The Riversleigh rainforest koala (''Nimiokoala greystanesi'') is an extinct

marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...

, closely related to the

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

koala The koala or, inaccurately, koala bear (''Phascolarctos cinereus''), is an arboreal herbivorous marsupial native to Australia. It is the only extant representative of the family Phascolarctidae and its closest living relatives are the ...

, that inhabited northwestern

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , established_ ...

in the early-middle

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...

(23–16 million years ago). Along with

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

sister A sister is a woman or a girl who shares one or more parents with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to refer to ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

Litokoala ''Litokoala'' is an extinct genus of marsupials, and along with '' Nimiokoala'', is closely related to the modern koala. The three genera may have diverged at an earlier date, although the drying of the continent and the expansion of ''Eucalyptu ...

'', the Riversleigh rainforest koala is the smallest representative of

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

'' Phascolarctide''.Black, K. (2014) 1192. Based on

cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived ch ...

analysis, ''Nimiokoala'' is one of the more

basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is nec ...

genera of ''Phascolarctide''. It died out due to

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

rendering the environment more arid. It probably had a more generalized diet than that of the modern species, but its exact food preferences are unknown.

Etymology

The generic name, ''Nimiokoala,'' is derived from the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

word ''Nimio'' "excessive" referring to its complex molar morphology relative to other koala species. The specific name, ''greystanesi,'' honors Greystanes High School.

History of research

As of 2013, the fossil record of extinct koalas consists of 163 specimens across 58 deposits in Riversleigh; 55 specimens are attributed to the Riversleigh rainforest koala. To date, a partial

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, t ...

has been found along with several lower jaws and isolated teeth. On the basis of these fossils, the dental apparatus of the animal has been completely restored. The species was named in 1997. Specimen QMF30482 was assigned as a

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

; its bones are stored in the

Queensland Museum The Queensland Museum is the state museum of Queensland, dedicated to natural history, cultural heritage, science and human achievement. The museum currently operates from its headquarters and general museum in South Brisbane with specialist m ...

Description

In the absence of postcranial fossils, the dimensions of the Riversleigh rainforest koala were calculated from measurements of its surviving teeth. It is estimated to have body length of about 25–30 cm (9.8-11.8 in), and a weight of about 3.5 kg (7.7 lb), one third the size of modern koalas and more than 10 times smaller than the largest known representative of ''Phascolarctide'' ('' Phascolarctos yorkensis''). Its muzzle was more prominent than that of modern koalas, resembling the

possum Possum may refer to: Animals * Phalangeriformes, or possums, any of a number of arboreal marsupial species native to Australia, New Guinea, and Sulawesi ** Common brushtail possum (''Trichosurus vulpecula''), a common possum in Australian urban a ...

snout. The teeth of the Riversleigh rainforest koala are selenodontal (crescent-shaped), with a numerous cusps and accessory shearing blades. The fossilized skull contains large

orbits In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a ...

and very large auditory bulliae relative to its size.

Ecology and behavior

In the early-middle Miocene, Riversleigh was covered with

tropical forests Tropical forests (a.k.a. jungle) are forested landscapes in tropical regions: ''i.e.'' land areas approximately bounded by the tropic of Cancer and Capricorn, but possibly affected by other factors such as prevailing winds. Some tropical fore ...

. More open areas, with karst soil, occurred at forest edges or freshwater streams and lakes. As the climate became more arid, with a more pronounced change of seasons, small species of koalas died out, including the Riversleigh rainforest koala. The small size of ''Nimiokoala'', which requires a proportionately more intensive diet, and large eye sockets, which indicate good night vision, suggest that this animal was much more mobile than the modern koala. The structure of the ear of ''Nimikoala'' corresponds to that which can be observed in modern koalas; in conjunction with the large auditory bulliae, it can be concluded that the Riversleigh rainforest koala was sensitive to and used low-frequency sounds for communication, including to attract mates.

References

{{Taxonbar, from1=Q1189148, from2=Q42602605 Pleistocene mammals of Australia Prehistoric vombatiforms Pleistocene marsupials Koalas Riversleigh fauna Fossil taxa described in 1997