Ukhaatherium
| Ukhaatherium | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Campanium | ||||||||||||
| 83.6 to 72 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Ukhaatherium | ||||||||||||
| Novacek , Rougier , Wible , McKenna , Dashzeveg & Horovitz , 1997 | ||||||||||||
Ukhaatherium is an extinct genus of mammals thatlivedin what is now East Asia during the Upper Cretaceous around 84 to 72 million years ago. It is best known from Ukhaa Tolgod , where it is occupied by eight individuals, six of them completely. In general, Ukhaatherium was quite small with a weight of around 32 g and resembled today's insectivores . In addition, it still had some primeval skeletal features.
features
Ukhaatherium was a small mammal, which weighed only around 32 g and its habitus resembled today's insectivores . Several individuals of adult animals have been handed down, so that a large part of the skeleton is known today. The curved course of the spine was typical, the number of dorsal or thoracic vertebrae was 19 to 20 and thus generally corresponded to all Theria . The cervical vertebrae were still relatively simple and had no or only very small, but mostly typical spinous processes for today's higher mammals . The musculoskeletal system was characterized by elongated bones in the upper arm and thigh . The extremities ended in five rays. Characteristic was the formation of a pouch bone on the pubic bone of the pelvis , which is a primeval feature and does not occur in higher mammals today, but is known in marsupials and even more primitive mammals. Ukhaatherium also showed some peculiarities in the construction of the skull , which differ from today's modern mammals, but which they also connect with them. The skull reached a length of 28 mm and was relatively narrow with only slightly developed zygomatic arches . The nasal bone , which reached far back, was original, but the fact that the edge of the orbit was already formed by part of the upper jaw was more modern . The dentition differed typically for early mammals from that of today's and was more extensive. Five incisors were found in the upper jaw and four in the lower jaw , while the number of premolars had already been reduced to four. Thus Ukhaatherium had a somewhat more modern set of teeth than the original member of Eutheria Eomaia , whereby, contrary to earlier assumptions, not the last (P5), but the middle premolar (P3) was reduced. The entire dental formula was accordingly: . In general the row of teeth was not yet closed, especially the formation of a diastema between the first and second premolar indicates the basal position of ukhaatherium . The canine was relatively large, the first premolar being larger than the second, while, as an advanced feature, the molars were partially elongated, with the largest reaching about 1.6 to 1.9 mm in length. The two rearmost premolars showed a clear molarization, thus resembling the rear molars and like them had a high and conically shaped enamel cusp .
Fossil finds
Since 1990, joint expeditions of the American Museum of Natural History and the Mongolian Academy of Sciences have taken place in the Gobis desert of southern Mongolia to study Upper Cretaceous and Lower Tertiary vertebrates. More than 500 skull remains of mammals alone were discovered at the Ukhaa Tolgod fossil site in the Nemegt Basin, which was discovered in 1993 , together with excellently preserved fossils of dinosaurs - skeletons, eggs and embryos -, birds and lizards . Outstanding were the finds of two mammalian genera from the early stage of the development of Eutheria , including the first known from Ukhaatherium and another from Zalambdalestes . These come from the Djadochta Formation , which is dated in the Campanian around 84 to 72 million years ago. In the following years at least eight different individuals of Ukhaatherium , six of them more or less completely represented by the body skeleton, could study the almost complete skeleton of this early modern mammal, which is one of the best known of all late Mesozoic mammals. Another early mammal was described with Maelestes in 2007, also based on finds from Ukhaa Tolgod.
Paleobiology
It was at Ukhaatherium as with many Cretaceous mammalian animals to a generalized in his physique animal with a quadrupedal, plantigrade (plantigrade) locomotion, adapted to the soil life, but without greater specialization in a very nimble or otherwise, such as grave lifestyle. The presence of the pouch bone on the pelvic girdle is sometimes associated with a shorter gestation period , so that less developed young animals were born, as is still the case with marsupials today. Today only pouches and cloisters have a pouch bone, which has no direct function in reproduction, but is an element of the musculoskeletal system, but restricts the birth canal due to its prominent position. The interpretation of the not fully developed offspring is also supported by the V-shaped formation of the ischial arch, a feature that is also present in marsupials and in today's physically small representatives of the Eutheria with less developed offspring. Only with the extension of the gestation period did the pouch bone recede and the ischial arch expanded.
