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''Hesperosaurus'' (meaning "western lizard", from Classical Greek ἕσπερος/''hesperos'' "western" and σαυρος/''sauros'' "lizard") is a herbivorous stegosaurian dinosaur from the Kimmeridgian epoch of the Jurassic period, approximately 156 million years ago. Fossils of ''Hesperosaurus'' have since 1985 been found in the state of Wyoming and Montana in the United States of America. The type species ''Hesperosaurus mjosi'' was named in 2001. It is from an older part of the Morrison Formation, and so a little older than other Morrison stegosaurs. Several relatively complete skeletons of ''Hesperosaurus'' are known. One specimen preserves the first known impression of the horn sheath of a stegosaurian back plate. ''Hesperosaurus'' was about six to seven metres long and two to three tonnes heavy. It was a member of the Stegosauridae, quadrupedal plant-eaters protected by vertical bony plates and spikes. It was closely related to ''Stegosaurus'' and was similar to it in having two rows of, possibly alternating, plates on its back and four spikes on its tail end. The plates on its back were perhaps not as tall, but were longer. It possibly had a deeper skull than ''Stegosaurus''. ==Discovery and species== In 1985, fossil hunter Patrick McSherry, at the ranch of S.B. Smith in Johnson County, Wyoming, found the remains of a stegosaur. As he had difficulty securing the specimen due to the hard rock matrix, he sought help from Ronald G. Mjos and Jeff Parker of Western Paleontological Laboratories, Inc. They, in turn, cooperated with paleontologist Dee Hall of Brigham Young University. At first, it was assumed it represented an exemplar of ''Stegosaurus''. However, Clifford Miles, while preparing the remains, recognised that they belonged to a species new to science. The type species ''Hesperosaurus mjosi'' was named and described in 2001 by Kenneth Carpenter, Clifford Miles, and Karen Cloward. The generic name is derived from the Greek ἕσπερος, ''hesperos'', "western", in reference to its location in the western United States. The specific name honours Mjos who, apart from his involvement in the process of collecting and preparing the holotype, also had a cast of it made, exhibited with the inventory number DMNH 29431 in the Denver Museum of Natural History. The holotype, HMNH 001 (later HMNS 14), was found in the Windy Hill Member, stratigraphic zone 1 of the lower Morrison Formation,〔Foster, J. (2007). "Appendix." ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World''. Indiana University Press. pp. 327–329.〕 dating from the early Kimmeridgian, about 156 million years old. In 2001, it represented the oldest known American stegosaur. It consists of a nearly complete skull and much of the skeleton. It includes the disarticulated elements of the skull, the rear lower jaws, an hyoid, thirteen neck vertebrae, thirteen back vertebrae, three sacrals, forty-four tail vertebrae, neck ribs, dorsal ribs, chevrons, a left shoulderblade, a complete pelvis, ossified tendons and ten neck and back plates. The skeleton was partly articulated and, in view of healed fractures, belongs to an old individual.〔 It was obtained by the Japanese Hayashibara Museum of Natural Science at Okayama. From 1995 onward at the Howe-Stephens Quarry in Big Horn County, Wyoming, named after the historic location of the Howe Ranch, once explored by Barnum Brown, and the new owner Press Stephens, Swiss palaeontologist Hans Jacob Siber excavated stegosaur specimens. The first was SMA 3074-FV01 (also SMA M04), a partial skeleton dubbed "Moritz" after ''Max und Moritz'' as an earlier ''Galeamopus'' sauropod skeleton from the site had been nicknamed "Max". In September 1995, specimen SMA 0018 (also mistakenly referred to as SMA M03) was uncovered, dubbed "Victoria" after the feeling of victory the exploring team felt when they discovered allosaurus "Big Al Two" after the original "Big Al" had been confiscated as federal property. It represents a rather complete skeleton with skull, also preserving skin and horn sheath impressions. A third specimen was found in 2002: SMA L02, dubbed "Lilly" after the sisters Nicola and Rabea Lillich assisting the excavations as volunteers. The specimens are part of the collection of the ''Aathal Dinosaur Museum'' in Switzerland. At first they were considered ''Stegosaurus'' exemplars. In 2001, initially only "Moritz" and "Lilly" were reclassified as cf. ''Hesperosaurus mjosi''.〔Siber, H.J., & Möckli, U., 2009, ''The Stegosaurs of the Sauriermuseum Aathal'', Aathal: Sauriermuseum Aathal, pp 56〕 In 2010, "Victoria" was by Nicolai Christiansen e.a. referred to ''Hesperosaurus mjosi''.〔N.A. Christiansen and E. Tschopp, 2010, "Exceptional stegosaur integument impressions from the Upper Jurassic Morrison Formation of Wyoming", ''Swiss Journal of Geosciences'' 103: 163-171〕 Carpenter had originally concluded that ''Hesperosaurus'' was a rather basal stegosaur. However, Susannah Maidment and colleagues in 2008 published a more extensive phylogenetic study in which it was recovered as a derived form, closely related to ''Stegosaurus'' and ''Wuerhosaurus''. They proposed that ''Hesperosaurus'' should be considered a species of ''Stegosaurus'', with ''Hesperosaurus mjosi'' becoming ''Stegosaurus mjosi''; at the same time ''Wuerhosaurus'' was renamed into a ''Stegosaurus homheni''. This has proven to be controversial. In biology the concept "genus" is not operationalised, i.e. generally no operational definition is given indicating in which way it could be empirically established whether a certain species belongs to a certain genus. Carpenter, considering the problem more of a philosophical than a scientific nature, in 2010 rejected the synonymy of ''Hesperosaurus'' with ''Stegosaurus'' stating that in his opinion ''Hesperosaurus'' was sufficiently different from ''Stegosaurus'' to be named a separate genus.〔Carpenter, K., 2010, "Species concept in North American stegosaurs", ''Swiss Journal of Geosciences'', 103(2): 155-162〕 Christiansen e.a. in 2010 judged likewise.〔 In 2015, additional specimens were reported: a concentration of at least five individuals discovered at the JRDI 5ES Quarry near Grass Range, Montana, en two individuals found in the Meilyn Quarry at Como Bluff.〔Saitta E.T., 2015, "Evidence for Sexual Dimorphism in the Plated Dinosaur ''Stegosaurus mjosi'' (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA", ''PLoS ONE'' 10(4): e0123503〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hesperosaurus」の詳細全文を読む スポンサード リンク
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