Losodokodon

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Losodokodon
Temporal occurrence
Upper Oligocene
27 to 24 million years
Locations
  • Lothidok (Kenya)
Systematics
Paenungulata
Tethytheria
Russell animals (Proboscidea)
Elephantimorpha
Mammutids (Mammutidae)
Losodokodon
Scientific name
Losodokodon
Rasmussen & Gutiérrez , 2009

Losodokodon is a now extinct genus from the order of the proboscis thatlivedin eastern Africa in the Upper Oligocene about 27 to 24 million years ago. She is only known for more than a dozen molar teeth. Their special chewing surface pattern, consisting of conical cusps arranged in pairs, refers Losodokodon to the Mammutidae family, an ancient group of proboscis that separated around 28 million years ago from the line that led to today's elephants . Losodokodon is the oldest member of the Mammutidae in phylogenetic terms. Its discovery means that the origin and development of the more modern proboscis, following from the primeval forms of the Eocene , must be set much earlier than previously assumed.

description

Losodokodon is a medium-sized representative of the proboscis, but it was smaller than the closely related Eozygodon . The genus has so far only been proven for a total of six molars, which include the rearmost right and second left, upper molar including a remnant of the palatine bone , three premolars and several fragments of another tooth. The rearmost molar was 9.2 cm long and 3.1 cm high, the second was slightly smaller at 8.5 cm in length and 2.8 cm in height. Both teeth had three highly conical enamel cusps arranged in pairs on the chewing surface , the respective pairs standing transversely to the longitudinal direction of the teeth. This gave the molars a trilophodontic structure, as is typical of the early relatives of modern elephants. The molars are thus more developed than the posterior molars of the earliest proboscis from the group of Plesielephantiformes, characterized by two pairs of cusps ( bilophodont ). At the cusps, the molars reached a width of over 5 cm. In contrast to other representatives of the mammoths, the molars were built much narrower overall. The humps of the individual pairs were connected to one another by a low bar and not, as in the case of the Eozygodon or Zygolophodon, by additional smaller, inner humps. Overall, the molars can thus be described as primitive zygodont , which is to be regarded as a preliminary stage for the typically zygodont to zygolophodont molars of the Mammutidae . The individual hump ridges were separated by deep, sometimes V-shaped valleys. Also in contrast to Eozygodon , the third row of cusps was not fully developed, but there was already a slight approach to a fourth on the rearmost molar, which shows the transition to Eozygodon with a fully formed fourth ridge, consisting of two cusps. The premolars were bilophodontically shaped (with only two rows of enamel cusps arranged in pairs), and the cusps also had a significantly more humped shape, so that the teeth appeared more bunodontic . At about 4.5 cm, they only reached about half the molar length.

Location

The Losodokodon finds come from the Eragaleit beds , a series of clay / silt stones and conglomerates, as well as other sediments deposited between two streams of the Kalakol basalts . The Kalakol basalts form the base of the Lothidok Mountains in northwestern Kenya , about 30 km west of Lake Turkana in the East African Rift , and are a total of 785 m thick. They consist of more than 20 individual streams between 4 and 60 m thick. Between the individual streams lie thin sediment packages , of which the Eragaleit beds are the most powerful and also represent the only fossil-bearing layer. It is located about 600 m below the upper edge of the Kalakol basalts. The great hardness of the basalts causes only a few erosive outcrops, most of them exist in the river valleys of the Nathuraa and Eragaleit . The largest outcrop, located in the Eragaleit valley, reaches a height of 50 m. The basalts were dated radiometrically using the potassium-argon method . The data for the basalt current below the Eragaleit beds are between 26.9 and 28 million years, for the one above between 24.01 and 24.04 million years. Thus, the finds can be dated to the late Oligocene . The Oligocene age of the finds is also indicated by some additional finds of typically African origin, such as Arsinoitherium and various representatives of the hyrax , as well as some primates such as Kamoyapithecus , which was one of the first fossils to be described from the Eragaleit beds . Altogether more than a dozen taxa on mammals are known, whereby the fauna has a transitional character with older forms from the outgoing Paleogene and new ones , which lead over to the following Neogene ( African Mid-Tertiary Event = AMTE). The basalts are overlaid by the Lothidok Formation , which is very rich in fossils and belongs to the Miocene .

