Euryzygoma dunense

Introduction

Euryzygoma dunense was a large, quadrupedal marsupial herbivore from the Pliocene of Australia. Euryzygoma had unusual, flaring cheekbones that may have been used either for storing food or as a sexual display. It was a browser, feeding on leaves and shrubs in a variety of habitats. Euryzygoma, a diprotodontine, was closely related to the largest known marsupial, the massive Diprotodon.

Identification

Euryzygoma is in the subfamily Diprotodontinae, distinguished from the subfamily Zygomaturinae in part by the structure of the diagnostic third premolar. In zygomaturines, this tooth is complex and multi-cusped, while in diprotodontines it is simpler in form. The third upper premolar (P3/) of Euryzygoma is structurally intermediate between the P3/ of Diprotodon and the P3/ of earlier 'nototheres', with an incipient horse-shoe shaped crown similar to the horse-shoe shaped P3/ of Diprotodon.

The snout of Euryzygoma was deep but very narrow. The cheekbones were exceptionally wide, the skull being wider than long (unusual in mammals) . These cheekbones are strongly developed in some specimens but less so in others, suggesting possible differences between the sexes (sexual dimorphism). The cheeks were formed by extensions (lateral processes) of the zygomatic arch. The ends of these processes are smooth bone rather than rugose, suggesting that Euryzygoma did not have horny growths. The interior of the skull was honeycombed by numerous air spaces surrounding the comparatively tiny brain cavity (seen in a damaged skull of Euryzygoma, the front half of the skull separated from the back half). The enamel of the molar teeth was rugose, as it is in Diprotodon.

Habitat

Early in the Pliocene, much of Queensland and New South Wales, like the rest of Australia, was warm temperate eucalypt forest, and rainfall was high. During the latter part of the Pliocene the climate cooled and the region became drier.

Distribution

In Queensland, Euryzygoma (E. dunense) is known from the Darling Downs in the southeast, and from Bluff Downs Station (Euryzygoma sp.) in northern Queensland. It is also known from X in northwest New South Wales.

Related to: Euryzygoma dunense was most closely related to a second, unnamed species of Euryzygoma from Bluff Downs. Euryzygoma may have been the ancestor of Diprotodon.

Feeding and diet

Euryzygoma was a browser, feeding on leafy vegetation. The bony extensions of its cheekbones may have supported fleshy cheek pouches for storing or processing food.

Other behaviours and adaptations

When the skull of Euryzygoma was first described, it was proposed that the extensions of the cheekbones might have supported food pouches, which in some mammals may be used to store food. One possible reason for this strategy is that Euryzygoma may have therefore spent less time near dangerous, crocodile-infested waterholes. These flaring cheeks may also have been used as a sexual display: some skulls of Euryzygomalack the flared cheekbones. Other diprotodontines (in particular, the Pleistocene Diprotodon) exhibit a high degree of sexual dimorphism (morphological differences between males and females), and it is possible that only one sex in Euryzygoma(probably male) had wide, flaring cheekbones. These strong bony extensions may have been used in male to male combat or as sexual displays.

Life history cycle

Diprotodontines, like all marsupials, began life as tiny neonates that completed their growth after birth in the mother's pouch. Living vombatiforms (koalas and wombats) have a backwardly-facing pouch, and it is probable that diprotodontoids had the same.

In Euryzygoma, it appears that one sex (probably male) had larger and more well developed cheekbones than the other (probably female). There was therefore possibly a high degree of morphological difference between the sexes (sexual dimorphism). In living sexually dimorphic mammals, breeding is usually polygynous (males mate with multiple females over the breeding season). Euryzygoma may also have used such a breeding strategy.

Fossils description

Euryzygoma dunense was iinitially described from isolated teeth, and when first published was put into the diprotodontoid genus Nototherium (De Vis 1887). A skull of Euryzygoma was unearthed in 1912 while workers were sinking a well on the Darling Downs near Brigalow, southeastern Queensland. Several additional fossil skulls have since been found, including a beautifully preserved skull found by R. A. Stirton at Chinchilla in southeast Queensland. Numerous other bones have been found, although a composite skeleton has not been reconstructed.

Evolutionary relationships

Diprotodontids first appear in the late Oligocene, about 25 million years ago. These early diprotodontids were probably descended from late Oligocene to early Miocene wynyardiids (small marsupials with a dentition intermediate between that of possums and diprotodontids) and were about the size of sheep.

The subfamily Diprotodontinae is a Pliocene-Pleistocene group. In addition to Euryzygoma, the subfamily includes Bematherium, Ngapakaldia and Pitikantia from the late Oligocene; Pyramios from the late Miocene, Euowenia, Meniscolophus, Nototherium and Diprotodon from the Pliocene-Pleistocene. Euryzygoma XX.

References

• Archer, M. 1977b. Origins and subfamilial relationships of Diprotodon (Diprotodontidae, Marsupialia). Memoirs of the Queensland Museum 18, 37-39.
• Black, K. (thesis)?
• Black, K. and Mackness, B. 1999. Diversity and relationships of diprotodontoid marsupials. Australian Mammalogy 21, 20-21; 34-45.
• De Vis, C. W. 1888b. On a supposed new species of Nototherium. Abstract of the Proceedings of the Linnean Society of New South Wales for December 28th, 1887: v.
• Longman, H. 1921. A new genus of fossil marsupials. Memoirs of the Queensland Museum 7, 65-80.

Further reading

• Aplin, K. 1983. Cheeky Giant of the Pliocene. Pp. 60-61 in Prehistoric Animals of Australia. Quirk, S. and Archer, M. (eds). Australian Museum, Sydney.
• 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.