Riversleigh Tube-nosed Bandicoot

Riversleigh Tube-nosed Bandicoot, Yarala burchfieldi is one of the oldest and smallest bandicoots known, as well as the most archaic. It is in its own family, Yaralidae, at the base of the bandicoot radiation, and is structurally intermediate between bandicoots and dasyurids. It would have foraged in the forest leaf litter for insects and may have been at least partly carnivorous, like the dasyurids. The skull of Yarala is one of the most perfectly preserved bandicoot fossils known.

Identification

Bandicoots are small mammals (averaging around one kilogram in weight) known only from Australia and New Guinea. Bandicoots and bilbies together make up the order Peramelemorphia. Bandicoots generally have long snouts, large ears, a pear-shaped body and thin tail. The limbs are relatively long and thin. Bandicoots and bilbies are both polyprotodont: they have several lower incisors in the lower jaw (unlike diprotodontians - kangaroos, wombats, possums and others - which only have two lower incisors). Dentally, therefore, bandicoots and bilbies most closely resemble the insectivorous/carnivorous dasyurids, the group that includes the Tasmanian Devil, quolls, antechinus and other small marsupicarnivores. However, the second and third toes on the hind feet of peramelemorphians are joined (syndactylous), an unusual condition seen only in diprotodontians (possibly an adaptation for climbing). This mix of characters has made it difficult to sort out the origins and phylogenetic relationships of peramelemorphians.

Yarala burchfieldi has two features of the skull known only in bandicoots: the jugal bone of the zygomatic arch is bifid, or Y-shaped, where it contacts the maxilla; and the nasal bones are narrow along their entire length. Yarala burchfieldi also has several dental features in common with other bandicoots (e.g., a posteriorly oriented preparacrista on M2/, posterolingual position of hypoconulids, and buccal position of the centrocrista: Muirhead and Filan 1995). However, certain advanced features that would clearly place Yarala in an existing family of bandicoots are absent. Yarala has several features are more primitive (plesiomorphic) than in other bandicoots, including certain dental features (e.g., the cristid oblique does not terminate against the trigonid lingual to the midpoint of the crown; postparacrista and premetacrista join to for a centrocrista and don't breach the ectoloph; and there is no metaconule (also the case in the bilbies) (Muirhead and Filan 1995). Primitive skull features include a short snout, a large alisphenoid that contacts the parietal (a bone of the side wall of the braincase), and differences in the size and position of certain foramina (openings in the skull for nerves and blood vessels).

Yarala burchfieldi is smaller than any known bandicoot. Skull measurements indicate that it was about the size a small antechinus (marsupial 'mouse').

Habitat

Riversleigh from the early to middle Miocene was mainly forested, with more open areas near the forest edges and freshwater streams or lakes in a karst (limestone) environment.

Distribution

Yarala burchfieldi is known only from the Riversleigh World Heritage Fossil Site in northwestern Queensland. It was a widespread species, found in at least ten sites. Yarala kida is known from the Kangaroo Well Local Fauna in the Northern Territory.

Feeding and diet

Yarala had a dentition that was in many ways intermediate between that of more advanced bandicoots and dasyurids (known collectively as marsupial carnivores). Its diet may therefore have been more like that of insectivorous-carnivorous dasyurids than other bandicoots, which usually have an insectivorous-omnivorous diet. It is believed that, because of its small size (similar to an small antechinus) and its diet, Yarala may have competed with small dasyurids for food.

Life history cycle

Bandicoots and bilbies are all small, terrestrial mammals that forage for insects, fungi and fruit at dusk, and sleep in nests by day. Bilbies are more carnivorous than bandicoots, sometimes taking small mammals and lizards. Living bandicoots and bilbies have one of the shortest gestation periods known to mammals: the Northern Brown Bandicoot and Long-nosed Bandicoot both have a gestation period of just 12.5 days. Most bandicoots and bilbies have eight teats, although litter sizes average just four.

