Subhyracodon

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Subhyracodon
Skeletal reconstruction of subhyracodon

Skeletal reconstruction of subhyracodon

Temporal occurrence
Lower Eocene ( Priabonian ) to Upper Oligocene ( Rupelian )
38 to 28.1 million years
Locations
  • North America
Systematics
Higher mammals (Eutheria)
Laurasiatheria
Unpaired ungulate (Perissodactyla)
Rhinocerotoidea
Rhinoceros (Rhinocerotidae)
Subhyracodon
Scientific name
Subhyracodon
Brandt , 1878

Subhyracodon is an extinct genus of rhinos and livedin North Americafrom the late Eocene to the early Oligocene 37 to 29 million years ago. The genus is one of the earliest representatives of this group of mammals, but, in contrast to numerous later forms, had no horn formation. The animals reached the size of today's cattle and lived in open woodlands or alluvial forests, where they mostly fed on soft vegetable food.

features

Subhyracodon comprised small to medium-sized rhinoceros representatives, which were very similar to their partially co-living relative Trigonias , but have somewhat more modern features. They reached a head-torso length of 210 cm and a shoulder height of around 120 cm. The weight is assumed to be 600 to 830 kg. Originally, as for Trigonias, a value of 250 kg was assumed, but this figure refers to representatives that are now assigned to other rhinoceros genera.

Subhyracodon skull

The skull was up to 49 cm long and was elongated and very flat. When viewed from above, it looked wedge-shaped with little protruding and laterally flattened cheekbones . The rear of the skull rose less steeply than in Trigonias . The occiput had only a slight bulge. It was also at right angles and clearly indented when viewed from above. The nasal bone was weak and elongated, but comparatively shorter than that of Trigonias . Signs of horn formation on the surface in the form of roughened surfaces as attachment points are not known. The intermaxillary bone also had an elongated shape and, like the upper jaw, lacks the characteristic shortenings of the more modern rhino shapes. Between these two bones and the nasal bone there was a large and extensive nasal space. The median jawbone and the nasal bone were not connected.

The lower jaw reached 40 cm in length and had a rather delicate structure with a weak, but up to 8 cm long symphysis . The maximum height of the jawbone was 4.8 cm. Opposite trigonias is already clearly reduced the teeth, the dental formula was for a full-grown animal: . The incisors were rather small and stood vertically in the upper jaw, in the lower jaw obliquely forward at an angle of 45 °. The exception was the lower second (outer) incisor, which was conical in shape and significantly enlarged, so that it reached a length of 2.4 cm. One canine was not developed. The diastema to the posterior teeth was up to 2.4 cm. The molars were very low-crowned ( brachyodont ), the premolars resembled the molars except for the first . The second was completely molarized, the two rear ones strongly molarized. The second molar was the largest tooth in the row of molars.

The body skeleton is also well known due to the numerous, partly complete skeletons. The spine consisted of 7 cervical, 18 thoracic, 5 lumbar, 6 sacrum and 21 caudal vertebrae. The tapir-like curved arrangement of the thoracic vertebrae was compensated for by the length of the spinous processes , which form an almost straight line. Overall, these were much slimmer than Trigonias . The spinous process of the first thoracic vertebra reached 16 cm in length. The humerus was also slimmer than the related Trigonias and had a length of 30 cm. The shaft of the 28 cm long spoke is clearly curved, while the ulna , 35 cm long, exceeded the upper arm. At 37 cm, the thigh bone was the largest long bone in the skeleton. The shin and fibula were not fused together. The former reached 28 cm, the latter 26 cm in length. Hands and feet each ended in three toes ( tridactyl ), making Subhyraracodon much more modern than Trigonias . As with all odd-toed ungulates , the main ray was the third, the metacarpal bone (Metacarpus III) was 14 cm and the metatarsal bone (Metatarsus III) 13 cm long.

Locations

Live reconstruction of subhyracodon by Charles R. Knight

Subhyracodon has so far only been found in North America , where the rhinoceros species has been found with numerous skeletal remains in the high plains . Significant fossils have been identified from the Chadron Formation in South Dakota , the Badlands National Park is significant here . There they are from the "Lower Titanotherium -bed" ( Lower Titanotherium bed ), a sequence consisting of volcanic and siliceous sediments (aeolian and fluviatil) inside molded by wind and water deposits, which the middle and late Eozän belong. Here Subhyracodon appears together with its relative Trigonias , but there are also frequent finds of the early horse Mesohippus and the huge, almost 2 t heavy Brontop , which belongs to the Brontotheria (originally Titanotheria). Other finds, including a mandible and postcranial skeletal parts, came to light in the Horsetrail Creek member of the Brule Formation in Logan County , Colorado . Other sites are in Saskatchewan and California . The most recent finds from the latter US state date to the Lower Oligocene and were discovered in the Vespe Formation in Ventura County .

Paleobiology

Most of the finds so far come from sites relatively close to water. Subhyracodon may have lived in riparian forests or in the transition areas to more open landscapes. The low-crowned teeth indicate a relative specialization in soft plant foods, but studies also showed a certain proportion of hard plant foods. It is believed that the rhinoceros genus ate fibrous plant parts and water-containing leaves. Further analyzes of isotopes of oxygen in the molars revealed a very high dependence on water, but the animals were not bound to a semi-aquatic life. This is shown by studies carried out in parallel on carbon isotopes , which speak for a life in more open landscapes, with which the animals probably preferred low trees and bush vegetation as a source of food.

