Haplocanthosaurus (meaning "simple spined lizard") is a genus of sauropod dinosaur. Two species, H. delfsi and H. priscus, are known from incomplete fossil skeletons. It lived during the late Jurassic period (Kimmeridgian stage), 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. Haplocanthosaurus specimens have been found in the very lowest layer of the Morrison Formation, along with Hesperosaurus, Eobrontosaurus, and Allosaurus jimmadensi.
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 adult, and 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.
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).
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 rebachisaurids.
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