Hyrachyus

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Hyrachyus
Skeleton of Hyrachyus

Skeleton of Hyrachyus

Temporal occurrence
Lower to Upper Eocene
50 to 42 million years
Locations
Systematics
Higher mammals (Eutheria)
Laurasiatheria
Unpaired ungulate (Perissodactyla)
Ceratomorpha
Hyrachyidae
Hyrachyus
Scientific name
Hyrachyus
Leidy , 1871

Hyrachyus is a now extinct representative of the early odd-toed ungulates , which waswidespreadin what is now North America and Eurasia from the end of the Lower to the beginning of the Upper Eocene 50 to 40 million years ago. The genus was only medium-sized, on average it reached the size of a wolf and lived in tropical to subtropical forests near the water. It is known from numerous, partly complete skeleton finds, the discoveries of which go back to the year 1870 in North America. The systematic position of Hyrachyus is controversial. Some researchers see the animal at the basis of the evolution of the rhinos , others at that of the tapirs . In general, Hyrachyus is considered a primitive representative of the Ceratomorpha , a unit consisting of these two odd-toed ungulate families.

features

Hyrachyus is a small to medium-sized representative of the early, ceratomorphic (i.e. related to the tapirs and rhinos ) odd ungulates and, with its characteristic arched back, was outwardly similar to the other primeval forms of this mammal order . In size, some species reached that of today's large wolf , while others were mustang-sized . For smaller representatives, a head-torso length of around 120 cm, a shoulder height of 90 cm and a weight of around 36 kg can be assumed.

The skull had lengths of 21 to 25 cm and an average skull height of 14.6 cm in the area of ​​the first premolar . In side view he showed a straight forehead line, which broke off downwards in the area of ​​the nasal bone . The occiput was slightly extended, on the parietal bone there was a slightly raised crest . It divided into two ribs of bones in front and behind. The anterior ran over the frontal bone , the posterior connected to the occipital bulge. The nasal bone was elongated and protruded from the intermaxillary bone . The very short interior of the nose, which only extended to about 2.5 mm in length and ended above the canine , proved to be striking . In addition, the inner space of the nose was limited at the back by the intermaxillary bone, which stood up steeply and was thus short in shape, whereby it was in contact with the nasal bone on the underside of the nasal bone, which is different from today's, but typical for primitive odd ungulates. The eye window was about 5.5 cm in diameter, the anterior margin was above the second molar . The infraorbital foramen was located above the third premolar approximately at the level of the lower orbital margin.

The lower jaw was strong and up to 20 cm long, its lower edge was almost straight. The horizontal body of the lower jaw increased slightly towards the rear and measured around 4 cm behind the last molar . The ascending branch was a good 9.3 cm high on the crown process and a good 7.6 cm high on the articular process and thus clearly exceeded the occlusal plane. The symphysis reached to the anterior border of the second premolar and was 4.5 cm long. The angular process was rounded and not very noticeable. Below the last two premolars there were two mental foramen of about the same size , and further openings were formed in the anterior part of the lower jaw. The teeth had the full dentition of the early higher mammals and had the following dental formula of: . The incisors were chisel-like in shape and small in shape. The canine was conical in shape and significantly enlarged, so that it was up to twice as high as the incisors. The typical diastema to the posterior dentition was 1.8 cm. In general, the molars had low ( brachyodontic ) crowns. The premolars were only slightly molarized except for the rear two, so that the first two appeared rather pointed. The two posterior premolars then had an individually formed and transverse tooth enamel ridge . The molars had a bilophodontic structure with two transverse melting ridges, as is typical of today's tapirs. The size of the teeth increased from front to back, so the anterior premolar was only 0.9 cm long, the rearmost molar, however, 2 cm. The triangular shape of the last molar was also characteristic.

Since there are numerous complete skeletal finds, the body skeleton is almost completely known. The spine consisted of 7 cervical, 18 thoracic, 7 lumbar and 5 sacral vertebrae, which reached a total length of 90 to 100 cm measured over the curvature. The number of tail vertebrae is not precisely documented. The foremost thoracic vertebrae had strong spinous processes up to 8 cm long . The musculoskeletal system resembled that of the other early odd ungulates. The humerus was 15 cm long in smaller forms and had a lightly pressed shaft and a low, hemispherical head that overhanged backwards. The ulna reached a length of around 16 cm and had a pronounced upper joint end ( olecranon ). The length of the thigh bone was 24 cm, its joint head sat on a short neck and was dominated by the Great Rolling Hill. The shaft was rather cylindrical, in the upper section there was a strong third rolling mound. The shin was 22 cm in length and was characterized by a slightly S-shaped curved shaft. The front feet with four and the rear feet with three toes were also striking, which is to be regarded as typical of primitive odd-toed ungulates. In contrast to today's tapirs, which are the only group of unpaired ungulates to have this feature, the three rays (II to IV) on the hind foot were almost equally long, but the middle one was significantly stronger. The forefoot had a similar structure, but here the outermost ray (V) was significantly shortened. The metapodia of the central rays were 7.8 cm long at the forefoot and 11.2 cm long at the rear foot.

