Rodents do not appear in the fossil record until about ten million years after the last non-avian dinosaur became extinct. However, the most diverse group of mammals co-existing with the dinosaurs in the Northern Hemisphere were the rodent-like multituberculates. In the Southern Hemisphere, there are only a very small number of multituberculate fossils known from South America, Madagascar, India, and Australia, and some of those records are doubtful.

 

The most advanced group of multituberculates were the cimolodontans, which thrived in the Northern Hemisphere beginning about 115 million years ago and only finally went extinct 30 million years after the dinosaurs had. 

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A single tooth formed the basis for the naming of a multituberculate from Australia, Corriebaatar marywaltersae, in 2009. Provisionally referred to the cimolodontans, the name was in honour of two Museums Victoria/Monash University volunteers, Dr. Corrie Williams who had found one of the few multituberculate teeth known from South America and Mary Walters, who had found the first Australian multituberculate fossil.

 

In 2017, a third Museums Victoria/Monash University volunteer, Wendy White, found a second specimen of Corriebaatar. Being more complete, that second fossil demonstrates unequivocally that Corriebaatar is a cimolodontan. This additional specimen, which included an almost complete lower jaw, thus formed the basis for a second, more detailed look at Corriebaatar.

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Recent dating by Dr Barbara Wagstaff, based on a study of fossil pollen in the deposit near Inverloch where the two specimens of Corriebaatar were collected, indicates that their age is about 15 million years older than the oldest cimolodontan known from the Northern Hemisphere.

 

A general view amongst palaeontologists is that the major groups of mammals originated in the Northern Hemisphere and dispersed to the Southern. The basis for this was the insight, advocated by Darwin amongst others, that the largest contiguous land mass was the natural theatre for the processes of evolution to occur most rapidly. Groups originating there subsequently then dispersed to the periphery in this view.

 

Given that the Australian cimolodontan is about 15 million years older than the oldest known cimolodontan in the Northern Hemisphere, one can only wonder whether this general view of the geographic direction of dispersal of major mammalian groups is truly the reality. Or did cimolodontans disperse from Australia to the Northern Hemisphere?

 

The study of this second specimen of Corriebaatar was initially carried out by Dr. Tom Rich of Museums Victoria and Patricia Vickers-Rich of Monash and Swinburne universities. Subsequently, they were joined by Dr. David Krause of the Denver Museum of Nature & Science, who has been studying multituberculates for more than four decades and thus brought to the investigation insights about the group that otherwise would have been overlooked.

The study entitled “Second specimen of Corriebaatar marywaltersae from the Lower Cretaceous of Australia confirms its multituberculate affinities ” appears in the international palaeontological journal Acta Paleontologica Polonica. Pol. 67 (1): 115-134, 2022.

Three women who were critical in the discovery of the multituberculate material related to this study include:  Dr Corrie Williams (Ph.D. Monash University, Australia), Ms Mary Walters (B.A., Monash University, Australia) and Wendy White (MSc., University of Manitoba, Canada)