is a dwarfed version of giant Pleistocene versions of Phascolarctos, but this isn’t borne out by the fact that P. cinereas actually lived alongside its giant relatives during the Pleistocene (Price 2008).

Koalas have traditionally been given their own ‘family’, but some authors have also opted to reflect the distinctive nature of the lineage by erecting a ‘superfamily’ (Phascolarctoidea) and ‘infraorder’ (Phascolarctomorphia).

The incredible marsupial lions

Thylacoleo pair as reconstructed by Frank Knight in the 1980s.

Thylacoleonids, marsupial lions or ‘thylacolions’ are among the most incredible of marsupials. Superficially possum-like features meant that they were regarded as members of Phalangeroidea for a few decades and I’ll admit that Thylacoleo does look – as many before have said – like a “giant murderous possum” in some artistic reconstructions (look at the photo of Phalanger gymnotis on p. 97 of the 1999 Walker’s Mammals of the Word: Sixth Edition, Volume 1 if you can). Though a few authors continued to hint at phalangeroid affinities for thylacoleonids as recently as the 1990s, cranial and other characters have generally led to their inclusion within vombatiforms, and as stem-members of the wombat lineage. Thylacoleo is thus better imagined as a “giant murderous wombat”, and I’m not the first to say that, either.

The largest and best known marsupial lion is of course the Pleistocene taxon Thylacoleo carnifex (first described by Richard Owen in 1858), but it isn’t the only one [T. carnifex skull shown here, photo by Brian Switek, from Laelaps]. We now know of two small (domestic cat-sized) Priscileo species from the Oligocene and Miocene, four Wakaleo species, also from the Oligocene and Miocene, and the additional Thylacoleo species T. crassidentatus and T. hilli from the Pliocene (two additional genera are also known but – so far as I can tell – have so far only been described in an unpublished 2007 thesis). The debate over the behaviour and feeding habits of Thylacoleo is well known and there’s no longer any substantive doubts over the specialised carnivory of these animals. Enormous shearing premolars give them one of the most specialised mammalian dentitions. Evidence from bite marks even shows that Thylacoleo ate (and preyed on?) the rhino-sized Diprotodon. In the hand, a pseudo-opposable thumb, enlarged thumb claw and slender metacarpals all suggest that Thylacoleo was capable of climbing, and its hindfoot anatomy is also consistent with climbing. By inference, smaller thylacoleonids were likely good climbers as well. Some authors have regarded it as a cursor but this seems odd given its rather stocky limb proportions.

The body size of Thylacoleo has been controversial and some authors have regarded it as a leopard- or lynx-sized animal of 40 or even 20 kg. These low estimates are difficult to accept given that its skull can be 26 cm long. Steve Wroe and colleagues concluded that an average mass for T. carnifex was most likely between 101-130 kg, with 164 kg being estimated for one individual (Wroe et al. 1999, Wroe 2000). It was massively robust and powerful, and certainly a capable predator of megafauna.

We have to stop here, but we’re far from finished. We’ll look at the remaining vombatiform groups in part II, coming soon. For previous Tet Zoo articles on marsupials and other metatherians, see…

Refs – -

Amrine-Madsen, H., Scally, M., Westerman, M., Stanhope, M. J., Krajewski, C. & Springe, M. S. 2003. Nuclear gene sequences provide evidence for the monophyly of australidelphian marsupials. Molecular Phylogenetics and Evolution 28, 186-196.

Archer, M., Arena, R., Bassarova, M., Black, K., Brammall, J., Cooke, B. M., Creaser, P., Crosby, K., Gillespie, A., Godthelp, H., Gott, M., Hand, S. J., Kear, B. P., Krikmann, A., Mackness, B., Muirhead, J., Musser, A., Myers, T., Pledge, N. S., Wang, Y. & Wroe, S. 1999. The evolutionary history and diversity of Australian mammals. Australian Mammalogy 21, 1-45.

