Sthenurinae

Australia: The Land Where Time Began

Sthenurinae (short-faced kangaroos) - subfamily of family Macropodidae

Of the extinctions that occurred during the Pleistocene were from the family Macropodidae (kangaroos). The subfamily Sthenurinae were completely lost and the surviving subfamily, Macropodinae, were severely reduced.

The main difference between the sthenurine kangaroos and living kangaroos was a skull that was short and deep, forearms that were long and heavily muscled, and the digits on the hind foot were greatly reduced, leaving a large 4th digit, in effect a single large toe. Their large, heavy skulls would have been capable of a powerful bite, but their teeth were not high-crowned and molar progression was lacking, so they were not adapted for grazing on abrasive plant material such as grass. It is believed they used their sturdy construction to feed by browsing on their hind legs, pulling branches close enough to get at the leaves, and probably not to feed by bending to eat foliage near the ground. On their forepaws the outer digits were reduced, but their 2nd and 3rd digits had elongated, having a hooked shape, that probably allowed them to pull branches of trees low enough to reach the leaves.

The sthenurines have been divided into 2 broad groups, based on their skulls and teeth, and the supposed ecology of the different groups (Prideaux, 2004). The genera Sthenurus and Metasthenurus appeared closer to the Macropodinae, having longer muzzles and higher-crowned cheek teeth than the other genera, so presumably had a less powerful bite than the other genera. Compared with the other genera that had relatively wider incisors, that would have allowed the stripping of more small leaves with each bite, possibly feeding on small-leaved shrubs such as chenopods. They mainly occupied the drier inland areas. The geographic range of S. tindalei and S. stirlingi matched the distribution of the chenopod shrublands of the present.

Simosthenurus and Procoptodon, the other group of genera, had very short faces with small incisors, allowing a very powerful bite. Combined with the large surface of their cheek teeth, these adaptations would have allowed them to feed on tough vegetation, though not on abrasive material such as grass. It is believed they fed on trees and shrubs, probably with large, tough leaves, using their hands to manipulate their food in a similar way to koalas. It is believed Simosthenurus species occupied areas that were more coastal than the species of Sthenurus, probably in woodlands and open forests.

There were some unusual features in one of the species of Simosthenurus, Si. maddocki, that had slender lower incisors, small cheek teeth and is believed to have had a long, possibly manipulative tongue. It has been suggested it may have eaten some fruit and possibly unusually soft browse (Johnson, 2006). Giraffes use their manipulative tongues to pluck small leaves from among the long thorns of acacia trees.

Procoptodon had similar features to Simosthenurus, very short, robust skulls and small incisors, but their cheek teeth were more like those found in grazing kangaroos (Sanson, 1982). The small size of their incisors suggest they were not grazers, as grass is difficult to gather with the forepaws, and they would have been restricted to small bites by the small size of their incisors. They would probably have had difficulty getting enough to eat if they grazed on grass. It has been suggested they may have had a mixed grazing/browsing method of feeding, possibly overlapping with the diet of Diprotodon, but using their upright stance to reach higher foliage (Prideaux, 2004). The largest of the kangaroos, P. goliah, is estimated to have weighed abut 250 kg and reached a height of 2-3 m.

The dry open woodlands are believed to be the environment in which the sthenurines arose during the Pliocene. The history of its evolution apparently coincided with the drying of Australia during the Pleistocene, when dry open woodlands and shrublands expanded, and during the Pleistocene its numbers and diversity greatly increased.

Some of the extinct species of macropodinae were similar to living kangaroos, including some species of Macropus that apparently looked similar to larger versions of red kangaroos and grey kangaroos, that are believed to have been grazers, that probably inhabited grassy woodlands and open plains.

There were also species that are believed to be browsers, such as Protemnodon, a largish species that had shorter feet and much longer, stronger forearms than modern species of kangaroo, having broad forepaws with long digits all of approximately the same length. P. anak had an unusually long neck and long, slender muzzle. It is believed to have possibly resembled a gerenuk, an antelope with a long neck. This antelope from Africa feeds by standing on its hind legs to reach the foliage of thorny acacia trees, resting its forelegs on the branches. Fossils of P. anak have been found with the remains of its stomach contents still in place, that consisted of large fragments of leaves and coarse twigs (Flannery, 1982).

P. hopei, from the high-elevation grasslands of New Guinea, had a dentition that suggested a diet of grass (Flannery, 1982).

Protemnodon, species had stocky hind limbs with short feet, which the author¹ suggests probably didn't benefit much from the elastic recoil mechanism that allows bounding of kangaroos of the present to be so energy efficient at high speed. Protemnodon could probably accelerate quickly, though at the cost of high energy consumption as is the case with Simosthenurus.

Sources & Further reading