Fossil finds

Fossil remains of Hyrachyus originate mainly from North America and Eurasia and date from the end of the Lower to the beginning of the Upper Eocene 50 to 42 million years ago. The finds from the Bridger Basin in the southwest of the US state of Wyoming are outstanding . They date to the end of the Lower Eocene (locally stratigraphically Bridgerian ). These include numerous, partly complete skulls and several articulated skeletons. A few finds could also be recovered from the Washakie Basin and the Wind River Basin , both also in Wyoming, and the Huerfano Basin in Colorado and are about similar old.

In Europe , finds of Hyrachyus are mainly known from the Middle Eocene. One of the most remarkable finds is a complete skeleton from the Messel Pit near Darmstadt, which dates back to around 47 million years ago. The remains from the Geiseltal in Saxony-Anhalt are also of great importance . Here at least 75 remains of skulls and teeth of all ages came to light, which are stratigraphically distributed over the lower and middle coal of the lignite seams there. Finds from France , such as the freshwater marl sands of Argenton and Bouxviller, and Great Britain were also reported. A lower jaw fragment has also come down to us from the Csordakút Basin in Hungary , which is one of the few Eocene land mammal fossils in the country. In contrast, only a few finds have been reported from Asia , such as an upper jaw with preserved last pre- and all three molars from the Irdin-Manha Formation of the Middle Eocene in Inner Mongolia . A lower jaw that in sandstones of mitteleozänen Oyake formation was discovered, is hyrachyus on the southern Japanese main island of Kyushu occupied.

Paleobiology

In general, Hyrachyus is a lightly built animal. The relatively long lower leg sections of both the front and rear limbs suggest a fast-paced ( cursorial ) gait. This is also supported by the high positions of the three rolling hills on the thigh bone and the generally narrow pelvis as well as the rather narrow joint rollers on the humerus. The neck of Hyrachyus was relatively long and roughly corresponded to the length of the head. The shape of the cervical vertebrae indicates that the neck was carried forward at an angle and that there was an angle of about 60 ° to the head. The dented vertebral heads, however, possibly only allowed a limited range of lateral movement of the neck. However, due to the large spinous processes of the anterior thoracic vertebrae, there was a strong neck musculature for raising and lowering the head. The light physique of Hyrachyus therefore probably caused fewer bone pathologies compared to the later heavy-weight odd-toed ungulates.

Hyrachyus lived in tropical and subtropical forests and marshlands of floodplains or on the banks of lakes. Leonard B. Radinsky noted in a study from 1967 a sexual dimorphism based on the upper molars, as these showed themselves to be very variable in size within the species. Studies on fossil material from the Geiseltal have shown that the genus has undergone hardly any dental morphological changes in the course of its tribal history, with the exception of a general increase in size, as has been demonstrated, for example, in unpaired ungulates of the same age such as the propalaeotherium, which is a related horse . However may exist differences between male and female animals in the design of the canine, the in the former conical ( caniniform is formed), but in the latter case the cutting teeth ( incisiviform ) is similar. Furthermore, the tooth change corresponds to that of the other higher mammals and begins with the eruption of the foremost molar, but an exchange of the first deciduous to the first permanent premolar was not detectable. It is assumed here that this tooth is either not changed or is changed very early in the youth stage. The very short interior of the nose suggests that Hyrachyus did not have a tapir-like trunk.