Asher, R., Horovitz, I., & Sánchez-Villagra, M. (2004). First combined cladistic analysis of marsupial mammal interrelationships Molecular Phylogenetics and Evolution, 33 (1), 240-250 DOI: 10.1016/j.ympev.2004.05.004

Black, K. 2007. Maradidae: a new family of vombatomorphian marsupial from the late Oligocene of Riversleigh, northwestern Queensland. Alcheringa 31, 17-32

Beck, R. M. D, 2008. A dated phylogeny of marsupials using a molecular supermatrix and multiple fossil constraints. Journal of Mammalogy 89, 175-189.

Horovitz, I. & Sánchez-Villagra, M. R. 2003. A morphological analysis of marsupial mammal higher-level phylogenetic relationships. Cladistics 19, 181-212.

Louys, J., Aplin, K., Beck, R. M. D. & Archer, M. 2009. Cranial anatomy of Oligo-Miocene koalas (Diprotodontia: Phascolarctidae): stages in the evolution of an extreme leaf-eating specialization. Journal of Vertebrate Paleontology 29, 981-992.

Munson, C. J. 1992. Postcranial description of Ilaria and Ngapakaldia (Vombatiformes, Marsupialia) and the phylogeny of the vombatiforms based on postcranial characters. University of California, Publications in Zoology 125, 1-99.

Murray, P. F. 1991. The Pleistocene megafauna of Australia. In Vickers-Rich, P., Monaghan, J. M., Baird, R. F. & Rich, T. H. (eds) Vertebrate Palaeontology of Australia. Pioneer Design Studio (Lilydale, Victoria), pp. 1071-1164.

Price, G. J. 2008. Is the modern koala (Phascolarctos cinereus) a derived dwarf of a Pleistocene giant? Implications for testing megafauna extinction hypotheses. Quaternary Science Reviews 27, 2516-2521.

Weisbecker, V. & Archer, M. 2008. Parallel evolution of hand anatomy in kangaroos and vombatiform marsupials: functional and evolutionary implications. Palaeontology 51, 321-338.

Wroe, S. 2000. Move over sabre-tooth tiger. Nature Australia 27, 44-51.

- ., Myers, T. J., Wells, R. T. & Gillespie, A. 1999. Estimating the weight of the Pleistocene marsupial lion, Thylacoleo carnifex (Thylacoleonidae: Marsupialia): implications for the ecomorphology of a marsupial super-predator and hypotheses of impoverishment of Australian marsupial carnivore faunas. Australian Journal of Zoology 47, 489-498.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. tai haku 6:04 am 10/26/2011

    Given the consistent fossil evidence for climbing and the fact it doesn’t sound particularly lion like in habit; I’d like to put forward the use of “Drop Bear” for Thylacoleo in future.

    Link to this
  2. 2. Matt vR 6:17 am 10/26/2011

    Motion seconded!
    Great article Darren. It’s surprising that predators arose from the lumbering Vombitformes. I’m wondering if the Tasmanian Devil shares this ancestry?
    On the surface they seem to share many of the physiological features of wombats. On second thoughts the idea that lumbering vombitformes supplemented their diets with scavenged meat slowly evolving hunting habits isn’t too much of a stretch.

    Link to this
  3. 3. barndad 6:40 am 10/26/2011

    Thylacoleo really is the stuff of nightmares. I think the fossil genus is Wakaleo not Wakeleo though.

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  4. 4. naishd 6:52 am 10/26/2011

    Thanks for comments. There’s some discussion of arboreality and when it likely evolved in vombatiforms in part II (alas, I’ve decided to avoid the drop-bear discussion, since I’m part of the global conspiracy to keep its true identity a secret). And, yes, my bad on mispelling of Wakaleo – now corrected.

    Darren

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  5. 5. josimo70 6:55 am 10/26/2011

    Hypercarnivory evolving from herbivory sounds umprecedented. Would it imply an omnivorous diet for stem Vombatomorphia, or even Vombatiformes? Placental counterparts of Vombatiformes branches were sloths, cats/civets, rhinos and marmots/gophers.