Systematics
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Internal systematics of Asioryctitheria according to Rook et al. 2013
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Ukhaatherium is a genus of the family of Asioryctidae within the group Asioryctitheria , very early representatives of the higher mammals that lived in the late Cretaceous period. Asioryctes is the closest related genus , both together form the subfamily of Asioryctinae, while Kennalestes is a bit outside of this group. These early mammals, largely confined to Asia, are characterized by the presence of four premolars. Originally all three genera were included in the parent group of Eutheria, but more recent studies see them more as an outgroup, which is why they are now classified as nonplacental Eutheria of lower rank. The Zalambdalestidae , which are already characterized by a further reduced set of teeth, are among the closest related groups .
Ukhaatherium was discovered in 1993; it was first described in 1997 by Michael J. Novacek and fellow researchers. The holotype (copy number PSS-MAE 10) is a skull with an almost complete skeleton. The only way is Ukhaatherium nessovi recognized. The generic name Ukhaatherium is made up of the Mongolian word Ukhaa ("brown") and the Greek word θηρίον ( thērion "animal"). The species name nessovi honors the paleontologist Lev Nessov for his pioneering work in researching the Mesozoic mammals of Uzbekistan and Kazakhstan.
Individual evidence
- ↑ Qiang Ji, Zhe-Xi Luo, Chong-Xi Yuan, John R. Wible, Jian-Ping Zhang and Justin A. Georgi: The earliest known eutherian mammal. Nature 416, 2002, pp. 816-822
- ^ A b J. David Archibald and Alexander O. Averianov: Late Cretaceous asioryctitherian eutherian mammals from Uzbekistan and phylogenetic analysis of Asioryctitheria. Acta Palaeontologica Polonica 51 (2), 2006, pp. 351-376
- ↑ a b c d Michael J. Novacek, Guillermo W. Rougier, John R. Wible, Malcolm C. McKenna, Demberelyin Dashzeveg and Inés Horovitz: Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia. Nature 389, 1997, pp. 483-486
- ↑ a b Inés Horovitz: Postcranial skeleton of Ukhaatherium nessovi (Eutheria, Mammalia) from the Late Cretaceous of Mongolia. Journal of Vertebrate Paleontology 23 (4), 2003, pp. 857-868
- ^ A b John R. Wible, Guillermo W. Rougier and Michael J. Novacek: Anatomical evidence for superordinal / ordinal Eutherian taxa in the Cretaceous. In: Kenneth D. Rose and J. David Archibald: The rise of placental mammals. Johns Hopkins University Press, 2005
- ^ A b John R. Wible, Guillermo W. Rougiers, Michael J. Novecek and Robert J. Asher: The Eeutherian mammal Maelestes gobiensis from the Late Cretaceous of Mongolia and the phylogeny of Cretaceous Eutheria. Bulletin of the American Museum of Natural History 327, 2009, pp. 1–123
- ^ A b Deborah L. Rook and John P. Hunter: Rooting Around the Eutherian Family Tree: the Origin and Relations of the Taeniodonta. Journal of Mammal Evolution 2013 doi : 10.1007 / s10914-013-9230-9
- ↑ Maureen A. O'Leary, Jonathan I. Bloch, John J. Flynn, Timothy J. Gaudin, Andres Giallombardo, Norberto P. Giannini, Suzann L. Goldberg, Brian P. Kraatz, Zhe-Xi Luo, Jin Meng, Xijun Ni, Michael J. Novacek, Fernando A. Perini, Zachary S. Randall, Guillermo W. Rougier, Eric J. Sargis, Mary T. Silcox, Nancy B. Simmons, Michelle Spaulding, Paúl M. Velazco, Marcelo Weksler, John R Wible and Andrea L. Cirranello: The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals. Science 339, 2013, pp. 662-667