Ukhaa Tolgod

from Wikipedia, the free encyclopedia

Coordinates: 43 ° 32 ′ 0 ″  N , 101 ° 34 ′ 0 ″  E

Map: Mongolia
marker
Ukhaa Tolgod​
Magnify-clip.png
Mongolia

Ukhaa Tolgod is an important fossil site in the Gobi Desert in the south of what is now Mongolia . It was discovered in 1993 by an international group of scientists during an expedition of the American Museum of Natural History to this region and has since been a central research task of this institution. The reconstructed landscape, which existed in the Upper Cretaceous 84 to 72 million years ago, according to the investigations represented a desert-like dune landscape with sparse vegetation. It was inhabited by numerous lizards , dinosaurs , early relatives of birds and early mammals . Ukhaa Tolgod is one of the richest sites of that geological epoch, especially for the early mammals it is the one with the highest density of finds from this period. The fossil finds are in the thousands with partially complete skeletons and thus give a detailed insight into the living environment of the late Cretaceous period.

Geographical location

The Ukhaa Tolgod site is located in southern Mongolia , about 650 km south of the capital Ulaanbaatar in the Nemegt basin in the Ömnö-Gobi-Aimag province . About 300 km to the northeast is the rugged rock formation Bajandsag , where numerous fossils have also been found. The Nemegt basin provides a after Oligozän resulting tectonic represents recess and is surrounded by a plurality of ridges. The northern border is formed by the elevations of the Gilbent Ulul Mountains , which are 3 km north of Ukhaa Tolgod. The name Ukhaa Tolgod is of Mongolian origin and means "brown hills", which refers to some distinctive, reddish-brown colored knolls, steep slopes and cliffs in the area. Characteristic elevations and typical find points are the "Sugar Mountains", the "Sphinx" or the "Camel Humps", other sites include the "Ankylosaur areas" or "Xanadu". The main find area comprises the upper 2.2 km of a long drainage basin that runs from northeast to southwest and has cut into the overlying loose sediments , but the entire scattering of finds at this drainage basin extends up to 7 km in length. The area of ​​the main discovery area covers around 4 km² and rises to around 1500 m above sea level, with an amphitheater opening directly to the west. In the immediate vicinity there are numerous other fossil sites with more or less the same age, such as the Little Ukhaa site, discovered in 1996, about 10 km to the west .

Geological structure

In Ukhaa Tolgod mainly sediments of the Djadochta Formation are exposed, which is to be placed in the Upper Cretaceous . The Djadochta Formation is up to 80 m thick and contains two strata, the lower Bayn-Dzak stratum and the upper Turgrugyin stratum . In the up to 75 m thick profile sections of Ukhaa Tolgod, the lower Bayn-Dzak stratum is largely exposed . In this, a total of five different types of facies can be distinguished on deposit units , which, however, are not always to be found in the same stratigraphic order at the individual exposure points . These deposits , however, dip uniformly at an angle of about 2.5 ° to the south, which can be explained by the later unfolding of the Gilbent Ulul. Often there are clearly cross-layered , matt-orange colored sandstones (type E-1 ) with an average thickness of 12 m and an indistinctly layered sandstone also slightly orange in color, but containing cross-layered concretions (type E-2 ). This second type of facy is 10 on average, but up to 24.5 m thick at maximum. In both is äolisch be transmitted dune sediments , with those of facies E-2 well later diagenetic were overprinted processes. Partly E-2 also represents the base sediment. Furthermore, a mostly 20 m thick, structureless and again pale orange colored sandstone appears, which was created by landslides on the leeward sides of the dunes (type S ). In contrast, a brown-colored and 20 cm thick layer of clay and siltstone , which had formed between the former dunes and was very thin , can be traced back to short-term puddles or small pools (type M ). Finally, there is also a rather thin band of conglomerates that are tinted green and usually form the upper end. This was obviously washed into the former dune field from higher areas in the vicinity (type K ). While the sand and clay / silt stones are very fine-grained, the conglomerate is coarser and individual components can reach a diameter of up to 85 cm. Most of the vertebrate finds come from the S facies .

Finds

The fossil preservation of the fauna is extraordinarily good and comprises more than 1000 skull finds, including numerous with partially completely handed down skeletons of lizards , dinosaurs , early birds and also early mammals . The fossil density is very high and reaches the highest concentration of the known sites of the late Mesozoic , especially in the early mammals . In addition, the vertebrate fossils give an insight into the way of life, as remains of fossilized eggs and embryos could be documented. In contrast to the well-preserved fauna, macroscopic plant remains have not survived, which may be related to the strong oxidation of the deposits. Only occasionally occurring rhizolites (fossilized root remains) indicate the former presence of plants.

