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Tetrapod Zoology

Tetrapod Zoology

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Marsupial tapirs, diprotodontids, wombats and others: the vombatiform radiation, part II

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Palorchestids (= 'marsupial tapirs'), as illustrated by Frank Knight for the 1985 book Kadimakara: Extinct Vertebrates of Australia.

ResearchBlogging.org

Time to finish up on those fantastic vombatiforms. Be sure to read part I first. In part I, we looked at koalas and marsupial lions, both of which seem to be outside Vombatoidea, the vombatiform clade that includes wombats and the superficially wombat-like, mostly terrestrial diprotodontids and kin. This current article surveys vombatoid diversity. Yes, wombats and such diprotodontids as Diprotodon are fairly familiar – but what about wynyardiids, maradids, ilariids?

Among the most peculiar vombatiforms and most peculiar of diprotodontians are the wholly extinct palorchestids, sometimes termed marsupial tapirs or tree-pushers. Seven species are known in two genera (Miocene-to-Pleistocene Palorchestes and Oligocene-Miocene Propalorchestes). They seem to be the sister-group of Diprotodontidae, with the palorchestid + diprotodontid clade being known as Diprotodontoidea (Archer et al. 1999) [see cladogram at bottom]. Palorchestes was originally interpreted (by Richard Owen in 1874) as a giant kangaroo and various reconstructions of its skull and life appearance perpetuated this idea until as recently as the 1950s.

Another palorchestid reconstruction, this time by Peter Schouten. Sorry about the small size.

A narrow and retracted nasal region suggest the presence of a trunk, a reasonably large tail is present (note that this isn’t shown in Knight’s reconstruction), the forelimbs were heavily muscled with a partially immobile elbow joint and near-solid ulna*, and the claws on both the hands and feet are large and laterally compressed. Palorchestes has thus been described as “a composite animal with tapir, chalicothere, pantodont, and slothlike features” (Murray 1984, p. 608) and as surely “enough to have inspired the legend of the bunyip – or at least a few nightmares among Australia’s first Aboriginal inhabitants” (Flannery & Archer 1985, p. 236). Knuckle-walking and digital hyperextension (where the claws are retracted like those of cats) have been suggested for palorchestids, as has the idea that they were koala-like or sloth-like climbers.

* The medullary cavity is strongly reduced and absent for much of the bone’s length.

The Miocene vombatiforms Ngapakaldia and Pitikantia have been regarded as palorchestids by some authors. Ngapakaldia at least was apparently superficially diprotodontid-like, but with hand and wrist features suggestive of a climbing ability. Some carbon-dated Palorchestes specimens have been dated to c. 36,000 years old and rock art supposed to depict this animal has been reported. One of those paintings is shown here (borrowed from here on History of Geology) - I’m unable to make much sense of it and don’t exactly find it convincing.

Apparently, there's a quadrupedal animal with a big nose in here somewhere.

Ilariids, wynyardiids, maradids

Among the most poorly known of vombatiforms are the Late Oligocene ilariids, known only for the two Ilaria species (I. illumidens and I. lawsoni) and Kuterintja ngama. Koobor (sometimes suggested to be a koala) has also been regarded as an ilariid by some authors. Ilariids are only known from teeth and fragmentary jaws, so there isn’t a great deal of interesting stuff to say about them. They possessed horizonally projecting, subcylindrical lower incisors (as is typical for diprotodontians) and especially tall-cusped premolars and molars suited for herbivory. Their molars were selenodont and hence koala-like. Munson (1992) found ilariids to be the sister-group to a wynyardiid + diprotodontoid clade, but they’ve also been hypothesised to be particularly close to wombats at times (Weisbecker & Archer 2008).

Namilamadeta skull, from Archer et al. (1991).