Systematics

Losodokodon is a genus within the order of the proboscis and, with its trilophodontic structure of the anterior molars, belongs to the group of elephantiformes , which with this characteristic distinguish themselves from the even older proboscid forms of the Plesielephantiformes with only two ridges on the first two molars. The construction of the molars with the zygodonten Kauflächenmuster refers the genus in the family of Mammutidae , an extinct group of distant relative of today's elephant (Elephantidae). Where Losodokodon as the oldest member of the undoubted Mammutiden. Is closely related Eozygodon that from the Lower Miozän is detected the eastern and southern Africa. The mammoths originated in Africa and in the course of their tribal history they also populated large parts of Eurasia and North America . At least in North America, they persisted with the genus Mammut until the end of the Pleistocene . According to molecular genetic studies, determined on finds of Pleistocene mammoth fossils in comparison with today's representatives of the proboscis and the mammoth , which is also extinct , the line of mammoths separated from the line leading to today's elephants around 24 to 28 million years ago.

Since the molars of Losodokodon are rather primitive and not as distinctly zygodontic as those of its successors, the discovery of this genus showed that the mammoths also emerged from a group of proboscis with bunodont teeth (i.e. teeth with a bumpy chewing surface pattern) as has been demonstrated for many other lines of proboscis. Only in the course of further development did the typical zygodontic to zygolophodontic tooth structure emerge. Losodokodon also closes a gap in the fossil record of the proboscis in Africa. In comparison to the numerous finds of the Eocene and the Lower Oligocene, only a few representatives of this order have been found from the Upper Oligocene . The few known forms of this time include Chilgatherium , a member of the Deinotheria, as well as Eritrea , a genus that may be at the base of the development to the Gomphotheria , and still a very early representative of Gomphotherium itself. With the discovery of Losodokodon is now the The phylogenetic division of the Gomphotherien and Mammutiden dates back far back to the Oligocene, and the origin of the Elephantiformes must be set much earlier than originally thought.

Discovery story

The first investigations on the Eragaleit beds were carried out by Camille Arambourg in the 1930s and published scientifically in 1933. This was followed by work at the University of California , beginning in 1948, during which numerous fossil material was discovered, including the upper jaw remains of a primate, which was scientifically described in 1980 as the first fossil from the Eragaleit beds under the name Proconsul hamiltoni (renamed Kamoyapithecus hamiltoni in 1995 ). The expeditions of Richard and Meave Leakey in the 1980s, during which extensive additional primate material could be discovered, are significant . In 1992 an extensive review of the geological situation of the Lothidok Mountains took place , which also included absolute age measurements. The extensive fossil material, most of which is kept in the Kenya National Museum , has been scientifically processed since 2006, and the independence of Losodokodon could also be recognized.

The first scientific description of Losodokodon was in 2009 by D. Tab Rasmussen and Mercedes Gutiérrez . The holotype (copy number KNM -LS 18244) includes the third right and second left maxillary molars with attached palatal bone. With Losodokodon losodokius only one species is known. The generic name Losodokodon is made up of “Losodok”, the local name and pronunciation of the Turkana for the Lothidok Mountains, and the Greek word ὀδούς ( odū́s “tooth”). The species addition also indicates the location of the find.

Individual evidence

  1. ↑ a b c d e D. Tab Rasmussen and Mercedes Gutiérrez: A Mammalian fauna from the Late Oligocene of Northwestern Kenya. Palaeontographica Department A 288 (1-3), 2009, pp. 1-52
  2. ↑ a b H. B. Boschetto, FH Brown and I. McDougall: Stratigraphy of the Lothidok Range, northern Kenya, and K / Ar ages of its Miocene primates. Journal of Human Evolution 22, 1992, pp. 47-71
  3. ^ Mercedes Gutiérrez and D. Tab Rasmussen: Late Oligocene Mammals from Northern Kenya. Journal of Vertebrate Paleontology 27 (3 suppl.), 2007, p. 85A
  4. ↑ Jehezekel Shoshani1, William J. Sanders and Pascal Tassy: Elephants and other Proboscideans: a summary of recent findings and new taxonomic suggestions. In: G. Cavarretta et al. (Eds.): The World of Elephants - International Congress. Consiglio Nazionale delle Ricerche, Rome, 2001, pp. 676-679
  5. ^ A b William J. Sanders, Emmanuel Gheerbrant, John M. Harris, Haruo Saegusa and Cyrille Delmer: Proboscidea. In: Lars Werdelin and William Joseph Sanders (eds.): Cenozoic Mammals of Africa. University of California Press, Berkeley, London, New York, 2010, pp. 161-251
  6. ↑ Nadin Rohland, Anna-Sapfo Malaspinas, Joshua L. Pollack, Montgomery Slatkin, Paul Matheus and Michael Hofreiter: Proboscidean Mitogenomics: Chronology and Mode of Elephant Evolution Using Mastodon as Outgroup. PLoS Biology 5 (1), 2007, p. E207