03 July 2014

Fossil (Gondwanan) Vertebrate(s) of the Month: Palorchestes & Diprotodon

'Giant Kangaroos & Wombats' by Charles R. Knight
Field Museum of Natural History
When the great paleoartist Charles R. Knight painted the mural at left in the 1920s, he thought he was depicting two unrelated marsupials from the Pleistocene of Australia.  Certainly the two animals would have been familiar to Knight's audience of natural history dilettantes at Chicago's Field Museum; they were among the first fossils ever to be described from Australia - by no less a luminary than Sir Richard Owen - and had become icons of Victorian paleontology.  This is particularly true of Diprotodon, the animal lumbering into view on the right hand side of the mural, which remains one of the most familiar members of the Australian megafauna.  Diprotodon is often referred to as a giant wombat, which is not too far from the truth, as its closest (though still somewhat distant) living relatives are koalas and wombats, and it certainly was a behemoth.  In fact, at around the same size as a large rhinoceros, it was the largest marsupial ever to have lived.  When Owen described Palorchestes 1873, it seemed like a similarly superlative animal: a giant kangaroo, hence its genus name, meaning 'Ancient Leaper' and its appearance in Knight's mural.  Owen was the greatest comparative anatomist of his day and once famously reconstructed the appearance of a moa on the basis of a single bone, but even the best get it wrong sometimes.  It took nearly a century, but in 1958 the Australian paleontologist Jack Woods recognized that the teeth of Palorchestes indicated that it was not, in fact, a kangaroo, but a fairly close relative of Diprotodon.  If an ancient Australia without giant kangaroos seems dull, don't fret: there WERE truly enormous kangaroos in the Pleistocene, and subsequent discoveries have shown that Palorchestes was even weirder than first thought.  The structure of its nasals suggests that the skull supported a tapir-like trunk to go along with its tapir-like teeth, though its elongated, powerful forelimbs, large claws, and grooved lower jaw apparently indicative of a long, flexible tongue impart an appearance that defies comparison to any living mammal (the closest comparisons that spring to mind are extinct ground sloths and chalicotheres).  Incidentally, palorchestid and diprotodontid fossils are often found near billabongs and other bodies of fresh water (possibly due to congregation and mass mortality during droughts), and it's been suggested that their fossils gave rise to the bunyip legend and thus, indirectly, the greatest childrens' book of all time.