Apatosaurus

***Apatosaurus*** (meaning "deceptive lizard") is a genus of 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* had an average length of 21–23 m (69–75 ft), and an average mass of 16.4–22.4 t (16.1–22.0 long tons; 18.1–24.7 short tons). A few specimens indicate a maximum length of 11–30% greater than average and a mass of approximately 33 t (32 long tons; 36 short 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*.

## 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*, reaching 21–23 m (69–75 ft) in length and 16.4–22.4 t (16.1–22.0 long tons; 18.1–24.7 short tons) in body mass. 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*. 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 rivaling the largest titanosaurs. However, the upper size estimate of OMNH 1670 is likely an exaggeration, with the size estimates revised in 2020 at 30 m (98 ft) in length and 33 t (36 short tons) in body mass based on volumetric analysis.

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 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) pubes and 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.

## *Apatosaurus* in The Amazing Dinosaur Adventure

Luke the *Apatosaurus*Luke's MotherLuke's FatherDallas the *Apatosaurus*