Apatosaurus And Trackway

''Apatosaurus'' (; meaning "deceptive lizard") 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 (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 , and an average mass of . A few specimens indicate a maximum length of 11–30% greater than average and a mass of approximately .

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 CM3018, the type specimen of ''A.louisae'', reaching in length and in body mass. A 2015 study that estimated the mass of volumetric models of ''Dreadnoughtus'', ''Apatosaurus'', and ''Giraffatitan'' estimates CM3018 at , similar in mass to ''Dreadnoughtus''. Some specimens of ''A.ajax'' (such as OMNH1670) represent individuals 1130% longer, suggesting masses twice that of CM3018 or , potentially rivaling the largest titanosaurs. However, the upper size estimate of OMNH1670 is likely an exaggeration, with the size estimates revised in 2020 at in length and 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 BYU17096, 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 15cervicals, 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.

Discovery and species

Initial discovery

The first ''Apatosaurus'' fossils were discovered by Arthur Lakes, a local miner, and his friend Henry C. Beckwith in the spring of 1877 in Morrison, a town in the eastern foothills of the Rocky Mountains in Jefferson County, Colorado. Arthur Lakes wrote to Othniel Charles Marsh, Professor of Paleontology at Yale University, and Edward Drinker Cope, a paleontologist based in Philadelphia, about the discovery until eventually collecting several fossils and sending them to both paleontologists. Marsh named ''Atlantosaurus montanus'' based on some of the fossils sent and hired Lakes to collect the rest of the material at Morrison and send it to Yale, while Cope attempted to hire Lakes as well but was rejected.Kohl, M. F., & McIntosh, J. S. 1997, Discovering Dinosaurs in the Old West: The field journals of Arthur Lakes. One of the best specimens collected by Lakes in 1877 was a well preserved partial postcranial skeleton, including many vertebrae, and a partial braincase ( YPM VP 1860), which was sent to Marsh and named ''Apatosaurus ajax'' in November 1877. The composite term ''Apatosaurus'' comes from the Greek words ' ()/' () meaning "deception"/"deceptive", and ' () 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, most likely that of the representative species ''Mosasaurus''. By the end of excavations at Lakes' quarry in Morrison, several partial specimens of ''Apatosaurus'' had been collected, but only the type specimen of ''A. ajax'' can be confidently referred to the species.

During excavation and transportation, the bones of the holotype skeleton were mixed with those of another Apatosaurine individual originally described as ''Atlantosaurus immanis''; as a consequence, some elements cannot be ascribed to either specimen with confidence. Marsh distinguished the new genus ''Apatosaurus'' from ''Atlantosaurus'' on the basis of the number of sacral vertebrae, with ''Apatosaurus'' possessing three and ''Atlantosaurus'' four. Recent research shows that traits usually used to distinguish taxa at this time were actually widespread across several taxa, causing many of the taxa named to be invalid, like ''Atlantosaurus''. Two years later, Marsh announced the discovery of a larger and more complete specimen (YPM VP 1980) from Como Bluff, Wyoming, he gave this specimen the name ''Brontosaurus excelsus''. Also at Como Bluff, the Hubbell brothers working for Edward Drinker Cope collected a tibia, fibula, scapula, and several caudal vertebrae along with other fragments belonging to ''Apatosaurus'' in 1877–78 at Cope's Quarry 5 at the site. Later in 1884, Othniel Marsh named ''Diplodocus lacustris'' based on a chimeric partial dentary, snout, and several teeth collected by Lakes in 1877 at Morrison.Mossbrucker, M. T., & Bakker, R. T. (October 2013). Missing muzzle found: new skull material referrable to Apatosaurus ajax (Marsh 1877) from the Morrison Formation of Morrison, Colorado. In ''Geological Society of America Abstracts with Programs'' (Vol. 45, p. 111). In 2013, it was suggested that the dentary of ''D. lacustris'' and its teeth were actually from ''Apatosaurus ajax'' based on its proximity to the type braincase of ''A. ajax''. All specimens currently considered ''Apatosaurus'' were from the Morrison Formation, the location of the excavations of Marsh and Cope.

