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.
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.
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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