Simosthenurus

Simosthenurus, also referred to as the short-faced kangaroo, is an extinct genus of megafaunal macropods that existed in Australia, specifically Tasmania, during the Pleistocene. Analysis of Simosthenurus fossils has contributed to the finding that there are three lineages of macropods: Sthenurinae, Macropodinae, and Lagostrophinae.[1] The genus Simosthenurus was among the sthenurines.

Simosthenurus Temporal range: Pleistocene
S. occidentalis skeleton at American Museum of Natural History
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Marsupialia
Order:	Diprotodontia
Family:	Macropodidae
Genus:	†Simosthenurus Tedford, 1966
Species
S. maddocki S. occidentalis S. antiquus S. baileyi S. brachyselenis S. eurykaphus S. pales S. tirarensis S. orientalis

The two most documented members of the genus are S. maddocki and S. occidentalis, though other species have also been discovered.

Palaeobiology

Restoration of S. occidentalis

Osteological information (predominantly cave floor surface finds[1]) has yielded that Simosthenurus is part of the same family as that of modern kangaroos.[2] However, modern kangaroos are plantigrade hoppers, using jumping as their means of locomotion, while Simosthenurus was a bipedal unguligrade, walking in a manner similar to that of hominids.[3] Although members of Simosthenurus were no taller than most modern species of kangaroo, their robust bones, broad pelvis, long arms and short necks were unique adaptations to their browsing mode of feeding. They had single-toed hind feet had small hoof-like nails more typical of animals adapted to moving over relatively flat terrain.[4] Simosthenurus is a highly distinct lineage of macropods, with no living descendants. However, it is possible that their closest living cousin is the banded hare-wallaby, which is now restricted to small isolated islands off the coast of Western Australia.[5]

S. occidentalis

S. occidentalis mtDNA sequences were obtained from fossils in Tasmanian caves; the fossils yielded radiocarbon dates between 46,000 and 50,000 years ago. The sequences obtained in this study were from fossils much older than any Australian fossils that previously yielded sequences.[1]

S. occidentalis was a leaf-eating marsupial, about the size of a modern grey kangaroo, though far more robust, with adults estimated to be about 118 kg. The species has been suggested to have used a bipedal striding gait when moving slowly. This is unlike modern kangaroos, which use pentapedal motion, or 'punting', pushing off their tail and forelimbs and swinging their hindlimbs forward when moving slowly.[1] It is thought that, by rearing up on their hind limbs and using their long arms and fingers, they could reach overhead to grasp high leaves and branches and pull them down to their mouth. They then would use their powerful jaws and striated teeth to grind the tough leaves.[4]

S. maddocki

An adult S. maddocki was smaller than S. occidentalis, weighing only 78 kg.[1] Like some other species from the same time period, they were apparently highly selective feeders. Local records indicate that the species was mainly located in southeastern Australia. It is uncommon to find fossils of this rare species, especially when compared to other Sthenurines.

Extinction

Skull of Simosthenurus gilli at the Melbourne Museum

S. occidentalis skeleton, Victoria Fossil Cave

There are several proposed causes of the extinction of Simosthenurus. The two most popular hypotheses include human involvement and climate change.

Human impact

One theory postulates that human impact caused it. There are fewer extinct megafaunal Tasmanian species compared to those of continental Australia. This is most likely due to humans arriving in mainland Australia first. The extinction of Simosthenurus may be attributable to human over-hunting or habitat alteration.[6] However, there is no archaeological evidence for interactions with humans, and the overlap of habitation in Australia and Tasmania of both humans and Simothenurus species, if there was one, would have been relatively short.[7]

Climate change

Another theory is that climate change caused the extinction of this genus. The last glacial period, popularly known as the Ice Age, has been linked with a severe reduction in several megafaunal populations, including Simosthenurus.[8]

References

1. Llamas; et al. (2014). "Late Pleistocene Australian Marsupial DNA Clarifies the Affinities of Extinct Megafaunal Kangaroos and Wallabies". Molecular Biology and Evolution. 32 (3): 574–584. doi:10.1093/molbev/msu338. PMID 25526902.
2. Prideaux, Gavin J.; Warburton, Natalie M. (2010). "An osteology-based appraisal of the phylogeny and evolution of kangaroos and wallabies (Macropodidae: Marsupialia)". Zoological Journal of the Linnean Society. 159 (4): 954–987. doi:10.1111/j.1096-3642.2009.00607.x.
3. Janis, CM; Buttrill, K; Figueirido, B (2014). "Locomotion in Extinct Giant Kangaroos: Were Sthenurines Hop-Less Monsters?". PLoS ONE. 9 (10): e109888. doi:10.1371/journal.pone.0109888. PMC 4198187. PMID 25333823.
4. "Extinct Animals- Simosthenurus occidentalis". www.parks.sa.gov.au. Archived from the original on September 5, 2007. Retrieved 2015-10-29.
5. "DNA Sheds Light on Mysterious Giant Kangaroo". Retrieved 2015-10-29.
6. Gillespie, Richard; Camens, Aaron B.; Worthy, Trevor H.; Rawlence, Nicolas J.; Reid, Craig; Bertuch, Fiona; Levchenko, Vladimir; Cooper, Alan (2012-03-22). "Man and megafauna in Tasmania: closing the gap". Quaternary Science Reviews. 37: 38–47. doi:10.1016/j.quascirev.2012.01.013.
7. Cosgrove, Richard; Field, Judith; Garvey, Jillian; Brenner-Coltrain, Joan; Goede, Albert; Charles, Bethan; Wroe, Steve; Pike-Tay, Anne; Grün, Rainer (2010-10-01). "Overdone overkill – the archaeological perspective on Tasmanian megafaunal extinctions". Journal of Archaeological Science. 37 (10): 2486–2503. doi:10.1016/j.jas.2010.05.009.
8. Webb, Steve (2008). "Megafauna demography and late Quaternary climatic change in Australia: A predisposition to extinction". Boreas. 37 (3): 329–345. doi:10.1111/j.1502-3885.2008.00026.x.