Tetrapod Zoology

Tetrapod Zoology

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


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).

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.

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

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