Scale reptiles and turtles

Skull of Estesia

The fauna of the scale reptiles is very rich with over 20 different genera and several thousand fossil remains, including numerous skulls. The most common are the Skink-like , which comprise nine genera, including a large number of representatives of the rail lizards . Among them are the small , narrow-headed gobinatus , which is also represented by a partial skeleton, and the larger Macrocephalosaurus , which had a skull up to 8 cm long. Some other genera, such as Eoxanta and Slavoia , have not yet been systematically assigned exactly, but may be close to the niggas or skinks . Remains of iguanas are almost as common as the Skink-like , of which eight genera have been identified so far, with agamas being represented by Mimeosaurus and Priscagama . The iguanas themselves are represented by several complete skulls from Temujinia . The discovery of an almost complete skeleton of the closely related Saichangurvel , the "beautiful lizard", which only lacks the remains of the right hind foot, is spectacular . The skull measured about 2.8 cm in length, the complete spine consisted of 54 individual vertebrae. With four genera a little less common are the creeping species . This may be the traditional with only sparse fossil remains, but in the relationship of the present-day monitor lizards as alternate end Aiolosaurus be assigned. Also in the family circle is the Estesia , which is quite large with a skull up to 15 cm long , as well as the much smaller Gobiderma , whose 5 cm long skull is characterized by a pockmarked roughened bone surface, which shows the original scaling. The last two belong to the group of carnivorous monsterosaurs, which is also related to the monitor lizards . In addition to the various groups of lizards, Zangerlia is also known as a representative of the turtles from the Halsberger turtle group and has a fairly complete skeleton including a tortoise shell around 20 cm long.

Dinosaurs and birds

Skull of Khaan from Ukhaa Tolgod
Skeleton of Citipati from Ukhaa Tolgod
Skeleton of Halszkaraptor from Ukhaa Tolgod

The dinosaur finds include more than 150 skeletons of at least eight genera, but there is still a great deal of undetermined and systematically not yet precisely assigned material. One of the largest representatives was Pinacosaurus , an armored herbivore about 5 m long that is part of the Ankylosauria . With the smaller Protoceratops an early representative of the horned dinosaur ( Ceratopsia ) is proven. Theropod dinosaurs are represented much more extensively . The Troodontidae are represented with several forms. Byronosaurus was first detected in Ukhaa Tolgod on the basis of an incomplete skull with a high number of teeth and some postcranial skeletal parts. A nest with six eggs also contained skeletal remains of two perinatal individuals (shortly before or after birth), whose assignment to Byronosaurus was only provisional; they probably belong to a different genus of troodontid dinosaurs. It is assumed that these are young animals from Almas , which was described using a partial skeleton from Ukhaa Tolgod. The find was associated with some eggshells, although it is unclear whether these were eggs laid by the animal or hunted prey. The genus is named after the legendary Mongolian snowman . Another troodontid form is present with a fragmentary upper jaw; it has not yet been possible to assign it more precisely. The Oviraptoridae , bird-like dinosaurs, include both Khaan and Citipati , both were also described for the first time with the help of finds from Ukhaa Tolgod. Three almost complete skeletons of each of the two genera have survived. Two of the Citipati skeletons lay in the hatching position, each over a nest of at least 15 or 12 eggs, long oval in shape and up to 18 cm in length. In addition, another nest was discovered with several fossil eggs in a circular arrangement, one of which had disintegrated into three parts and still contained the embryonic bones. On the basis of this find it could be shown that the growing embryos of the Oviraptoridae occupy a similar position in the egg as those of today's crocodiles with the head in front of the knee, which differs somewhat from those of the birds , which keep their head tucked under their arm. Tsaagan in turn represents a representative of the Dromaeosauridae and was described on the basis of an approximately 20 cm long skull with eight attached vertebrae. Halszkaraptor is to be classified in the broader family circle . The genus was described in 2017 and is based on an almost complete skeleton. Like today's waterfowl, the duck-sized animals were adapted to a semi-aquatic way of life. They used their front legs to swim and walked ashore on their back legs. Possibly they went in search of food as ambulance hunters. A complete foot with three long and a fourth, significantly reduced toes totaling 30 cm in length is still available from Kol , probably the largest member of the Alvarezsauridae family to date , who doubles all other members of this group due to the length of the metatarsal bone Size exceeded. This genus was also found for the first time in Ukhaa Tolgod. At least 15 remains of Shuvuuia are to be placed in the same group , here a 17 cm long and very lightly built skull served to define the genus. Furthermore, the entire finds include several, not precisely defined genera from the group of Ornithomimidae as well as several undefined large Theropoda.

