Birkamys
| Birkamys | ||||||||||||
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Lower jaw of Birkamys |
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| Temporal occurrence | ||||||||||||
| Upper Eocene to Lower Oligocene | ||||||||||||
| 34 to 28 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Birkamys | ||||||||||||
| Sallam & Seiffert , 2016 | ||||||||||||
Birkamys is an extinct genus from the group of porcupine relatives . Within this, it is regarded as a very original form of the Phiomorpha , which includes today's pipe rats and rock rats . Finds of Birkamys come from the important fossil deposit of the Fayyum area in northern Egypt as well as from individual other sites in the region. There are predominantly parts of the skull and the dentition, which have come down to us in a highly fragmented manner. The animals were comparatively small, characteristic of porcupinesReminiscent skull and lower jaw structure as well as a large opening on the palate . As a specialty, as with some of today's Phiomorpha, the premier teeth were not replaced during individual development, so that the milk teeth remained permanently in the dentition. The genus was introduced in 2016.
features
Birkamys is a small representative of the Hystricognathi and one of the smallest known from the paleogene of the Afro-Arab region. There are mainly fragmented skull and dentition remains, but most of the known skull finds are severely crushed. The rostrum was long and narrow. The median jawbone occupied most of the anterior region and extended up to half of the diastema between the anterior and posterior teeth. There was a large opening on the roof of the mouth that arose from the incisive foramen and took up the entire space down to the last two premolars . It is sometimes referred to as the "anterior palatal window". As with other members of the hystricognathi, the infraorbital foramen was very large and also acted as an anchor point for the masseter muscle . It was roughly level with the third premolar. Directly underneath there was a small hump, to the side of which the anterior zygomatic arch section began. A part of the ascending branch is missing from the lower jaws found so far. In general, he was slim. The large angular process protruding to the side made it appear completely hystricognathic. The lower edge of the horizontal bone body was strongly curved and was completely occupied by the alveolus of the first incisor . Two massive bone ribs are separated from the angular process, which framed the masseteric fossa above and below and also served as the attachment point for the masseter muscle. The mental foramen was oval in shape and opened approximately at the level of the last premolar. The articular process only slightly exceeded the chewing surface of the molars , but was again lower than the crown process. The mandibular foramen was located in the area between the articular and crown processes.
The teeth consist of one incisor, two (above) or one (below) premolars and three molars for each arch. The incisors were designed as typical incisor teeth , the lower one had an oval cross-section with lateral narrowing. Tooth enamel was only formed on the front. To the posterior teeth there was a wide diastema, which in the lower jaw took up half the horizontal bone and was deeply indented. With the help of computed tomographic examinations it was possible to prove that in Birkamys the milk premolars existed permanently in the dentition and were therefore not replaced in the course of individual development. The trait is also known from some of today's Phiomorpha. Other early Phiomorpha such as Turkanamys or Acritophiomys, however, still showed a change of teeth in the premolar teeth. The molars of rodents have a complex structure with several humps and ridges or yokes ( called lophen ) running between them , the individual shapes differ in the number of cusps and ridges. The anterior upper premolar (dp3) was small and nail-like, the last premolar (dp4) had a square outline and four cusps on top, a rectangular shape on the bottom and five cusps on the chewing surface. The upper molars were all more or less rectangular in shape. They also had four cusps (Para-, Meta-, Proto- and Hypoconus), which stood together in pairs and were connected to one another by ridges oriented at right angles to the longitudinal axis of the teeth. The protoloph ran between the para- and protoconus and the hypoloph between the meta- and hypoconus. Laterally on the outside of the tooth edge there was another bar, the anteroloph in front of the protoloph and the posteroloph behind the metaloph. This made the upper molars appear tetralophodontic (with four ridges). On the lower molars there were also four large cusps (meta-, ento-, proto- and hypoconid) arranged in pairs, a fifth cusp, the hypoconulid, rose on the rear edge of the tooth. The tongue-side bumps were smaller than the lip-side. A total of three ridges ran between the humps: the metalophulid (between the meta- and protoconid), the hypolophid (between the hypo- and entoconid) and the posterolophid (on the hypoconulid). An additional second Metalophulid between the actual and the Metalophulid Hypolophid was at Birkamys formed only rudimentary and incomplete, unlike, for example at Phiomys and Acritophiomys with a strong and complete additional bar; In addition, Acritophiomys had the mesolophid, another ridge on the foremost molar. The first molar was the largest in the upper dentition, its dimensions were 1.07 mm in length and 1.13 mm in width. In contrast, the second molar in the lower dentition represented the largest tooth in the rear row of teeth. It was 1.1 mm long and 1.06 mm wide. The rearmost molar was relatively small, which differs from Phiomys .
