Apatosaurus

Apatosaurus, whose name means "deceptive lizard," was a plant-eating sauropod from the Late Jurassic of North America. The first species, *Apatosaurus ajax*, was named by Othniel Charles Marsh in 1877, and a second, *Apatosaurus louisae*, was described by William H. Holland in 1916. These animals lived roughly 152 to 151 million years ago, during the late Kimmeridgian to early Tithonian ages, and their fossils come from the Morrison Formation in what are now Colorado, Oklahoma, New Mexico, Wyoming, and Utah. On average, *Apatosaurus* reached a certain length and mass, though a few specimens suggest individuals could be 11 to 30 percent larger, with a correspondingly greater mass.

This dinosaur belongs to the diplodocid family, but differs from *Diplodocus* itself mainly in having sturdier neck bones and leg bones. Its cervical vertebrae are shorter and heavier than those of *Diplodocus*, and its leg bones are thicker despite being longer, making *Apatosaurus* a more robust animal. Its tail was held off the ground during normal walking. Each forelimb had one claw, while each hindlimb had three. The skull, long mistaken for that of *Camarasaurus*, actually resembles *Diplodocus* more closely. *Apatosaurus* was a generalist feeder that probably held its head up high. Its vertebrae were lightened by air sacs that left them full of holes. Like other diplodocids, its tail might have been used as a whip to make loud sounds or, as recently suggested, as a sensory organ.

The skull was confused with those of *Camarasaurus* and *Brachiosaurus* until 1909, when the holotype of *A. louisae* was found along with a complete skull just a few meters from the front of the neck. Henry Fairfield Osborn disagreed with this link and mounted an *Apatosaurus* skeleton with a *Camarasaurus* skull cast. Skeletons were displayed with speculative skull casts until 1970, when McIntosh showed that sturdier skulls once assigned to *Diplodocus* were more likely from *Apatosaurus*.

Within the family Diplodocidae, *Apatosaurus* is one of the more primitive genera, with only *Amphicoelias* and possibly an unnamed genus being more basal. The subfamily Apatosaurinae was named in 1929 but not used validly until a 2015 study. Only *Brontosaurus* also belongs to this subfamily; other genera are either synonyms or reclassified as diplodocines. *Brontosaurus* was long considered a junior synonym of *Apatosaurus*, with its type species reclassified as *A. excelsus* in 1903. A 2015 study argued that *Brontosaurus* is a valid genus distinct from *Apatosaurus*, though not all paleontologists agree. In the Late Jurassic of North America, *Apatosaurus* lived alongside dinosaurs like *Allosaurus*, *Camarasaurus*, *Diplodocus*, and *Stegosaurus*.

*Apatosaurus* was a large, long-necked, four-legged animal with a long, whip-like tail. Its forelimbs were slightly shorter than its hindlimbs. Most size estimates come from specimen CM3018, the type of *A. louisae*, which reached a certain length and body mass. A 2015 volumetric study estimated CM3018 at a mass similar to *Dreadnoughtus*. Some *A. ajax* specimens, such as OMNH1670, represent individuals 11 to 30 percent longer, suggesting masses twice that of CM3018, potentially rivaling the largest titanosaurs. However, the upper size estimate for OMNH1670 is likely exaggerated; a 2020 volumetric analysis revised it to a smaller length and mass.

The skull is small relative to the body. The jaws contain spatulate teeth suited for eating plants. The snout is squared, similar to other diplodocoids, with only *Nigersaurus* having a squarer skull. The braincase is well preserved in specimen BYU17096, which also preserved much of the skeleton; a phylogenetic analysis found its shape similar to other diplodocoids. Some *Apatosaurus* skulls have been found with teeth still in place. Those teeth with exposed enamel show no scratches, but instead a sugary texture and little wear.

The neck vertebrae are deeply split, with neural spines forming a wide trough, giving the neck a broad, deep shape. The vertebral formula for the *A. louisae* holotype is 15 cervicals, 10 dorsals, 5 sacrals, and 82 caudals, though the number of tail vertebrae can vary even within a species. The cervical vertebrae of *Apatosaurus* and *Brontosaurus* are stouter and more robust than those of other diplodocids, and Charles Whitney Gilmore found them most similar to *Camarasaurus*. They also have cervical ribs that extend lower than in diplodocines, and the vertebrae and ribs are narrower toward the top of the neck, making the neck nearly triangular in cross-section. In *A. louisae*, the first cervical bones (atlas and axis) are nearly fused. The dorsal ribs are not fused or tightly attached to their vertebrae but are loosely connected. *Apatosaurus* has ten dorsal ribs on each side. The large neck was filled with weight-saving air sacs. Like its close relative *Supersaurus*, *Apatosaurus* has tall neural spines that make up more than half the height of the vertebrae.