Just five modern species of monotreme (or egg-laying mammal) have been described:
- Platypus (Ornithorhynchus anatinus)
- Short-beaked echidna (Tachyglossus aculeatus)
- Three species of long-beaked echidna (Zaglossus bruijni, Z. bartoni and Z. attenboroughi)
The platypus lives in Australia, long-beaked echidnas are found in New Guinea, and short-beaked echidnas occur in both Australia and New Guinea.
Based mainly on fossil remains found at Lightning Ridge (in New South Wales) and Dinosaur Cove and Flat Rocks (in Victoria), monotremes appear to have been a fairly diverse and important component of the Australian mammal fauna in the early Cretaceous period (roughly 110 million years ago). Living alongside these early monotremes were dinosaurs, turtles, lungfish and the now extinct ausktribosphenid mammals. These fossils date from a time when Australia was located far south of its current position and was joined to Antarctica as part of eastern Gondwana.
The only monotreme fossils found to date outside Australia belong to Monotrematum suderamericanum , described from teeth found in Patagonia (southern Argentina) that have been dated to about 62 million years ago. It is presumed that this discovery reflects the fact that monotremes dispersed to other parts of Gondwana after evolving in Australia.
The earliest known monotreme which unequivocally resembled what we think of as a platypus (based on finding a nearly complete, platypus-like bill) has been named Obdurodon dicksoni and dates from approximately 15-20 million years ago. O. dicksoni was a bit bigger than the modern platypus and had a larger bill and more powerful jaw muscles relative to the size of its head. In contrast to the current living form, adults also appear to have retained true teeth in the form of relatively thin-enamelled, six-rooted molars. The earliest known remains of the living species have been dated to around 100,000 years ago.
Investigating the evolutionary relationship between echidnas and the platypus has been hampered by the fact that the earliest known echidna fossils are only about 13 million years old. Based on patterns of genetic divergence, it has been hypothesized that the two groups began evolving independently as recently as 19-48 million years ago. Ironically, one of the most primitive physical features of monotremes – the typically reptilian design of the bones in the shoulder region – may explain why both the platypus and echidnas have survived so well. Although their limbs extend out from the body in a nearly horizontal plane and are primarily limited to rotational movements, the structure of their shoulder girdle also provides exceptional strength and ability when swimming (platypus) or digging (echidnas).
Flannery, T.F. and Groves, C.P. (1998). A revision of the genus Zaglossus (Monotremata, Tachyglossidae), with description of new species and subspecies. Mammalia 62: 367-396.
Musser, A.M. (1998). Evolution, biogeography and palaeoecology of the Ornithorhynchidae. Australian Mammalogy 20: 147-162.
Phillips, M.J., Bennett, T.H. and Lee, M.S.Y. (2009). Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas. PNAS Early Edition. http://www.pnas.org/cgi/doi/10.1073/pnas.0904649106