Epirigenys
| Epirigenys | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Upper Oligocene | ||||||||||||
| 28 million years | ||||||||||||
| Locations | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Epirigenys | ||||||||||||
| Lihoreau , Boisserie , Manthi & Ducrocq , 2015 | ||||||||||||
Epirigenys is an extinct genus in the Anthracotheriidae family. It is alsoclose tothe hippos and is seen as their sister group. However, it is only known from a few molars and a remains of the lower jaw that were found in eastern Africa . The fossil remains date to the beginning of the Upper Oligocene and are therefore around 28 million years old. The first scientific description of the genus took place in 2015. With the discovery of Epirigenys , the previously only suspected relationship between the anthracotheria and the hippopotamus could be confirmed. In addition, it suggests that the hippos originated in Africa and shows that this group has a long history of evolution on the continent.
description
Epirigenys represented a medium-sized representative of the Anthracotheriidae and had an estimated body weight of 78 to 91 kg. However, the genus is only known through a left lower jaw fragment with the attached teeth from the last premolar to the second molar and over two dozen isolated teeth and tooth fragments. With the exception of a single, spatula-shaped incisor , all of these are remnants of the posterior dentition. The molars showed features that are typical of both the anthracotheria and the early hippos . They had low ( brachyodonte ) tooth crowns and a chewing surface pattern that mediated between bunodont (bumpy) and selenodont (with ridges running along the tooth axis) ( bunoselenodont ). The cusps were broadly arched with blunt-shaped ridges in between, which made them not quite as pronounced as in the closely related Bothriogenys . The upper molars were characterized by five main cusps, three of which formed a unit in the rear tooth area. Both the early hippos and the early anthracotheria had a similar tooth design. The cingulum of the upper molars, a tongue-side bulge that attaches to the tooth base, was significantly higher in Epirigenys than in Bothriogenys - about halfway up the cusp tips - and thus also resembled the early hippos. In contrast to the hippopotamuses, however, there were some additional smaller cusps on the cheek side of the molars, such as the parastyle and the mesostyle. The lower molars had a significantly wider and shorter shape compared to Bothriogenys , which is particularly similar to the hippopotamus on the second posterior molar. Overall, the lower back molars had only four low cusps with slightly indented valleys in between. The upper second molar was 2.0 cm long and 2.1 cm wide, the lower second molar was 1.8 cm long and 1.2 cm wide.
Reference
All previously known finds of Epirigenys were discovered on the Lokone Hills (Lokone Horst) in the Lokichar Basin southwest of Lake Turkana in northern Kenya . The Lokichar Basin is now around 615 m above sea level and is characterized by a very dry climate with around 180 mm of precipitation annually. It belongs to a series of five south-north oriented basins in the East African Rift (Lokichar, North Lokichar, North Kerio, Lothidok and Turkana Basins), in which sediments from the Paleogene to the Pliocene were deposited. They owe their emergence to the drift movements that also led to the formation of the Great African Rift Valley. Intensive seismic investigations revealed a semi-trench-shaped structure for the Lokichar Basin, 60 km long, 30 km wide and 7 km deep. In the west the basin is bounded by the Lokichar fold ( Lokichar Fault ), in the east the chain of hills of the Lokone Horst ( Lokone Fault ) form the boundary. The structurally similar North Kerio Basin extends beyond the range of hills .
7 km thick basin fill superimposed on Precambrian formed gneisses and amphibolites . It consists of a series of coarse and fine sandstones , some of which are enriched with kerogen . With the sandstone layers, sometimes several hundred meters thick, a distinction can be made between Lokone sandstone and Auwerwe sandstone formations . The older Lokone Sandstone Formation in turn consists of the Loperot Shale Member and the Lokone Shale Member . The lower stratum is roughly Eocene to Oligocene in age, the upper series dates from the Upper Oligocene to the Lower Miocene . The sandstones were deposited in an extensive freshwater lake, the size of which was at least 50 km long and 20 km wide. It existed under humid climatic conditions with annual rainfall of at least 1000 mm. Pollen analyzes showed that the bank areas of the lake were covered with forests with partly deciduous trees and with damp rainforests. The landscape at that time was roughly similar to that which exists today on the southern edge of the Congo Basin .
So far, several fossil-bearing sites in the Lokichar Basin are known (LOK 1, 2, 12 and 13), each of which belongs to the Lokone Shale Member . They are all located on the eastern edge of the basin in the area of the Lokone Horst. Locations LOK 1, 2 and 12 are associated with coarse to very coarse, partially firmly bonded, quartz-rich sandstones that are several decimeters thick and may be traced back to former rivers in the lake. LOK 13 corresponds to fine-grained, mollusc-rich sandstones that indicate an original shore area. More than 100 fossil remains were found in the sites. The extensive mammal material, mostly isolated teeth, belongs to five animal orders. In addition to Epirigenys , which is by far the most common mammal in Lokons and occurs in both sites, rodents such as Metaphiomys , original carnivores such as Apterodon from the Creodonta group or the primitive proboscis Phiomia were discovered. There are also fish and turtles. Two other layers of finds in the hanging wall of those already mentioned do not contain any mammalian material, but crocodiles appear there. The composition of the fauna suggests a position in the Upper Oligocene and is therefore around 28 million years old. Along with the Fayyum in Egypt, Chilga in Ethiopia or Lothidok, which is also located in northern Kenya, Lokone is one of the few sites with an Oligocene period that are known to date from Africa.
Systematics
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Internal systematics of the hippopotamoid according to Lihoreau et al. 2015
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Epirigenys is a genus from the family of Anthracotheriidae , an extinct group of mammals , as the closest relatives of hippopotamus be considered (Hippopotamidae). Both families, the Anthracotheriidae and the Hippopotamidae, together form a monophyletic group that bears the name Hippopotamoidea and thus has the status of a superfamily. As today's hippos, representing the order of the cloven-hoofed animals also (Artiodactyla), the sister group of whales (Cetacea) within its parent taxon Cetartiodactyla represent, they are again under the group of Cetancodonta summarized. Within the anthracotheria, Epirigenys is close to the primitive forms from the group of Bothriodontinae , to which Bothriogenys also belongs. Due to the common combination of features of these early anthracotheria and those of the hippopotamuses occurring on the molars, Epirigenys is regarded as a sister group of the hippos according to phylogenetic studies. However, the deep embedding of the hippos within the anthracotheria means that the latter are to be regarded as paraphyletic .
The origin of the hippopotamus was unclear for a long time. In addition, there was a serious time gap between the first appearance of the whales in the Lower Eocene around 53.5 million years ago and that of the hippos in the Lower Miocene around 21 million years ago, which led to the assumption of a line of spirits lasting over 30 million years . In addition to a derivation of the hippopotamus from the pig species (Suina) or the extinct Cebochoeridae , primitive even-toed ungulates from the Eocene, the Anthracotheriidae were the preferred group of origin. These occurred for the first time at the end of the Middle Eocene in Eurasia and soon spread to North America and Africa , where they were first detected in the Upper Eocene with Bothriogenys in the Fayyum. In general, the anthracotheria can be described as relatively large, pig-like animals, which were characterized above all by a typical bunodont (humped) to selenodont (with crescent moon-shaped longitudinal ridges) pronounced chewing surface pattern on the surfaces of the molars. The selenodontia of the molars increased significantly in the course of the history of the Anthracotherien tribes, as well as the shape of the skull with wider snouts became significantly more hippopotamus. The hippos, on the other hand, never developed a clearly selenodontic tooth pattern and largely retained the bumpy chewing pattern. Both groups, however, have the reduction of the fifth main tubercle on the upper molars in the course of development.
The central point of discussion was whether the hippos were descended from an older ( bunoselenodont ) or a younger ( selenodont ) development phase of the anthracotheria. Sometimes a relatively late branching was assumed and the resulting regression of the selenodontic chewing surface design in hippos was explained by the more plastic behavior of the mammalian teeth during evolution. The question also arose whether the origin of the hippos, whose oldest fossil record comes from Africa, was possibly in Eurasia and whether immigration to Africa only took place with the creation of the land bridge over the closing Tethys Ocean in the Lower Mioocene. The discovery of Epirigenys now shows that the ancestry of the hippopotamus can be derived from rather primitive anthracotheria. She was also able to prove that the hippos originated in Africa and developed from the descendants of the first immigrants of the Anthracotherien, who reached the continent from Eurasia long before the land bridge was formed. Accordingly, the hippos are an integral part of the African fauna and remained endemic there for a long time . Only in the course of the Upper Miocene did they conquer large areas of Eurasia. Last but not least, the permanent integration of the hippopotamus into the line of development of the anthracotheria shortened the time gap between the first appearance of the whales and the ancestors of the hippopotamus (Hippopotamoidea) to around 10 million years.