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  6. 6. molloch 7:16 am 10/26/2011

    Tasmanian Devils, along with Thylacines, quolls, dunnarts, antechinus and phascogales belong to the order Dasyuromorphia, as opposed to the order Diprotodontia (above). Thylacolonids have a completely different dentition to Dasyuromorphs, which are polyprotodont (many front teeth). One cool thing about Thylacoleo is that it has evolved a predatory lifestyle from an obviously diprotodont ancestry – the incisors have become morphologically similar to canines (caniform). Dasyuromorphs have the boring old killer canines, similar to eutherian carnivores, Thylacoleo still had canines – but they were small and peg like, similar to the koala’s.

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  7. 7. Jerzy New 9:50 am 10/26/2011

    Indeed strange that wombat lineage produced specialized large herbivores and carnivores, and phalanger lineage produced specialized small forms.

    I wonder if there is something in marsupial physiology which makes it difficult to switch from small to large body size? Something how young get to or grow in pouch?

    Link to this
  8. 8. Jerzy New 9:50 am 10/26/2011

    Cannot wait to see that Thylacoleo cave painting again! BTW, apparently it is in remote location and few people seen it firsthand.

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  9. 9. Cameron McCormick 10:23 am 10/26/2011

    more than 2 million animals killed for their skins in 1924 alone.

    This website notes that 800,000 were slaughtered in a month in Queensland. Not only is it shocking that any koalas are left, it’s amazing there were ever that many – are there any estimates of pre-exploitation numbers and densities?

    Some are about half as big as the living species while one (P. yorkensis, originally given its own genus: Cundokoala) was about twice the size of it.

    Prehistoric mammals of Australia and New Guinea clarifies that it is twice the weight. Damn… and I had just pictured a koala the size of a bear.

    Link to this
  10. 10. naishd 11:44 am 10/26/2011

    A reasonable amount of ink has been spilt on the subject of pre-exploitation Koala numbers. The massive numbers present in the late 19th century are generally thought to represent an irruption, perhaps (it is suggested) resulting from a decline in Aboriginal hunting of the species. However, changes in land-use during this time resulted in tree-felling and, apparently, lots of eucalypt re-growth, and some authors have argued that this better explains the boom in numbers. We’re talking about populations in the millions. Not only were millions being killed during the first two decades of the 20th century for their fur, it’s said that diseases including mange were killing “millions” too, though there seems to be uncertainty over whether the alleged die-offs were anything other than natural amounts of mortality from tick paralysis and just poor condition. A good review of this whole subject can be found in…

    Gordon, G. & Hrdina, F. 2005. Koala and possum populations in Queensland during the Harvest Period, 1906-1936. Australian Zoologist 33, 69-99.

    Darren

    Link to this
  11. 11. Heteromeles 12:27 pm 10/26/2011

    wakaleo=”canoe lion?”

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  12. 12. naishd 12:31 pm 10/26/2011

    Wakaleo = ‘small lion’, with ‘waka’ being from the dialect of the Dieri people.

    Darren

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  13. 13. Ranjit Suresh 3:45 pm 10/26/2011

    I’m not convinced by that the cave painting from the Kimberley region really does depict Thylacoleo. Considering its phylogeny and the fact that it was most closely related to wombats and was not at all close to thylacines, it’d be surprising that it shared with that species a striped pattern. Striped species are rare among mammals. What are the odds that the two major marsupial predators from the Pleistocene had the same coat pattern?

    I think the painting might be just an artistically distorted thylacine.

    Link to this
  14. 14. josimo70 4:23 pm 10/26/2011

    What is known about New Guinea and Tasmania paleobiogeography? Why such a non-carnivore clade evolved to hypercarnivory if there’s a lot of Dasyuromorphians virtual competitors. Could some of these groups haved evolved insulated, maybe in tropical New Guinean forests?