Systematics

Internal systematics of the North American rhinos according to Prothero 2005
  Rhinocerotidae  

 Uintaceras


   

 Teletaceras


   

 Penetrigonias


   

 Trigonias


   

 Amphicaenopus


   

 Subhyracodon


   

 Diceratherium


   

 Skinneroceras


   

 Menoceras


   


 Floridaceras


   

 Aphelops


   

 Galushaceras


   

 Peraceras





   

 Teleoceras




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Subhyracodon is an extinct genus of rhinos and is part of the primitive subfamily of the Diceratheriinae . These were mainly common in North America. Within the subfamily, the rhinoceros genus belongs to the Dicerathini tribe . The namesake for both the subfamily and the tribe is Diceratherium , which was characterized by two horns on the nose.

Several types of subhyracodon have been described, the following are valid today:

  • S. kewi Stock , 1933
  • S. mitis ( Cope , 1875)
  • S. occidentalis ( Leidy , 1850)

Here, S occidentalis the type of shape and was larger than the other two species. The most recent form from the late Oligocene is S. kewi . Other species described, such as S. trigonodus , S. metalophus , S. gridleyi and S. hesperius are now considered synonyms of S. occidentalis . The rhinoceros representative originally referred to as S. tridactylum is now assigned to Diceratherium .

Johann Friedrich von Brandt

The rhinos discovered in North America were originally assigned to the genus Rhinoceros by the American paleontologist Joseph Leidy in 1850 , and in the following year he assigned them to Aceratherium . The first description of subhyracodon was made in 1878 by the German naturalist Johann Friedrich von Brandt , of the taxon but as a subgenus of the rhino species mite aceratherium and aceratherium occidentalis looked. The publication was in Latin and was published by a Russian publisher. In 1881 the American zoologist Edward Drinker Cope assigned Aceratherium mite to the genus Caenopus , which he had newly named ; it is believed that Cope was unfamiliar with Brandt's description. American researchers used the term Caenopus for almost half a century , but in 1927 Horace Elmer Wood pointed out in 1927 that subhyracodon takes precedence as it is the older name and raised subhyracodon to the genus level. However, it was still some time before the name caught on in North America as it was widely viewed as poorly chosen. The name Subhyracodon suggests a close position to the Hyracodontidae , a group related to the rhinos, but Brandt had clearly described a representative of the rhinos. The name is from the Latin prefix sub ( "under") and the scientific name for Hyracodon together, which in turn connect from the current name for the hyrax ( Hyrax , actually Hyracoidea) and the Greek word ὀδούς ( odoús "tooth" ) represents. Caenopus, on the other hand, comes from the Greek words καινος ( kainos "new") and πούς ( poús "foot") and refers to the more modern, tridactyle hand or forefoot design of the rhinoceros genus.

Subhyracodon first appeared in the late Eocene around 37 million years ago (locally stratigraphically outgoing chadronium ). It is one of the oldest rhinos in North America, only Teletaceras and Uintaceras are older . The oldest representative is S. ociidentalis . In the Lower Oligocene (locastratigraphically lower Arikareeum ) about 30 million years ago, the rhinoceros genus died out and was replaced by Diceratherium . At that time it was possibly only distributed with S. kewi in what is now the western part of North America. Thereby forming subhyracodon with diceratherium likely precursor forms of the later modern Rhinocerotinae to which they belong today's rhinos.

Web links

Commons : Subhyracodon  - collection of images, videos and audio files

Individual evidence

  1. ^ A b Alfred J. Mead and William P. Wall: Dietary Implications of Jaw Biomechanics in the Rhinocerotoids Hyracodon and Subhyracodon from Badlands National Park, South Dakota. National Park Service Paleontological Research 3, 1998, pp. 18–22 ( [1] )
  2. Christine M. Janis: Evolution of horns in ungulates - ecology and palaeoecology. Biological Review (Cambridge Philosophical Society) 57 (2), 1982, pp. 261-317
  3. ^ A b c d e William Berryman Scott: Part V: Perissodactyla. In: William Berryman Scott, Glenn Lowell Jepsen, and Albert Elmer Wood (Eds.): The Mammalian Fauna of the White River Oligocene .. The American Philosophical Society, Philadelphia, Pennsylvania, 1941, pp. 775-821
  4. a b Wendy A. Schultz: Body size evolution in Leptomeryx and Rhinocerotinae (Subhyracodon and Trigonias) across the Eocene-Oligocene (Chadronian-Orellan) boundary. University of Nebraska, Lincoln, 2009
  5. a b c d e f g Donald R. Prothero: The evolution of North American rhinoceroses. Cambridge University Press, 2005, pp. 1-219
  6. a b Alessandro Zanazzi and Matthew J. Kohn: Ecology and physiology of White River mammals based on stable isotope ratios of teeth. Palaeogeography, Palaeoclimatology, Palaeoecology 257, 2008, pp. 22-37
  7. Kurt Heissig and Oldřich Fejfar: The fossil rhinos (Mammalia, Rhinocerotidae) from the Lower Miocene of Tuchorice in northwestern Bohemia. Sborník Národního Muzea v Praze. Acta Musei Nationalis Pragae (series B, Natural History) 63 (1), 2007, pp. 19-64
  8. Horace Elmer Wood: Some early tertiary rhinoceroses and hyracodonts. Bulletin of American Paleontology 13 (50), 1927, pp. 166-265
  9. 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