Cetiosauriscus (/ˌsiːtioʊˈsɔːrɪskəs/ SEE-tee-oh-SOR-iss-kəs)[3] is a genus of sauropod dinosaur that lived between 166 and 164 million years ago during the Callovian (Middle Jurassic Period) in what is now England.

This was changed in 1927, when Friedrich von Huene found NHMUK PV R3078 and the C. leedsi type specimen to be too different from Cetiosaurus, warranting its own genus, which he named Cetiosauriscus, meaning "Cetiosaurus-like".

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.

The type species of Cetiosaurus has changed throughout history because of incomplete remains and the taxon's significance, and many aspects of its anatomy and relationships are still uncertain.

Cetiosaurus was originally named to include C. medius, C. brevis, C. brachyurus and C. longus, which span from the Middle Jurassic to the Early Cretaceous of various localities across England.

[4] Another English taxon, Ornithopsis hulkei, was named in 1870 by palaeontologist Harry Govier Seeley for vertebrae from the Early Cretaceous Wessex Formation, younger than the existing species of Cetiosaurus.

[10] The sauropod fossil today known as Cetiosauriscus stewarti was discovered in May 1898 by clay workers in the area around Fletton to the south of Peterborough and east of the Great Northern Railway line.

Pits in this region expose the fossil-rich sedimentary rocks of the marine Oxford Clay, which is of middle Callovian age and today regarded as one of the classic geological formations of British palaeontology.

In mid-August, after some cleaning and repairing of the specimen, geologist Henry Woodward visited Eyebury and produced a life-sized drawing of the remains for presentation at the British Association for the Advancement of Science Meeting.

[7] The amount of material made NHMUK PV R3078 the most complete sauropod specimen from the United Kingdom, comparable only later to the "Rutland Dinosaur" (referred to Cetiosaurus) discovered in 1967.

[7][10] A tail tip (NHMUK PV R1967) from the same locality, but a different individual was thought by palaeontologist Alan Charig in 1980 to belong to Cetiosauriscus.

The assignment of NHMUK PV R1967 to Cetiosauriscus was considered unlikely in alternate studies by palaeontologists Friedrich von Huene, Paul Upchurch and Darren Naish because of the lack of overlap and uncertain phylogenetic positions.

The known material, discovered in Late Jurassic (Kimmeridgian) deposits in the Reuchenette Formation of Switzerland, includes dorsal and caudal vertebrae, forelimb bones, and a hindlimb and partial pes, from at least two individuals.

[19] Similarities such as the anatomy of the caudal vertebrae were suggested by Christian Meyer and Basil Thüring in 2003 to support the referral of greppini to Cetiosauriscus.

[23] The species Cetiosaurus longus, named in 1842 by Owen, was referred to the genus Cetiosauriscus without comment by sauropod palaeontologist John Stanton McIntosh in 1990.

[4] Cetiosaurus glymptonensis, named on the basis of nine middle-distal caudal centra from the Forest Marble Formation of Oxfordshire, England, was referred to Cetiosauriscus by McIntosh in 1990.

[4] "Cetiosaurus" glymptonensis is considered to be Eusauropoda incertae sedis by Upchurch and Martin (2003), Weishampel and colleagues (2004) and Whitlock (2011), and is in need of a new genus name because it has a single diagnostic feature, the lateral ridges.

[4][15][20] In 1980, Charig described a specimen of indeterminate diplodocid from the Early Cretaceous of England, and re-examined the holotype of Cetiosauriscus leedsii to compare its characteristics.

The specific name was chosen to honour Sir Ronald Stewart, the chairman of the London Brick Company that owned the clay pit the fossils had been found in.

[25] The only specimen that can confidently be assigned to C. stewarti is the holotype NHMUK PV R3078, although it is possible that isolated teeth from the Oxford Clay could be from the taxon.

Both the short, but wider-than-tall, centra preserve traces of the sideways projections (transverse processes) found in other vertebrae, which are very low on the sides compared to following caudals.

Moving towards the end of the tail the centra reduce in size and the transverse processes shrink until they are completely absent, with the neural spines becoming shorter, thinner, and more slanted.

The subfamily, including the other taxa Cetiosaurus, Haplocanthosaurus, Dystrophaeus, Elosaurus and Rhoetosaurus, was founded upon the general basal features of elongate cervicals and shortened dorsals—both opisthocoelous, amphicoelous caudals that are rod-shaped distally, paired sternal plates, an ilium lacking the postacetabular process (region of the ilium behind the ischium joint and acetabulum), a very wide pubis, wide distal ischium, significantly shorter forelimb than hindlimb, fibula lacking the middle muscle attachment, and long metacarpals and short metatarsals.

[1][30] David S. Berman and McIntosh in 1978 referred Cetiosauriscus to the family Diplodocidae along with multiple other genera; Diplodocus, Apatosaurus, Barosaurus, Mamenchisaurus Dicraeosaurus and Nemegtosaurus.

The results of the favoured cladogram of Tschopp et al. is shown below:[28] Shunosaurus lii Spinophorosaurus nigerensis Omeisaurus Mamenchisaurus Cetiosauriscus stewarti Barosaurus affinis Jobaria tiguidensis Amphicoelias latus Lourinhasaurus alenquerensis Camarasaurus Turiasaurus riodevensis Losillasaurus giganteus Brachiosaurus sp.

[35] It has been suggested that the biconvex distal caudal vertebrae in sauropods were used for making whip-like cracking noise, being thin and delicate and not intended for impact, as the joints would be very vulnerable to damage rendering them useless.

[37] The single specimen is known from the Lower Member of the Oxford Clay Formation, along with multiple other dinosaur genera and many other groups of animals, in the biozone of the index fossil Kosmoceras jason.

[7][17] The Oxford Clay Formation is a marine deposit of southern and middle England, known for the high-quality preservation of some fossils and the large diversity of taxa.

[17] Hundreds of invertebrates are known from the marine deposits, including bivalves, gastropods, scaphopods, ammonites, teuthoids, a nautiloid, foraminifera, coelenterates, bryozoans, brachiopods, annelids, crustaceans, ostracods, cirripedes and echinoderms.

Fish are known from the clades Elasmobranchii, Chimaera, and Actinopterygii, and the ichthyosaur Ophthalmosaurus, the plesiosaurs Cryptoclidus, Muraenosaurus, Tricleidus, Liopleurodon, Peloneustes, Pliosaurus and Simolestes, the crocodilians Metriorhynchus and Steneosaurus, and the pterosaur Rhamphorhynchus were all present.