Aktautitan
| Aktautitan | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Middle Eocene ( Irdinmanhum ) | ||||||||||||
| 46.5 to 41.1 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Aktautitan | ||||||||||||
| Mihlbachler , Lucas , Emry & Bayshahsov , 2004 | ||||||||||||
Aktautitan represents a representative of the Brontotherien , a primitive group of the odd ungulates . It lived in the Middle Eocene between 46 and 41 million years ago in what is now Central Asia and has been identified via two almost complete skeletons from eastern Kazakhstan . Here was one Aktautitan the largest known Brontotherien and had two small bony horns, but pointed, in contrast to other major forms significantly reduced in length limbs on, giving it a hippo-like gave body shape. To what extent the Brontotherien representative also practiced a similar, semi-aquatic way of life is not clear. A speciesis known with Aktautitan hippopotamopus .
features
Aktautitan was one of the largest known representatives of the Brontotherium and was comparable to Megacerops or Embolotherium in terms of size and body weight . Like these, it had a massive physique reminiscent of rhinos , but its very shortened limbs, reminiscent of those of hippos , were characteristic.
The massive skull reached a length of 74 to 85 cm. In contrast to the related embolotherium, the zygomatic arches were not very strongly protruding laterally, but rather parallel and rather slim in shape. The forehead line was clearly saddled and corresponds to that of other horn-bearing Brontotheria. However, the occiput did not yet have the strong extension of the later Brontotherien and was designed rather at right angles. The characteristic bony horn, formed from outgrowths from the posterior end of the nasal bone and overlapping those of the anterior part of the frontal bone , formed a raised bone ridge that began above the orbit and protruded forward at an angle of 45 °. On the uppermost point of this back, which was more than 20 cm away from the eye window, there was a pair of clear, rounded bone swellings. The sloping bone ridge was possibly a pre-form of the massive, battering ram-like horn formation in Embolotherium . The anterior end of the nasal bone fell off clearly in front of the horns and was relatively thin. The eye window itself was above the second molar . The interior of the nose between the nasal bone and the median jawbone was extremely extensive and reached down to the last premolar . Parasagittal bones running laterally on the parietal bones were also typical of Aktautitan .
The up to 66 cm long lower jaw was strong but narrow and up to 11 cm high in the area of the last molar. The symphysis extended to the last premolar. The teeth had the full Säugetierbezahnung with the following dental formula of: . The incisors had a spherical to clearly conical shape and were rather small. They formed a closed dental arch, which was closed off by the canine tooth , which was also conical-pointed but varied in size . Behind it was a small diastema about 2 cm long. The teeth of the rear row of teeth increased in size from front to back and were equipped with low dental crowns ( brachyodont ). The two posterior premolars were clearly molarized, similar to the posterior molars. These also showed significant longitudinal stretching, so that the last molar reached a length of up to 12 cm. The W-shaped course of the tooth enamel on the chewing surface of the upper molars was also striking, which is a general characteristic of Brontotheria.
The postcranial skeleton is largely known in full, but the spine in particular has only been passed on in a highly fragmented manner due to the preservation conditions. The musculoskeletal system showed clearly shortened limbs. The shoulder blade was elongated and clearly triangular, the humerus was short and massive and clearly constricted on the shaft. The length was 44 cm and was therefore significantly shorter than the comparably large Megacerops , whose upper arm was 61 cm long. The radius and ulna had a long, narrow, uncurved shape, the former being 28 cm long. The thigh bone was the longest limb bone with 51 cm, here too the differences to Megacerops with 81 cm are considerable. The lower end of the thigh had a typical asymmetry of the knee joint , comparable to other heavily built mammals, but did not achieve the same clarity as in the rhinoceros. The shin also showed compression and was 35 cm long. The front limbs ended in four-rayed hands, the back in three-rayed feet, typical of primitive odd- toed ungulates . The middle ray ( Metapodium III) was most strongly developed, the Metacarpus III 12 cm, the Metatarsus III 11 cm long; the comparable elements were almost twice as long in Megacerops .
References
Finds of Aktautitan have so far only been discovered in eastern Kazakhstan . They come from the Aktauberg near Kyzyl Murun in the Ili basin south of Lake Balkhash , which is one of the southern foothills of the Djungarian Alatau . There they were discovered in the Kyzylbulak Formation , a 150 m thick rock deposit made up of alternating sand , silt and clay , which belongs to the Middle Eocene (locally stratigraphically called Irdinmanhum ). The fossils concentrated on a 50 cm thick layer of clay / silt stones of greenish color, which was called the brontothere bone bed . This fossil deposit had already been discovered in 1996, and individual bones could also be recovered. Two largely complete skeletons and a third skull were finally discovered.
Paleobiology
As with other Brontotheria also known, with Aktautitan a certain sexual dimorphism can be proven on the basis of the canines. Possibly in males these were designed much larger than in females and protruded prominently over the incisors. This canine tooth dimorphism, which was pronounced in genera with small or nonexistent horns, such as Parvicornus or Rhinotitan , was partially lost in later forms with large horn formations in favor of a dimorphism of these horns.
The sediments in which the skeletons were found are made up of fine and thin layers that show no evidence of soil formation . They are interpreted as the deposit products of a shallow, stagnant body of water or swamp in which the animals perished. In an area of fine-grained sandstone only 20 cm above the Brontotherien find layer, about 100 footprints were found showing the footprints of large, quadruped animals. They have a diameter of around 20 cm and are around 10 cm deep. The distance from the front to the rear leg is 120 cm, the lateral distance between the legs about 40 cm. Due to the structure and size of the limbs, they may come from Aktautitan , but it would also be possible to assign them to the Sharamynodon , a large rhinoceros relative belonging to the Amynodontidae, which was also found there .