Fossil finds
Extensive finds from Birkamys came to light in the Fayyum area in northern Egypt in North Africa and came from the site L-41 in the lower section of the Gebel Qatrani formation . The site is about 46 m above the base of the rock unit and, according to paleomagnetic investigations, dates to the Upper Eocene ; the absolute age is around 34 million years. L-41 is the oldest site within the formation. It was discovered in 1983 during field investigations by a team of researchers led by Elwyn L. Simons , a more precise idea followed later by D. Tab Rasmussen, among others . It consists of greenish to greenish gray colored clay and silt stones . These are in turn embedded in a sequence of banded sands. The fine-grained deposits can be seen as relics of a former lake. It is characterized by a high proportion of evaporites and a high concentration of sodium chloride . As a result, the overall structure of the site deviates noticeably from the other, more coarse clastic, fluvial deposits of the Gebel-Qatrani Formation. As a further specialty, a high number of skulls with partly associated parts of the body skeleton can be highlighted. L-41 has so far provided fossil remains of fish, amphibians, reptiles, birds and mammals. Among other things, the high variety of snakes , primates , rodents and bats is remarkable. Due to the good preservation of the remains of smaller vertebrates, the site L-41 is of great importance for the Fayyum region. The rodents have hundreds of skeletal elements, including fossils from Birkamys and fossils from Mubhammys , Acritophiomys and Gaudeamus . Strikingly, so far all remains belong to the Hystricognathi, only from the older Qasr-el-Sagha formation in the Fayyum are also remains of thorntail squirrels . However, evidence of the Phiomorpha is missing here. From Birkamys the skull, lower jaw and teeth have been found mainly parts.
Apart from the Fayyum, Birkamys is possibly documented from the Samlat Formation on the extreme western edge of the Sahara near the city of Ad-Dakhla in Western Sahara . The sandy deposits there are the remains of an estuary that existed at the beginning of the Lower Oligocene around 33 million years ago. The mostly isolated teeth are on average slightly larger than those from the Fayyum, which is why the reference to Birkamys is not clear.
Systematics
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Internal systematics of the hystricognathi according to Sallam and Seiffert 2016
To clarify the phylogenetic position, the Caviomorpha and Hystricidae according to Sallam et al. 2011 added |
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Internal systematics of the hystricognathi according to Marivaux and Boivin 2019
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Birkamys is an extinct genus from the group of Hystricognathi within the suborder of porcupine relatives (Hystricomorpha). The Hystricognathi form a diverse group of Eurasian , African and American representatives of the rodents (Rodentia), which include the porcupines (Hystricidae), the guinea pig relatives (Caviomorpha) and the Phiomorpha . Characteristic features are found primarily in the structure of the skull and the lower jaw, in the latter, for example, through the position of the angular process. The entire group may have its origins in Asia , where it was recorded in the Middle Eocene almost 50 million years ago. The present-day African and American distribution areas were reached by the first immigrants at the transition to the Upper Eocene around 40 million years ago.
By designing the molars with three or four ridges, Birkamys is one of the developed representatives of the Hystricognathi, whose original tooth pattern included more ridges. This is expressed, among other things, by the reduction of the middle bar, the mesoloph / mesolophid, which is characteristic of the phiomorpha, among other things. In addition, the milk premolars remain in the teeth in Birkamys , which also agrees with some other Phiomorpha. As a result, a closer relationship to today's cane rats (Thryonomyidae), rock rats (Petromuridae) and sand graves (Bathyergidae) can be assumed. The view is congruent to molecular genetic analyzes, according to which the Phiomorpha in the Upper Eocene had split off from the Caviomorpha around 36 million years ago, while the Hystricidae had already followed their own evolutionary path. According to phylogenetic studies, Mubhammys , whose fossil remains were also found in the Fayyum area , is considered to be the closest relative . An exact assignment to a specific family within the Phiomorpha has not yet been made. The two genera form the sister group to the later Phiomorpha including the recent forms.
According to this view belongs Birkamys next Mubhammys and Acritophiomys the oldest known representatives of Phiomorpha. The discovery of Mubhammys and Birkamys indicates that the suppression of the permanent premolar as a characteristic of the Phiomorpha began as early as the turn of the Eocene to the Oligocene and therefore very early in terms of genealogy . It also proves that the extraordinarily large palatal window is an inclusive characteristic of various early pliomorpha and thus also appeared very early in the development of the hystricognathi. The small size of Birkamys suggests that there must have been rapid changes in size, including dwarfing , within the group over the course of the Eocene. According to a phylogenetic study from 2019, a position of Birkamys together with Mubhammys in a family of Phiocricetomys is also possible. The common group is called "phiocricetomyins" and is characterized by extremely low-crowned molars with flat transverse ridges. Originally considered as members of the Myophiomyidae , the investigation classifies the "phiocricetomyins" outside the Phiomorpha as basal hystricognathi, so the suppression of the permanent premolars would have arisen several times within the porcupine relatives.
