Danjiangia
| Danjiangia | ||||||||||||
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Lower jaw fragment of Danjiangia lambdodon |
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| Temporal occurrence | ||||||||||||
| Lower Eocene | ||||||||||||
| 56 to 52 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Danjiangia | ||||||||||||
| Wang , 1995 | ||||||||||||
Danjiangia is a genus of the extinct Brontotherien family. It is one of the oldest representatives of the group and appeared at the transition from the Paleocene to the Eocene around 56 million years ago. Fossil remains are known from two areas in eastern Asia . They consist of a partial skull and several mandibular fragments. The animals still had a relatively original tooth structure with simple premolars and molars . The first description of Danjiangia dates back to 1995. Initially, however, the genus was associated with other original odd-toed ungulates .
features
Danjiangia was a small representative of the Brontotherien. Several fragments of teeth plus a damaged partial skull are present. The partial skull shows only a few characterizing features due to the damage. He had a relatively low skull and an elongated rostrum . The upper jaw stood upright, the orbit was with its front edge above the first molar . The infraorbital foramen opened at the level of the third premolar . At the rostrum, the nasal interior possibly extended to the canine . Only the horizontal bone body of the lower jaw has survived. It was long and low, but increased in height towards the back. The symphysis at the anterior end, connecting the two halves of the jaw, extended to the second premolar and was narrow. The mandibular foramen was below the last molar, the mental foramen was duplicated.
As in the majority of the Brontotheria, the dentition was unreduced and thus consisted of the complete dentition of the higher mammals , which consists of 44 teeth. The dental formula was accordingly: . Both the upper and lower incisors formed a semicircular arc and were separated from each other by short diastemas . In general, they were small in size. In the upper row of teeth they had an elliptical cross-section, in the lower they were spatulate and directed forward. The last incisor in each case had an additional cusp. The canine was free from the last incisor. It was highly conical in shape and protruded slightly to the side, and in the upper dentition it curved backwards. Except for the last lower one, the premolars hardly resembled the molars. They were characterized by pointed bumps, of which three were formed on the third top, otherwise one each. The latter can be seen as a relatively original feature. On the upper molars, the W-shaped shear ridge (ectoloph), typical of Brontotheria, was visible on the cheek side. Its width took up about a third to two fifths of the entire tooth width. The individual humps of the Ectoloph, such as the Meta- and Paraconus, were relatively isolated, so that the Ectoloph was not continuous, which in turn is a clear difference to Eotitanops . The transverse melt strips were rather weakly developed, but visible, a characteristic that is even less developed in Lambdotherium and completely absent in Eotitanops . The ledges were also quite low and not directly connected to the ectoloph. On the tongue side, the two main cusps, protoconus and hypoconus, pointed slightly backwards. There was a cingulum, a low bulge of tooth enamel, on the anterior and posterior edges of the teeth. The lower molars had a double V-shaped outline and were characterized by high transverse ridges (proto-, meta- and hypolophid). The hypolophid in particular was more developed than Lambdotherium . In the upper jaw, the length of the three posterior premolars was 2 cm and that of the molars 2.7 cm. The corresponding values for the lower row of teeth were 2 cm and 3.1 cm.
References
The Danjiangia fossil finds are spread over two areas in East Asia . The oldest find in terms of genealogy , a lower jaw fragment discovered in 1976, was reported from the Lingcha Formation south of Lingcha in the Hengyang Basin in the southern Chinese province of Hunan . The approximately 5200 km² large basin is known for its wealth of palaeogenic fossils. Finds were published here as early as the mid-1940s that were considered to be the oldest mammal remains in southern China at the time. The fossils come mainly from the widespread Red beds (also called Hengyang Red beds or Hengyang sandstones ). Stratigraphically , two fauna horizons can be distinguished within the Red beds : The lower one contains numerous reptiles as well as finds of the Pantodonta , a primitive branch of the higher mammals , and from a geological point of view belongs to the Limuping Formation . The upper one lies in the Lingcha Formation and includes numerous groups of mammals, including insectivores , rodents , primates , predators , ungulates, and various extinct lines such as the Leptictida or Cimolesta . After isotope examinations and analyzes of magnetostratigraphy , the upper find horizon can be placed in the transition area from the Paleocene to the Eocene around 56 million years ago (locally stratigraphically Bumbanium ). The lower one belongs to the end of the Paleocene (local stratigraphic Gashatum ). A skull with its associated lower jaw, in turn, can be assigned to the Yuhuangding Formation in the Liguanqiao Basin in the central Chinese province of Hubei . They came to light there in 1992 from the middle section of the rock unit, which is generally considered to have a position in the Lower Eocene.
