Apatosaurus

***Apatosaurus*** (/əˌpætəˈsɔːrəs/; meaning "deceptive lizard") is a genus of extinct sauropod dinosaurs 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. They lived about 152 to *151 million years ago* (mya), during the early Tithonian age, and are now known from fossils in the Morrison Formation of modern-day Colorado, Oklahoma, and Utah, in the United States. *Apatosaurus* had an average length of 21–22.8 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 32.7–72.6 t (32.2–71.5 long tons; 36.0–80.0 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 skull of *Apatosaurus*, 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.

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. Until 1970, *Apatosaurus* skeletons were mounted with speculative skull casts, 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. While 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 as synonyms or reclassified as diplodocines. *Brontosaurus* has long been considered a junior synonym of *Apatosaurus*; its only species was reclassified as *A. excelsus* in 1903. However, the 2015 study concluded that *Brontosaurus* was a valid genus of sauropod distinct from *Apatosaurus*. Nevertheless, not all paleontologists agree with this division. As it existed in North America during the late Jurassic, *Apatosaurus* would have lived aside 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*. In 1936, this was measured to be 21.8 m (72 ft), by measuring the vertebral column. Current estimates are similar, which find 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* (like 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 comparison with the size of the animal. The jaws are lined with spatulate (chisel-like) teeth suited to a herbivorous diet. The snout of *Apatosaurus* and similar diplodocoids are 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 similar morphology 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, 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 the vertebrae and ribs 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, instead being 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 neural tall 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 its 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; 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 more slender 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.

## Discovery and species

The name *Apatosaurus ajax* was coined in 1877 by Othniel Charles Marsh, Professor of Paleontology at Yale University, based on a nearly complete skeleton (holotype, YPM 1860) discovered the eastern foothills of the Rocky Mountains in Gunnison County, Colorado. The composite term *Apatosaurus* comes from the Greek words *apatē* (ἀπάτη)/*apatēlos* (ἀπατηλός) meaning "deception"/"deceptive", and *sauros* (σαῦρος) meaning "lizard"; thus, "deceptive lizard". Marsh gave it this name based on the chevron bones, which are dissimilar to those of other dinosaurs; instead, the chevron bones of *Apatosaurus* showed similarities with those of mosasaurs. During excavation and transportation, the bones of the holotype skeleton were mixed with those of another *Apatosaurus* individual originally described as *Atlantosaurus immanis*; as a consequence, some elements cannot be ascribed to either specimen with confidence anymore. Marsh distinguished the new genus *Apatosaurus* from *Atlantosaurus* on the basis of the number of sacral vertebrae, with *Apatosaurus* possessing three and *Atlantosaurus* four. Two years later, Marsh announced the discovery of a larger and more complete specimen at Como Bluff, Wyoming. Marsh decided to give this specimen a new name, because conventions and the relative sparse fossil record at that time meant that features then used to distinguish genera and species have now been found to be more widespread among sauropods. He named the new species *Brontosaurus excelsus*. All specimens currently considered *Apatosaurus* were from the Morrison Formation, the location of the excavations of Marsh and his rival Edward Drinker Cope.

Another specimen, in American Museum of Natural History and under the specimen number 460, which is occasionally assigned to *Apatosaurus*, is very complete; only the head, feet, and sections of the tail are missing, and it was the first sauropod skeleton mounted. The specimen was found north of Medicine Bow, Wyoming in 1898 by Walter Granger, and took the entire summer to extract. To complete the mount, sauropod feet that were discovered at the same quarry and a tail fashioned to appear as Marsh believed it should—which had too few vertebrae—were added. In addition, a sculpted model of what the museum thought the skull of this massive creature might look like was made. This was not a delicate skull like that of *Diplodocus*—which was later found to be more accurate—but was based on "the biggest, thickest, strongest skull bones, lower jaws and tooth crowns from three different quarries". These skulls were likely those of *Camarasaurus*, the only other sauropod for which good skull material was known at the time. The mount construction was overseen by Adam Hermann, who failed to find *Apatosaurus* skulls. Hermann was forced to sculpt a stand-in skull by hand. Osborn said in a publication that the skull was "largely conjectural and based on that of *Morosaurus*" (now *Camarasaurus*).