Systematics

Position of Hyrachyus within the Tapiromorpha according to Holbrook and Lapergola 2011
  Odd-toed ungulates  

 Outgroup 


  Tapiromorpha  

 Isectolphidae


   
  Ceratomorpha  

 Helaletidae


   

 Hyrachyus


   

 Tapiridae


   

 Rhinocerotidae


Template: Klade / Maintenance / 3


  Ancylopoda  

 Chalicotheriidae


   

 Lophiodontidae






Template: Klade / Maintenance / Style

Hyrachyus is a now extinct genus , which is often referred to the family of Helaletidae , a fossil group that is also closely related to today's tapirs . The Helaletidae, which include Helaletes and Heptodon , are considered the forerunners of the tapirs and are therefore part of the superfamily of the Tapiroidea and the suborder Ceratomorpha , which includes the tapirs as well as the rhinos . It is also part of the intermediate order Tapiromorpha , which also includes the extinct Chalicotheria . However, it is scientifically controversial whether Hyrachyus is actually in the line of development of the tapirs (Tapiroidea) or the rhinoceros ( Rhinocerotoidea ). The reason for this debate is the characteristics of the maxillary molars, which have tapir-like, low, parallel, transverse enamel ridges, but with their slightly high crown and the triangular shape of the last molar are reminiscent of rhinos. For this reason, Hyrachyus is sometimes placed in an independent family, the Hyrachyidae, but Leonard B. Radinsky pointed out as early as 1967 that there were too few distinguishing features to the Helaletidae to justify an independent family. Investigations into the microstructure of the tooth enamel again showed similarities to early representatives of the rhinos and deviations from those of the tapirs and could speak for a closer position to the rhinocerotoidea.

Several types of Hyrachyus are known, including the following:

  • H. eximus Leidy , 1871
  • H. minimus ( Fischer , 1829)
  • H. modestus ( Leidy , 1870)
  • H.stehlini ( Depéret , 1904)

The exact number of species is unknown. While Horace Elmer Wood still listed numerous types of Hyrachyus in his general arrangement from 1934, Leonard B. Radinsky clearly summarized these in 1967, which later met with criticism. Above all, a lack of revision of the genus is criticized, during which an exact phylogenetic position of Hyrachyus should be worked out.

Joseph Leidy (1823-1891)

The first description of Hyrachyus was in 1871 by the American paleontologist Joseph Leidy on the basis of a lower jaw fragment, which represents a young animal. The year before, he had placed a single tooth at Lophiodon , which he later considered to belong to Hyrachyus . Especially in the 1870s there were numerous new descriptions of species of Hyrachyus in North America , but these are largely invalid and either represent existing species or belong to the closely related genus Helaletes . In Europe too, numerous representatives were established at the end of the 19th century and some were given the alternative spelling Hyrachius . These later turned out to be members of the genera Tapirus or Protapirus . In terms of research history, the oldest finds clearly relating to Hyrachyus date from 1829 and were discovered in Argenton in France. These were initially considered to be the remains of Lophiodon , later of Chasmotherium . In 1966 and 1967, these and other related fossils were referenced in independently conducted studies of Hyrachyus . The lectotypes (specimen numbers USNM 660 and 661) comprise a left lower jaw with the four milk premolars and the foremost molar of a young animal and an isolated molar, both of which were already used by Leidy to identify the genus Hyrachyus . The finds come from the lower Bridger Formation and were found at Smith'Fork in the Bridger Basin in the southwestern part of the US state Wyoming . They are now kept at the National Museum of Natural History in Washington, DC .

literature

  • Bin Bai, Jin Meng, Yuan Qing Wang, Hai Bing Wang and Luke T. Holbrook: Osteology of the Middle Eocene ceratomorph Hyrachyus modestus (Mammalia, Perissodactyla). Bulletin of the American Museum of Natural History 413, 2017, pp. 1-68
  • Luke T. Holbrook: Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 132, 2001, pp. 1-54
  • Leonard B. Radinsky: Hyrachyus, Chasmotherium, and the Early Evolution of Helaletid Tapiroids. American Museum Novitates 2313, 1967, pp. 1-23

