Homogalax
| Homogalax | ||||||||||||
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Skull of Homogalax |
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| Temporal occurrence | ||||||||||||
| Lower Eocene ( Wasatchian ) | ||||||||||||
| 56 to 47.8 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Homogalax | ||||||||||||
| Hay , 1899 | ||||||||||||
Homogalax is a very early representative of the odd ungulate, which is now extinct. It was proven on the basis of several fossil finds from the Northwest of the USA , with the majority of the remains from the state of Wyoming . The finds date to the Lower Eocene between 56 and 48 million years ago. In general, Homogalax was very small and reached the weight of today's white bearded peccaries with a maximum of 15 kg. In tribal history , the genus represents a predecessor of the lines of development that led to today's rhinos and tapirs (collectivelycalled Ceratomorpha ). In contrast to these, Homogalax was adapted for rapid locomotion.
features
Homogalax was a small representative of the very early odd-toed ungulates and belonged to a predecessor of the line that led to today's rhinos and tapirs . In general it resembled other, primeval forms of this mammal order such as Hyracotherium or Sifrhippus , some well-studied members of the early equine species , which shows the very basal position within the odd ungulates. Based on various skeletal elements, a body weight of 9.5 to 15.1 kg can be assumed. Is known homogalax of many fossil record, but only partially include complete material. Underneath is a heavily weathered skull, which, however, shows few skeletal features. This is around 15 cm long and relatively flat, and overall it was similar to that of Hyracotherium . The short central jaw bone , which rose steeply and came into contact with the nasal bone , was typical , which is not the case with today's ceratoromorphic ungulate ungulates (tapirs and rhinos). The ascending central jawbone formed the back of the interior of the nose, which is also unknown in today's odd-toed ungulates. As a result, the nasal bone had only a short extension to the front and only slightly protruded over the canine . The occiput was clearly short in shape, while the zygomatic arch was not very protruding and rather parallel to the skull.
The bit includes the complete early Higher mammals and had the following dental formula of: . The incisors were chisel-shaped and small, but varied in size. In general, the third (outermost) incisor became the largest. The canine was usually long and pointed in shape and was somewhat pressed on the side. Only rarely did a short, less than 1 cm long diastema occur in the posterior dentition ; others could be observed between the last incisor and the canine and between the first two premolars . The premolars themselves were completely unmolarized - so they did not resemble the molars - and usually only had a raised enamel cusp on the chewing surface. The molars were characterized by low ( brachyodont ) tooth crowns and two tapir-like transverse enamel ridges ( bilophodont ), which could also be inclined in the lower jaw. The length of the teeth increased towards the back. The first premolar was around 0.6 cm long, the last molar up to 1.7 cm long. A complete skeleton is not available, but some parts of the musculoskeletal system have survived. Particularly noteworthy are the long bones . The humerus was built long and narrow and measured about 13 cm. The thigh bone , on the other hand, reached a length of 15 cm and typically had a third trochanter (rolling mound), which is typical for odd ungulates, but was not yet so prominent in Homogalax . The shin became as long as the thigh bone . The forefoot consisted of four rays with a strong central ray ( metacarpus III). The laterally attached rays (Metacarpi II and IV), however, were somewhat reduced in length, the outermost ray (Metacarpus V) greatly reduced. The hind foot, however, had only three rays, but also had a strong central ray ( metatarsus III). Compared to the anterior feet, the posterior ones were around 30% longer (metacarpus III 5.1 cm to metatarsus III 6.7 cm). The individual toe links also proved to be extremely long. Four-pronged front and three-pronged hind feet are typical of primitive odd-toed ungulates and are now only found in tapirs .
Fossil finds
Finds of Homogalax are largely only known from North America and come from the Lower Eocene 56 to 48 million years ago (locally stratigraphically called Lower Wastachian ). They show that Homogalax was a regular, albeit not frequent, element of the fauna at that time. In the US state of Wyoming in particular , numerous fossils have been recovered. The remains from the Willwood Formation of the Bighorn Basin are of great importance, including more than 100 pieces of fossil from several individuals, including a complete skull, which is one of the few surviving from Homogalax . This also includes the majority of the known postcranial skeleton finds, such as individual long bones, a complete foot and hand skeleton and parts of the pelvis and shoulder blade . Other remains have come down to us from the Washakie Basin , as well as from the Powder River Basin ; Both sites, however, contained mostly only isolated teeth. Outside Wyoming, remains from the Golden Valley Formation in North Dakota have been reported. Far away from these sites, the northernmost finds of Homogalax were stored in the Margaret Formation on Ellesmere Island in the far north of Canada , they include, among other things, individual dentition fragments.
Paleobiology
Based on the teeth, only a few changes in size and shape can be made out in the course of the tribal history of Homogalax , so that the genus hardly developed any further. However, the canine varies in size, which can be interpreted as sexual dimorphism . However, the measured data obtained are not unambiguous, as there are also intermediate lengths. The long construction of the lower sections of the limbs, especially the hind legs, but also individual bones, here especially the tarsal bones , shows adaptations to a fast-paced way of life ( cursorial ). Among other things, specially developed joint surfaces on the ankle bone ensured that the foot was stable in the longitudinal direction and did not shear sideways at high walking speed. A permanently fast locomotion within the odd ungulate is generally believed to be an ancient characteristic and is only practiced today by the highly specialized representatives of horses .