In 1903, Elmer Riggs published a study that described a well-preserved skeleton of a diplodocid from the Grand River Valley near Fruita, Colorado, Field Museum of Natural History specimen P25112. Riggs thought that the deposits were similar in age to those of the Como Bluff in Wyoming, from which Marsh described *Brontosaurus*. Most of the skeleton was found, and after comparison with both *Brontosaurus* and *Apatosaurus ajax*, Riggs realized that the holotype of *A. ajax* was immature, and thus the features distinguishing the genera were not valid. Since *Apatosaurus* was the earlier name, *Brontosaurus* should be considered a junior synonym of *Apatosaurus*. Because of this, Riggs recombined *Brontosaurus excelsus* as *Apatosaurus excelsus*. Based on comparisons with other species proposed to belong to *Apatosaurus*, Riggs also determined that the Field Columbian Museum specimen was likely most similar to *A. excelsus*.

Despite Riggs' publication, Henry Fairfield Osborn, who was a strong opponent of Marsh and his taxa, labeled the *Apatosaurus* mount of the American Museum of Natural History *Brontosaurus*. Because of this decision the name *Brontosaurus* was commonly used outside of scientific literature for what Riggs considered *Apatosaurus*, and the museum's popularity meant that *Brontosaurus* became one of the best known dinosaurs, even though it was invalid throughout nearly all of the 20th and early 21st centuries.

It was not until 1909 that an *Apatosaurus* skull was found during the first expedition, led by Earl Douglass, to what would become the Carnegie Quarry at Dinosaur National Monument. The skull was found a short distance from a skeleton (specimen CM 3018) identified as the new species *Apatosaurus louisae*, named after Louise Carnegie, wife of Andrew Carnegie who funded field research to find complete dinosaur skeletons in the American West. The skull was designated CM 11162; it was very similar to the skull of *Diplodocus*. Another smaller skeleton of *A. louisae* was found nearby CM 11162 and CM 3018.[30] The skull was accepted as belonging to the *Apatosaurus* specimen by Douglass and Carnegie Museum director William H. Holland, although other scientists—most notably Osborn—rejected this identification. Holland defended his view in 1914 in an address to the Paleontological Society of America, yet he left the Carnegie Museum mount headless. While some thought Holland was attempting to avoid conflict with Osborn, others suspected Holland was waiting until an articulated skull and neck were found to confirm the association of the skull and skeleton. After Holland's death in 1934, museum staff placed a cast of a *Camarasaurus* skull on the mount.

While most other museums were using cast or sculpted *Camarasaurus* skulls on *Apatosaurus* mounts, the Yale Peabody Museum decided to sculpt a skull based on the lower jaw of a *Camarasaurus*, with the cranium based on Marsh's 1891 illustration of the skull. The skull also included forward-pointing nasals—something different to any dinosaur—and fenestrae differing from the drawing and other skulls.

No *Apatosaurus* skull was mentioned in literature until the 1970s when John Stanton McIntosh and David Berman redescribed the skulls of *Diplodocus* and *Apatosaurus*. They found that though he never published his opinion, Holland was almost certainly correct, that *Apatosaurus* had a *Diplodocus*-like skull. According to them, many skulls long thought to pertain to *Diplodocus* might instead be those of *Apatosaurus*. They reassigned multiple skulls to *Apatosaurus* based on associated and closely associated vertebrae. Even though they supported Holland, it was noted that *Apatosaurus* might have possessed a *Camarasaurus*-like skull, based on a disarticulated *Camarasaurus*-like tooth found at the precise site where an *Apatosaurus* specimen was found years before. On October 20, 1979, after the publications by McIntosh and Berman, the first true skull of *Apatosaurus* was mounted on a skeleton in a museum, that of the Carnegie. In 1998, the Felch Quarry skull that Marsh included in his 1896 skeletal restoration was suggested to belong to *Brachiosaurus* instead. In 2011, the first specimen of *Apatosaurus* where a skull was found articulated with its cervical vertebrae was described. This specimen, CMC VP 7180, was found to differ in both skull and neck features from *A. louisae*, but shared many features of the cervical vertebrae with *A. ajax*. Another well-preserved skull is Brigham Young University specimen 17096, a well preserved skull and skeleton, with a preserved braincase. The specimen was found in Cactus Park Quarry of western Colorado.

