Palorchestes azeal

Palorchestes azael was an unusual marsupial herbivore with retracted nasal bones on the skull, suggesting that it may have had a small trunk like that of tapirs. Palorchestes also had powerful forelimbs and large, compressed claws that it may have used to pull up shrubs or tear at the bark of trees. It was a browser, probably feeding on leaves or roots.

Identification

Palorchestes was first identified from its teeth, which like those of other diprotodontoids resembled the teeth of kangaroos (both groups have two transverse lophs on the molar teeth). Palorchestes was therefore first described as an enormous kangaroo (e.g., Owen 1877, Tate 1948). It was not recognized as a diprotodontoid until the late 1950s; it lacks a masseteric foramen through the lower jaw, a key feature of kangaroos and wallabies (Macropodidae) (Woods 1958). (link to AM site)

The teeth of Palorchestes are large and high-crowned, with well-developed fore-, mid- and hindlinks (the development of distinct midlinks characterize the genus), V-shaped transverse valleys, lingual cingula, and absence of parastyles on M1-2/. It has been noted that the molars of Palorchestes azeal are quite variable in morphology. Molars may be more complex anteriorly, with the development of additional midlinks that are absent in more posterior molars.

The nasal bones of the skull of Palorchestes (known from P. azeal and P. painei) are short and retracted, suggesting the presence of a tapir-like trunk. The infraorbital foramina are also very large, another indication that Palorchestes had a long, narrow, well-ennervated snout possibly developed into a trunk. There are extensive areas on the lower jaw for the attachment of powerful jaw musculature. Palorchestes also had huge, compressed claws and powerful front limbs (adaptations to tearing bark or digging roots). The complete skeleton has not yet been described.

Palorchestes differs from species of Propalorchestes (one of which was the probable ancestor of Palorchestes) in dental features. However, the general form of the diagnostic third premolar does not vary widely between Propalorchestes and Palorchestes. The P3/ is bicuspid and triangular in both genera, but is wider than long in Propalorchestes (Pr. ponticulus). Palorchestes had a longer upper and lower jaws (and therefore snout).

Palorchestes azeal differed from earlier species of Palorchestes (P. x, and P. anulus) in size (being the largest species in the genus) and in its dentition (it had well developed anterior and middle links on molars, and the upper third premolar, P3/ is simple, with three cusps).

Palorchestes azeal was most closely related to other species of Palorchestes, of which there were five (P. anulus, P. painei, P. selestiae, P. pickeringi and P. parvus). Of these, P. azeal appears to be closest to P. parvus. Together with the older Propalorchestes, Ngapakaldia and Pitikantia, they make up the family Palorchestidae (one of two families in the suborder Diprotodontoidea, which includes the Pleistocene Diprotodon). The closest living relatives of palorchestids are wombats.

Habitat

Tasmania, Palorchestes would have lived near a marsh or shallow lake surrounded by eucalypt woodland.

Distribution

Palorchestes azeal is the most widely distributed palorchestid, known from a variety of habitats. Fossils of P. azeal are known from Bluff Downs in south-eastern Queensland, from Naracoorte Caves in South Australia and Gippsland in Victoria (the type locality). Pleistocene Palorchestes species have been recovered from the Darling Downs, southeastern Queensland.

Feeding and diet

Like all diprotodontoids, Palorchestes was an herbivore. Its high-crowned teeth were well suited to handling coarse or abrasive vegetation. It would have had a long, prehensile tongue, used in living herbivores like giraffes to manipulate leaves and other vegetation. Palorchestes had large, compressed claws that could have been used to dig roots and tubers.

Life history cycle

As in all marsupials, Palorchestes would borne tiny, poorly formed young that would have completed their development in the mother's pouch. Like their relatives the wombats, palorchestids probably had a backward-facing pouch (a legacy of their burrowing ancestry). Palorchestids are rare in the fossil record, suggesting that they were probably solitary animals. Little else is known of their life history.