Systematics
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Position of Homogalax within the Tapiromorpha according to Holbrook and Lapergola 2011
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Homogalax is a genus from the order of the odd ungulate . Within this, it represents a very original representative, which is usually placed in the Isectolophidae family . The Isectolophidae form the most primitive branch of the intermediate order of the Tapiromorpha , a morphologically diverse group that includes the extensive relatives of today's tapirs (Tapiroidea) and rhinoceros (Rhinocerotoidea), but also the fossilized chalicotheria (Chalicotherioidea). As a sister group , the Isectolophidae stand opposite all other members of this kinship group and thus include their oldest members in terms of their phylogenetic history . Within the Isectolophidae, Homogalax and its sister taxon Cardiolophus are again considered to be very basal, while Isectolophus , which gives the family its name , but also Meridiolophus are much closer to the other Ceratomorpha , the common group of rhinos and tapirs. Some researchers see the family as a non-closed unit ( paraphyletic ), but others disagree because of some tooth characteristics.
Today a species of the genus Homogalax is recognized: H. protapirinus ( Wortman , 1896). Other species such as H. tapirinus and H. primaevus have been described in the past, but are synonymous with H. protapirinus . Species in Asia were also named, such as H. namadicus and H. wutuensis , but these are now to be regarded as representatives of earlier Chalicotheria such as Protomoropus . Another species, H. reliquius , could later be identified as a representative of Isectolophus .
The systematic position of Homogalax was very controversial in the history of research. Early finds, which included two severely chewed jaw fragments from early Eocene deposits in the US state of New Mexico , were made in 1875 by Edward Drinker Cope on Orohippus tapirinus , a very early horse representative . Two years later she referred Cope to Hyracotherium , another very original horse ancestor. In 1880 Jacob L. Wortman discovered extensive finds in Eocene deposits of the Bighorn Basin in Wyoming , which are reminiscent of that early tapir. In the following year Cope established the genus Systemodon due to the lack of or only slightly developed, but typical for Hyracotherium, diastema between the front and rear teeth and elevated S. tapirinum (formerly Orohippus tapirinus ) to the type of the genus. Henry Fairfield Osborn recognized Systemodon as the ancestor of the tapirs eleven years later . Wortman established the new species S. protapirinum and S. primaevus in 1896 , but at the same time referred S. tapirinum again to Hyracotherium and thus excluded the type species from the genus. Thereupon Oliver Perry Hay proposed the name Homogalax in 1899 in order to avoid taxonomic and systematic problems; Since then, Systemodon has been partially synonymous with Homogalax . The name Homogalax comes from the Greek ( ομογάιαξ ) and means something like "stepbrother". The type material (copy number AMNH 4460) includes an upper jaw with the preserved row of teeth from the first pre- to the last molar and a lower jaw, which is only missing the front premolar.
Individual evidence
- ↑ a b c d Kenneth D. Rose: Skeleton of early Eocene Homogalax and the origin of Perissodactyla. Palaeovertebrata 25 (2-4), 1996, pp. 243-260
- ^ A b c d Luke T. Holbrook: Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 132, 2001, pp. 1-54
- ↑ a b Luke T. Holbrook, Spencer G. Lucas and Robert J. Emry: Skulls of the Eocene Perissodactyls (Mammalia) Homogalax and Isectolophus. Journal of Vertebrate Paleontology 24 (4), 2004, pp. 951-956
- ^ A b c d Leonard B. Radinsky: Origin and Early Evolution of North American Tapiroidea. Peabody Museum of Natural History Yale University Bulletin 17, 1963, pp. 1-106
- ^ A b 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
- ↑ Jaelyn J. Eberle and David A. Eberth: Additions to the Eocene Perissodactyla of the Margaret Formation, Eureka Sound Group, Ellesmere Island, Arctic Canada. Canadian Journal of Earth Sciences 52, 2015, pp. 123-133
- ↑ a b 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
- ↑ Chow Minchen and Li Chuan-kuei: Homogalax and Heptodon of Shantung. Vertebrata Palasiatica 9 (1), 1965, pp. 15-22
- ↑ JJ Hooker and D. Dashzeveg: The origin of chalicotheres (Perissodactyla, Mammalia). Palaeontology 47 (6), 2004, pp. 1363-1386
- ↑ Spencer G. Lucas, Luke T. Holbrook and Robert J. Emry: Isectolophus (Mammalia, Perissodactyla) from the Eocene of the Zaysan basin, Kazakstan and its biochronological significance. Journal of Vertebrate Paleontology 23 (1), 2003, pp. 238-243
- ^ Henry Fairfield Osborn and Jacob L. Wortman: Fossil mammals of the Wasatch and Wind River beds. Collection of 1891. Bulletin of the American Museum of Natural History 4, 1892, pp. 81–148 (pp. 124–126)
- ^ Jacob L. Wortman: Species of Hyracotherium and allied perissodactyls from the Wasatch and Wind River beds of North America. Bulletin of the American Museum of Natural History 8, 1896, pp. 81-110
- ↑ Oliver Perry Hay: On the names of certain North American fossil vertebrates. Science 9, 1899, pp. 593-594
