Stylolophus

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Stylolophus
Temporal occurrence
Lower Eocene
56 to 56 million years
Locations
Systematics
Afrotheria
Paenungulata (without rank)
Tethytheria
Embrithopoda
Stylolophidae
Stylolophus
Scientific name of the  family
Stylolophidae
Gheerbrant , 2020
Scientific name of the  genus
Stylolophus
Gheerbrant , Schmitt & Kocsis , 2018

Stylolophus is a genus from the extinct and little researched mammal order of the Embrithopoda, whichbelongsto the closer relationship of the proboscis and manatees and whose most famous member is the gigantic arsinoitherium . The genus occurred at the beginning of the Lower Eocene around 55 million years ago in northern Africa . Finds are documented from Morocco and consist of skull parts and remains of teeth. They were discovered in the important fossil site of the Ouled Abdoun Basin . It is the smallest and oldest representative of the order to date. It differs from other members of the group, in addition to the smaller body size, mainly through teeth with low crowns and less specialized and more original chewing surface patterns. The finds received their first scientific description in 2018. Two types are known.

features

Stylolophus was a small representative of the Embrithopoda. Based on the size of the teeth, a body weight of 20 to 40 kg was determined for smaller individuals, for larger animals the corresponding values ​​are 60 to 128 kg with an estimated body length of 1 to 1.5 m. So far, several crushed skulls and fragments of teeth have been proven. A total length of 22 cm for smaller and 30 cm for larger representatives can be reconstructed for them. More or less the rostral sections of the skull are preserved. The median jawbone was high and long, and probably broadly connected to the nasal bone . As a result, the upper jaw probably had no part of the interior of the nose. In the case of Arsinoitherium , there was no contact between the middle jawbone and the frontal bone in Stylolophus . On the upper jaw, the zygomatic arch started relatively close above the row of teeth. It began approximately at the level of the second and third molars and thus shifted a little further back than with Namatherium . The infraorbital foramen opened above the second or third premolar . The anterior margin of the orbit was correspondingly at the fourth premolar, which is more advanced than in Namatherium and Arsinoitherium , but roughly corresponds to the position in Palaeoamasia . The nasal bone, which was relatively elongated, had extremely large paranasal sinuses . Obviously, these are homologous formations to the anterior bony horns in Arsinoitherium . Most of the anterior sections of the lower jaw are known, the ascending branch has only survived in fragments. The horizontal bone body was relatively low, but varied intraspecifically. The symphysis at the anterior end reached back to the second premolar. A single mental foramen lay below the second premolar and opened wide forward. The mandibular foramen was slit-shaped and clearly formed behind the row of teeth, but in a very low position. It led into the bone at an angle of around 45 °. Shape, position and development are very similar to Palaeoamasia .

The dentition consisted of the complete teeth of the higher mammals with a total of 44 teeth. It consists of three incisors , one canine, four premolars and three molars per half of the jaw. Unique features can be found on the front teeth. The two inner, lower incisors protruded at an angle, so they were procumbent , and they were relatively large. In addition, the innermost incisor had an open root, which can be interpreted as a sign of hypertrophy with high ( hypsodontic ) tooth crowns. Occasionally, such features can still be found in Arsinoitherium , the terminal form of the Embrithopoda, but here the incisors were usually relatively small. In agreement with other early representatives of the Embrithopoda from the group of the Palaeoamasidae, the molars had relatively low ( brachyodonte ) tooth crowns, in contrast to the more developed forms of the Arsinoitheriidae . As with all members of the Embrithopoda, they marked a chewing surface with two transverse tooth ridges ( bilophodont ). In the case of Stylolophus , however , this feature was still quite originally designed. This is expressed in the fact that the two main cusps on the cheek side of the tooth (para- and metaconus) were not yet shifted so clearly towards the tongue side. Furthermore, there were individual secondary shear bars between these, whereby the ectoloph, the cheek-side cutting edge of the tooth, exhibited a conspicuously W-shaped course. The trait is known as bilambdodont and is also found in some immediate ancestors of the Paenungulata such as Ocepeia and Abdounodus . Due to the little lingual displacement of the para- and metaconus in Stylolophus , the cusp structures there, such as the pseudohypoconus and the protoconus, were not yet as strongly reduced as in later Embrithopoda. The anterior premolars were also not so clearly molarized. The second premolar in each case had only one root in the lower jaw, but two in the upper jaw. The roots of the two anterior molars of the upper jaw were separated below the para- and protoconus.

Fossil finds

The Ouled Abdoun Basin in Morocco

The previously known material from Stylolophus came to light in the Ouled Abdoun Basin in Morocco . The basin extends around 70 km south of Casablanca and is particularly important for its rich phosphate deposits . These had formed in the transition from the Cretaceous to the early Paleogene . The phosphate-containing sediments reach a thickness of 30 to 300 m from north to south and cover a period of around 25 million years ( Maastrichtian to Ypresian 72 to 48 million years ago), but they are not continuously developed. The deposition sequences of the Ouled Abdoun Basin are among the most extensive of their kind in the area of ​​the former Tethys Ocean . A relatively shallow and warm sea close to the mainland can be assumed as the educational environment. The very rich fossil material includes marine life and also terrestrial vertebrates. A total of around 330 species have been recorded so far, including the oldest remains of Cenozoic mammals in all of Africa. The Ouled-Abdoun-Basin gained fame among other things for the primeval trunk animals that appear here , such as Eritherium , Phosphatherium and Daouitherium , which are among the earliest forms of all. In addition, there are also relatively original mammals such as Ocepeia and Abdounodus , which are close to the Afrotheria. Most of the fossils are found in phosphate mining. This takes place in several mines spread across the Ouled Abdoun Basin.

