Apatosaurus

***Apatosaurus*** (/əˌpætəˈsɔːrəs/; meaning "deceptive lizard") is a genus of herbivorous sauropod dinosaur that lived in North America during the Late Jurassic period. Othniel Charles Marsh described and named the first-known species, ***A. ajax***, in 1877, and a second species, ***A. louisae***, was discovered and named by William H. Holland in 1916. *Apatosaurus* lived about 152 to 151 million years ago (mya), during the late Kimmeridgian to early Tithonian age, and are now known from fossils in the Morrison Formation of modern-day Colorado, Oklahoma, New Mexico, Wyoming, and Utah in the United States. *Apatosaurus louisae* had an average length of 21–23 m (69–75 ft), and an average mass of 16.4–22.4 tonnes (16.1–22.0 long tons; 18.1–24.7 tons). A few specimens indicate a maximum length of 11–30% greater than average of approximately 30 m (98 ft) and a mass of approximately 33-38.2 tonnes (32-37.6 long tons; 36-42.1 tons).

The cervical vertebrae of *Apatosaurus* are less elongated and more heavily constructed than those of *Diplodocus*, a diplodocid like *Apatosaurus*, and the bones of the leg are much stockier despite being longer, implying that *Apatosaurus* was a more robust animal. The tail was held above the ground during normal locomotion. *Apatosaurus* had a single claw on each forelimb and three on each hindlimb. The *Apatosaurus* skull, long thought to be similar to *Camarasaurus*, is much more similar to that of *Diplodocus*. *Apatosaurus* was a generalized browser that likely held its head elevated. To lighten its vertebrae, *Apatosaurus* had air sacs that made the bones internally full of holes. Like that of other diplodocids, its tail may have been used as a whip to create loud noises, or, as more recently suggested, as a sensory organ.

The skull of *Apatosaurus* was confused with that of *Camarasaurus* and *Brachiosaurus* until 1909, when the holotype of *A. louisae* was found, and a complete skull just a few meters away from the front of the neck. Henry Fairfield Osborn disagreed with this association, and went on to mount a skeleton of *Apatosaurus* with a *Camarasaurus* skull cast. *Apatosaurus* skeletons were mounted with speculative skull casts until 1970, when McIntosh showed that more robust skulls assigned to *Diplodocus* were more likely from *Apatosaurus*.

*Apatosaurus* is a genus in the family Diplodocidae. It is one of the more basal genera, with only *Amphicoelias* and possibly a new, unnamed genus more primitive. Although the subfamily Apatosaurinae was named in 1929, the group was not used validly until an extensive 2015 study. Only *Brontosaurus* is also in the subfamily, with the other genera being considered synonyms or reclassified as diplodocines. *Brontosaurus* has long been considered a junior synonym of *Apatosaurus*; its type species was reclassified as *A. excelsus* in 1903. A 2015 study concluded that *Brontosaurus* is a valid genus of sauropod distinct from *Apatosaurus*, but not all paleontologists agree with this division. As it existed in North America during the late Jurassic, *Apatosaurus* would have lived alongside dinosaurs such as *Allosaurus*, *Camarasaurus*, *Diplodocus*, and *Stegosaurus*.