Individual evidence

  1. ^ A b c Edward Drinker Cope: On the Osteology of the Extinct Tapiroid Hyrachyus. Proceedings of the American Philosophical Society 13, 1873, pp. 212-224
  2. ↑ a b c d e f Horace Elmer Wood: A revision of the Hyrachyidae. Bulletin of the American Museum of Natural History 67, 1934, pp. 181-295
  3. ^ A b Kelsey T. Stilson, Samantha SB Hopkins and Edward Byrd Davis: Osteopathology in Rhinocerotidae from 50 Million Years to the Present. PLoS ONE 11 (2), 2016, p. E0146221 doi: 10.1371 / journal.pone.0146221
  4. ↑ a b c d e f g Kerstin Hlawatsch and Jörg Erfurt: Tooth morphology and stratigraphic distribution of Hyrachyus minimus (Perissodactyla, Mammalia) in the Eocene Geiseltalschichten. In: Jörg Erfurt, Lutz Christian Maul (Hrsg.): 34th meeting of the working group for vertebrate paleontology of the paleontological society March 16 to March 18, 2007 in Freyburg / Unstrut. Hallesches Jahrbuch für Geoswissenschaften BH 23, 2007, pp. 161–173
  5. ^ Luke T. Holbrook: Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 132, 2001, pp. 1-54
  6. ↑ a b c d Bin Bai, Jin Meng, Yuan Qing Wang, Hai Bing Wang and Luke T. Holbrook: Osteology of the Middle Eocene ceratomorph Hyrachyus modestus (Mammalia, Perissodactyla). Bulletin of the American Museum of Natural History 413, 2017, pp. 1-68
  7. ↑ a b Karl-Heinz-Fischer: The tapiroid perissodactyls from the Eocene brown coal of the Geiseltal. Geologie 45, 1964, pp. 1-101
  8. ↑ a b c Jens Lorenz Franzen: Hyrachyus minimus (Mammalia, Perissodactyla, Helaletidae) from the Middle Eocene oil schist of the "Messel Pit" near Darmstadt (Germany, S-Hessen). Senckenbergiana lethaea 61 (3/6), 1981, pp. 371-376
  9. ↑ a b c d e f Leonard B. Radinsky: Hyrachyus, Chasmotherium, and the Early Evolution of Helaletid Tapiroids. American Museum Novitates 2313, 1967, pp. 1-23
  10. ↑ Ch. Depéret: Sur les caractères et les affinités du genre Chasmotherium Rütimeyer. Bulletin de la Société géologique de France 4 (4), 1904, pp. 569-587 ( [1] )
  11. ↑ a b László Kocsis: Vertebrates remains of the Middle Eocene Csordakút basin, Hungary. In: The 7th European Workshop on Vertebrate Palaeontology - Sibiu (Romania) - July 2-7, 2002. Bucharest 2002, p. 23
  12. ^ Leonard B. Radinsky: Early Tertiary Tapiroidea of ​​Asia. Bulletin of the Merican Museum of Natural History 129, 1965, pp. 181-264
  13. ↑ Kazunori Miyata: First record of a primitive rhinocerotoid Hyrachyus from the Middle Eocene of Japan. In: Erin Maxwell, Jessica Miller-Camp and Robert Anemone (Eds.): Society of Vertebrate Paleontology October / November 2013, Abstracts of papers of the 73rd annual meeting. Los Angeles, 2013, p. 178
  14. ↑ a b Meinolf Hellmund: Tooth emergence and replacement in the European Hyrachyus minimus (Fischer, 1829) (Mammalia, Perissodactyla) from the Geiseltal Fossillagerstätte - a further example for 'Schultz's rule' in ungulates. New Yearbook for Geology and Paleontology Essays 282 (2), 2016, pp. 157–180
  15. ^ Luke T. Holbrook and Joshua Lapergola: A new genus of Perissodactyl (Mammalia) from the Bridgerian of Wyoming, with comments on basal Perissodactyl phylogeny. Journal of Vertebrate Paleontology 31 (4), 2011, pp. 895-901
  16. ↑ Donald R. Prothero, Earl Manning and C. Bruce Hanson: The phylogeny of the rhinocerotoidea (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 87, 1986, pp. 341-366
  17. ^ A b c Robert M. Schoch: A review of the Tapiroids. In: Donald R. Prothero and Robert M. Schoch (Eds.): The evolution of Perissodactyls. New York and Oxford, 1989, pp. 298-320
  18. ↑ John M. Rensberger: Evidence from the enamel microstructure for reversals in ditary behavior in the transition from primitive Ceratomorpha to Rhinocerotoidea. Bulletin of Carnegie Museum of Natural History 36, 2004, pp. 199-210
  19. ^ Joseph Leidy: Report on the vertebrate fossils of the Tertiary formations of the West. United States Geological Survey of Wyoming and portions of contiguous Territories 2d (4th) Annual Report, Washington, 1871, pp. 340-370 (p. 357) ( [2] )
  20. ^ Joseph Leidy: Remarks on a collection of fossils from the western territories. Proceedings of the Academy of Natural Sciences of Philadelphia 22, 1870, pp. 109-110 ( [3] )
  21. ^ Karl-Heinz-Fischer: On the systematic position of Chasmotherium RÜTIMEYER 1862 (Mammalia, Perissodactyla). Reports of the German Society for Geological Sciences 12A (5), 1967, pp. 595-600.

Web links

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