Irenolophus
| Irenolophus | ||||||||||||
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Skull of Irenolophus , holotype specimen of I. qii |
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| Temporal occurrence | ||||||||||||
| Lower to Middle Eocene | ||||||||||||
| 52 to 41.1 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Irenolophus | ||||||||||||
| Bai , Meng , Mao , Zhang & Wang , 2019 | ||||||||||||
Irenolophus is a genus of the extinct family of the Deperetellidae , which are distant relatives of today's tapirs . The genus lived in the Lower and Middle Eocene around 52 to 41 million years ago in what is now East Asia . Evidence is available from Inner Mongolia . They include a complete but severely crushed skull with a lower jaw as well as other bits and pieces and individual foot bones. They are medium-sized tapir-like animals, in which the nasal bone was still relatively long and the interior of the nose was very small. In the case of the tapirs, the former is greatly reduced, the latter significantly enlarged, both of which are due to the shape of the trunk . Further features can be found in the simply designed pre - molar teeth and the characteristically built molar teeth with two strong transverse ridges. The scientific introduction of Irenolophus took place in 2019. There are currently two types.
features
Irenolophus was a medium-sized representative of the Tapiroidea. The genus has been handed down with a complete skull and several remains of the jaw. The skull shows lateral indentations, which were caused by sediment load , and it is severely splintered. Its total length is 24 cm, of which the snout area took up around 14 cm, making it a good 27% longer than the brain skull section. Overall, the skull had an arched profile. The middle jaw bone was not only in contact with the upper jaw but also with the nasal bone , which corresponds to the situation with Heptodon , but differs from Colodon , in which the latter does not apply. The interior of the nose was very short and ended, as in Heptodon, above the gap between the canine and the anterior premolar . In Colodon and Lophialetes, however, it reached back above the molar row . The upper jaw was high, the infraorbital foramen formed here opened above the second and third premolars, comparable to other Tapriver relatives, but in a lower position than is known from the heptodon and colodon . The nasal bone was still quite extensive and protruded beyond the rearmost incisor. Later forms such as Colodon had significantly shorter nasal bones. Two ribs of bone rose on the flat frontal bone, which merged backwards to form a distinct crest . The orbit was round and quite large, but smaller than in Lophioletes . The back of the skull is badly damaged, but the articular surfaces on the occiput were wide apart at the top and close together at the bottom. The palate on the underside of the skull extended to the second molar, in accordance with heptodon . The external ear canal opened downwards. On the lower jaw there was a slender, horizontal body of bone that only increased slightly backwards in height. The height under the second premolar was 2.8 cm and under the first molar 3.2 cm. The symphysis at the anterior end extended to the first premolar and was relatively narrow. It protruded upwards at an angle. In contrast to the low bone body, the crown process rose up to 12 cm. Its leading edge was almost perpendicular to the chewing plane of the teeth. The posterior part is damaged in most of the lower jaw finds. A relatively complete specimen measured 21 cm in length.
The bit consisted of the complete dentition of higher mammals with a total of 44 teeth and the following dental formula : . The anterior set of teeth is hardly known; based on the alveoli , the rearmost incisor in both the upper and lower rows of teeth was smaller than the two anterior ones. The upper canine was isolated, separated by a short diastema from the third incisor and a long one measuring up to 2.4 cm from the first premolar. In the lower jaw there was only a gap between the canine and the anterior premolar. The canine tooth had an oval cross-section in both rows and was probably quite large. The premolars were only slightly molarized, so they looked only slightly like the molars. The molars had a strictly bilophodontic structure with two transverse ridges, which is typical of the Tapir relationship. Striking for the Deperetellidae, the two ridges of the upper molars (Metaloph and Protoloph) together formed a U, whereby the anterior ridge merged almost continuously into an additional shear ridge, the centrocrista, which was integrated into the structure. The union was not yet as clearly developed as with Teleolophus . The metaconus, which had completely disappeared on the rear molar in the latter genus, was still visible in Irenolophus . There was also a metaconus on the two anterior molars, which was shifted towards the tongue according to other representatives of the Deperetellidae. On the lower molars, the two ridges (protolophid and hypolophid) ran almost parallel and were slightly inclined so that they were oriented backwards on the tongue side. The hypoconulid was missing on the third molar, a small side hump at the rear end of the tooth, which formed a small third ridge in Lophialetes and was relatively narrow in Heptodon . The premolar row was around 4 to 4.7 cm long in the upper and lower jaw, the values for the molar row were around 5 to 5.8 cm each.
What is remarkable about the carpal bones found is that the joint facet of the hookbone connecting to the outer (fifth) metacarpal bone was extremely large. This suggests that the outer finger ray of the hand (ray V) was not yet as strongly reduced as is the case with today's tapirs .