Birds appear rather seldom and have been handed down with two genera so far. A special feature is Apsaravis , which is relatively closely related to modern birds (Neornithes) and is proven to have a very well-preserved skeleton from Ukhaa Tolgod, but which has only a fragmented skull. It is noteworthy that this bird was possibly able to fly in the manner of the neornithes, which was somewhat different from that of the original birds, which was determined on the basis of the limb bones. Another early bird, Gobipteryx , is placed among the enantiornithes , an extinct primitive group of birds that still had teeth and hand claws. From Gobipteryx are Ukhaa Tolgod far only a few skull fragments, including a front portion of a head, was discovered.

Mammals

Reconstruction of Zalambdalestes

The finds of early mammals that were discovered here are also significant, of which more than 500 skulls are available, the largest known number of all sites from this geological epoch. The most original representatives are those of the Allotheria , especially the mostly mouse-sized Multituberculata , of which five genera have been identified so far. These include Kryptobaatar , Tombaatar and Djadochtatherium . It turns Tombataar , the "Great Hero", already one of the major representatives of multituberculata is and was first discovered in Ukhaa Tolgod; its description is based on a 4 cm long front skull fragment. Furthermore, with Deltatheridium a member of the marsupials has been proven. However, the finds of the more modern mammals are to be emphasized, of which three genera are known, all of which resemble modern insectivores in their habitus ; two of these genera had been described directly with the help of fossil material from Ukhaa Tolgod. Ukhaatherium from the group of Asioryctitheriidae was a small animal with a skull length of 2.8 cm and a weight of around 32 g. At least eight individuals are known of this, six of them almost completely. Maelestes reached a similar size , but so far only an incomplete individual has come down to us. Already significantly greater contrast was Zalambdalestes from the family of Zalambdalestidae cm with a length of head 5, from which a complete skeleton exists. Both Ukhaatherium and Zalambdalestes developed a pouch-bone which is actually characteristic of marsupials and even more primitive mammals , which is missing in all modern mammals today. Originally, the three genera mentioned were placed at the base of the development of the higher mammals , but a study from 2013 states that these are now to be addressed as nonplacental Eutheria of lower rank.

Age position

To date, no radiometric measurements from the Ukhaa Tolgod deposits are available. From a biostratigraphic point of view, most of the dinosaur finds can be dated to the Campanium , a stage of the Upper Cretaceous , and are therefore between 84 and 72 million years old. However, compared to similar finds from Central Asia and North America , a slightly older age is possible. Investigations on magnetostratigraphy on the fossil-bearing strata, which were carried out on the Bajandsag , but in the same stratigraphic unit of the Bayn-Dzak strata , show a normal polarization of the earth's magnetic field at the base of the strata , while a reverse polarization was found above. The normal state of the earth's magnetic field was only restored in the upper layer areas. This reverse polarization phase in the sediment sequence can be correlated with Chron 34n , which occurred 83.5 million years ago, while the upper sequences are to be placed in the transition from Chron 33 to Chron 32 between 75 and 71 million years ago.

Landscape reconstruction

A former dune landscape of a desert can be reconstructed from the geological findings . The dunes were only sparsely covered with vegetation , which can be seen from the rhizolites (fossilized parts of the roots) in the cross-layered sandstones ( facade type E-1 ). These dunes existed mainly during a rather dry phase and were more or less mobile. In contrast, the dunes of the weakly stratified sandstones (Facia type E-2 ) had a rather stable location. Since the majority of vertebrate fossils occur in the unlayered sandstones (type S ), it can be assumed that they perished when sand slipped on the dune slopes and were buried so quickly, which can also be deduced from the lack of gnawing traces by scavengers. Original assumptions about the death of the animals by strong sandstorms could also be contradicted. These mass slides may have been caused by brief, heavy rains. The rain also caused the formation of small pools and puddles between the dunes, which is deduced from the clay / silt stones (type M ), which were an important prerequisite for life under these extremely dry conditions.

