Australia: The Land Where Time Began

A biography of the Australian continent 

Lower Cretaceous Mammals

Deposits from the Aptian-Albian age are the only ones in Australia that contain evidence of possible mammals. The material from the Aptian age is known only from the Wonthaggi Formation. The 4 taxa identified are Corriebaatar marywaltersae, Ausktribosphenos nyktos, Bishops whitmorei and Teinolophus trusleri. These mammals were about the size of a mouse with a skull length of about 25-30 mm, based on isolated jaws. They represent separate lineages. Corriebaatar marywaltersae, the most recent find, is a multituberculate. This group of mammals mainly from the Mesozoic was cosmopolitan and have been well-known from the Cretaceous of the Northern hemisphere. Constituting its own unique family, its recognition is highly significant as it predates other multituberculates from the Lower Cretaceous of Gondwana, being found in South America, indicating a possible migration from that continent to Australia before the cooling occurred when Australia was near the South Pole.

Ausktribosphenos and Bishops have been described as eutherians (placentals) based on several characteristics present in placentals but not in marsupials. The characteristics used in this assignment to the placentals are characteristic wear patterns on the teeth, a dentition having 3 molars with a last premolar that is molariform, whereas in marsupials there are 4 molars and a premolar that is not molariform, and the jaw joint structure is more like that seen in placentals than in marsupials. Others have suggested that they are sufficiently distinct to be given their own radiation, the Ausktribosphenida that should be placed with the monotremes in the Australosphenida, a clade that is exclusive to Gondwana, being distinct from both placentals and marsupials that are members of the Boreosphenida, a Northern Hemisphere radiation. If they are indeed placentals they would push back the arrival of placentals in Australia by more than 60 million years. The next oldest placentals in Australia, bats, are found in the Murgon site in Queensland of Palaeogene age, have been in Australia since about 54 Ma.

Teinolophos trusleri from the Wonthaggi Formation is a monotreme. Included in the Monotremata such as Kryoryctes cadburyi from the Eumeralla Formation, Steropodon galmani and Kollikodon ritchiei from the Griman Formation. It has been suggested that the monotremes probably arose in Australia in the Mesozoic, from where they migrated across Antarctica to South America in the latest Cretaceous. Ornithorhynchid monotremes were present in Patagonia in the earliest Palaeogene. The smallest known monotreme is Teinolophos trusleri with a mandible that was 25 mm long. It has an unusual jaw that suggests it probably had a powerful bite.

Monotreme taxa from the Albian were large-bodied, being about 400 mm long, that appear to have been terrestrial burrowers or aquatic. An incomplete humerus is the basis for Kryoryctes being structurally similar to those of modern echidnas, Tachyglossidae, though it appears to be more primitive but with a comparable burrowing lifestyle. Opal mines at Lightning Ridge have produced jaws of Steropodon and Kollikodon, both of which are believed to have been aquatic. Steropodon, with a molar structure that was similar to ornithorhynchids, has been placed with Teinolophos the most primitive monotremes. Based on its unusual molars that developed into bulbous crushing plates, Kollikodon has been placed in a separate group, Kollikodontidae. Kollikodon is believed to have fed on molluscs (shelled invertebrates), possibly detecting prey by use of electrosensory organs in the snout.

More unusual mammal fossils have recently been found in the Griman Creek formation and the Toolebuc Formation. Among the fossils from the  Griman Creek Formation is a bizarre tooth from what the authors3 suggest may be a dryolestid, and an incomplete jaw that is estimated to have probably been about 80 mm long with 3 molar sockets, similar to that of Ausktribosphenids. The material from the Toolebuc formation is a tiny vertebra that is marsupial-like and an incisor tooth. The authors suggest that though none can be diagnosed further at this point their recognition indicates there may be much mammal material to be discovered from the Mesozoic of Australia.





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                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading