The Morrison Formation is one of the most wonderful slices of prehistoric time found anywhere in the world. Parts of this Late Jurassic record pop up all over the American west, from Montana to Texas, and the sequence harbors wonderful bonebeds such as those at Dinosaur National Monument, Utah, and Bone Cabin Quarry, Wyoming. Yet, while the upper part of the Morrison has yielded splendid specimens of famous dinosaurs such as Apatosaurus, Stegosaurus, Allosaurus and more, the lower part of the formation contains a gaggle of puzzling dinosaurs. Haplocanthosaurus is one of these enigmas.
When discussing any geologic formation, it’s easy to talk about it as if it’s just a narrow slice of time. Yet distinct formations can record many millions of years of evolution and extinction. The Morrison Formation, for one, records about 10 million years of Jurassic history, from about 156 to 146 million years ago. And the dinosaurs paleontologists find near the top are not the same as the ones they found lower down in the formation.
Haplocanthosaurus, one of the long-necked sauropods, was part of the lower Morrison fauna. The 50-foot herbivore trod the Jurassic landscape around 155 million years ago and lived alongside the equally unfamiliar forerunners of famous dinosaurs. The stegosaur Hesperosaurus, the slender Allosaurus “jimmadseni” and hefty Eobrontosaurus also lived during this earlier portion of Morrison time.
Despite the fact that the dinosaur was named in 1903, however, paleontologists are still confounded by Haplocanthosaurus. The mid-sized sauropod appears to have been a close relative of the extremely common, blunt-headed dinosaur Camarasaurus. Frustratingly, though, Haplocanthosaurus is extremely rare, and no one has found the dinosaur’s skull just yet. With a skull, the dinosaur’s relationships and biology would come into sharper focus, but no such luck.
Haplocanthosaurus is a symbol of how much we still have to learn about even long-known dinosaurs. The lower part of the Morrison Formation, in particular, seems to be filled with strange dinosaurs that may offer clues about how the exceptionally rich fauna of the later Morrison–filled with sauropods and knife-toothed predators–evolved. Were Hesperosaurus, Eobrontosaurus, Allosaurus “jimmadseni” and Haplocanthosaurus ancestral to any of the later forms? Or did they fall away as new species migrated into the same habitats from elsewhere? The depths of the Morrison Formation still hold Jurassic mysteries worth investigating.