Haplocanthosaurus, meaning “simple sniped lizard”, is a genus of sauropod dinosaur from the Late Jurassic Period (156 to 144 million years ago). It was found in the Kimmeridgian/Tithonian stages. The type species, H. priscus, was discovered by a young college student named Edwin Delfs in Colorado. The species, H. delfsi, was also unearthed by Delfs in the same Morrison Formation.

This dinosaur was previously known as the Haplocanthus until 1903 when Hatcher believed there had been a fish species already given the name Haplocanthus, thus he changed it to Haplocanthosaurus. Years later it was discovered that the actual fish genus was Haplacanthus and not Haplocanthus (slight spelling variation), making the original name okay for the dinosaur. However, by this time the new name had stuck in the science world and the ICZN kept Haplocanthosaurus as the name of the dinosaur.

This was one of the smallest sauropods found in the Morrison Formation. While most found here were well over 70 feet long, the Haplocanthosaurus measured only 49 feet in length and weighed about 14 tons. There are only four known specimens, and only one fairly complete specimen that was able to be mounted. The mounted version is located at the Cleveland Museum of Natural History and is featured with a replica head, as the actual skull was never recovered.

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