Another group generally included within Vombatoidea is the wholly extinct Wynyardiidae, currently known from seven species in three genera (Wynyardia, Muramura and Namilamadeta). The first wynyardiid to be recognised – Early Miocene Wynyardia bassiana from Tasmania – was discovered early on in the history of Australian palaeomammalogical research: some time round about 1860. It actually wasn’t named or described until 1901, partly because it was assumed to be the skeleton of a modern species, not a fossil one (Aplin & Rich 1985). For a long time, it was the only pre-Pleistocene Australian marsupial fossil known to science. However, numerous Late Oligocene and additional Early Miocene wynyardiid fossils have been discovered since the 1970s, many of them at Riversleigh. Wynyardiids are postcranially similar to primitive wombats, and some experts have reconstructed them as wombat-like, terrestrial herbivores. Others, however, have suggested that Wynyardia at least was a generalist arboreal herbivore (Macphail 1996). Wynyardia was perhaps similar in size to a large possum while forms like Namilamadeta were badger- or wombat-sized.

A slender, gracile vombatiform dentary from the Late Oligocene of Queensland was named as the new species Marada arcanum by Black (2007) and suggested to be distinctive enough to warrant the creation of a new ‘family’, Maradidae [holotype dentary shown here, from Black (2007)]. Very little is known of Marada, but its combination of features led Black (2007) to suggest that it might be closely related to wynyardiids or diprotodontoids. Weakly developed lophodonty on its low-crowned teeth suggest that it was a browser. The dentary is about 15 cm long, so it was a reasonably large animal.

Diprotodontines and zygomaturines

Diprotodontids (this term not to be confused with Diprotodontia) are best known for the gigantic Pleistocene form Diprotodon, often imagined to look something like a rhino-sized wombat (always always always [well, virtually always] shown with shaggy brown hair). Diprotodon was indeed very large with a mass of perhaps 2700 kg or so: it was the largest marsupial that ever lived. Peculiar, elevated crests around the external nostrils suggest that it had a highly specialised nose. Some people have illustrated it with a short, tapir-like trunk while others think that it had an enormous, koala-like rhinarium. Spatulate upper incisors and long, chisel-like lower ones might be browsing adaptations for a diet of tough, spiky plants. Its presence across continental Australia shows that it was a habitat generalist, able to make a living in woodlands, semi-arid plains and other regions (Price 2008).

Diprotodon skeleton, photo by John O'Neill, from wikipedia.

Like most diprotodontids, Diprotodon has proportionally small hands and feet and highly inflexible wrists and ankles. Massive, sometimes extensively fused ankle bones took much of the animal’s weight (within diprotodontids as a whole, this is most extensively expressed in Euowenia from the Pliocene), some of the hand bones were also peculiarly enlarged and suited for weight-bearing, and the stout, columnar limbs were clearly specialised for graviportality. At least some diprotodontids might have been able to stand bipedally when foraging (Camens & Wells 2010).

Of course, Diprotodon is merely one of several closely related forms, the oldest of which are from the Late Oligocene. Others include Pitikantia, Bematherium, Pyramois, Meniscolophus, Euowenia and Euryzygoma. These are all diprotodontine diprotodontids. Some (like Euryzygoma) possess flaring bony cheek processes and enormous cranial sinuses. Sexual dimorphism is apparently present in Neohelos and Diprotodon. In Diprotodon, what appear to be two sexes were long regarded as two species (D. optatum is the big one; D. minor is the smaller one). It seems that this animal lived in small, gender-segregated herds (Price 2008).

Zygomaturus: skeletal reconstruction by Peter Murray, life restoration by Nobu Tamura.

Diprotodontidae also includes the more diverse zygomaturine radiation, the members of which include Silvabestius, Alkwertatherium, Nimbadon, Kolopsoides, Plaisiodon, Kolopsis and Kukaodonta. These have mostly been interpreted as forest-dwelling, terrestrial herbivores but the presence of flexible, possum-like hands and curved hand claws in sheep-sized Nimbadon suggest that this form at least was capable of climbing (Weisbecker & Archer 2008). The best known zygomaturine – Zygomaturus – has a raised, flaring nasal region where two roughened lumps project above the nostrils. Both might have supported keratinised pads or horns, and indeed Zygomaturus has often been illustrated with two short horns on its snout tip. Dwarf zygomaturines known from the Pleistocene of New Guinea (Hulitherium and Maokopia) have been interpreted both as relicts from an old (Miocene) invasion and as more recently evolved, highly specialised taxa.