The remains of Stylolophus have been found in different areas of the Ouled Abdoun Basin. The fossils defining the genus were found in the northeastern part, more precisely in Grand Daoui and Sidi Chennane. They were stored there in the area between the so-called Bed I and Bed II (the find area is more precisely called Intercalary phosphate beds II / I ), which in turn are located above the very fossil-rich deposits of the Paleocene . The find section is characterized by a phosphate- containing limestone , from which, among other things, phosphatherium was documented. It is possible that individual finds can also be assigned directly to Bed I , which consists of phosphate-containing sands and marls . In both cases, the finds indicate an age at the beginning of the Lower Eocene (Ypresian) around 55 million years ago. They are made up of individual parts of the skull and fragments of teeth. Other fossils have been made in the same region, but their exact location is unknown. Presumably they came to light stratigraphically somewhat higher sections ( Sillon A and Sillon B ), which also contain phosphate-rich sands and belong to the Ypresium. This is mainly a crushed skull made up of the snout area and adjacent parts of the brain skull.

Paleobiology

On the basis of the lower jaw, which has so far only been documented for the smaller representatives of Stylolophus , differently developed forms can be recognized. In some specimens, the horizontal bone body is very low with a height of 1.8 to 2.6 cm below the second molar. Others are up to 29% higher based on the characteristic. It may be a sexual dimorphism with stronger lower jaws in males.

Systematics

Internal structure of the Embrithopoda according to Gheerbrant et al. 2020
 Embrithopoda  

 Stylolophus


   

 Palaeoamasia


   

 Hypsamasia


   

 Crivadiatherium


   

 Namatherium


   

 Arsinoitherium


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Template: Klade / Maintenance / Style

Stylolophus is a genus from the extinct group of Embrithopoda . The Embrithopoda occurred in the Eocene and Oligocene in large parts of what is now Africa and western Eurasia . But they were neither particularly diverse nor very numerous. In general, these are relatively large animals, their most important and best-known representative is the huge arsinoitherium , which at its time was one of the largest land-living mammals along with some members of the proboscis . The closest relatives of the Embrithopoda are the proboscis and the manatees , which is supported by the middle jawbone , which is extended backwards and in contact with the frontal bone . All three groups together form the superordinate taxon of the Tethytheria , which in turn form the Paenungulata together with the sleepers . The latter are placed in the superordinate order of Afrotheria , one of the four main lines within the higher mammals .

In addition to Stylolophus , five other genera are distinguished within the Embrithopoda, which are divided into two families : the Palaeoamasidae and the Arsinoitheriidae . The former are a rather primitive strand of the group that has so far only been handed down from the Eurasian part of the distribution, but usually only bits of teeth are present. The Arsinoitheriidae contain the more modern forms, including the terminal form Arsinoitherium , which are characterized by high-crowned teeth. They are largely occupied from Africa and the Arabian Peninsula . Stylolophus can be classified as particularly original within the Embrithopoda, evidence of this can be found in the comparatively small body size and the very low-crowned molars. In particular, the design of the molars with their bilambdodontic chewing surface pattern, the cusps that have only slightly migrated on the cheek side and the hardly reduced cusps on the tongue side suggest an early split in the tribal history of the Embrithopoda. In phylogenetic studies, Stylolophus is therefore a sister form to all other representatives of the Embrithopoda. Based on the original characteristics, the Stylolophidae family of its own was introduced for Stylolophus in 2020.

The genus Stylolophus was first scientifically described in 2018 by Emmanuel Gheerbrant and fellow researchers . The research group referred to the finds from the Ouled Abdoun Basin in Morocco . The holotype (copy number OCP DEK / GE 668) was a right lower jaw with the roots of the first two incisors , the alveoli of the third incisor, the canine and the second premolar as well as the contained first premolar and the complete row of teeth from the third premolar to the last molar selected. The length of the molar row in the piece is 38.5 mm. Together with other found objects such as a left lower jaw branch and a partial skull, it can be assigned to a smaller individual. The material in the region which was found intercalary beds II / I . The generic name Stylolophus is of Greek origin and is made up of the words στύλος ( stylos ) for “column” or “pillar” and λόφος ( lophos ) for “hill” or “comb”, it refers to the special tooth structure of the upper jaw molars.