Fossils description

Yarala burchfieldi is known from a partial skull and fragmentary upper and lower jaws. It is the most complete fossil bandicoot known.

Evolutionary relationships

The origins of Peramelemorphia are debated, and the relationships of bandicoots and bilbies to other marsupials are uncertain (and probably quite distant). Peramelemorphians may have evolved from dasyurids, as suggested by the teeth of Yarala and by the polyprotodont dentition of other peramelemorphians. Alternatively, the unusual syndactyly of both peramelemorphians and diprotodontians suggests that these two groups may have had a common origin. Some have even suggested that peramelemorphians may have had a separate origin from other Australian marsupials, evolving from a South American marsupial ancestor before dispersing to Australia.

Among living bandicoots, the Australian and New Guinea bandicoots (Peramelidae and Peroryctidae) are more closely related to each other than either is to the Bilby (Macrotis lagotis), the sole living member of Thylacomiyidae.

Yaralidae is the fourth peramelemorphian family, with two extinct species of Yarala (Y. burchfieldi and Y. kida). Yarala is a key to understanding bandicoot evolution because it appears to be at the base of Peramelemorphia. Dental and cranial characters both place Yarala within the order Peramelemorphia. Yarala is the most primitive known bandicoot (its name comes from the Wanyi [Aboriginal dialect], meaning 'root of tree'). However, it lacks certain shared features that are present in all other bandicoots and does not have a close relationship to any known group, and is probably not an ancestor of later bandicoots. A second, slightly older species, Yaralakida, has been proposed as a possible ancestor of Y. burchfieldi.

A new superfamily, Yaraloidea, has been proposed (Muirhead 2000). The high number and diversity of bandicoots in the Cainozoic of Australia implies an ancient origin for the group, although some molecular studies have a younger origin in the late Oligocene or early Miocene, at least for living bandicoot species. Yaraloids may be important in the biocorrelation of late Oligocene to early Miocene Australian mammalian faunas because of their diversity and wide distribution.

References

• Groves, C. P. and Flannery, T. F. 1990. Revision of the families and genera of bandicoots. Pp. 1-11 in Seebeck, J. H., Brown, P. R., Wallis, R. L. and Kemper, C. M. (eds) Bandicoots and Bilbies. Surrey Beatty and Sons, Sydney.
• Meredith, R., Westerman, M. and Springer, M. S. 2008. A timescale and phylogeny for 'Bandicoots' (Peramelemorphia: Marsupialia) based on sequences for five nuclear genes. Molecular Phylogenetics and Evolution 47, 1-20.
• Muirhead, J. and Filan, S. L. 1995. Yarala burchfieldi, a plesiomorphic bandicoot (Marsupialia, Peramelemorphia) from Oligo-Miocene deposits of Riversleigh, northwestern Queensland. Journal of Palaeontology 69, 127-134.
• Muirhead, J. 2000. Yaraloidea (Marsupialia, Peramelemorphia), a new superfamily of marsupial and a description and analysis of the cranium of the Miocene Yarala burchfieldi. Journal of Paleontology 74, 512-523.
• Pacey, T. L., Baverstock, P. R. and Jerry, D. R. 2001. Phylogenetic relationships of the Bilby, Macrotis lagotis (Peramelemorphia: Thylacomiyidae), to the Bandicoots - DNA sequence evidence. Molecular Phylogenetics and Evolution 21, 26-31.
• Schwartz, L. R. S. 2006. A new species of bandicoot from the Oligocene of northern Australia and implications of bandicoots for correlating Australian Tertiary mammal faunas. Palaeontology 49, 991-998.

Further reading

• Archer, M., Hand, S. J. and Godthelp, H. 1994. Riversleigh: The Story of Animals in Ancient Rainforests of Inland Australia. Reed Books, Chatswood.
• Long, J. A. et al. 2002. Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution. Johns Hopkins University Press, Baltimore.