Where the wombats are

Photo by Stygiangloom, from wikipedia.

And, finally, we come to the wombats (or Vombatidae, sometimes Phascolomyidae in older literature), represented today by the Common or Coarse-nosed wombat Vombatus ursinus and the two or three endangered hairy-nosed wombats (Lasiorhinus) [L. latifrons shown here]. Despite appearances, wombats aren’t closely related to koalas (though some authors have suggested that they are: a subject I’ll cover in another article). All are stout-bodied, burrow-dwelling herbivores with forelimb anatomy indicative of a scratch-digging lifestyle. They can weigh as much as 38 kg and possess a superficially rodent-like, root-less dentition (they give nasty bites) and small cheek pouches.

Wombat droppings are square and also among the driest reported for any mammal. Wombats mate with the female lying on her side while the male crouches over her in normal quadrupedal pose (Taylor 1993). Wombat burrows can be as long as 30 m and, in the Southern hairy-nosed wombat L. latifrons, large warrens are constructed and remain in use for generations. As many as ten animals share the same warren, but they seem to interact infrequently and to feed solitarily (Johnson 2001).

The oldest wombat is currently Rhizophascolonus from the Early Miocene. Early wombats have relatively short-crowned teeth with closed roots while later ones have tall-crowned teeth with open, continuously growing roots (and there are intermediate ones, with tall crowns but closed roots). These teeth allowed wombats to exploit grasses, and (along with macropods) they’re among the few diprotodontian lineages that diversified in the arid grassland habitats that spread across Australia during the Pliocene and Pleistocene.

Phascolonus skeletal reconstruction, by Peter Murray.

Diprotodon and its kin were not giant wombats, but truly giant members of this group did evolve. Both Phascolonus (with spatulate incisors) and Ramsayia (with narrow incisors) stood about 70 cm tall at the shoulder; the former has an estimated mass of 250 kg. Intermediate-sized (e.g., Phascolomys) and small (Vombatus hacketti) fossil wombats are known as well. Whether giant wombats were burrow-dwelling has been controversial. They possess some burrowing adaptations but don’t seem as well adapted for burrowing in digit morphology as extant forms. Woolnough & Steele (2001) argued that giant wombats were unlikely to be burrowers mostly on the basis of their size and the energetics involved, but this ignores the fact that some surprisingly large mammals (notably some ground sloths) are or were capable burrowers. However, Warendja from the Miocene and Pleistocene has thin, convex skull roof bones that seem incompatible with burrowing. Because it occupies a ‘basal’ position on the wombat cladogram (e.g., Archer 1984, Archer et al. 1999), it might be that burrowing characterises the clade that include extant wombats and their close relatives, rather than wombats as a whole (Phascolonus and Ramsayia are deeply nested within this clade, incidentally).

Of tails and trees

There are a few things worth saying about ancestral conditions in vombatiforms before we wrap up here. For all their morphological and behavioural diversity, all vombatiforms seem to have been mostly similar in basic body shape – they’re all rather barrel-chested, typically short-headed, quadrupedal marsupials with diprotodont dentition and diastema between the incisors and premolars. Palorchestids seem to be the most aberrant members of the group, with their long, narrow muzzles and highly apomorphic forelimbs. If the reasonably well-known Ngapakaldia is a primitive palorchestid, then we already know of taxa that seem intermediate in skull form between diprotodontids and highly modified palorchestids like Palorchestes.

Simplified vombatiform cladogram, ilariids and various other taxa not shown.