Almost all modern paleontologists agreed with Riggs that the two dinosaurs should be classified together in a single genus. According to the rules of the ICZN (which governs the scientific names of animals), the name *Apatosaurus*, having been published first, has priority as the official name;*Brontosaurus* was considered a junior synonym and was therefore long discarded from formal use. Despite this, at least one paleontologist—Robert T. Bakker—argued in the 1990s that *A. ajax* and *A. excelsus* were in fact sufficiently distinct for the latter to merit a separate genus.

In 2015 Emanuel Tschopp, Octávio Mateus, and Roger Benson released a paper on diplodocoid systematics, and proposed that genera could be diagnosed by 13 differing characters, and species separated based on 6. The minimum number for generic separation was chosen based on the fact that *A. ajax* and *A. louisae* differ in 12 characters, and *Diplodocus carnegiei* and *D. hallorum* differ in 11 characters. Thus, 13 characters were chosen to validate the separation of genera. The 6 differing features for specific separation were chosen by counting the number of differing features in separate specimens generally agreed to represent one species, with only one differing character in *D. carnegiei* and *A. louisae*, but five differing features in *B. excelsus*. Therefore, Tschopp *et al.* argued that *Apatosaurus excelsus*, originally classified as *Brontosaurus excelsus*, had enough morphological differences from other species of *Apatosaurus* that it warranted being reclassified as a separate genus again. The conclusion was based on a comparison of 477 morphological characteristics across 81 different dinosaur individuals. Among the many notable differences are the wider—and presumably stronger—neck of *Apatosaurus* species compared to *B. excelsus*. Other species previously assigned to *Apatosaurus*, such as *Elosaurus parvus* and *Eobrontosaurus yahnahpin* were also reclassified as *Brontosaurus*. Some features proposed to separate *Brontosaurus* from *Apatosaurus* include: posterior dorsal vertebrae with the centrum longer than wide; the scapula rear to the acromial edge and the distal blade being excavated; the acromial edge of the distal scapular blade bears a rounded expansion; and the ratio of the proximodistal length to transverse breadth of the astragalus is 0.55 or greater. Paleontologist Michael D'Emic made a critique. Palaeontologist Donald Prothero criticized the mass media reaction to this study as superficial and premature, concluding that he would keep “Brontosaurus” in quotes and not treat the name as a valid genus.

### **Valid species**

Multiple species of *Apatosaurus* have been designated from scant material. Marsh named as many species as he could, which resulted in many being based upon fragmentary and indistinguishable remains. In 2005, Paul Upchurch and colleagues published a study that analyzed the species and specimen relationships of *Apatosaurus*. They found that *A. louisae* was the most basal species, followed by FMNH P25112, and then a polytomy of *A. ajax*, *A. parvus*, and *A. excelsus*. Their analysis was revised and expanded with many additional diplodocid specimens in 2015, which resolved the relationships of *Apatosaurus* slightly differently, and also supported separating *Brontosaurus* from *Apatosaurus*.

* ***Apatosaurus ajax*** was named by Marsh in 1877 after Ajax, a hero from Greek mythology. Marsh designated the incomplete, juvenile skeleton YPM 1860 as its holotype. The species is less studied then *Brontosaurus* and *A. louisae*, especially because of the incomplete nature of the holotype. In 2005, many specimens in addition to the holotype were found assignable to *A. ajax*, YPM 1840, NSMT-PV 20375, YPM 1861, and AMNH 460. The specimens date from the late Kimmeridgian to the early Tithonian ages. In 2015, only the *A. ajax* holotype YPM 1860 assigned to the species, with AMNH 460 found either to be within *Brontosaurus*, or potentially its own taxon. However, YPM 1861 and NSMT-PV 20375 only differed in a few characteristics, and cannot be distinguished specifically or generically from *A. ajax*. YPM 1861 is the holotype of *"Atlantosaurus" immanis*, which means it might be a junior synonym of *A. ajax*.
* ***Apatosaurus louisae*** was named by Holland in 1916, being first known from a partial skeleton that was found in Utah. The holotype is CM 3018, with referred specimens including CM 3378, CM 11162, and LACM 52844. The former two consist of a vertebral column; the latter two consist of a skull and a nearly complete skeleton, respectively. Its specimens all come from the late Kimmeridgian of Dinosaur National Monument. In 2015, Tschopp et al. found the type specimen of *Apatosaurus laticollis* to nest closely with CM 3018, meaning the former is likely a junior synonym of *A. louisae*.

