Nimbacinus dicksoni

Introduction

Nimbacinus dicksoni was a small, fox-sized thylacine, a carnivorous marsupial distantly related to the 'Tasmanian Tiger' (Thylacinus cynocephalus). The near-complete skeleton of Nimbacinus from the Miocene of Riversleigh is the only skeleton of a thylacine species other than Thylacinus discovered to date. Thylacines were the main mammalian predators of the Miocene, with over ten species known from northern and central Australia.

Identification

Thylacines were small to mid-sized, quadrupedal marsupial predators. All thylacines had long snouts, three premolars in each jaw, and molar teeth specialized for carnivory (the cusps and crests reduced and/or elongated to form cutting blades on the molars). Over ten species are now known from northern and central Australia. They ranged in size from those the size a quoll to species of Thylacinus larger than T. cynocephalus. Thylacines were generally quite similar to one another, differing mainly in their dentitions. These dental differences may reflect differences in diet, although all were at least to some extent carnivorous.

Skulls of fossil thylacines are exceedingly rare. To date, there are only three species - Nimbacinus dicksoni, Mutpuracinus archibaldi and Badjcinus turnbulli - known from fossil crania. Thylacinid skulls are anatomically conservative and have changed little over time. Early thylacines shared many features with dasyurids (quolls and the Tasmanian Devil) although even the oldest thylacines show dental specializations towards greater carnivory.

Nimbacinus was smaller and more generalized than T. cynocephalus but more advanced in some features than earlier Miocene thylacinids such as Badjcinus. The teeth of Nimbacinus were relatively unspecialized within the thylacine group but its middle ear region was more specialized than that of earlier thylacines like Mutpuracinus archibaldi, being intermediate between Mutpuracinus and the much later Thylacinus. Aside from a suite of differences in the dentitions of Nimbacinus and T. cynocephalus, just a single phylogenetically important skull feature separates the two species: a fossa for the lower canine bordered by the anterolateral processes of the maxilla and premaxilla (bones of the upper snout).

The skeleton of Nimbacinus has not yet been described. There are no other associated postcranial remains of thylacines other than those of the Pleistocene to Recent T. cynocephalus, the last and most specialized of the thylacines (and presumably the most dog-like).

Habitat

Riversleigh during 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. Bullock Creek, the most northerly Cainozoic vertebrate fossil site on the Australian continent, was an area of freshwater streams and lakes during the middle Miocene. The climate was marked by high temperatures and strong seasonality, with long periods of low rainfall (one of the earliest fossil sites to show increasing aridity in Australia).

Distribution

Nimbacinus dicksoni has been found at both the Riversleigh World Heritage Fossil Site in northwestern Queensland, and the Bullock Creek locality in the Northern Territory. These two sites have several species in common, including a second small thylacine, Mutpuracinus archibaldi and a second possible species of Nimbacinus, N. richi.

Feeding and diet

Like the Tasmanian Thylacine and other marsupial predators, Nimbacinus would have preyed on mammals, birds and other vertebrates. Its relatively small size suggests a diet of small vertebrates. However, a study of bite force in fossil mammals found that Nimbacinus dicksoni had a high bite force quotient (BFQ), enabling it to take comparatively large prey.

Life history cycle

Like all marsupials, Nimbacinus would have had tiny, hairless young that developed to maturity in a pouch after birth. The Tasmanian thylacine hunted at night or during the early morning hours, like most carnivorous marsupials, and Nimbacinus probably did the same. Nimbacinus would have been a top predator within its size range. Like the Tasmanian thylacine and quolls ('native cats'), Nimbacinus was probably not a swift runner. Like T. cynocephalus, it may have instead relied on persistence and stamina.

Fossils description

Nimbacinus dicksoni was first described in 1990 from upper and lower jaw fragments and isolated teeth. In 1996, a complete skull and lower jaw (described in 2001), together with an associated, near-complete skeleton (as yet undescribed), were discovered at Riversleigh.

A second species of Nimbacinus, N. richi, has been described from Bullock Creek (Murray and Megirian 2000), However, the validity of this as a separate species has been questioned by Wroe and Musser (2001), who state that the specific differences described (the relative sizes of the entoconid) might be attributed to variation within N. dicksoni.

Evolutionary relationships

The discoveries of so many new thylacine species over the past two decades re-ignites the debate over thylacine relationships and over who their immediate ancestors were. The traditional view has been that thylacines were descended from a dasyurid ancestor perhaps during the Oligocene. The first phylogenetic analysis incorporating data from Nimbacinus dicksoni found an alternative result, that thylacinids were actually the older and more 'primitive' group and that dasyurids were a newer and more specialized group (Wroe and Musser 2001). The results of a subsequent analysis of thylacine relationships (in a paper describing another Miocene thylacine, Mutpuracinus) retrieved the traditional traditional arrangement, with thylacines above dasyurids in the dasyuromorphian tree (Murray and Megirian 2006). No further studies based on firsthand investigation of fossil thylacines have been published. The question of thylacinid relationships must therefore be considered unresolved. Study of the Nimbacinus skeleton will undoubtedly shed light on the question of thylacine affinities.

References

• Muirhead, J. and Archer, M. 1990. Nimbacinus dicksoni, a plesiomorphic thylacine (Marsupialia, Thylacinidae) from Tertiary deposits of Queensland and the Northern Territory. Memoirs of the Queensland Museum 28, 203-221.
• Muirhead, J. 1992. A specialized thylacinid, Thylacinus macknessi (Marsupialia: Thylacinidae) from the Miocene deposits of Riversleigh, northwestern Queensland. Australian Mammalogy 15, 67-76.
• Muirhead, J. 1997. Two new early Miocene thylacines from Riversleigh, northwestern Queensland. Memoirs of the Queensland Museum 41, 367-377.
• Muirhead, J. and Wroe, S. 1998. A new genus and species, Badjicinus turnbulligen. et sp. nov. (Thylacinidae: Marsupialia), from the late Oligocene of Riversleigh, northern Australia, and an investigation of thylacinid phylogeny. Journal of Vertebrate Paleontology 18, 612-626.
• Murray, P. F. and Megirian, D. 2000. Two new genera and three new species of Thylacinidae (Marsupialia) from the Miocene of the Northern Territory. The Beagle, Records of the Museums and Art Galleries of the Northern Territory 16, 145-162.
• Murray, P. F. and Megirian, D. 2006. Cranial morphology of the Miocene thylacinid Mutpuracinus archibaldi (Thylacinidae, Marsupialia) and relationships within the Dasyuromorphia. Pp. 229-276 in Reed, L., Bourne, S., Megirian, D., Prideaux, G., Young, G. and Wright, A. (eds) Proceedings of CAVEPS 2005, Alcheringa Special Issue 1.
• Wroe, S. and Musser, A. M. 2001. The skull of Nimbacinus dicksoni (Thylacinidae: Marsupialia). Australian Journal of Zoology 49, 487-514.
• Wroe, S., McHenry, C. and Thompson, J. 2005. Bite Club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa. Proceedings of the Royal Society B doi:10.1098/rspb.2004.2986.

Further reading

• 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.