Haplocanthosaurus was one of the smallest sauropods of the Morrison. While some Morrison sauropods could reach lengths of over 20 meters (or over 70 feet), Haplocanthosaurus wasn't nearly as large, and reached a total length of 14.8 meters (49 feet) and an estimated weight of 12.8 metric tons.
There are four known specimens of Haplocanthosaurus: one of H. delfsi, and three of H. priscus. Of these, the type of H. delfsi is the only one complete enough to mount. The mounted specimen of H. delfsi now stands in the Cleveland Museum of Natural History, albeit with a completely speculative replica skull, as the actual skull was not recovered. Present in stratigraphic zones 1, 2, and 4. If the specimen of "Morosaurus" agilis is included, Haplocanthosaurus is also known from fragments of a skull and cervicals 1-3.
Haplocanthosaurus is known from many elements, mostly of vertebra. In the middle and cervical caudals of Apatosaurus, [Camarasaurus]], Cetiosaurus and Haplocanthosaurus, the intraprezygapophyseal lamina is separate from the root of the neural canal by a vertical midline lamina. In the last few caudals and the most cranial dorsals, the lateral edge of the prezygapophyseal lamina becomes widened and roughened. Hatcher (1901) interpreted this as forming the attachment area for the muscles from which the scapular blade was suspended.
The dorsoventrally elongate oval outlines are characteristic of Haplocanthosaurus with only Camarasaurus also possessing them. The parapophyses remaining as oval facets on the craniolateral margin, and the sacral spines 1-3 fused are also found in both Haplocanthosaurus and Camarasaurus.
The Cetiosaurus specimen OUMNH J13695 has a low horizontal ridge on each of its lateral surfaces, creating a slightly subhexagonal transverse cross-section, and that feature is also seen on Cetiosauriscus, the anterior caudals of Haplocanthosaurus, and caudals 15-30 of Omeisaurus. Also, the area around the periphery of each articullar face is flattened, creating a ‘bevelled’ appearance, and also occurs in Haplocanthosaurus and Cetiosauriscus.
Haplocanthosaurus is distinguished by dorsal vertebra lacking cranial centrodiapophyseal laminae. Also, it is distinguished by elongate intrapostzygapophyseal laminae, dorsoventrally directed dorsal transverse processes that approach the height of the neural spines, and the distal end of the scapular blade being dorsally and ventrally expanded
Haplocanthosaurus priscus was originally named Haplocanthus priscus by John Bell Hatcher in 1903. Soon after his original description, Hatcher came to believe the name Haplocanthus had already been used for a genus of acanthodian fish (Haplacanthus, named by Louis Agassiz in 1845), and was thus preoccupied. Hatcher re-classified his sauropod later in 1903, giving it the new name Haplocanthosaurus. However, the name was not technically preoccupied at all, since there was a variation in spelling: the fish was named Haplacanthus, not Haplocanthus. While Haplocanthus technically remained the valid name for this dinosaur, Hatcher's mistake was not noticed until many years after the name Haplocanthosaurus had become fixed in scientific literature. When the mistake was finally discovered, a petition was sent to the ICZN (the body which governs scientific names in zoology), which officially discarded the name Haplocanthus and declared Haplocanthosaurus the official name (ICZN Opinion #1633).
Originally described as a "cetiosaurid", José Bonaparte decided in 1999 that Haplocanthosaurus differed enough from other sauropods to warrant its own family, the Haplocanthosauridae.
Phylogenetic studies have failed to clarify the exact relationships of Haplocanthosaurus with any certainty. Studies have variously found it to be more primitive than the neosauropods, a primitive macronarian (related to the ancestor of more advanced forms such as Camarasaurus and the brachiosaurids), or a very primitive diplodocoid, more closely related to Diplodocus than to titanosaurs, but more primitive than rebbachisaurids Template:Clade
2011, an analysis by Whitlock recovered Haplocanthosaurus as the basalmost member of the Diplodocoidea, corresponding to the third hypothetical position on the cladogram above.