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  15. 15. josimo70 4:28 pm 10/26/2011

    Thinking better, chimps and early humans are occasional carnivores, evolved from omnivorous/frugivorous ancestors and close relatives of herbivorous gorillas.

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  16. 16. John Harshman 5:22 pm 10/26/2011

    Koalas famously possess human-like fingerprints and I’m sure I’ve heard it said that a Koala fingerprint left at a human crime scene would seriously and unquestionably be assumed to be that of a human.

    So you’re saying that a koala could commit the perfect crime without wearing gloves? I smell a future episode of CSI.

    When you run out of marsupials, how about a series on fossil lemurs?

    Link to this
  17. 17. David Marjanović 12:16 am 10/27/2011

    Phascolarctos cinereus is heterodactyl (I and II opposing the others), not zygodactyl (I and IV opposing II and III), if we really want to apply terms from bird anatomy to a mammal.

    the incisors have become morphologically similar to canines (caniform)

    Caniniform.

    Link to this
  18. 18. Jerzy New 4:12 am 10/27/2011

    BTW
    “non-vombatoid vombatomorphian vombatiforms”
    - this pass as a joke, but
    “selenodont molars (meaning that the cusps are lunate when seen in occusal view, with the concave side of the curve facing outwards)”
    - here you overdid your hair dryer.

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  19. 19. Jerzy New 4:16 am 10/27/2011

    One question is why Thylacoleo needed such a good climbing adaptations, if its prey was terrestrial? I wonder if it cached prey on trees to avoid competition with Megalania? Can special abilities to drag prey on trees be deduced from the bones?

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  20. 20. BilBy 4:59 am 10/27/2011

    Leopards might be a good analogy. Good climbers, cache their prey up trees. Might be stretch with a diprotodont though. Saw a ‘life size’ reconstruction of the dropbear Thylacoleo carnifex at the West Australian Museum – was surprised at how small it looked, can’t imagine it being more than 30-40 kg max. Over 100 kg? Ooer. Looking forward to macropod stuff

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  21. 21. naishd 5:14 am 10/27/2011

    Thanks for all the comments. The cave painting that Ranjit refers to (comment 13: see the image here) should indeed be looked at with a critical eye, but I don’t find it objectionable that it might be a Thylacoleo: yeah, it could be a badly rendered thylacine, but transverse striping across the hindquarters evolved convergently in bandicoots (Perameles), numbats and kangaroos (Lagostrophus), so it isn’t out of question that it could have evolved in a vombatiform.

    On koala hand anatomy, several sources describe the digital configuration as ‘zygodactyl’ but, David is right (comment 17), this is not really an appropriate term.

    Jerzy (comment 18) – what’s wrong with the description of selenodont molars? And even “non-vombatoid vombatomorphian vombatiforms” should make sense if you look at the cladogram :)

    Darren

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  22. 22. Dartian 9:31 am 10/27/2011

    Darren:
    Numerous fossil koalas (about 18 species) are known [...] None of these are much different from Phascolarctos and indicate that this group has been rather conservative during its history

    How well are the postcrania of those fossil taxa known? Are there any indications that any of them were secondarily(?) terrestrial?

    Cameron:
    Damn… and I had just pictured a koala the size of a bear.

    Perhaps not quite the size of a bear, but if it really did have a body mass of about 40 kg, Phascolarctos yorkensis would have been about as heavy as a female orangutan.

    Link to this
  23. 23. Heteromeles 9:40 am 10/27/2011

    Too bad, it appears that I’ve lost my copy of the paper on the “drop bear” (T. carnifex) that discussed how the animal’s size has shrunk over the last century, from mega-predator to something leopard-sized or smaller (I believe Tim Flannery was guilty of the smallest reconstruction size). The paper showed a comparison of the skulls of drop bears with leopards, Smilodons, and lions, and its skull is very clearly in the lion range. I don’t remember why researchers kept claiming smaller and smaller sizes for this animal, but as with claims about it being an herbivore, one should check the data before believing reconstructions of this species.