Research history

The first investigations in the region were carried out by the Soviet Academy of Sciences between 1946 and 1949 , during which some important dinosaur fossils from the Upper Cretaceous were discovered, such as Tarbosaurus and Saurolophus . These were followed by Polish- Mongolian expeditions from 1963 to 1971 and a Japanese- Mongolian expedition from 1992 . In 1991, the Academy of Sciences of Mongolia and the American Museum of Natural History began joint explorations in southern Mongolia, held nearly 70 years after the famous Central Asiatic Expeditions of the American Museum of Natural History to the Gobi Desert in the 1920s . In the course of this, Ukhaa Tolgod was discovered in 1993, which preserved a previously unknown wealth of fossils from the late Mesozoic Era. The field investigations and the evaluation of the entire find material from the following almost 20 years, but also the geological framework conditions and all other interpretations continue to this day.

Individual evidence

  1. ↑ a b c Demberelyin Dashzeveg, Michael J. Novacek, Mark A. Norell, James M. Clark, Luis M. Chiappe, Amy Davidson, Malcolm C. McKenna, Lowell Dingus, Carl Swisher and Perle Altangerel: Extraordinary preservation in a new vertebrate assemblage from the Late Cretaceous of Mongolia. Nature 374, 1995, pp. 446-449
  2. ↑ a b c d e f g h Lowell Dingus, David B. Loope, Demberlyin Dashzeveg, Carl C. Swisher III, Chuluun Minjin, Michael J. Novacek and Mark A. Norell: The Geology of Ukhaa Tolgod (Djadokhta Formation, Upper Cretaceous , Nemegt Basin, Mongolia). American Museum Novitates 3616, 2008, pp. 1-40
  3. ↑ a b Gao Keqin and Mark A. Norell: Taxonomic composition and systematics of Late Cretaceous lizard assemblages from Ukhaa Tolgod and adjacent localities, Mongolian desert. Bulletin of the American Museum of Natural History, Number 249, 2000, pp. 1-118
  4. ↑ Mark A. Norell, Gao Keqin and Jack Conrad: A New Platynotan Lizard (Diapsida: Squamata) from the Late Cretaceous Gobi Desert (O¨ mno¨gov), Mongolia. American Museum Novitates 3605, 2007, pp. 1-22
  5. ↑ a b Demberelyin Dashzeveg, Lowell Dingus, David P. Loope, Carl C. Swisher III, Togtokh Dulam and Mark R. Sweeney: New Stratigraphic Subdivision, Depositional Environment, and Age Estimate for the Upper Cretaceous Djadokhta Formation, Southern Ulan Nur Basin, Mongolia. American Museum Novitates 3498, 2005, pp. 1-31
  6. ^ A b David P. Loope, Lowell Dingus, Carl C. Swisher III and Chuluun Minjin: Life and death in a Late Cretaceous dune field, Nemegt Basin, Mongoliua. Geology 26 (1), 1998, pp. 27-30
  7. ↑ Jack L. Conrad and Mark A. Norell: A Complete Late Cretaceous Iguanian (Squamata, Reptilia) from the Gobi and Identification of a New Iguanian Clade. American Museum Novitates 3584, 2007, pp. 1-47
  8. ↑ Hong-Yu Yi and Mark A. Norell: New materials of Estesia mongoliensis (Squamata: Anguimorpha) and the evolution of venom grooves in lizards. American Museum Novitates 3767, 2013, pp. 1-31
  9. ^ Walter G. Joyce and Mark A. Norell: Zangerlia ukhaachelys, New Species, a Nanhsiungchelyid Turtle from the Late Cretaceous of Ukhaa Tolgod, Mongolia. American Museum Novitates 3481, 2005, pp. 1-19
  10. ^ Robert V. Hill, Lawrence M. Witmer, and Mark A. Norell: A New Specimen of Pinacosaurus grangeri (Dinosauria: Ornithischia) from the Late Cretaceous of Mongolia: Ontogeny and phylogeny of Ankylosaurs. American Museum Novitates 3395, 2003, pp. 1-29
  11. ^ Mark A. Norell, Peter J. Makovicky and James M. Clark: A new troodontid theropod from Ukhaa Tolgod, Mongolia. Journal of Vertebrate Palaeontology 20 (1), 2000, pp. 7-11
