page_title: Haplocanthosaurus
section_title: Classification
hierarchical_section_title: Haplocanthosaurus / Classification
caption_attribution_description: English: Haplocanthosaurus skeletal mount at the Cleveland Museum of Natural History
context_page_description: Haplocanthosaurus is a genus of intermediate sauropod dinosaur. Two species, H. delfsi and H. priscus, are known from incomplete fossil skeletons. It lived during the late Jurassic period, 155 to 152 million years ago. The type species is H. priscus, and the referred species H. delfsi was discovered by a young college student named Edwin Delfs in Colorado, USA. Haplocanthosaurus specimens have been found in the very lowest layer of the Morrison Formation, along with Hesperosaurus, Brontosaurus yahnahpin, and Allosaurus jimmadseni.
context_section_description: 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.
  In 2005, Darren Naish and Mike Taylor reviewed the various proposed positions of Haplocanthosaurus in their study of diplodocoid phylogeny. They found it could be a non-neosauropod eusauropod, a basal macronarian, or a basal diplodocoid. In 2011, an analysis by Whitlock recovered Haplocanthosaurus as the basalmost member of the Diplodocoidea, the third potentiality of Taylor & Naish. In 2015, a specimen-level phylogenetic analysis was published, finding Haplocanthosaurus to be a confirmed diplodocoid, either very basal, or more derived than rebbachisaurids. Their implied weighting cladogram is shown below.
caption: Incorrectly mounted H. delfsi skeleton, Cleveland Museum of Natural History
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