Dwarf thorntail squirrel ( anomalurus pusillus )
|Gervais , 1849|
The shape of a thorntail squirrel has astonishing similarities to that of a flying squirrel . Despite this outward resemblance, thorntail squirrels are neither part of the squirrels nor are they related to them. With the exception of the deviating thorntail bill, all species have a flight membrane . On closer inspection, there is a difference to the gliding squirrels: While they have a sickle-shaped bone on the wrist that tightens the gliding skin, thorntail squirrels have a widened ulna on which a cartilaginous rod is seated that takes on this function. From this it can already be seen that the gliding membrane of gliding squirrels and thorn-tailed squirrels developed independently of each other, so that it is an example of convergent evolution .
The thorntail squirrel is named after a row of hairless scales on the underside of the tail. Each of these scales has a horny thorn. The scales occupy the front third of the tail, their number is between twelve and eighteen, depending on the species. The function of this device is obviously a better hold, which the animals find in the branches when they anchor themselves with the thorns in the bark. The size varies considerably: the glide billets have a head body length of only 6 cm, while some of the largest thorntail squirrels reach dimensions of 45 cm, to which the same tail length is added.
Way of life
Thorntail squirrels are inhabitants of the rainforests, where they are perfectly adapted to a life in the treetop region with their pointed claws, the aforementioned horn scales and the sliding membrane. These animals probably only accidentally end up on the forest floor, and there they move very awkwardly. They climb a vertical tree trunk almost like a caterpillar : they hammer the claws of the front legs into the bark and then pull the back of the body afterwards.
The sliding behavior is similar to that of the flying squirrels. Thorn-tailed squirrels repel themselves from a high branch and open their sliding skin, which they then carry up to 100 m, according to unconfirmed reports even up to 250 m. Often, however, a much shorter flight is sufficient to reach a neighboring tree.
All thorntail squirrels are nocturnal . They sleep through the day in tree hollows. The food is nuts, fruits and leaves. Occasionally they also eat insects.
The classification of the thorntail squirrel in the rodent system has long been completely puzzling and is still far from being clarified. A classification with the squirrels , as the name of the group seems to suggest, was ruled out early on, since there are no similarities beyond the sliding skin, and this too is very different from that of the sliding squirrels on closer inspection. Above all, the shape of the skull is unique among rodents. In the search for similarities with other rodents, they finally found the jumping hare , completely different from the external form , which showed similarities with the thorntail squirrels in anatomical details of the middle ear and the carotid artery . This hypothesis, first proposed in 1985, was initially questioned due to a lack of fossil evidence, but is now valid thanks to the molecular genetic analyzes by Montgelard & al. as secured. Thorntail squirrels and spring hares are therefore considered sister groups and are grouped together in a common taxon, thorntail squirrel relatives (Anomaluromorpha). Both groups separated from one another very early, probably in the Eocene .
The question of which relationships the Anomaluromorpha have to other rodents is currently still in the realm of pure speculation. At the moment the taxon is largely isolated within the rodents.
Three genera of the thorntail squirrel are known:
The latter genus with only one species is very different due to the lack of a flying membrane, but due to the characteristic horny scales on the underside of the tail there is no doubt that it belongs to the thorn-tailed squirrels. Sliding and thorntail bilks are sometimes united in a common subfamily Zenkerellinae (or Idiurinae), which the Anomalurinae with the only genus Anomalurus face.
While there is no evidence of common ancestors of the spring hares and thorntail squirrels, as mentioned, fossil thorntail squirrels have been documented since the early Eocene , which makes them one of the oldest known rodent taxa. The fossils of the Zegdoumyidae, which are closely related to the thorn-tailed squirrels, are also known from the Eocene. Of the thorntail squirrels themselves, the genus Nementchamys is known from the Eocene and the genus Paranomalurus from the Oligocene and Miocene . The genera living today have been handed down since the Miocene. While all of these fossils were found in Africa, which is also home to today's thorntail squirrels, the remains of a rodent called pondaungimys have recently been found in Southeast Asia. Other finds that have not yet been assigned with absolute certainty also allow the conclusion that thorn-tailed squirrels were once at home in Asia.
- Ronald M. Nowak: Walker's Mammals of the World. 2 volumes. 6th edition. Johns Hopkins University Press, Baltimore MD et al. 1999, ISBN 0-8018-5789-9 .
- Bernhard Grzimek : Grzimeks animal life. Encyclopedia of the Animal Kingdom. Volume 11: Mammals. Part 2. Weltbild Verlag, Augsburg 2000, ISBN 3-8289-1603-1 .
- Malcolm C. McKenna, Susan K. Bell: Classification of Mammals. Revised Edition. Above the species level. Columbia University Press, New York NY 2000, ISBN 0-231-11013-8 .
- Claudine Montgelard, Sophie Bentz, Claire Tirard, Olivier Verneau, François M. Catzeflis: Molecular Systematics of Sciurognathi (Rodentia): The Mitochondrial Cytochrome b and 12S rRNA Genes Support the Anomaluroidea (Pedetidae and Anomaluridae). In: Molecular Phylogenetics and Evolution. Vol. 22, No. 2, February 2002, pp. 220-233, doi : 10.1006 / mpev.2001.1056 .