Second Dinosaur Rush and skull issue

After the end of the Bone Wars, many major institutions in the eastern United States were inspired by the depictions and finds by Marsh and Cope to assemble their own dinosaur fossil collections.Brinkman, P. D. (2010). ''The second Jurassic dinosaur rush''. University of Chicago Press. The competition to mount the first sauropod skeleton specifically was the most intense, with the American Museum of Natural History, Carnegie Museum of Natural History, and Field Museum of Natural History all sending expeditions to the west to find the most complete sauropod specimen, bring it back to the home institution, and mount it in their fossil halls. The American Museum of Natural History was the first to launch an expedition, finding a well preserved skeleton (AMNH 460), which is occasionally assigned to ''Apatosaurus'', is considered nearly 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 shouldbut which had too few vertebraewere 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 accuratebut 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 had 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 known as the Carnegie Quarry at Dinosaur National Monument. The skull was found a short distance from a skeleton (specimen CM3018) 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 CM11162; it was very similar to the skull of ''Diplodocus''. Another smaller skeleton of ''A.louisae'' was found nearby CM11162 and CM3018. The skull was accepted as belonging to the ''Apatosaurus'' specimen by Douglass and Carnegie Museum director William H. Holland, although other scientistsmost notably Osbornrejected 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 nasalssomething unusual for any dinosaurand fenestrae differing from both 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 October20, 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, it was suggested that the Felch Quarry skull that Marsh had included in his 1896 skeletal restoration instead belonged to ''Brachiosaurus''. This was supported in 2020 with a redescription of the brachiosaurid material found at the Felch Quarry.

Recent discoveries and reassessment

In 2011, the first specimen of ''Apatosaurus'' where a skull was found articulated with its cervical vertebrae was described. This specimen, CMCVP7180, 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 in western Colorado. In 2013, Matthew Mossbrucker and several other authors published an abstract that described a premaxilla and maxilla from Lakes' original quarry in Morrison and referred the material to ''Apatosaurus ajax.''

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 paleontologistRobert T. Bakkerargued 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 thirteen differing characters, and species separated based on six. The minimum number for generic separation was chosen based on the fact that ''A.ajax'' and ''A.louisae'' differ in twelve characters, and ''Diplodocus carnegiei'' and ''D.hallorum'' differ in eleven characters. Thus, thirteen characters were chosen to validate the separation of genera. The six 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 etal. 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 widerand presumably strongerneck 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 bearing a rounded expansion; and the ratio of the proximodistal length to transverse breadth of the astragalus 0.55 or greater. Sauropod expert Michael D'Emic pointed out that the criteria chosen were to an extent arbitrary and that they would require abandoning the name ''Brontosaurus'' again if newer analyzes obtained different results.D'Emic, M. 2015
"Not so fast, Brontosaurus"
Time.com Mammal paleontologist 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.Prothero, D. 2015
"Is "Brontosaurus" Back? Not So Fast!"
Skeptic.com.

Valid species

Many 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 FMNHP25112, 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 YPM1860 as its holotype. The species is less studied than ''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'', YPM1840, NSMT-PV 20375, YPM1861, and AMNH460. The specimens date from the late Kimmeridgian to the early Tithonian ages. In 2015, only the ''A.ajax'' holotype YPM1860 assigned to the species, with AMNH460 found either to be within ''Brontosaurus'', or potentially its own taxon. However, YPM1861 and NSMT-PV 20375 only differed in a few characteristics, and cannot be distinguished specifically or generically from ''A.ajax''. YPM1861 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 CM3018, with referred specimens including CM3378, CM11162, and LACM52844. The former two consist of a vertebral column; the latter two consist of a skull and a nearly complete skeleton, respectively. ''Apatosaurus louisae'' specimens all come from the late Kimmeridgian of Dinosaur National Monument. In 2015, Tschopp etal. found the type specimen of ''Apatosaurus laticollis'' to nest closely with CM3018, 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 ''Morosaurus'' 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. Many 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 etal. 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 etal. 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).

Paleobiology

It was believed throughout the 19th and early 20th centuries that sauropods like ''Apatosaurus'' were too massive to support their own weight on dry land. It was theorized that they lived partly submerged in water, perhaps in swamps. More recent findings do not support this; sauropods are now thought to have been fully terrestrial animals. A study of diplodocid snouts showed that the square snout, large proportion of pits, and fine, subparallel scratches of the teeth of ''Apatosaurus'' suggests it was a ground-height, nonselective browser. It may have eaten ferns, cycadeoids, seed ferns, horsetails, and algae. Stevens and Parish (2005) speculate that these sauropods fed from riverbanks on submerged water plants.

