Thingodonta
Encyclopedia
Thingodonta is the colloquial name given to a bizarre order of extinct Australia
n marsupial
s, first described in 1988 and known only from the Oligo
-Miocene
deposits of Riversleigh
, northwestern Queensland
, Australia.
A single genus
, Yalkaparidon (from an aboriginal word for boomerang
, alluding to the boomerang shape of its molar
s when seen in occlusal view, and the Greek word for tooth) and two species
, Y. coheni and Y. jonesi, have so far been described. Numerous isolated teeth and jaw
bones of Yalkaparidon are known, but only a single skull
(of Y. coheni) has so far been recovered.
These specimens of Yalkaparidon exhibit a fascinating melange of characters: the molars are zalambdodont (a distinctive tooth
type also found in the marsupial mole
Notoryctes, the living placental 'insectivore
s' Solenodon, tenrecs and golden mole
s, as well as a number of fossil
groups); the incisor
s are very large and hypselodont (open-rooted and hence ever-growing, similar to those of rodent
s); the basicranial region of the only known skull is very primitive, somewhat similar to those of plesiomorphic bandicoot
s. The zalambdodont molars appear to link it to notoryctid marsupial moles, but detailed study of the teeth of these two groups suggests that they have evolved independently, and Yalkaparidon is anatomically otherwise very different from the marsupial moles. The incisors resemble those of diprotodontians, but no other features convincingly support this relationship, and the convergent evolution
of such incisors in South American 'pseudodiprotodont' groups (such as caenolestids and polydolopimorphians) suggests that Yalkaparidon and diprotodontians may have evolved similar incisors independently. Basicranial similarities to bandicoots most likely represent shared plesiomorphic characters, and hence are not indicative of a close relationship.
For these reasons, Yalkaparidon is currently placed in its own family
, Yalkaparidontidae, and order
, Yalkaparidontia; this placement would make this the only order of Australian marsupials known to have gone extinct. However, Frederick Szalay suggested in his 1994 book 'Evolutionary History of the Marsupials and an Analysis of Osteological Characters' that Yalkaparidon is indeed a diprotodontian (as evinced by its incisors), albeit one that retains a highly primitive basicranium.
The exact function of its unusual dentition
remains obscure, and suggestions that it may have fed on worm
s (based on the similarities of its molars to those of worm-eating tenrecs), caterpillar
s or egg
s are tenuous. However, its source of food presumably had a hard outer covering (necessitating use of the large incisors) but relatively soft interior, as zalambdodont molars cannot crush food items. The possibility that it was a "mammalian woodpecker" similar to the Aye-aye
and Striped Possum
has been raised.
Future insights into the evolutionary relationships, functional morphology
and lifestyle of Yalkaparidon must await more detailed study. It remains one of the most enigmatic members of Australia's unique mammalian fauna
.
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
n marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s, first described in 1988 and known only from the Oligo
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
-Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
deposits of Riversleigh
Riversleigh
Riversleigh, in North West Queensland, is Australia's most famous fossil site. The 100 km² area has fossil remains of ancient mammals, birds and reptiles of Oligocene and Miocene age...
, northwestern Queensland
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...
, Australia.
A single genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, Yalkaparidon (from an aboriginal word for boomerang
Boomerang
A boomerang is a flying tool with a curved shape used as a weapon or for sport.-Description:A boomerang is usually thought of as a wooden device, although historically boomerang-like devices have also been made from bones. Modern boomerangs used for sport are often made from carbon fibre-reinforced...
, alluding to the boomerang shape of its molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
s when seen in occlusal view, and the Greek word for tooth) and two species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, Y. coheni and Y. jonesi, have so far been described. Numerous isolated teeth and jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
bones of Yalkaparidon are known, but only a single skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
(of Y. coheni) has so far been recovered.
These specimens of Yalkaparidon exhibit a fascinating melange of characters: the molars are zalambdodont (a distinctive tooth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
type also found in the marsupial mole
Mole (animal)
Moles are small cylindrical mammals adapted to a subterranean lifestyle. They have velvety fur; tiny or invisible ears and eyes; and short, powerful limbs with large paws oriented for digging. The term is especially and most properly used for the true moles, those of the Talpidae family in the...
Notoryctes, the living placental 'insectivore
Insectivore
An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....
s' Solenodon, tenrecs and golden mole
Golden mole
Golden moles are small, insectivorous burrowing mammals native to southern Africa. They form the family Chrysochloridae, and are taxonomically distinct from the true moles which they resemble due to convergence...
s, as well as a number of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
groups); the incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s are very large and hypselodont (open-rooted and hence ever-growing, similar to those of rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s); the basicranial region of the only known skull is very primitive, somewhat similar to those of plesiomorphic bandicoot
Bandicoot
Bandicoots are a group of about 20 species of small to medium-sized, terrestrial marsupial omnivores in the order Peramelemorphia.- Etymology :...
s. The zalambdodont molars appear to link it to notoryctid marsupial moles, but detailed study of the teeth of these two groups suggests that they have evolved independently, and Yalkaparidon is anatomically otherwise very different from the marsupial moles. The incisors resemble those of diprotodontians, but no other features convincingly support this relationship, and the convergent evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of such incisors in South American 'pseudodiprotodont' groups (such as caenolestids and polydolopimorphians) suggests that Yalkaparidon and diprotodontians may have evolved similar incisors independently. Basicranial similarities to bandicoots most likely represent shared plesiomorphic characters, and hence are not indicative of a close relationship.
For these reasons, Yalkaparidon is currently placed in its own family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
, Yalkaparidontidae, and order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
, Yalkaparidontia; this placement would make this the only order of Australian marsupials known to have gone extinct. However, Frederick Szalay suggested in his 1994 book 'Evolutionary History of the Marsupials and an Analysis of Osteological Characters' that Yalkaparidon is indeed a diprotodontian (as evinced by its incisors), albeit one that retains a highly primitive basicranium.
The exact function of its unusual dentition
Dentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...
remains obscure, and suggestions that it may have fed on worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s (based on the similarities of its molars to those of worm-eating tenrecs), caterpillar
Caterpillar
Caterpillars are the larval form of members of the order Lepidoptera . They are mostly herbivorous in food habit, although some species are insectivorous. Caterpillars are voracious feeders and many of them are considered to be pests in agriculture...
s or egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
s are tenuous. However, its source of food presumably had a hard outer covering (necessitating use of the large incisors) but relatively soft interior, as zalambdodont molars cannot crush food items. The possibility that it was a "mammalian woodpecker" similar to the Aye-aye
Aye-aye
The aye-aye is a lemur, a strepsirrhine primate native to Madagascar that combines rodent-like teeth and a special thin middle finger to fill the same ecological niche as a woodpecker...
and Striped Possum
Striped Possum
The Striped Possum is a member of the Petauridae family, one of the marsupial families. The species is black with three white stripes running head to tail, and its head has white stripes that form a 'Y' shape...
has been raised.
Future insights into the evolutionary relationships, functional morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
and lifestyle of Yalkaparidon must await more detailed study. It remains one of the most enigmatic members of Australia's unique mammalian fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
.