Comparison of A. ajax
(orange) and A. louisae
(red) with a human (blue) and
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
 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
skull, specimen CMC VP 7180
The skull is small in relation to the size of the animal. The jaws are lined with spatulate (chisel-like) teeth suited to an
 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.
Cervical vertebra of A. ajax
1860) in side and anterior view
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
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
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.
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
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
 The first three toes had claws on each hindlimb. The phalangeal formula is 2-1-1-1-1, meaning the innermost finger (phalanx) on the forelimb has two bones and the next has one.
 The single manual claw bone (
ungual) is slightly curved and squarely
truncated on the anterior end. The pelvic girdle includes the robust
ilia, and the fused (co-ossified)
ischia. The femora of Apatosaurus are very stout and represent some of the most robust femora of any member of Sauropoda. The tibia and fibula bones are different from the slender bones of Diplodocus but are nearly indistinguishable from those of Camarasaurus. The fibula is longer and slenderer than the tibia. The foot of Apatosaurus has three claws on the innermost digits; the digit formula is 3-4-5-3-2. The first metatarsal is the stoutest, a feature shared among diplodocids.