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Grallator
''Grallator'' is an ichnogenus (form taxon based on footprints) which covers a common type of small, three-toed print made by a variety of bipedal theropod dinosaurs. ''Grallator''-type footprints have been found in formations dating from the late Triassic through to the early Cretaceous periods. They are found in the United States, Canada, Europe, Australia and China but are most abundant on the east coast of North America, especially the Triassic and Early Jurassic formations of the northern part of the Newark Supergroup.〔Weishampel, D.B. & L. Young. 1996. Dinosaurs of the East Coast. The Johns Hopkins University Press〕〔Xing Li-da, Harris, J.D., Feng Xiang-yang, and Zhang Zhi-jun. (2009). "Theropod (Dinosauria: Saurischia) tracks from Lower Cretaceous Yixian Formation at Sihetun Village, Liaoning Province, China and possible track makers." ''Geological Bulletin of China'', 28(6): 705-712.〕 The name ''Grallator'' translates into "stilt walker", although the actual length and form of the trackmaking legs varied by species, usually unidentified. The related term "Grallae" is an ancient name for the presumed group of long-legged wading birds, such as storks and herons. These footprints were given this name by their discoverer, Edward Hitchcock, in 1858.〔
''Grallator'' footprints are characteristically three-toed (tridactyl) and range from 5 to 15 centimeters (or 2 to 6 inches) long. Though the tracks show only three toes, the trackmakers likely had between four and five toes on their feet. While it is usually impossible to match these prints with the exact dinosaur species that left them, it is sometimes possible to narrow down potential trackmakers by comparing the proportions in individual ''Grallator'' ichnospecies with known dinosaurs of the same formation. For example, ''Grallator'' tracks identified from the Yixian Formation may have been left by ''Caudipteryx''.〔
==Newark Supergroup tracks==
The most famous, and archetypal tracks that conform to the ''Grallator'' type are those found on the East Coast of North America, specifically from the Late Triassic to Early Jurassic Newark Supergroup. These footprints were likely made by an unidentified, primitive dinosaur similar to ''Coelophysis''.〔 The Newark Supergroup footprints show digits II, III and IV, but no trace of the shorter digits I and V which would likely have been present in a dinosaur of this stage. The outer two digits would have been stubby and ineffective, not touching the ground during walking or running.〔 Despite losing most of their effectiveness, dinosaur evolution had not yet removed the these digits to fully streamline the foot. This is known because rare specimens are found with traces of these outer digits. Digits II, III and IV have 3, 4 and 5 phalanges respectively, giving ''Grallator'' a ?-3-4-5-? digital formula.
Although the Newark Supergroup ''Grallator'' tracks were made by a bipedal saurischian dinosaur, they can easily be mistaken for those of the late Triassic ichnogenus ''Atreipus''.〔Safran, J. and Rainforth, E.C. (2004). ("Distinguishing the tridactyl dinosaurian ichnogenus ''Atreipus'' and ''Grallator'': where are the latest Triassic Ornithischia in the Newark Supergroup?" ) Abstracts with Programs, Geological Society of America, 36(2): 96.〕 The trackmaker of ''Atreipus'' prints was a quadrupedal ornithischian. The reason for this similarity is a lack of divergence in the foot evolution of the two distinct groups of dinosaurs: ornithischians and saurischians.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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