  12. ^ Gabe S. Bever and Mark A. Norell: The Perinate Skull of Byronosaurus (Troodontidae) with Observations on the Cranial Ontogeny of Paravian Theropods: American Museum Novitates 3657, 2009, pp. 1-51
  13. Jump up ↑ Rui Pei, Mark A. Norell, GS Bever, Michael Pittman and Xing Xu: Osteology of a New Late Cretaceous Troodontid Specimen from Ukhaa Tolgod, Ömnögovi Aimag, Mongolia. American Museum Novitates 3889, 2017, pp. 1-47
  14. Jump up ↑ Mark A. Norell and Sunny H, Hwang: A Troodontid Dinosaur from Ukhaa Tolgod (Late Cretaceous Mongolia). American Museum Novitates 3446, 2004, pp. 1-9
  15. ↑ James M. Clark, Mark A. Norell and Rinchen Barsboöd: Two new oviraptorids (Theropoda: Oviraptorosauria), Upper Cretaceous Djadokhta Formation, Ukhaa Tolgod, Mongolia. Journal of Vertebrate Paleontology 21 (2), 2001, pp. 209-213
  16. ↑ James M. Clark, Mark A. Norell and Luis M. Chiappe: An Oviraptorid Skeleton from the Late Cretaceous of Ukhaa Tolgod, Mongolia, Preserved in an Avianlike Brooding Position Over an Oviraptorid Nest. American Museum Novitates 3265, 1999, pp. 1-36
  17. ↑ Mark A. Norell, Amy M. Balanoff, Daniel E. Barta and Gregory M. Erickson: A Second Specimen of Citipati osmolskae Associated with a Nest of Eggs from Ukhaa Tolgod, Omnogov Aimag, Mongolia. American Museum Novitates 3899, 2018, pp. 1-44
  18. ↑ Mark A. Norell, James M. Clark and Luis M. Chappe: An Embryonic Oviraptorid (Dinosauria: Theropoda) from the Upper Cretaceous of Mongolia. American Museum Novitates 3315, 2001, pp. 1-20
  19. ↑ Mark A. Norell, James M. Clark, Alan H. Turner, Peter J. Makovicky, Rinchen Barsbold, and Timothy Rowe: A New Dromaeosaurid Theropod from Ukhaa Tolgod (Ömnögov, Mongolia). American Museum Novitates, Number 3545, 2006, pp. 1-51
  20. ↑ Andrea Cau, Vincent Beyrand, Dennis FAE Voeten, Vincent Fernandez, Paul Tafforeau, Koen Stein, Rinchen Barsbold, Khishigjav Tsogtbaatar, Philip J. Currie and Pascal Godefroit: Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs. Nature 552, 2017, pp. 395-399
  21. ↑ Alan H. Turner, Sterling J. Nesbitt and Mark A Norell: A Large Alvarezsaurid from the Late Cretaceous of Mongolia. American Museum Novitates, Number 3548, 2009, pp. 1-14
  22. ↑ Luis M. Chiappe, Mark A. Norell and James M. Clark: The skull of a relative of the stem-group bird Mononykus. Nature 392, 1998, pp. 275-278
  23. ^ Mark A. Norell and Julia A. Clarke: Fossil that fills a critical gap in avian evolution. Nature 409, 2001, pp. 181-184
  24. ↑ Luis M. Chiappe, Mark A. Norell and James Clark: A New Skull of Gobipteryx minuta (Aves: Enantiornithes) from the Cretaceous of the Gobi Desert. American Museum Novitates 3346, 2001, pp. 1-19
  25. ↑ Guillermo W. Rougiers, Michael J. Novacek and Demberelyin Dashzeveg: A New Multituberculate from the Late Cretaceous Locality Ukhaa Tolgod, Mongolia. Considerations on Multituberculate Interrelationships. American Museum Novitates 3191, 1997, pp. 1-26
  26. ↑ a b Michael J. Novacek, Guillermo W. Rougier, John R. Wible, Malcolm C. McKenna, Demberelyin Dashzeveg and Inés Horovitz: Epipubic bones in eutherian mammalsfrom the Late Cretaceous of Mongolia. Nature 389, 1997, pp. 483-486
  27. ↑ John R. Wible, Guillermo W. Rougiers, Michael J. Novacek 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
  28. ↑ John R. Wible, Michael Novacek and Guillermo W. Rougier: New data on the skull and dentition in the mongolian Late Cretaceous Eutherian mammal. Bulletin of the American Museum of Natural History 281, 2004, pp. 1-144
  29. ↑ 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