Australia: The Land Where Time Began

A biography of the Australian continent 

The ektopodontids had teeth that were so unusual that they were originally thought to be a type of multituberculate animal, that were widespread in the Northern Hemisphere in the Mesozoic and early Cainozoic. It was eventually realised the orientation of their teeth was unlike any known multituberculate. They were referred to the monotremes when they were first studied scientifically in 1967, but they were assigned to the phalangerids in 1972 when more primitive phalangerids were discovered. Though not common in the known fossil record, they persisted until the Pleistocene. The reason for their decline is uncertain, suggestions being the arrival of rodents in the Pliocene, that would have competed strongly if the ektopodontids were in fact seed-eaters. Another suggestion is possibly a result of the contraction of rainforests during the Pleistocene (Flannery).

There were 3 genera and 5 species of ektopodontids, all of which had the characteristic short snout and forward-facing eyes, believed to indicate they were nocturnal and probably arboreal, and the distinctive molars of the group were subtriangular to rectangular. These molars had transverse ridges that were divided into patterns of radially or longitudinally arranged struts or ribs. They had structurally similar upper and lower molars with complex occlusal relations. As with other diprotodontidians, ektopodontids has 2 forward-facing incisors. The upper molars had 3-4 roots while the lower molars had 2.

Chunia is unique among the ektopodontids in having a network or radiating ribs and struts on the longitudinal cusps, 5 or 6 widely spaced cusps on the upper molars, with a narrow, shallow transverse depression between the crests. C. omega is a bit younger than C. illuminata and differs from it in having a higher number of ribs and struts. The sublophodont teeth of Chunia resemble those of cuscuses from the rainforests of Australia and New Guinea.

It is believed Chunia lived in the wet woodland that surrounded a large lake. The rodent-like incisors suggest they may have been granivores, but a wide variety of suggestions have been made concerning the diet of ektopodontids - seeds, nuts, fruit, leaves, caterpillars, and if it had an aquatic lifestyle, aquatic invertebrates.

Ektopodon stirtoni, a phalangeroid from the Etadunna Formation, Lake Palankarinna, in South Australia to the east of Lake Eyre, from the Middle Tertiary that is related to the modern brush-tailed possum.

Fossils of closely related, or conspecific animal have been found in the deposits of Lake Pinpa, Namba Formation, Tarkarooloo Basin, South Australia.

The ancestral ektopodontids are believed to have possibly been phalangerids (cuscuses and brush-tailed possums), suggesting they may have been a group of more specialised phalangerids. They were originally thought to have been related to monotremes, and another controversial suggestion is that they were related to koalas (phascolarctids). Their remains have been found in deposits from the Late Oligocene of central Australia to the Pleistocene of Victoria.

The trend in ektopodontids evolution was towards increasing complexity in the number of cusps, crests, struts and ribs.

Sources & Further reading

1. Peter F. Murray & Patricia Vickers-Rich, Magnificent Mihirungs: The Colossal Flightless Birds of the Australian Dreamtime, Indiana University Press, 2004
2. Michael Archer, Suzanne J. Hand & Henk Godthelp, 2000, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland

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• Marsupials