Marsupialia

The kangaroos doubtless represent a ground-living group de scended from arboreal ancestors; in the most primitive living type, the musk-kangaroo (Hypsiprymnus), the hind-foot still bears the stamp of the foot of the tree-living phalanger, in which the fourth digit is much enlarged, the second and third digits are reduced and syndactylous, the first digit is still present, and separate friction pads are retained under the digits. In the more advanced kangaroos the first digit is lost and so are the friction pads. For further evolution of this family see KANGAROO.

The

wombats (Phascolomys) are sometimes called "native badgers" on account of their robustness and burrowing habits, but they far excel the true or placental badgers in strength and in ability to dig deep tunnels with great rapidity. Their skulls are rodent-like in so far as they have only a single pair of long curved incisors in the upper jaw as well as in the lower; their molar teeth also have long-curved crowns and widely open roots. Each upper molar crown consists essentially of a pair of V's in tandem and this arrangement appears to be derivable from the double-V molars of the native bear (Phascolarctos) with which Phascolomys is in many ways rather closely related.

Fossil Marsupials of Australia.

During the Pleistocene or glacial epoch when northern Europe and northern North America were repeatedly covered by continental glaciers, Australia re mained free of glaciers except in the vicinity of its highest moun tains. While the mastodon, the mammoth, the cave-bear and other animals dominated Europe, Australia had her own peculiar mam malian fauna which has left its bones in cave deposits of eastern Australia and in ancient lake basins of the interior. In the cave fauna are the scattered bones of giant kangaroos of many extinct species, of the lion-like Thylacoleo, of the marsupial wolf and many others. Around the margins of the lakes, however, lived still stranger beasts, the clumsy diprotodonts and nototheres. In general body form these resembled huge rodents; the name "marsupial rhinoceros" applied to one notothere seems highly inappropriate even if the animal did have a hump on its nasal bones. In general the molar teeth of the diprotodonts each bear two sharp cross-crests, not unlike those of the extinct proboscidean Dinotherium. They appear to be an offshoot of the phalanger

stock, distantly related to the kangaroos, wombats and native bears. The fossil diprotodont Wynyardia, described by Sir Bald win Spencer from the Tertiary beds of Table Cape, Tasmania, was formerly regarded as of Oligocene age but is now known to be much younger (Pliocene). The skull is thoroughly phalangerid in type.

Fossil Marsupials of South America.--We

have already noted that the family of American opossums, appearing late in the Cretaceous in North America, and continuing there well into Oligocene, somehow found its way into South America at least by Lower Miocene and possibly earlier. But along with these purely American fossils there have been found in the Santa Cruz (Lower Miocene) formation of Patagonia the fossil repre sentatives of two families that have a strangely Australian ap pearance.

The first are unquestionably carnivorous marsupials, the several genera ranging from the size of a large opossum to that of a hyaena. Of these the largest and most famous form, called Bor hyaena, figured in scientific literature as a kind of link between the extinct creodonts of the Eocene epoch of North America and the existing marsupial wolf of Australia. But in every funda mental feature they are true carnivorous marsupials. W. D. Matthew even holds that the special resemblances between the Patagonian borhyaenids and the Australian carnivorous mar supials are likewise due in part to parallel evolution, the bor hyaenids being predatory derivatives of the American opossum stock, the Australian thylacines of the related Australian dasyurid stock, both derived ultimately from Cretaceous opossums of the northern hemisphere. But H. E. Wood has shown that, apart from the rather close resemblances in the borhvaenid dentition to that of the Australian thylacines, there are several curious resemblances in the backbone, pelvis and limbs, which tend to link borhyaenids with thylacines and contrast them both with opossums, so that to attribute this all to parallelism, plus descent from a common didelphid ancestor, seems to be essentially a petitio principii, especially as the now rather numerous known opossums from Cretaceous to Recent times show not the slightest tendency to vary in the borhyaenid-thylacine direction.