page_title: Apatosaurus
section_title: Description
hierarchical_section_title: Apatosaurus / Description
caption_attribution_description: Apatosaurus ajax Marsh, 1877 sauropod dinosaur skull from the Jurassic of Utah, USA (public display, CMC VP 7180, Cincinnati Museum of Natural History &amp; Science, Cincinnati, Ohio, USA). This is one of the few Apatosaurus/Brontosaurus sauropod dinosaur skulls in existence. It is articulated with several anterior neck vertebrae, which verifies its generic identity. It's likely species ID is Apatosaurus ajax (see Barrett et al., 2011). Classification: Animalia, Chordata, Vertebrata, Dinosauria, Sauropodomorpha, Diplodocidae Stratigraphy: Brushy Basin Member, Morrison Formation, Upper Jurassic Locality: Emery County, Utah, USA Reference cited: Barret, P.M., G.W. Storrs, M.T. Young &amp; L.M. Witmer. 2011. A new skull of Apatosaurus and its taxonomic and palaeobiological implications. p. 5 in 59th Annual Symposium of Vertebrate Palaeontology and Comparative Anatomy, Abstracts of Presentations, Lyme Regis, Dorset, UK, September 12th-17th 2011. Sauropod dinosaurs were the largest terrestrial animals ever. They all have the same basic body plan: large body with four walking legs, very long neck &amp; tail, and a small head relative to body size. Sauropods were herbivores, and are often perceived as holding their heads &amp; necks up high to reach vegetation normally out of reach to other organisms. Modern reconstructions of many sauropod species depict them with heads and necks held close to the horizontal, or at low angles above the horizontal.
context_page_description: Apatosaurus 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, 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 had an average length of 21–22.8 m, and an average mass of 16.4–22.4 t. A few specimens indicate a maximum length of 11–30% greater than average and a mass of 32.7–72.6 t.
  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.
context_section_description: Apatosaurus was a large, long-necked, quadrupedal animal with a long, whip-like tail. Its forelimbs were slightly shorter than its hindlimbs. Most size estimates are based on specimen CM 3018, the type specimen of A. louisae. In 1936 this was measured to be 21.8 m (72 ft), by measuring the vertebral column. Current estimates are similar, finding that the individual was 21–22.8 m (69–75 ft) long and had a mass of 16.4–22.4 t (16.1–22.0 long tons; 18.1–24.7 short tons). A 2015 study that estimated the mass of volumetric models of Dreadnoughtus, Apatosaurus, and Giraffatitan estimates CM 3018 at 21.8–38.2 t (21.5–37.6 long tons; 24.0–42.1 short tons), similar in mass to Dreadnoughtus. Past estimates have put the creature's mass as high as 35.0 t (34.4 long tons; 38.6 short tons). Some specimens of A. ajax (such as OMNH 1670) represent individuals 11–30% longer, suggesting masses twice that of CM 3018 or 32.7–72.6 t (32.2–71.5 long tons; 36.0–80.0 short tons), potentially rivalling the largest titanosaurs.
  The skull is small in relation to the size of the animal. The jaws are lined with spatulate (chisel-like) teeth suited to an herbivorous diet. The snout of Apatosaurus and similar diplodocoids is squared, with only Nigersaurus having a squarer skull. The braincase of Apatosaurus is well preserved in specimen BYU 17096, which also preserved much of the skeleton. A phylogenetic analysis found that the braincase had a morphology similar to those of other diplodocoids. Some skulls of Apatosaurus have been found still in articulation with their teeth. Those teeth that have the enamel surface exposed do not show any scratches on the surface; instead, they display a sugary texture and little wear.
  Like those of other sauropods, the neck vertebrae are deeply bifurcated; they carried neural spines with a large trough in the middle, resulting in a wide, deep neck. The vertebral formula for the holotype of A. louisae is 15 cervicals, 10 dorsals, 5 sacrals, and 82 caudals. The caudal vertebra number may vary, even within species. The cervical vertebrae of Apatosaurus and Brontosaurus are stouter and more robust than those of other diplodocids and were found to be most similar to Camarasaurus by Charles Whitney Gilmore. In addition, they support cervical ribs that extend farther towards the ground than in diplodocines, and have vertebrae and ribs that are narrower towards the top of the neck, making the neck nearly triangular in cross-section. In Apatosaurus louisae, the atlas-axis complex of the first cervicals is nearly fused. The dorsal ribs are not fused or tightly attached to their vertebrae and are instead loosely articulated. Apatosaurus has ten dorsal ribs on either side of the body. The large neck was filled with an extensive system of weight-saving air sacs. Apatosaurus, like its close relative Supersaurus, has tall neural spines, which make up more than half the height of the individual bones of its vertebrae. The shape of the tail is unusual for a diplodocid; it is comparatively slender because of the rapidly decreasing height of the vertebral spines with increasing distance from the hips. Apatosaurus also had very long ribs compared to most other diplodocids, giving it an unusually deep chest. As in other diplodocids, the tail transformed into a whip-like structure towards the end.
  The limb bones are also very robust. Within Apatosaurinae, the scapula of Apatosaurus louisae is intermediate in morphology between those of A. ajax and Brontosaurus excelsus. The arm bones are stout, so the humerus of Apatosaurus resembles that of Camarasaurus, as well as Brontosaurus. However, the humeri of Brontosaurus and A. ajax are more similar to each other than they are to A. louisae. In 1936 Charles Gilmore noted that previous reconstructions of Apatosaurus forelimbs erroneously proposed that the radius and ulna could cross; in life they would have remained parallel. Apatosaurus had a single large claw on each forelimb, a feature shared by all sauropods more derived than Shunosaurus. The first three toes had claws on each hindlimb. The 
caption: A. ajax skull, specimen CMC VP 7180
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