Fossil finds
The previously known fossil material of Irenolophus was for the most part discovered in the Arshanto Formation , and to a lesser extent in the Irdin Manha Formation . Both rock units represent important fossil deposits and are of central importance for East Asian biostratigraphy , as individual sections were named after them. They are exposed in the Erlian Basin in Inner Mongolia , and important sites can be found south of the city of Erlian . Initial research was carried out in the region in the 1920s during several expeditions to the American Museum of Natural History , each led by William Diller Matthew and Walter W. Granger . The Arshanto formation reaches a thickness of over 35 m. It rests on the Nomogen Formation and begins at the base with coarser fluviatile and partly cross-layered sediments of gray color, which gradually merge into finer sandstones upwards . The upper section is formed by reddish colored clay deposits. The Irdin Manha Formation, in turn, covers the Arshanto Formation. Their thickness is only 10 to 15 m. It shows itself as a series of gray-colored clays, sands and gravels in which numerous channels are embedded. The formations belong to the Lower and Middle Eocene , so they arose between 52 and 41 million years ago today (locally stratigraphically the Arshantum 52 to 47 million years ago and the Irdinmanhum 47 to 41 million years ago).
Both rock units contain several fossil areas that stand out due to their general wealth. Significant remains occur only among the odd-toed ungulates . From the other Tapir relationship (Tapiroidea) Heptodon from the Arshanto- as well as Desmatotherium and Lophialetes from the Irdin-Manha formation are documented. The former also comes from the important basal form Hyrachyus . The other rhino relatives (Rhinocerotoidea) include Pappaceras and Forstercooperia from the group of Indricotheriidae , which are distributed over both deposit units , or Rostriamynodon from the group of Amynodontidae with remains from the Irdin-Manha formation. The evidence for the Brontotheriidae is also outstanding , as Microtitan (Arshanto Formation) as well as Protitan , Metatelmatherium and Gnathotitan (all Irdin-Manha Formation) have been reported from here. In addition, there are numerous finds from younger rock units that cover the Upper Eocene and Lower Oligocene . The finds of Irenolophus scatter through most of the Arshanto Formation and can still be found in isolated cases in the lower parts of the Irdin Manha Formation. So far, a complete skull with lower jaw and individual foot bones have been recovered from the Huheboerhe discovery area. They were located in the base area of the Arshanto formation and served to define the genus Irenolophus . In addition, various lower and upper jaws, isolated teeth and other isolated limb fragments from the remaining areas of the rock units came to light. They are spread over various areas of discovery, such as Huheboerhe and Wulanboerhe and Nuhertingboerhe.
Systematics
Irenolophus is a genus of the extinct family of the Deperetellidae . The group belongs to the further relationship of the tapirs (Tapiridae), the so-called Tapiroidea . The Deperetellidae are largely restricted to the Eocene of Asia. Together with the Helaletidae and Lophialetidae, they form a rather primitive kinship of early tapir-like forms. In a further perspective, the Tapiroidea and thus also the Deperetellidae belong to the Ceratomorpha , the common group of tapirs and rhinoceroses (Rhinocerotidae) or to the higher-order community of the Tapiromorpha . Traditionally, the tapir-rhinoceros line is considered to be a sister group of the Hippomorpha , the equine relationship, within the order of the odd ungulate (Perissodactyla) .
The Deperetellidae were originally established in 1965 by Leonard B. Radinsky . They are characterized by their complete dentition of the higher mammals with a diastema behind the canine , their only partially molarized premolars and their very high-crowned and extremely bilophodontic molars . The two ridges form a U-shaped course on the upper molars together with the paraconus, while the metaconus is reduced in size or has completely disappeared. Another peculiarity of the Deperetellidae can be found in the molarization mode of the upper premolars, which was somewhat different from that of the tapirs and some related groups as well as the majority of the rhinoceros community. The posterior tooth ridge (metaloph) did not arise from the hypoconus, a large tooth cusp, which was newly forming on the rear edge of the tooth, but from the migration of individual, already existing cusps, namely the protoconus and the paraconule, towards the tongue and backwards. This is somewhat reminiscent of the process with horses. In 1965 Radinsky referred only two genera, Deperetella and Teleolophus, to the group, which were gradually joined by others such as Pachylophus and Bahinolophus . The origin of the Deperetellidae is not entirely clear. The complete skull of Irenolophus shows with the only short nasal interior, the low-seated foramen infraorbitale and the occurrence of the first premolar very basal characteristics, which most members of the Helaletidae with the exception of Heptodon lack. The recession of the nasal cavity and the shortening of the nasal bone in later tapir-like forms are obviously related to the development of a trunk . For the time being it is therefore assumed that the Deperetellidae emerged from early Helaletiden tapirs. Within the Deperetellidae, Irenolophus can again be viewed as a basal form. The conclusion arises from the relatively short row of premolars compared to the row of molars, as the former increases in length in later forms.
The genus Irenolophus was first scientifically described in 2019 by Bin Bai and research colleagues . The basis for this was a skull including lower jaw and some hand bones from the base area of the Arshanto formation in the Huheboerhe discovery area in the Erlian Basin in Inner Mongolia (copy number IVPP V 25831). The generic name is derived from the name Iren for the Erlian basin and from the Greek word λόφος ( lophos ) for "hill" or "ridge". The latter part of the word refers to the pronounced ridges of the molars.
Two species can be distinguished within the genus Irenolophus :
Bai and colleagues listed the species Irenolophus qii in their first description in 2019. With the specific epithet , the authors honored the researcher Tao Qi for his meritorious work in the Erlian Basin. As early as 1987, Qi had published an extensive article on the mammalian fauna of the Arshanto Formation. In this he introduced Teleolophus primarius as a new representative of the Deperetellidae. The found material of this form, various dentition and foot bones, also occurs in Huheboerhe and also in Wulanboerhe, but spreads more through the Arshanto Formation and extends into the lying layers of the Irdin-Manha Formation . In contrast to other members of the genus Teleolophus , Teleolophus primarius shows some more original tooth features such as the non-forked paralophid of the lower premolars or an entoconid, a large tooth hump, which is also missing here. Bai and colleagues therefore transferred Teleolophus primarius to the genus Irenolophus . In addition, they combined some fossil remains, which Radinsky had mentioned in 1965 under Teleolophus cf medius and which exist from the same localities, with Irenolophus primarius . Compared to Irenolophus qii , Irenolophus primarius shows a longer premolar row in relation to the molar row (the ratio of premolar to molar row is 0.79 for I. qii , 0.82 for I. primarius ). A possibly third type of Irenolophus could represent a right lower jaw branch , which came to light from the Nuhertingboerhe region also in the base area of the Arshanto formation. In comparison to the type species Irenolophus qii , individual deviations in the tooth morphology and in the position of the mandibular foramen can be seen . However, since only the last molar is complete here and there is not enough material for comparison, a third type was not set up for the time being.
literature
- Bin Bai, Jin Meng, Fang-Yuan Mao, Zhao-Qun Zhang and Yuan Qing Wang: A new early Eocene deperetellid tapiroid illuminates the origin of Deperetellidae and the pattern of premolar molarization in Perissodactyla. PLoS ONE 14 (11), 2019, p. E0225045, doi: 10.1371 / journal.pone.0225045
Individual evidence
- ↑ a b c d e f g h i j Bin Bai, Jin Meng, Fang-Yuan Mao, Zhao-Qun Zhang and Yuan Qing Wang: A new early Eocene deperetellid tapiroid illuminates the origin of Deperetellidae and the pattern of premolar molarization in Perissodactyla . PLoS ONE 14 (11), 2019, p. E0225045, doi: 10.1371 / journal.pone.0225045
- ↑ Jin Meng, Yuanqing Wang, Xijun Ni, K. Christopher Beard, Chengkai Sun, Qian Li, Xun Jin and Bin Bai: New Stratigraphic Data from the Erlian Basin: Implications for the Division, Correlation, and Definition of Paleogene Lithological Units in Nei Mongol (Inner Mongolia). American Museum Novitates 3570, 2007, pp. 1-31
- ↑ Bin Bai, Yuan-Qing Wang, Fang-Yuan Mao and Jin Meng: New material of Eocene Helaletidae (Perissodactyla, Tapiroidea) from the Irdin Manha Formation of the Erlian Basin, Inner Mongolia, China and comments on Related Localities of the Huheboerhe Area . American Museum Novitates 3878, 2017, pp. 1-44
- ↑ Bin Bai, Yuanqing Wang, Qian Li, Haibing Wang, Fangyuan Mao, Yanxin Gong and Jin Meng: Biostratigraphy and Diversity of Paleogene Perissodactyls from the Erlian Basin of Inner Mongolia, China. American Museum Novitates 3914, 2018, pp. 1-60
- ^ A b Leonard B. Radinsky: Early Tertiary Tapiroidea of Asia. Bulletin of the American Museum of Natural History 129 (2), 1965, pp. 183-263
- ^ Robert M. Schoch: A review of the Tapiroids. In: Donald R. Prothero and RM Schoch (Eds.): The evolution of the Perissodactyls. New-York 1989, pp. 298-320
- ↑ Jerry J. Hooker and Demberelyin Dashzeveg: The origin of chalicotheres (Perissodactyla, Mammalia). Palaeontology 47 (6), 2004, pp. 1363-1386
- ^ 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
- ^ Luke Holbrook: The Identity and Homology of the Postprotocrista and its Role in Molarization of Upper Premolars of Perissodactyla (Mammalia). Journal of Mammalogy 22, 2015, pp. 259-269, doi: 10.1007 / s10914-014-9276-3
- ↑ Yongsheng Tong and Lei Yizhen: Fossil tapiroids from the upper Eocene of Xichuan, Henan. Vertebrata PalAsiatica 22 (4), 1984, pp. 269-280
- ↑ Takeshi Tsubamoto, Naoko Egi, Masanaru Takai, Chit Sein and Maung Maung: Middle Eocene ungulate mammals from Myanmar: A review with description of new specimens. Acta Palaeontologica Polonica 50 (1), 2005, pp: 117-138 ( [1] )
- ^ Leonard B. Radinsky: Origin and Early Evolution of North American Tapiroidea. Peabody Museum of Natural History Yale University Bulletin 17, 1963, pp. 1-106.
- ↑ Tao Qi: The Middle Eocene Arshanto fauna (Mammalia) of Inner Mongolia. Annals of the Carnegie Museum 56, 1987, pp. 1–73 ( [2] )
