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Monotreme Fossils


The time and place of monotreme origin is largely unknown. Most fossils have been found in Australia from the Cretaceous, however, in 1992 a platypus tooth was discovered in Argentina, suggesting that they were once distributed across Southern Gondwana.

The first monotreme fossil ever found in the 1970s and was Tertiary in age. Over the years a number of this animal's bones have been uncovered, identifying it as an extinct form of Platypus, Obdurodon insignis. More recently an almost compete skull has been discovered housing many teeth, from New South Wales. This new species was named Obdurodon dicksoni and proves that in extinct forms of Platypus teeth were present, implying that their loss is a derived feature. The snout of this species was similar to that of the extant form, however, questions remain over whether it was aquatic as a consequence of it's close association with a fossilised terrestrial fauna.

Two platypus skulls. The larger is from an extinct Miocene genus, the Obdurodon. The smaller skull is that of the modern day platypus. Ornithorynchus anatinus. Taken with permission from The Rise of Mammals, Benton 1991.

 

The oldest fossil monotremes come from the Lightening Ridge opal fields of New South Wales, Australia. A 100 million year old opalized jaw fragment from the monotreme Steropodon galmani has been found containing three distinctive teeth remarkably similar to those of the modern juvenile platypus. From the dimensions of the lower jaw it is proposed that the animal was about the size of a cat, this would therefore make it the largest mammal to have existed during the Mesozoic.
A second jaw was then also found in deposits of a similar age and identified as being that of Kollikodon ritchiei. These two fossils suggest that the mammalian Order had already separated and diversified by the Early Cretaceous.

 

Representation of the size of the extinct echidna Megalibgwilia ramsayi. Reconstruction taken from Naracoorte Caves website.

During the Miocene and Pleistocene the existence of giant echidnas is evident. Three extinct species are known; two assigned to the genus Megalibgwilia. Several practically complete skulls have been found in caves in South Australia of the species Megalibgwilia ramsayi.


 




Author: Catrin Roberts
Last updated: 23.11.83
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