Systematics
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Internal systematics of the early Brontotheriidae according to Mihlbachler 2008
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Danjiangia is a genus from the extinct family of the Brontotheriidae (originally Titanotheriidae) within the order of the odd ungulate . The Brontotherien are placed in the distant relationship of today's horses due to certain tooth morphological characteristics . They occurred predominantly in the Eocene of North America and Eurasia and were very diverse. Especially the late forms, such as Megacerops as a character form of the entire group, sometimes reached huge dimensions and were characterized by bony horn formation on the nose. In contrast, Danjiangia is to be seen as a very early representative. According to phylogenetic studies, it shares some similarities with Lambdotherium , which in turn is optionally assigned a position within the (early) Brontotheria or within the independent family of Lambdotheriidae .
The first scientific description of Danjiangia goes back to Wang Yuan from 1995. Wang presented a skull and a lower jaw from the Yuhuangding Formation in the central Chinese province of Hubei , which is considered a holotype (copy number IVPP V-10842). The genus was named after the village of Danjiangkou, in the vicinity of which the finds were made. Together with the genus, Wang established the species D. pingi .
In his first description, Wang saw Danjiangia as the original representative of the Chalicotheriidae , a group of extinct odd-toed ungulates, for which a closer relationship to the tapirs and rhinos is being considered today. The inclusion of Danjiangia in the Chalicotherien was already questioned in 1998 by K. Christopher Beard , who interpreted the genus as an ancestral form of the Brontotherien. Jerry J. Hooker in 2003 and Matthew C. Mihlbachler in 2008 in his comprehensive review of Brontotherien took a similar view . The decisive factor here was the construction of the upper molars. Li Chuan-kuei and colleagues had already presented the lower jaw find from the Lingcha formation in the southern Chinese province of Hunan in 1979 , but at that time assigned it to Propachynolophus from the group of the Palaeotheriidae , close relatives of horses. The actual systematic assignment of the lower jaw was debated in the following period, as some researchers considered this to be typical for chalicotheria. It was only in 2018 that a research group around Bai Bin recognized a stronger relationship between the Lingcha lower jaw and Danjiangia and moved all of the find material to the Brontotheria. Due to certain deviations in the tooth characteristics, they created a new species for the Lingcha lower jaw with D. lambdodon . As a result, two types of Danjiangia are recognized today:
The reference from Danjiangia to the Brontotherien shifts the origin of the group to the Paleocene - Eocene border almost 56 million years ago. It is also the first record of Brontotherien from the eastern part of Eurasia . North American finds of Eotitanops or Lambdotherium are a little younger with an age of 53 to 50 million years. Thus Danjiangia as a representative of the Brontotherien has a similar temporal range as Sifrhippus as a representative of the Equidae or some Tapiromorpha such as Meridiolophus and Orientolophus . According to individual phylogenetic studies, it would be possible to derive the Brontotheria from a family group of the Palaeotheriidae, with Eurasia as a probable place of origin.
literature
- Bin Bai, Yuan-Qing Wang and Jin Meng: The divergence and dispersal of early perissodactyls as evidenced by early Eocene equids from Asia. Communications Biology 1, 2018, p. 115 doi: 10.1038 / s42003-018-0116-5
- Yuan Wang: A new primitive chalicothere (Perissodactyla, Mammalia) from the early Eocene of Hubei, China. Vertebrata PalAsiatica 33, 1995, pp. 138-159
Individual evidence
- ↑ a b c d e f Yuan Wang: A new primitive chalicothere (Perissodactyla, Mammalia) from the early Eocene of Hubei, China. Vertebrata PalAsiatica 33, 1995, pp. 138-159
- ↑ a b c d e f Bin Bai, Yuan-Qing Wang and Jin Meng: The divergence and dispersal of early perissodactyls as evidenced by early Eocene equids from Asia. Communications Biology 1, 2018, p. 115 doi: 10.1038 / s42003-018-0116-5
- ↑ Chung-Chien Young: Note on the first Eocene mammal from South China. American Museum Novitates 1268, 1944, pp. 1-3
- ^ A b Suyin Ting, Gabriel J. Bowen, Paul L. Koch, William C. Clyde, Yuanqing Wang, Yuan Wang and Malcolm C. McKenna: Biostratigraphic, chemostratigraphic, and magnetostratigraphic study across the Paleocene-Eocene boundary in the Hengyang basin, Hunan, China. In: Scott L. Wing, Philip D. Gingerich, Birger Schmitz and Ellen Thomas (eds.): Causes and consequences of globally warm climates in the early Paleogene. Geological Society of America Special Papers 369, 2003, pp. 521-535
- ↑ a b c 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
- ^ Donald R. Prothero, Evolutionary Transitions in the Fossil Record of Terrestrial Hoofed Mammals. Evo Edu Outreach 2, 2009, pp. 289-302, doi : 10.1007 / s12052-009-0136-1 .
- ↑ JJ Hooker and D. Dashzeveg: The origin of chalicotheres (Perissodactyla, Mammalia). Palaeontology 47 (6), 2004, pp. 1363-1386
- ↑ Li Chuan-kuei, QiuZhan-xiang, Yan De-fah and Xie Shu-hua: Notes on some early Eocene mammalian fossils of Hengtung, Hunan. Vertebrata PalAsiatica 17, 1979, pp. 71-80