Australia: The Land Where Time Began
Thingodintans - Yalkaparidontids See Australian Palaeohabitats Inferred from Mammal History
This marsupial group are an enigma. They appeared in the Oligocene-Miocene fossil record in the Riversleigh fossil deposit, but their previous fossil record is unknown. They went as suddenly as they appeared. Their known fossil record begins and ends in Riversleigh. They are known from whole skulls, and this has shown them to be highly specialised feeders. The high degree of specialisation requires a long period of evolution, but their connection to any possible ancestral line is unknown. Because the head is the only part of the body known, what their lifestyle was like can only be speculated on, particularly as there are no known animals they can be compared to.
There is only 1 genus known. The name of the genus, Yalkaparidon derives from a aboriginal word meaning boomerang, referring to the occlusal shape of their molars. A single skull of one of the 2 known species, Yalkaparidon coheni, the other being Yalkaparidon jonesi, but many isolated teeth have been found. The skull shows a bizarre mixture characters, molars of a marsupial mole, the rodent-like incisors of a diprotodont, the skull base like that of a type of bandicoot. It is reminiscent of the mixture of characters found in the platypus, though not as extreme.
No known living mammal has a dentition like that of this group. The molars are zalamdodont, a tooth type found in the marsupial mole, Notoryctes, placental insectivores Solendon, tehrecs and golden moles, and some fossil groups. The very large incisors are of a type called hypselodont, similar to those of rodents the roots being open-ended, so grow continuously. The basicranial region of the skull shows some similarity to that of plesiomorphic bandicoots. The Madagascan Streaked Tenrec has the closest teeth to that of Thingodontans.
The molars appear to link them to the marsupial mole, but when the teeth were closely studied they were seen to have evolved independently. The known skull is very different from that of the marsupial mole. The incisors are like those of the diprotodonts, but the rest of the skull is different. Convergent evolution has occurred in the pseudodiprotodontid groups in South America, such as caenolestids and polydolopimorphians, suggest that they may also have evolved their incisors independently of the diprotodontids. The basicranial similarities with bandicoots appears to be shared plesiomorphic characters, in which case the 2 groups may not be closely related.
The food source of the Yalkaparidontids can only be speculated about. The incisors suggest it ate tough food, as rodents can, but its molars suggest it ate soft food. Suggestions for food items are worms, eggs or caterpillars. It appears to have eaten food that had a hard outer surface and a soft interior. If the post-cranial skeleton is discovered it may give a clue about what sort of food it may have eaten. It has been suggested that if they ate worms it might explain the absence of long-beaked echidnas from the Riversleigh deposits, the echidnas flourishing only when the thingodontans became extinct. In New Guinea the long-beaked echidnas are specialised as worm feeders.
The group has been placed in its own family Yalkaparidondontidae, order Yalkaparidontia. If this is correct they would be the only marsupial order known to have gone extinct. At least one author has claimed that based on osteological analysis, it is a diprotodontian with a particularly primitive basicranium.
They are a very ancient group of Australian mammals that probably had differentiated while Australia was still part of Gondwana. There are a number of suggestions as to why no record of them have been found elsewhere in Australia, such as the Oligocene-Miocene deposits of central Australia. One suggestion is that the continent was divided into zoogeographic regions separated, at least partially, by ecological barriers, separating the northern areas, such as Riversleigh, from the central areas. This might explain the degree of endemism that is apparent at Riversleigh, for at least some of the components of the local fauna. Among the Riversleigh genera that appear to display high levels of endemism are Yalkaparidon, Yingabalanara, Ekaltadeta and Wabularoo. Some were present in 2 regions, Wakaleo, Wakiewakie, Namilamadeta, and Ngapakaldia. It is possible the tropical rainforests of the north were refugia for archaic groups like Thingodonta, which may become extinct elsewhere in the central parts of the continent.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading