Magnosaurus (meaning 'large lizard') was a genus of basal tetanuran theropod dinosaur from theMiddle Jurassic of England. It is based on fragmentary remains and has often been confused with or included in Megalosaurus.In 1923, Friedrich von Huene named Megalosaurus nethercombensis from a partial skeleton (OUM J12143) from theAalenian-Bajocian-age Middle Jurassic Inferior Oolite, found in the nineteenth century by W. Parker near Nethercomb, north ofSherborne, in Dorset, England. The material included partialdentaries, dorsal and caudal vertebrae, a partial ilium, a partial rightpubis, internal casts of the femora, and tibiae, from a possiblyjuvenile individual. Huene interpreted it as a more primitive species of Megalosaurus.In 1926, he named the tooth species Megalosaurus lydekkeri for a specimen, BMNH 41352, from the Lower Lias (Lower Jurassic) of England that Richard Lydekker had first described in 1888.Finally, in 1932, he created the genus Magnosaurus for M. nethercombensis, referred M. lydekkerito it, and created a third species, M. woodi, for the genus. M. woodi was based on a tibia (BMNH R.3542) from the Lower Lias, which he simultaneously and accidentally also named Sarcosaurus andrewsi; the latter name was in 1956 by von Huene given priority. Even more confusing, in the same 1932 publication von Huene renamed Sarcosaurus woodi into Magnosaurus woodi.Until the 1990s, the genus had been ignored as a species of Megalosaurus. However, with growing concern over what exactly is constituted by Megalosaurus, Magnosaurus has been generally separated as its own genus. Also, there are morphological differences: for example, possible Megalosaurus tibiae are compressed at the far end, unlike those ofMagnosaurus. Rauhut (2003) considered it and Eustreptospondylus to be the same genus, because the two share a similarly expanded front tip of the dentary and enlarged third dentary tooth. He therefore renamed Eustreptospondylus oxoniensis a Magnosaurus oxoniensis but this has not been generally followed. Reviews have found it to most likely be a basal tetanuran, probably a megalosaurid.A detailed redescription by Roger Benson in 2010 concluded Magnosaurus was valid taxon, a megalosauroid megalosaurid, and at about 175 million years old the oldest certain known member of the Tetanurae.

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