Fossils description

The genus Palorchestes first appears in the early late Miocene of Riversleigh in northwestern Queensland (P. annulus, known from just a single, isolated M1/). The lineage persisted until the late Pleistocene, when the last known palorchestids, Palorchestes azeal (known only from the late Pleistocene) became extinct.

Evolutionary relationships

Palorchestids were diprotodontoids, differing sufficiently from Diprotodontidae (Diprotodontinae + Zygomaturinae) to warrant a separate family, Palorchestidae. Although the monophyly of this family has been questioned, a recent, large-scale phylogenetic analysis strongly supports the separation of palorchestids at the familial level.

The oldest occurrence of the genus Palorchestes (P. anulus) is from the early late Miocene of Riversleigh, followed by P. x from the late Miocene Beaumaris Local Fauna, Victoria.

Propalorchestes, a more archaic palorchestid and most likely ancestral to Palorchestes, is known from the late Oligocene to middle Miocene of Bullock Creek, Northern Territory and Riversleigh, northwestern Queensland. Its molar teeth are structurally intermediate between X and species of Palorchestes. ?? The first molar of the oldest known species of Palorchestes, the transitional form P. anulus, is structurally intermediate between the molars of Propalorchestes and those of later species of Palorchestes. Palorchestes (P. painei) co-occurs with Propalorchestes (Pr. Xx) at the late Miocene Alcoota site in the Northern Territory.

The six species of Palorchestes did not overlap in time. Over time, they became larger and their teeth became more high-crowned and complex.

References

• Anderson, C. 1933. The fossil mammals of Australia. Proceedings of the Linnean Society of New South Wales 59, ix-xxv.
• Archer, M. and Bartholomai, A. 1978. Tertiary mammals of Australia: an overview. Alcheringa 2, 1-19.
• Banks, M. R., Colhoun, E. A. and van den Geer, G. 1976. Late Quaternary Palorchestes azeal (Mammalia, Diprotodontidae) from northwestern Tasmania. Alcheringa 1(2), 159-166.
• Bartholomai, A. 1978. The rostrum in Palorchestes Owen (Marsupialia: Diprotodontidae); results of the Ray E. Lemley expedition, Part 3. Memoirs of the Queensland Museum 18, 145-149.
• Black, K. and Mackness, B. 1999. Diversity and relationships of diprotodontoid marsupials. Australian Mammalogy 21, 20-21; 34-45.
• Black, K (thesis)
• Davis, A. and Archer, M. 1997.
• Glauert 1912b, 1926
• Hocknull, S. A. 2005. A small adult Palorchestes (Marsupialia, Palorchestidae) from the Pleistocene of the Darling Downs, southeast Queensland. Memoirs of the Queensland Museum (Proceedings of the Conference of Australasian Vertebrate Evolution, Palaeontology and Systematics) Vol. 51 (1), 202.
• Long, J. A. et al. 2002. Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution. Johns Hopkins University Press, Baltimore, 240 pp.
• Murray, P. F. 1991. Chapter 24: The Pleistocene megafauna of Australia. Pp. 1071-1164 in Vickers-Rich, P., Monaghan, J. M., Baird, R. F. and Rich, T. H. (eds) Vertebrate Palaeontology of Australasia. Pioneer Design Studio, Melbourne
• Owen, R. 1873b. On the fossil mammals of Australia. Part IX. Family Macropodidae: Genera Macropus, Pachysaigon, Leptosaigon, Procoptodon, and Palorchestes. Philosophical Transactions of the Royal Society of London 164, 783-803.
• Pledge, N. 1991.
• Tate 1948
• Tedford, R., Banks, M., Kemp, N., McDougall, I. and Sutherland, F. 1975. Recognition of the oldest fossil marsupials from Australia. Nature 255, 141-142.
• Woods 1958
• Woodburne, M. A. 1967b. Three new diprotodontids from the Tertiary of the Northern Territory, Australia. Bureau of Mineral Resources, Geology and Geophysics Bulletin, No. 85, 53-103.

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.