The remarkably short limbs suggest whether Aktautitan followed a semi-aquatic way of life comparable to the hippos . Such a body structure, which is similar to the hippopotamus, has arisen several times and independently within the ungulates and is called hippo-ecomorph . It is believed that it developed on the basis of comparable ecological demands, for example it is also found in some extinct rhinos such as Teleoceras and Chilotherium, but also in the tapirs . However, this did not always lead to a water-related way of life, as the fossil rhinos, for example, developed short legs in order to be able to take food from the ground as highly specialized grazers ( grazer ) with horizontal head postures. Aktautitan , however, like other Brontotherien and as shown by macroscopic traces on the teeth, was a leaf eater ( browser ). Possibly the short legs helped locomotion in swampy or soft to damp terrain, as they, in contrast to long limbs, reduced the sometimes fatal stuckness.
Systematics
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Internal systematics of the Embolotheriita according to Averianov et al. 2018
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The genus Aktautitan belongs to the family of Brontotheriidae (originally Titanotheriidae), which is a group of mammals from the order of the odd-toed ungulates that is now extinct . Due to certain dental morphological features, the family is considered distant relatives of today's horses . Within the Brontotheriidae, Aktautitan is a member of the subfamily of the Brontotheriinae and the intermediate tribus of the Embolotheriita. In a first analysis, Matthew C. Mihlbachler placed Aktautitan in the subfamily of the Embolotheriinae defined by Henry Fairfield Osborn . This subfamily was originally introduced in 1929 only for Embolotherium . In 2008, however, Matthew Mihlbachler moved this to the level of the intermediate tribus. Today, in addition to Embolotherium and Aktautitan , the Embolotheriita include other large genera such as Metatitan and Gnathotitan , which mostly had a former Asian distribution. They face the intermediate tribus Brontotheriita with Megacerops as a sister group.
The first description of the genus Aktautitan was made by Matthew C. Mihlbachler and research fellow in 2004. In previous publications the remains were considered to Brontotherien genus Titanium construed belong, but which differs by a differently structured front teeth and a different structure of the bony horns. The only recognized species is the Aktautitan hippopotamopus . The generic name Aktautitan refers on the one hand to the Aktauberg as a find area and on the other hand to the size of the animal with the Greek word τιτάν ( titan "titanium" or "giant"), a name addition often used in Brontotherien. The species name hippopotamopus is made up of the Greek words ἱπποπόταμος ( hippopotamos " hippopotamus ") and πούς ( poús "foot") and refers to the hippopotamus-like leg shape. The holotype specimen (specimen number KAN N2 / 875) comprises an almost complete skeleton, only part of the right forefoot and rear foot missing.
literature
- Matthew C. Mihlbachler, Spencer G. Lucas, Robert J. Emry and Bolat Bayshashov: A New Brontothere (Brontotheriidae, Perissodactyla, Mammalia) from the Eocene of the Ily Basin of Kazakstan and a Phylogeny of Asian `` Horned '' Brontotheres. American Museum Novitates 3439, 2004, pp. 1-43
Individual evidence
- ↑ a b c d e f g h i Matthew C. Mihlbachler, Spencer G. Lucas, Robert J. Emry and Bolat Bayshashov: A New Brontothere (Brontotheriidae, Perissodactyla, Mammalia) from the Eocene of the Ily Basin of Kazakstan and a Phylogeny of Asian '' Horned '' Brontotheres. American Museum Novitates 3439, 2004, pp. 1-43
- ^ A b Matthew C. Mihlbachler: Species taxonomy, phylogeny, and biogeography of the Brontotheriidae (Mammalia: Perissodactyla). Bulletin of the American Museum of Natural History 311, 2008, ISSN 0003-0090 , pp. 1-475
- ↑ Spencer George Lucas, Bolat Bayshahshov, Lyubov A. Tyutkova, Ayzhan K. Zhamangara and Bolat Z. Aubekerov: Mammalian biochronology of the Paleogene-Neogene boundary at Aktau Mountain, eastern Kazakhstan. Paläontologische Zeitschrift 71 (3/4), 1997, pp. 305-314
- ^ Matthew C. Mihlbachler: A New Uintan Horned Brontothere from Wyoming and the Evolution of Canine Size and Sexual Dimorphism in the Brontotheriidae (Perissodactyla: Mammalia). Journal of Vertebrate Paleontology 31 (1), 2011, pp. 202-214
- ↑ a b Robert J. Emry and Spencer G. Lucas: New ceratomorphs (Mammalia, Perissodactyla) footprints from the Eocene of the Ily Basin, Kazakstan. Journal of Vertebrate Paleontology 23 (suppl.), 2003, p. 44A
- ↑ Mark T. Clementz, Patricia A. Holroyd, and Paul L. Koch: Identifying Aquatic Habits Of Herbivorous Mammals Through Stable Isotope Analysis. Palaios 23 (9), 2008, pp. 574-585
- ↑ Alexander Averianov, Igor Danilov, Wen Chen and Jianhua Jin: A new brontothere from the Eocene of South China. Acta Palaeontologica Polonica 63. 2018 doi: 10.4202 / app.00431.2017
- ^ Henry Fairfield Osborn: Embolotherium, gen. Nov., Of the Ulan Gochu, Mongolia. American Museum Novitates 353, 1929, pp. 1–20 ( digitallibrary.amnh.org )
- ^ Robert J. Emry and Spencer G. Lucas: Brontothere (Mammalia, Perissodactyla) footprints from the Eocene of the Ily Basin, Kazakstan. Journal of Vertebrate Paleontology 22 (suppl.), 2002, p. 51A