There are two species within the genus:

S. minor forms the type species of Stylolophus and was defined at the same time as the genus. In addition, it is the older and smaller representative, whose row of molars on the skull reaches a length of 40 mm. This is also referred to by the specific epithet minor , which comes from Latin and translates as "smaller". The skull find from the higher elevations - probably Sillon A or Sillon B - of the Ouled-Abdoun Basin represents a larger individual. The total length of the upper molar row is 55.3 mm, which makes a difference of 30 to 40% compared to S. minor . In addition, individual differences in the tooth features can be seen. The first authors of the genus suspected that it is another separate species. This was named S. major in 2020 .

literature

  • Emmanuel Gheerbrant, Arnaud Schmitt and László Kocsis: Early African Fossils Elucidate the Origin of Embrithopod Mammals. Current Biology 28 (13), 2018, pp. 2167–2173, doi: 10.1016 / j.cub.2018.05.032
  • Emmanuel Gheerbrant, Fatima Khaldoune, Arnaud Schmitt and Rodolphe Tabuce: Earliest embrithopod mammals (Afrotheria, Tethytheria) from the early Eocene of Morocco: anatomy, systematics and phylogenetic significance. Journal of Mammalian Evolution, 2020, doi: 10.1007 / s10914-020-09509-6

Individual evidence

  1. ↑ a b c d e f g Emmanuel Gheerbrant, Arnaud Schmitt and László Kocsis: Early African Fossils Elucidate the Origin of Embrithopod Mammals. Current Biology 28 (13), 2018, pp. 2167–2173, doi: 10.1016 / j.cub.2018.05.032
  2. ↑ a b c d e f g Emmanuel Gheerbrant, Fatima Khaldoune, Arnaud Schmitt and Rodolphe Tabuce: Earliest embrithopod mammals (Afrotheria, Tethytheria) from the early Eocene of Morocco: anatomy, systematics and phylogenetic significance. Journal of Mammalian Evolution, 2020, doi: 10.1007 / s10914-020-09509-6
  3. ↑ a b c Emmanuel Gheerbrant, Mbarek Amaghzaz, Baadi Bouya, Florent Goussard and Charlène Letenneur: Ocepeia (Middle Paleocene of Morocco): The Oldest Skull of an Afrotherian Mammal. PLoS One 9 (1), 2014, p. E89739, doi: 10.1371 / journal.pone.0089739
  4. ^ A b Emmanuel Gheerbrant, Andrea Filippo and Arnaud Schmitt: Convergence of Afrotherian and Laurasiatherian Ungulate-Like Mammals: First Morphological Evidence from the Paleocene of Morocco. PLoS ONE 11 (7), 2016, p. E0157556, doi: 10.1371 / journal.pone.0157556
  5. ↑ Emmanuel Gheerbrant, Jean Sudre, Henri Cappetta, Mohamed Iarochene, Mbarek Amaghzaz and Baâdi Bouya: A mew large mammal from the Ypresian of Morocco: Evidence of surprising diversity of early proboscideans. Acta Palaeontologica Polonica 47 (3), 2002, pp. 493-506
  6. ↑ Emmanuel Gheerbrant: Paleocene emergence of elephant relatives and the rapid radiation of African ungulates. PNAS. 106 (6), 2009, pp. 10717-10721
  7. ^ Cyrille Delmer: Reassessment of the generic attribution of Numidotherium savagei and the homologies of lower incisors in proboscideans. Acta Palaeontologica Polonica 54 (4), 2009, pp. 561-580
  8. ^ A b Johan Yans, M'Barek Amaghzaz, Baâdi Bouya, Henri Cappetta, Paola Iacumin, László Kocsis, Mustapha Mouflih, Omar Selloum, Sevket Sen, Jean-Yves Storme and Emmanuel Gheerbrant: First carbon isotope chemostratigraphy of the Ouled Abdounate phosphate basin , Morocco; implications for dating and evolution of earliest African placental mammals. Gondwana Research 25, 2014, pp. 257-269
  9. ↑ a b Lászlό Kocsis, Emmanuel Gheerbrant, Mustapha Mouflih, Henri Cappetta, Johan Yans and Mbarek Amaghzaz: Comprehensive stable isotope investigation of marine biogenic apatite from the late Cretaceous – early Eocene phosphate series of Morocco. Palaeogeography, Palaeoclimatology, Palaeoecology 394 (2014) 74-88
  10. ↑ Emmanuel Gheerbrant, Jean Sudre, Henri Cappetta, Cécile Mourer-Chauviré, Estelle Bourdon, Mohamed Iarochene, Mbarek Amaghzaz and Baâdi Bouya: Les localités à mammifères des carrières de Grand Daoui, bassin des Ouled Abdoun, Maroc, Yprésien des lieuxatétat ét . Bulletin de la Societe Geologique de France 174 (3): 279-293
  11. ^ Nicholas Court: The skull of Arsinoitherium (Mammalia, Embrithopoda) and the higher order interrelationships of Ungulates. Palaeovertebrata 22 (1), 1992, pp. 1-43
  12. ↑ Rodolphe Tabuce, Robert J. Asher and Thomas Lehmann: Afrotherian mammals: a review of current data. Mammalia 72, 2008, pp. 2-14, doi: 10.1515 / MAMM.2008.004
  13. ↑ Sevket Sen: Dispersal of African mammals in Eurasia during the Cenozoic: Ways and whys. Geobios 46, 2013, pp. 159-172