Australia: The Land Where Time Began
A 100 million year old monotreme, an early platypus, Steropodon galmani, is the earliest mammal found in Australia. The fossil is an opalised jaw of Cretaceous age from Lightning Ridge, New South Wales. It is from the Lower Cretaceous (Albian) Griman Creek Formation. Being about the size of a house cat it is one of the largest known Mesozoic mammals, in size it overlapped with some of the smaller dinosaurs. The well-developed teeth of this animal already display the characteristic shape of monotremes.
The only living monotremes are the platypus and 2 species of echidna.
The term monotreme refers to one hole, being the cloaca, and this is indeed one of two defining features of this group. The cloaca is a common point of exit for all urinary, defecatory and reproductive systems. All monotremes reproduce by laying eggs, and nourishing their young with milk which is secreted through mammary glands located in their skin.
The 2 types of monotreme are the platypus and the echidna. Extant species are only found in Australia and New Guinea although fossil teeth from an extinct Platypus species has also been found in Argentina, revealing that they were once distributed over Southern Gondwana.
The monotremes have a distinct gait, reminiscent of the reptilian species, due to the positioning of the legs on the side of the body as opposed to underneath it. All individuals have a 15 mm spur above the heel of each hind leg, which whilst appearing vestigial in the echidna, houses a potent toxin (produced by the crural gland in the thigh), in male platypuses. It is the only Australian mammal known to be venomous. The males will use these spurs as weapons if they feel threatened, and inject poison into their foe. Venom production peaks with the breeding season, suggesting a use in sexual competition.
The early mammalian tree split into 3 main groups (monotremes, placentals and marsupials). It is now thought that the first group to split off were the Monotremes. However, they have left us scant information in the fossil record, and what little evidence there is suggests that this group diverged in the Early Cretaceous period. After this point, it would appear that the Monotremes underwent evolution (molecular and morphological) at a much slower pace than is known for the therian mammals.
The earliest fossil find for this group were from the Lightning Ridge Opal Fields of NSW, and consisted of the lower jaw bone of a 100 million years ago species, Steropodon galmani one of the first mammalian species known from the Mesozoic Era. These finds, and other like it, indicate that Monotremes first evolved in Australia (during Late Jurassic or early Cretaceous), prior to spreading to South America and Antarctica. Both of these land masses were at this point, obviously connected to Australia.
A jawbone of a primitive platypus, Teinolophos trusleri, that had already evolved the flat beak and aquatic lifestyle, has been found at Flat Rocks, Victoria,. It has been dated to the Aptian part of the Lower Cretaceous, about 123Ma, a time when it was believed the early mammals were unspecialised insectivores that survived by staying insignificant, keeping out of the way of the ruling dinosaurs. Maybe it could evolve on the then Australian Peninsula of Gondwana, where for some reason that is still debated, dinosaurs were late arriving, and possibly in lower numbers than elsewhere in Gondwana. See the Peninsula Effect.
A fossil platypus from the Early Miocene, Obdurodon dicksoni, has been found in the Riversleigh Fossil Site, northwest Queensland. Another species, Obdurodon insignis, from the Oligocene deposit at Lake Palankarinna, and also from an Oligocene deposit in the Lake Frome Embayment. Unlike the vestigial teeth of the modern platypus, the Obdurodon species had well developed teeth of a distinctive shape. Apart from these teeth, the central Australian deposits have only produced isolated teeth, a fragment of a lower jaw and a part of a pelvis.
The Australian fossil record also includes some specimens of Giant Echidnas from the Pleistocene era. These were as large as sheep, and have been found in Western Australia (in Mammoth Cave), and constitute the largest monotremes ever discovered. (Zaglossus hacketti).
The extant platypus is the only surviving descendant of Obdurodon. It had specialised its dentition to the point where its teeth have almost disappeared completely, as well reduced its size, and simplified most of its cranial anatomy. It is a sad fact of evolution that when an animal specialises in a particular direction, as the platypus has, it tends to be found to be in decline. The present populations of the platypus seems to be sufficient to believe it is not heading for extinction, but the habitat is has specialised to live in, permanent river systems, would no doubt be safe if humans were not present on the continent, but not when the rivers are being affected by deforestation in both rainforests and open forests, and the addition of all sorts of pollution, such as fertilisers and pesticides, and other chemicals used by the people living along the rivers it inhabits. One result of deforestation is the increase in the destructiveness of floods that can destroy the burrow systems of the platypus. Any surviving platypuses might dig a new burrow system, but in the mean time they would be exposed to predation.
Letters to Nature Nature 377, 418 - 420 (05 October 2002); doi:10.1038/377418a0
Kollikodon ritchie of the family Kollikodontidae (Montremata), from Lightning Ridge, New South Wales, is Australia's second mammal from the Mesozoic. The morphology of its molars was extremely bunodont (premolars or molars having cusps that are low and rounded). The presence of 2 families of monotreme in the Early Cretaceous, that differed distinctly from each other, indicates that the Monotremata arose long before the Cretaceous. At least in the Australian part of what was then eastern Gondwana it was very diverse. As 4 families are known from Australia, it seems likely that the Monotremata arose and radiated in the Australian and/or adjacent portion of Antarctica. Later a single dispersal took the ornithorhynchid monotremes to South America some time during or prior to the Palaeocene. It has been suggested that the kollikodontids are a sister group of the steropodontid-ornithorhynchid-tachyglossid clade. Both K. ritchie and Steropodon galmani are among the largest mammals known from the Mesozoic
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