The cladogram below is the result of an analysis by Tschopp, Mateus, and Benson (2015). The authors analyzed most diplodocid type specimens separately to deduce which specimen belonged to which species and genus.

### **Reassigned species**

* *Apatosaurus grandis* was named in 1877 by Marsh in the article that described *A. ajax*. It was briefly described, figured, and diagnosed. Marsh later mentioned it was only provisionally assigned to *Apatosaurus* when he reassigned it to his new genus *Morosaurus* in 1878. Since *Morvosaurus* has been considered a synonym of *Camarasaurus*, *C. grandis* is the oldest-named species of the latter genus.
* *Apatosaurus excelsus* was the original type species of *Brontosaurus*, first named by Marsh in 1879. Elmer Riggs reclassified *Brontosaurus* as a synonym of *Apatosaurus* in 1903, transferring the species *B. excelsus* to *A. excelsus*. In 2015, Tschopp, Mateus, and Benson argued that the species was distinct enough to be placed in its own genus so they reclassified it back into *Brontosaurus*.
* *Apatosaurus parvus*, first described from a juvenile specimen as *Elosaurus* in 1902 by Peterson and Gilmore, was reassigned to *Apatosaurus* in 1994, and then to *Brontosaurus* in 2015. Multiple other, more mature specimens were assigned to it following the 2015 study.
* *Apatosaurus minimus* was originally described as a specimen of *Brontosaurus* sp. in 1904 by Osborn. In 1917, Henry Mook named it as its own species, *A. minimus*, for a pair of ilia and their sacrum. In 2012, Mike P. Taylor and Matt J. Wedel published a short abstract describing the material of "A." *minimus*, finding it hard-to-place among either Diplodocoidea or Macronaria. While it was placed with *Saltasaurus* in a phylogenetic analysis, it was thought to represent instead some form with convergent features from many groups. The study of Tschopp *et al.* did find that a camarasaurid position for the taxon was supported, but noted that the position of the taxon was found to be highly variable and there was no clearly more likely position.
* *Apatosaurus alenquerensis* was named in 1957 by Albert-Félix de Lapparent and Georges Zbyweski. It was based on post cranial material from Portugal. In 1990, this material was reassigned to *Camarasaurus*, but in 1998 it was given its own genus, *Lourinhasaurus*. This was further supported by the findings of Tschopp *et al.* in 2015, where *Lourinhasaurus* was found to be sister to *Camarasaurus* and other camarasaurids.
* *Apatosaurus yahnahpin* was named by James Filla and Patrick Redman in 1994. Bakker made *A. yahnahpin* the type species of a new genus, *Eobrontosaurus* in 1998, and Tschopp reclassified it as *Brontosaurus yahnahpin* in 2015.

## Classification

*Apatosaurus* is a member of the family Diplodocidae, a clade of gigantic sauropod dinosaurs. The family includes some of the longest creatures ever to walk the earth, including *Diplodocus*, *Supersaurus*, and *Barosaurus*. *Apatosaurus* is sometimes classified in the subfamily Apatosaurinae, which may also include *Suuwassea*, *Supersaurus*, and *Brontosaurus*. Othniel Charles Marsh described *Apatosaurus* as allied to *Atlantosaurus* within the now-defunct group Atlantosauridae. In 1878, Marsh raised his family to the rank of suborder, including *Apatosaurus*, *Atlantosaurus*, *Morosaurus* (=*Camarasaurus*) and *Diplodocus*. He classified this group within Sauropoda, a group he erected in the same study. In 1903, Elmer S. Riggs said the name Sauropoda would be a junior synonym of earlier names; he grouped *Apatosaurus* within Opisthocoelia. Sauropoda is still used as the group name. In 2011, John Whitlock published a study that placed *Apatosaurus* a more basal diplodocid, sometimes less basal than *Supersaurus*. Cladogram of the Diplodocidae after Tschopp, Mateus, and Benson (2015).