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  24. 24. naishd 9:54 am 10/27/2011

    Fossil koalas are mostly known from skull and jaw fragments, and teeth, but it seems assumed based on the information from these parts that the postcrania wouldn’t have been all that different from that of Phascolarctos and hence suggestive of a similar, arboreal lifestyle. I haven’t seen anyone suggest that even giant Pleistocene koalas were semi-terrestrial or terrestrial.

    As for the shrinking Thylacoleo, the paper you want (Heteromeles, comment 23) is…

    Wroe, S., Myers, T. J., Wells, R. T. & Gillespie, A. 1999. Estimating the weight of the Pleistocene marsupial lion, Thylacoleo carnifex (Thylacoleonidae: Marsupialia): implications for the ecomorphology of a marsupial super-predator and hypotheses of impoverishment of Australian marsupial carnivore faunas. Australian Journal of Zoology 47, 489-498.

    It used to be findable online for free. Anyway, Flannery gave a 1991 estimate of 50-70 kg, then a 1997 estimate of 40-60 kg, while Webb published a 1998 suggestion of 20 kg. Quantitative analysis by Wroe et al. definitely puts Thylacoleo carnifex in the mass range of female (or small male) lions and tigers.

    Darren

    Link to this
  25. 25. Heteromeles 11:19 am 10/27/2011

    Thanks. The article is still available for free.

    Link to this
  26. 26. David Marjanović 4:02 pm 10/27/2011

    One question is why Thylacoleo needed such a good climbing adaptations, if its prey was terrestrial?

    Perhaps to ambush it from above? Drop-bear…

    Quantitative analysis by Wroe et al. definitely puts Thylacoleo carnifex in the mass range of female (or small male) lions and tigers.

    Is that the 265 kg estimate that I remember?

    Link to this
  27. 27. Heteromeles 8:46 pm 10/27/2011

    I keep thinking about vombatiforms as Australian martial arts for rotund, near-sighted people. Horribly off-topic I know, but I can’t shake the image.

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  28. 28. falcon121 10:12 pm 10/27/2011

    According to Wikipedia, wombats can be surprisingly agrgressive–i think the advice was somewhere along the lines of if a human makes a wombat angry, it might be best for that human to climb a tree until it goes away. Considering this is an animal the size of the average dog it doesnt sound like bad advice either …

    And marsupial lions could climb …

    Link to this
  29. 29. Jerzy New 8:30 am 10/28/2011

    @Ranjit
    Numbat and toolache wallaby are other examples of unrelated barred marsupials.

    @Darren
    I mean text got too dry to read (I admit it is subjective).

    [serious mode off] Coming to think about it, T.carnifex doesn’t show adaptations in the ischio-natal region indicative of gravitational predation. More likely candidate is Plesiorycteropus, but it is on the wrong continent.

    Link to this
  30. 30. Dartian 11:33 am 10/29/2011

    David:
    Is that the 265 kg estimate that I remember?

    Female lions or tigers don’t grow quite that big. In fact, 265 kg is huge even for a male; in the wild, only exceptionally large male Siberian tigers might reach such a weight.

    Link to this
  31. 31. naishd 12:07 pm 10/29/2011

    An estimate of 265 lb is kicking around in a few places on the web – and 265 lb is 120 kg: within the mass range for Thylacoleo calculated by Wroe et al. I wonder, therefore, if David just mis-remembered the units.

    Darren

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  32. 32. David Marjanović 10:39 pm 10/30/2011

    Ha! Probably I did, thanks!

    I now remember the source: the New Scientist article from probably around 2002 about how lots of extinct Australian mammals and birds (dromornithids, or rather Demon Ducks of Doom) were now interpreted as scary predators. It included a mention of Th. carnifex now being thought to be an extremely muscular animal much heavier than previous estimates.

    Link to this

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