They lived during the Late Jurassic (Kimmeridgian stage), 155 to 152 million years ago in North America.

[2] The type species is H. priscus, named in 1903 John Bell Hatcher , and the referred species H. delfsi was discovered by a young college student named Edwin Delfs in Colorado, United States and described by Jack McIntosh and Williams in 1988.

Haplocanthosaurus specimens have been found in the very lowest layer of the Morrison Formation, along with Hesperosaurus mjosi, Brontosaurus yahnahpin, and Allosaurus jimmadseni.

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.

Also, the area around the periphery of each articular face is flattened, creating a ‘bevelled’ appearance, and also occurs in Haplocanthosaurus and Cetiosauriscus.

Originally described as a "cetiosaurid", José Bonaparte decided in 1999 that Haplocanthosaurus differed enough from other sauropods to warrant its own family, the Haplocanthosauridae.

[14] In 2005, Darren Naish and Mike Taylor reviewed the various proposed positions of Haplocanthosaurus in their study of diplodocoid phylogeny.

[15] In 2011, an analysis by Whitlock recovered Haplocanthosaurus as the basalmost member of the Diplodocoidea, the third potentiality of Taylor & Naish.

[16] In 2015, a specimen-level phylogenetic analysis was published, finding Haplocanthosaurus to be a confirmed diplodocoid, either very basal, or more derived than rebbachisaurids.