Skinneroceras
| Skinneroceras | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Early to Oligocene (Early Arikareeum ) | ||||||||||||
| 29.7 to 28 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Skinneroceras | ||||||||||||
| Prothero , 2005 | ||||||||||||
Skinneroceras is an extinct genus of rhinos thatlivedin North America around 30 million years ago. So far, it has only been proven by a few fossil finds, but had a relatively small body structure. The rhino shape may not have had horns.
features
Skinneroceras was a very small rhinoceros shape, about the size of Subhyracodon ; but so far it is known only from a few finds. The skull reached 41 cm in length and was relatively slender, with little protruding zygomatic arches . In side view it showed only a slight saddle on the forehead line in contrast to the more concave course of the later rhinos. The occiput was narrow and only slightly drawn out. Striking parasagittal bones that were much more pronounced than those of its close relatives began to appear on the temporal bones. The nasal bone was rather slender, but elongated and showed no signs of roughening at the point of attachment of the horns. The intermaxillary bone also had a long, narrow shape, with the nasal cavity between this and the nasal bone extending to the third premolar .
The lower jaw was 34 cm long and had a long and narrow symphysis measuring up to 6 cm . The underside of the symphysis pointed upwards at an angle, thus forming a slight “chin”. The teeth were reduced compared to those of Diceratherium and Subhyracodon , in particular the first incisor in each case was formed in the upper jaw. In the lower jaw, the second incisor was characterized by an enlarged and conically pointed shape. There was generally no canine , the existing diastema to the posterior teeth was around 4.5 cm long. The anterior premolar was very small, the three posterior ones increased in size and had a shape clearly similar to the molars , i.e. they were molarized. The rear molars largely resembled those of Diceratherium . The last molar had a slightly trapezoidal shape and was 4 cm long.
Fossil finds
Finds of Skinneroceras are rather rare. Most and most significant fossils come from the base area of Roundhouse Rock in Morrill County in the western US state of Nebraska . This belongs to the White River Formation and dates to the middle Oligocene (locally stratigraphically called the earliest Arikareeum ) almost 30 million years ago. The fossils include a complete skull with an associated lower jaw, further lower jaw fragments and the first two cervical vertebrae. Additional find material in the form of isolated molars comes from greenish-colored siltstones of the Cabbage Patch beds of western Montana , which, according to paleomagnetic measurements, belong to the transition from the middle to the late Oligocene (27.5 million years before today).
Paleobiology
The completely traditional skull together with the associated lower jaw is attributed to a female animal due to the small size of the lower second incisors and the missing horn attachment points. The small body size is atypical for post-Eocene rhinos, possibly this is to be associated with a subsequent dwarfing. In general, remains of Skinneroceras are rare, it is assumed that it was a short-lived and rarely occurring rhinoceros representative. Accompanying finds of the flora allow a habitat interspersed with sequoias and more open grasslands to be reconstructed. Based on other found remains of fauna, such as turtles and snakes , a mild climate with, compared to today's conditions, on average warmer temperatures can be assumed. In addition, the conditions were rather semi-arid , but due to seasonal changes there could also be heavier precipitation.
Systematics
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Internal systematics of the North American rhinos according to Prothero 2005
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Skinneroceras is a genus of the rhinoceros family . It belongs to the subfamily Diceratheriinae within the rhinoceros . These represent a relatively original group in terms of phylogeny, whose character form Diceratherium had two paired horns on the nasal bone. It is largely only known from North America . In addition to the two genera, Subhyracodon also belongs to this subfamily.
The genus Skinneroceras is monotypical, the only species is Skinneroceras manningi . The genus name refers to Morris Skinner , an American paleontologist who discovered the type material, and the Greek word κέρας ( kéras "horn"). The species name manningi in turn honors the American paleontologist Earl Manning and his services to research into fossil rhinos; in addition, he had first recognized that fossils belonged to an independent genus. The holotype comprises a complete skull with a lower jaw (specimen number F: AM 111843).
Individual evidence
- ↑ a b c d e f g Donald R. Prothero: The evolution of North American rhinoceroses. Cambridge University Press, 2005, pp. 1-219
- ↑ a b Kurt Heissig: The American genus Penetrigonias Tanner & Martin, 1976 (Mammalia: Rhinocerotidae) as a stem group elasmothere and ancestor of Menoceras Troxell, 1921. Zitteliana A 52, 2012, pp. 79-95
- ^ A b Donald R. Prothero and Donald L. Rasmussen: New giant rhinoceros from the Arikareean (Oligocene-Miocene) of Montana; South Dakota and Wyoming. In: Spencer George Lucas et al. (Ed.): Neogene Mammals. New Mexico Museum of Natural History and Science Bulletin 44, 2008, pp. 323-329