1. Procoptodon goliah
     Habitat    Physiology    Appearance    Mobility    Diet and molar patterns    Possible factors for extinction    Environmental factors  

2. References

• P. browneorum (Merrilees, 1968)
• P. cegsai (Pledge, 1992)
• P. gilli (Merrilees, 1968)
• P. goliah (Owen, 1845) (type species)
• P. maddocki (Flannery & Hope, 1984)
• P. mccoyi (Turnbull, Lundelius & Tedford, 1992)
• P. oreas (De Vis, 1895)
• P. otuel Owen, 1874
• P. pusio Owen, 1874
• P. rapha Owen, 1874
• P. texasensis Archer, 1978
• P. williamsi Prideaux, 2004

Procoptodon^[1] is a genus of giant short-faced kangaroo living in Australia during the Pleistocene epoch. P. goliah, the largest-known kangaroo that ever existed, stood approximately {{convert|2|m|ft|1|abbr=on}}.^[2] They weighed about {{convert|200|-|240|kg|abbr=on}}.^[2][3][5] Other members of the genus are smaller, however; Procoptodon gilli is the smallest of all of the sthenurine kangaroos, standing approximately {{cvt|1|m}} tall.

Procoptodon goliah

Habitat

Physiology

Appearance

Mobility

The genus is paraphyletic, derived from Simosthenurus.^[22]{{rp|285}}

Diet and molar patterns

These animals lived alongside modern species of kangaroo, but specialised on a diet of leaves from trees and shrubs.^[5] Their robust skull architecture and shortened face has been thought to be related to increased masseter muscles used to chew foods. Dental microwear of P. goliah supports a browsing diet. Large bicuspids, crenulated dental crowns, and a massive bony jaw present in the fossil evidence of P. goliah would have been required to process and digest a substantial amount of leafy fodder.^[6] Stable isotopic data suggested its diet consisted of plants utilising a C4 photosynthetic pathway, typically associated with grasses. In this case, however, chenopod saltbushes found throughout semi-arid Australia were considered a more likely source of the C4 signature. [An intensification in aridity during the second half of the Pleistocene propagated the evolutionary progression of the P. goliah to adapt to an abundance of dry vegetation. Evidence that P. goliah was the most widely distributed species among the Pleistocene macropodids throughout the continent shows that this species was adapted to a tougher diet than any other extinct Pleistocene sthenurine.^[10]{{rp|xvii}} Similar to Macropus giganteus, Procoptodon has molar patterns that indicate they had a similar, grassy, herbivorous diet (as opposed to leaves) and were grazers, but determining specific diets and preference of extinct herbivores is admittedly difficult.^[11] Through the study of isotopic composition of Procoptodon goliah tooth enamel, in addition to biomechanical bone features, dietary clues and feeding behavior have been deduced.^[12] The osteological features provide evidence of Procoptdon goliah{{'}}s ability to handle fibrous vegetables and salt consumption. This, in turn, leads to the belief that the species needed to be close to a water source to deal with salt intake; however, at the same time some theories are beginning to arise that limb-remains indicate the ability to travel distances to and from water sources.^[12]

Possible factors for extinction

The genus was present until at least about 50,000 years ago before going extinct, although there is some evidence they may have survived to as recently as 18,000 years ago. Its extinction may have been due to climate shifts during the Pleistocene,^[2] or to human hunting.^[9] Those who support the hypothesis of a human-mediated extinction process cite that the arrival of humans to continental Australia occurred at approximately the same time as the disappearance of this species.^[13] More evidence that this extinction was facilitated by human interaction is that the time period in which the extinction occurred was characterised by a relatively stable climate.^[13] However, there is no evidence of predation on or consumption of P. goliah by humans in the fossil record.^[13]

New evidence has been found however that shows humans arrived 10,000–15,000 years prior to extinction so human-mediated extinction is no longer the major hypothesis.^[14]

Fossils of giant short-faced kangaroos have been found at the Naracoorte World Heritage fossil deposits in South Australia, Lake Menindee in New South Wales, Darling Downs in Queensland, and at many other sites. A full-size, lifelike replica is on permanent display with other ancient native Australian animals at the Australian Museum.^[13]

Some researchers have speculated that the replacement of nutritious, fire-sensitive plants with less flammable, less nutritious flora brought about by human, fire-based deforestation in Australia played a large role in P. goliah and other Australian mega-fauna extinctions about 50kya. However, the diet of P. goliah, primarily chenopods and Atriplex in particular, were less flammable and remained largely unaffected by fire. These diet patterns disprove theories that the P. goliah's extinction was due in large part to a reduction in food supply from fire.^[9] At the same time, it is important to note that because of the kangaroos elongated breeding cycles, their ability to increase population numbers after human predation was highly limited.^[12]

Environmental factors

Kangaroos living in dry, arid environments have been shown to exhibit higher densities of tooth enamel, caused by indirect hydration through consuming herb-enriched plants. Lower levels of this enamel in P. goliah teeth found in areas with similar environmental parameters compared to the modern grazing kangaroo suggests that it relied far more heavily on free-standing water sources such as lakes and streams.^[9]

Given the larger size of P. goliah and its tendency to favor larger, freestanding water sources, episodic droughts accumulating 55 kya in the southern interior region of Australia would have certainly affected Goliath populations. Yet records show that such droughts had characterised this region for the previous 7 million years, with P. goliah surviving multiple intensely dry episodes during this period. Any period of significant decreases in rainfall did not occur until 5,000–10,000 years after the approximate extinction of the P. goliah 45–50 kya, 20 ky before the last glacial maximum of high aridity. These factors disprove speculations that such droughts could have played a significant role in the extinction of P. goliah.^[9]

Dental microwear of P. goliah supports a browse diet; however, stable isotopic data suggested its diet consisted of plants utilising a C4 photosynthetic pathway, typically associated with grasses. In this case, however, chenopod saltbushes found throughout semi-arid Australia were considered a more likely source of the C4 signature. Due to the saltbushes being their main food source, Procoptodon goliah was heavily dependent on free-standing water to help promote the processing of its salt-laden diet.^[15][16]

There is evidence that supports both of the claims that the extinction of P. goliah may have been due to climate shifts during the Pleistocene^[2] or to human hunting.^[15] P. goliah, depending heavily on free-standing water, was more vulnerable to drought. This can explain why the red kangaroo survived the increasing aridity and Procoptodon goliah did not. However, there is also evidence that suggests that humans could have a significant influence in the extinction of P. goliah. P. goliah's need for a constant free-standing source of water, plus its height and its common habitat in open shrub lands, made the P. goliah more noticeable to the human hunters, thus making it vulnerable to humans who were also water-bound like the Procoptodon.^[16]

References

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