Because koalas, diprotodontids and wombats are short-tailed or tailless, it’s easy to assume that ancestral vombatiforms, vombatoids and diprotodontoids were similar. However, the presence of large, sometimes muscular tails in thylacoleonids and palorchestids (plus their presence in the vombatiform out-groups) implies that long tails were ancestrally present through the clade, and lost independently in koalas, diprotodontids and wombats at least. Strong adaptations for terrestriality in diprotodontids and wombats have also led to the general idea that vombatiforms as a whole are terrestrial marsupials, with koalas being specialised, arboreal descendants of terrestrial ancestors. However, the presence of climbing adaptations in koalas, thylacoleonids, vombatiform out-groups (the ‘possum’ lineages) and also in some zygomaturine diprotodontids implies that arboreality may have been primitive for vombatiforms (Weisbecker & Archer 2008). If so, then the terrestrial diprotodontoids and wombats have an arboreal ancestry.

And that’s it… I mean, as goes a very brief, whistle-stop tour of vombatiform diversity. Needless to say, there’s tons more to say, and I hope to elaborate on various of the groups discussed here in time. For previous Tet Zoo articles on marsupials and other metatherians, see...

PS - Tet Zoo ver 3 currently gets way less hits than Tet Zoo ver 2 (look at the NBN ratings to see progress). Not sure what I can do about that - any advice? Does it mean that lots of readers have just given up on Tet Zoo since the move, or is it that ver 2 still gets huge numbers of hits due to google?

Refs - -

Aplin, K. A. & Rich, T. H. 1985. Wynyardia bassiana Spencer, 1901. The Wynward marsupial. In Rich, P. V. & van Tets, G. F. (eds) Kadimaka. Extinct Vertebrates of Australia. Princeton University Press (Princeton, New Jersey), pp. 219-224.

Archer, M. 1984. The Australian marsupial radiation. In Archer, M. & Clayton, G. (eds) Vertebrate Zoogeography & Evolution in Australasia, pp. 633-808. Hesperian Press, Carlisle.

- ., 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.

- ., Hand, S. J. & Godthelp, H. 1991. Riversleigh: the Story of Animals in Ancient Rainforests of Inland Australia. Reed Books, Kew (Victoria).

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

Camens, A., & Wells, R. (2009). Palaeobiology of Euowenia grata (Marsupialia: Diprotodontinae) and its Presence in Northern South Australia Journal of Mammalian Evolution, 17 (1), 3-19 DOI: 10.1007/s10914-009-9121-2

Flannery, T. F. & Archer, M. 1985. Palorchestes. Large and small palorchestids. In Rich, P. V. & van Tets, G. F. (eds) Kadimaka. Extinct Vertebrates of Australia. Princeton University Press (Princeton, New Jersey), pp. 234-239.

Johnson, C. 2001. Wombats. In Macdonald, D. (ed). The New Encyclopedia of Mammals. Oxford University Press, pp. 856-857.

Macphail, M. K. 1996. A habitat for the enigmatic Wynyardia bassiana Spencer, 1901, Australia’s first described Tertiary land mammal? Alcheringa 20, 227-243.

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. 1984. Extinctions downunder: a bestiary of extinct Australian Late Pleistocene monotremes and marsupials. In Martin, P. S. & Klein, R. G. (eds) Quaternary Extinctions: A Prehistoric Revolution. University of Arizona Press (Tucson), pp. 600-628.

Price, G. J. 2008. Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen 1838 (Diprotodontidae, Marsupialia). Zoological Journal of the Linnean Society of London 153, 389-417.

Taylor, R. J. 1993. Observations on the behaviour and ecology of the Common wombat Vombatus ursinus in northeast Tasmania. Australian Mammalogy 16, 1-7.

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

Woolnough, A. P. & Steele, V. R. 2001. The palaeoecology of the Vombatidae: did giant wombats burrow? Mammal Review 31, 33-45.

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

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