The only known fossil includes most of the rear half of a skeleton as well as a hindlimb (NHMUK R3078). Found in Cambridgeshire in the 1890s, it was described by Arthur Smith Woodward in 1905 as a new specimen of the species "Cetiosaurus leedsi". This was changed in 1927, when Friedrich von Huene found NHMUK R3078 and the "C. leedsi" type specimen to be too different from "Cetiosaurus", warranting its own genus, which he named "Cetiosauriscus", meaning ""Cetiosaurus"-like". "Cetiosauriscus leedsi" was referred to the sauropod family Diplodocidae because of similarities in the tail and foot, and had the dubious or indeterminate species "Cetiosauriscus" "greppini", "C." "longus", and "C." "glymptonensis" assigned to it. In 1980, Alan Charig named a new species of "Cetiosauriscus" for NHMUK R3078 because of the lack of comparable material to the type of "C. leedsi"; this species was named Cetiosauriscus stewarti. Because of the poor state of preservation of the "Cetiosauriscus leedsi" fossil, Charig sent a petition to the International Commission on Zoological Nomenclature to instead make "C. stewarti" the type species. "Cetiosauriscus stewarti" became the oldest confirmed diplodocid until a phylogenetic analysis published in 2003 instead found the species to belong to Mamenchisauridae, and followed by studies in 2005 and 2015 that found it outside Neosauropoda, while not a mamenchisaurid proper.