Limusaurus
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| Upper Jurassic ( Oxfordium ) | ||||||||||||
| 163.5 to 157.3 million years | ||||||||||||
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| Limusaurus | ||||||||||||
| Xu et al. , 2009 | ||||||||||||
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Limusaurus is a genus of dinosaurs from the group of Ceratosauria within the theropods . So far, fragmentary to complete skeletons of at least 15 specimens have been found that come from the Upper Jurassic of Xinjiang in China . Limusaurus was a two-legged herbivore about 1.7 meters long,which was characterized by toothless jaws that opened into a beak. It is the only known representative of the ceratosaurs that was not predominantly carnivorous. Limusaurus wasfirst scientifically described in 2009 with the only species, Limusaurus inextricabilis .
The anatomy of this animal's five-pointed hand, which shows a greatly reduced first finger, caused a scientific dispute. This could indicate that the typical three-pronged hand of the more advanced theropods was created by reducing the first and fifth rays, and not, as previously assumed, by reducing the fourth and fifth rays. If this hypothesis proves to be true, the three-pronged hand of advanced theropods would consist of fingers 2, 3 and 4, which corresponds to the constellation that today's birds have.
description
Limusaurus was a relatively small ceratosaur. The holotype specimen is estimated to be 1.7 meters long; another specimen was 15% larger. Both specimens were young adults, as indicated by the fusion of the bone sutures. In contrast to all other known ceratosaurs, the jaws were toothless and ended in a fully developed beak (Rhamphotheca) - this feature was previously only known from a few coelurosaurs . The skull was shorter than that of other ceratosaurs and only half as long as the thighbone (femur). The arms were short, as is typical of ceratosaurs. The legs, on the other hand, were relatively long, with the lower leg and foot together making up about 120 to 130% of the length of the thigh bone. Such leg proportions, designed for fast running, are also present in some Coelurosaurs, and in the latter and Limusaurus have probably developed independently of each other ( convergent evolution ). Other unique features include the wide and short nasal bone , which is only a third of the length of the roof of the skull , and the very short first toe. As with other theropods, the latter did not touch the ground and was only 17% of the length of the third metatarsal in Limusaurus .
Paleobiology
Limusaurus is the only known herbivorous theropod outside of the Coelurosauria. The small head, the long neck, the toothless jaws and clusters of small, rounded stones that were discovered in the abdominal region of the skeletons and are interpreted as gastrolites (stomach stones) indicate a herbivorous diet . Gastroliths are used in the stomach of today's herbivorous birds to cut up food. The physique of Limusaurus - especially the small head with the toothless jaws and the large eye sockets, the long neck and the long hind legs - shows clear similarities with two only distantly related groups, the Shuvosauridae and the Ornithomimosauria . These parallels point to the occupation of similar ecological niches by these not closely related animals and are considered to be an example of convergent evolution .
Controversy on the evolution of the bird's hand
Advanced theropods, like modern birds, have three fingers, while the original theropods had five fingers. According to the doctrine, the three-pronged theropod hand was formed from a lateral reduction of the fingers: The fifth and fourth fingers have receded, leaving only the first, second and third fingers. This hypothesis is based on anatomical features of the phalanx, which can be observed in both the original, five-pointed hand and the more modern, three-pointed hand. However, the hand of today's birds consists of the second, third and fourth fingers, what studies on avian embryos suggest: the primordial dalen finger approaches these embryos still show the original, five-pointed pattern, with the three fingers develop from the middle three of these approaches.
Limusaurus has a severely reduced first finger, indicating a bilateral (two-sided) reduction in fingers, with the first and fifth fingers receding, with fingers 2, 3, and 4 being the longest. This discovery could indicate that the hand of the more modern theropods, as well as birds, was formed by bilateral reduction, and that the three fingers were actually the second, third, and fourth fingers, rather than the first, second, and third fingers. This hypothesis of bilateral reduction was already proposed by Thulborn and Hamley (1982), but received little attention in scientific circles. According to this hypothesis, the apparent contradiction between the finger identity of theropods and birds would be resolved.
History of discovery and naming
Limusaurus fossils come from the Shishugou Formation (Wucaiwan area) in the Junggar Basin of the Chinese Autonomous Region of Xinjiang . So far, the skeletons of at least 15 Limusaurus individuals have been discovered, some or all of which have been preserved and for the most part articulated (in the original anatomical association). The skeletons were found in a vertical position, embedded in several small, 1 to 2 meter deep pits, which are filled with a deformed mudstone ("mudstone"). Researchers suspect that these pits are step seals from very large sauropods, such as Mamenchisaurus . The sauropods, which waded over a water-saturated plain and partially sank into the ground, would have liquefied the ground, so that small theropods got stuck and sank.
Limusaurus was first scientifically described in 2009 by Xu Xing and colleagues in the journal Nature . The first description is based on an almost complete skeleton ( holotype , specimen number IVPP V 15923) and another, skullless skeleton (specimen number IVPP V 15924). The name Limusaurus inextricabilis ( lat. Limus - "mud", saurus - "lizard", inextricabilis - "unable to free itself") means something like "mud lizard that cannot free itself". The name indicates the fact that the animals got stuck in the mud and died.
Systematics
Limusaurus was a basal ceratosaur. Xu Xing (2009) combines Limusaurus and Elaphrosaurus , a Ceratosauria from the Tendaguru Formation , into a common clade. Similarities with Elaphrosaurus are the long neck, rather untypical for Ceratosauria , the slim, graceful and long- legged body, but Limusaurus was significantly smaller than Elaphrosaurus .
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Individual evidence
- ^ A b David A. Eberth, Xing Xu , James M. Clark : Dinosaur Death Pits from the Jurassic of China. In: Palaios. Vol. 25, No. 1/2, 2010, ISSN 0883-1351 , pp. 112–125, doi: 10.2110 / palo.2009.p09-028r
- ↑ a b c d e f g h i Xu Xing, James M. Clark, Jinyou Mo, Jonah Choiniere, Catherine A. Forster, Gregory M. Erickson, David WE Hone, Corwin Sullivan, David A. Eberth, Sterling Nesbitt, Qi Zhao, Rene Hernandez, Cheng-kai Jia, Feng-lu Han, Yu Guo: A Jurassic ceratosaur from China helps clarify avian digital homologies. In: Nature . Vol. 459, No. 7249, 2013, pp. 940-944, doi: 10.1038 / nature08124 .
- ^ Richard A. Thulborn, Tim L. Hamley: The Reptilian Relationships of Archeopteryx. In: Australian Journal of Zoology. Vol. 30, No. 4, 1982, ISSN 0004-959X , pp. 611-634, doi: 10.1071 / ZO9820611 .
- ↑ Werner Janensch : A set up and reconstructed skeleton of Elaphrosaurus bambergi. With an addendum on the osteology of this coelurosaur. In: Werner Janensch: (Ed.): Scientific results of the Tendaguru expedition 1909–1912. New series series 1: The reptiles of the Tendaguru strata of German East Africa (= Palaeontographica. Supplement 7, series 1, part 1, ISSN 0085-4611 ). Delivery 3. Schweizerbart, Stuttgart 1929, pp. 279–286.