Discovery story
The first fossil-bearing find layers in Lokone (LOK 1) were discovered by Meave Leakey in 1994 and 1995 , whereby the partially preserved lower jaw came to light during this first investigation. Then the research group Lokone Paleontological Research Project (LPRP) was founded, which carried out an initial intensive exploration of the region in November and December 2007. As a result, three more layers (LOK 2, 12 and 13) could be recorded. A large number of fossil material also came to light, which also included other remains of Epirigenys . The scientists involved published a first preliminary report on the fossils found in 2010. In this they assigned the tooth remnants, which are now referred to Epirigenys , to the Anthracotheriidae, but, due to their design, attested a closer relationship to the Bothriodontinae and especially to Bothriogenys .
The first scientific description of Epirigenys followed in 2015 by Fabrice Lihoreau and fellow researchers. The left branch of the lower jaw with the preserved teeth from the last premolar to the second molar was selected as the holotype (specimen number KNM -LH 30375), which is one of the first discoveries. It is kept in the Kenya National Museum . So far, one species is known with Epirigenys lokonensis . The generic name Epirigenys is made up of the Turkana word epiri for "hippopotamus" and the Greek word γένειον or γένῠς ( geneion or genys ). This is a play on words, since the word syllable -γεν- ( -gen- ) can mean “origin” or “line”, but the word genys also means “chin” or “lower jaw”. In both cases, the generic name alludes to the close relationship to the hippos. The species name lokonensis is a reference to the location of the fossils.
Individual evidence
- ↑ a b c d e f g Fabrice Lihoreau, Jean-Renaud Boisserie, Fredrick Kyalo Manthi and Stéphane Ducrocq: Hippos stem from the longest sequence of terrestrial cetartiodactyl evolution in Africa. Nature Communications 6, 2015 doi : 10.1038 / ncomms7264
- ^ A b Annie Vincens, Jean-Jacques Tiercelin and Guillaume Buchet: New Oligocene – early Miocene microflora from the southwestern Turkana Basin Palaeoenvironmental implications in the northern Kenya Rift. Palaeogeography, Palaeoclimatology, Palaeoecology 239, 2006, pp. 470-486
- ↑ a b c d Stéphane Durocq, Jean-Renaud Boisserie, Jean-Jaques Tiercelin, Cyrille Delmer, Eraldine Garcia, Manthi Fredrick Kyalo, Meave G. Leakey, Laurent Marivaux, Olga Otero, Stéphane Peigné, Pascal Tassy and Fabrice Lihoreau: New Oligocene Vertebrate Localities from Northern Kenya (Turkana Basin). Journal of Vertebrate Paleontology 30 (1), 2010, pp. 293-299
- ↑ MR Talbot, CK Morley, J.-J. Tiercelin, A. Le Hérissé, J.-L. Potdevin and B. Le Gall: Hydrocarbon potential of the Meso-Cenozoic Turkana Depression, northern Kenya. II. Source rocks: quality, maturation, depositional environments and structural control. Marine and Petroleum Geology 21, 2004, pp. 63-78
- ^ Meave Leakey , Ari Grossman, Mercedes Gutiérrez and John G. Fleagle : Faunal Change in the Turkana Basin during the Late Oligocene and Miocene. Evolutionary Anthropology 20, 2011, pp. 238-253
- ^ A b c Jean-Renaud Boisserie, Fabrice Lihoreau and Michel Brunet : The position of Hippopotamidae within Cetartiodactyla. PNAS 102 (5), 2005, pp. 1537-1541
- ↑ a b c Maeva Orliac, Jean-Renaud Boisserie, Laura MacLatchy and Fabrice Lihoreau: Early Miocene hippopotamids (Cetartiodactyla) constrain the phylogenetic and spatiotemporal settings of hippopotamid origin. PNAS 107 (26), 2010, pp. 11871-11876