The genus Birkamys was first scientifically described in 2016 by Hesham M. Sallam and Erik R. Seiffert . The basis for this was the find material from the site L-41 in the Fayyum area in Egypt . The holotype (copy number CGM 66000) is a partial skull consisting of the rostrum in which the incisors and both rows of teeth from the third premolar to the third molar are preserved. The name Birkamys is derived from the Arabic word birka for "swamp" or "lake" and from the Greek word μῦς ( mŷs ) for "mouse". Birka refers to the reference L-41 . The only known species is B. korai . The specific epithet honors Mahmoud Kora for his contributions to the paleontology and stratigraphy of Egypt. Some researchers suggest that Birkamys is synonymous with Neophiomys , an early representative of phiocricetomyin rodents. However, more revealing fossil material from Neophiomys is required to confirm the assumption .
literature
- Hesham M. Sallam and Erik R. Seiffert: New phiomorph rodents from the latest Eocene of Egypt, and the impact of Bayesian "clock" -based phylogenetic methods on estimates of basal hystricognath relationships and biochronology. PeerJ 4, 2016, p. E1717 doi: 10.7717 / peerj.1717
Individual evidence
- ^ Albert E. Wood: The African Oligocene Rodentia. In: Elwyn L. Simons and Albert E. Wood (Eds.): Early Cenozoic Mammalian Faunas Fayum Province, Egypt. Bulletin of the Peabody Museum of Natural History 28, 1968, pp. 23-105
- ↑ a b c d e f g h Hesham M. Sallam and Erik R. Seiffert: New phiomorph rodents from the latest Eocene of Egypt, and the impact of Bayesian "clock" -based phylogenetic methods on estimates of basal hystricognath relationships and biochronology. PeerJ 4, 2016, p. E1717 doi: 10.7717 / peerj.1717
- ^ Albert E. Wood and RW Wilson: A Suggested Nomenclature for the Cusps of the Cheek Teeth of Rodents. Journal of Paleontology 10 (5), 1936, pp. 388-391
- ^ A b Hesham M. Sallam, Erik R. Seiffert and Elwyn L. Simons: A basal phiomorph (Rodentia, Hystricognathi) from the late Eocene of the Fayum Depression, Egypt. Swiss Journal of Palaeontology 131, 2012, pp. 283-301
- ↑ a b Laurent Marivaux, El Mabrouk Essid, Wissem Marzougui, Hayet Khayati Ammar, Sylvain Adnet, Bernard Marandat, Gilles Marzeraud, Rodolphe Tabuce and Monique Vianey Liaud: A new and primitive species of Protophiomys (Rodentia, Hystricognathi) from the late middle Eocene of Djebel el Kébar, Central Tunisia. Palaeovertebrata 38 (1), 2014, p. E2
- ^ Elwyn L. Simons: Description of two genera and species of Late Eocene Anthropoidea from Egypt. PNAS 86, 1989, pp. 9956-9960
- ↑ D. Tab Rasmussen and Elwyn L. Simons: The oldest hyracoids (Mammalia: Pliohyracidae): new species of Saghatherium and Thyrohyrax from the Fayum. New Yearbook for Geology and Paleontology Abhandlungen 182, 1991, pp. 187-209
- ↑ Laurent Marivaux, Sylvain Adnet, Mohamed Benammi, Johan Yans and Mouloud Benammi: Earliest Oligocene hystricognathous rodents from the Atlantic margin of Northwestern Saharan Africa (Dakhla, Marocco): systematics, paleobiogeographical, and paleoenvironmental implications. Journal of Vertebrate Paleontology 37 (5), 2017, p. E1357567 doi: 1080 / 02724634.2017
- ↑ a b Hesham M. Sallam, Erik R. Seiffert and Elwyn L. Simons: Craniodental Morphology and Systematics of a New Family of Hystricognathous Rodents (Gaudeamuridae) from the Late Eocene and Early Oligocene of Egypt. PLoS ONE 6 (2), 2011, p. E16525 doi: 10.1371 / journal.pone.0016525
- ↑ a b c Laurent Marivaux and Myriam Boivini: Emergence of hystricognathous rodents: Palaeogene fossil record, phylogeny, dental evolution and historical biogeography. Zoological Journal of the Linnean Society, 2019, pp. 1-36
- ^ A b Laurent Marivaux, Monique Vianey-Liaud, Jean-Loup Welcomme and Jean-Jaques Jaeger: The role of Asia in the origin and diversification of hystricognathous rodents. Zoologica Scripta 31, 2002, pp. 225-239
- ↑ René Lavocat: Rodentia and Lagomorpha. In: Vincent J. Maglio and HBS Cooke (Eds.): Evolution of African Mammals. Harvard University Press, 1978, pp. 69-89
- Jump up ↑ Pauline Coster, Mouloud Benammi, Vincent Lazzari, Guillaume Billet, Thomas Martin, Mustafa Salem, Awad Abolhassan Bilal, Yaowalak Chaimanee, Mathieu Schuster, Xavier Valentin, Michel Brunet and Jean-Jacques Jaeger: Gaudeamus lavocati sp. nov. (Rodentia, Hystricognathi) from the early Oligocene of Zallah, Libya: first African caviomorph? Natural Sciences 97 (8), 2010, pp. 697–706
- ^ A b Hesham M. Sallam, Erik R. Seiffert, Michael E. Steiper, and Elwyn L. Simons: Fossil and molecular evidence constrain scenarios for the early evolutionary and biogeographic history of hystricognathous rodents. PNAS 106 (39), 2009, pp. 16722-16727 doi: 10.1073 / pnas.0908702106