A 2015 study of the necks of ''Apatosaurus'' and ''Brontosaurus'' found many differences between them and other diplodocids, and that these variations may have shown that the necks of ''Apatosaurus'' and ''Brontosaurus'' were used for intraspecific combat. Various uses for the single claw on the forelimb of sauropods have been proposed. One suggestion is that they were used for defense, but their shape and size make this unlikely. It was also possible they were for feeding, but the most probable use for the claw was grasping objects such as tree trunks when rearing.

Trackways of sauropods like ''Apatosaurus'' show that they may have had a range of around per day, and that they could potentially have reached a top speed of per hour. The slow locomotion of sauropods may be due to their minimal muscling, or to recoil after strides. A trackway of a juvenile has led some to believe that they were capable of bipedalism, though this is disputed.

Neck posture

Diplodocids like ''Apatosaurus'' are often portrayed with their necks held high up in the air, allowing them to browse on tall trees. Some studies state diplodocid necks were less flexible than previously believed, because the structure of the neck vertebrae would not have allowed the neck to bend far upward, and that sauropods like ''Apatosaurus'' were adapted to low browsing or ground feeding.

Other studies by Taylor find that all tetrapods appear to hold their necks at the maximum possible vertical extension when in a normal, alert posture; they argue the same would hold true for sauropods barring any unknown, unique characteristics that set the soft tissue anatomy of their necks apart from that of other animals. ''Apatosaurus'', like ''Diplodocus'', would have held its neck angled upward with the head pointing downward in a resting posture. Kent Stevens and Michael Parrish (1999 and 2005) state ''Apatosaurus'' had a great feeding range; its neck could bend into a U-shape laterally. The neck's range of movement would have also allowed the head to feed at the level of the feet.

Matthew Cobley ''et al.'' (2013) dispute this, finding that large muscles and cartilage would have limited movement of the neck. They state the feeding ranges for sauropods like ''Diplodocus'' were smaller than previously believed, and the animals may have had to move their whole bodies around to better access areas where they could browse vegetation. As such, they might have spent more time foraging to meet their minimum energy needs. The conclusions of Cobley ''etal.'' are disputed by Taylor, who analyzed the amount and positioning of intervertebral cartilage to determine the flexibility of the neck of ''Apatosaurus'' and ''Diplodocus''. He found that the neck of ''Apatosaurus'' was very flexible.

Physiology

Given the large body mass and long neck of sauropods like ''Apatosaurus'', physiologists have encountered problems determining how these animals breathed. Beginning with the assumption that, like crocodilians, ''Apatosaurus'' did not have a diaphragm, the dead-space volume (the amount of unused air remaining in the mouth, trachea, and air tubes after each breath) has been estimated at for a specimen. Paladino calculates its tidal volume (the amount of air moved in or out during a single breath) at with an avian respiratory system, if mammalian, and if reptilian.

On this basis, its respiratory system would likely have been parabronchi, with multiple pulmonary air sacs as in avian lungs, and a flow-through lung. An avian respiratory system would need a lung volume of about compared with a mammalian requirement of , which would exceed the space available. The overall thoracic volume of ''Apatosaurus'' has been estimated at , allowing for a , four-chambered heart and a lung capacity. That would allow about for the necessary tissue. Evidence for the avian system in ''Apatosaurus'' and other sauropods is also present in the pneumaticity of the vertebrae. Though this plays a role in reducing the weight of the animal, Wedel (2003) states they are also likely connected to air sacs, as in birds.

James Spotila ''et al.'' (1991) concludes that the large body size of sauropods would have made them unable to maintain high metabolic rates because they would not have been able to release enough heat. They assumed sauropods had a reptilian respiratory system. Wedel says that an avian system would have allowed it to dump more heat. Some scientists state that the heart would have had trouble sustaining sufficient blood pressure to oxygenate the brain. Others suggest that the near-horizontal posture of the head and neck would have eliminated the problem of supplying blood to the brain because it would not have been elevated.

James Farlow (1987) calculates that an ''Apatosaurus''-sized dinosaur about would have possessed of fermentation contents, though he cautions that the regression equation being used is based on living mammals which are much smaller and physiologically different. Assuming ''Apatosaurus'' had an avian respiratory system and a reptilian resting-metabolism, Frank Paladino etal. (1997) estimate the animal would have needed to consume only about of water per day.

Growth

A 1999 microscopic study of ''Apatosaurus'' and ''Brontosaurus'' bones concluded the animals grew rapidly when young and reached near-adult sizes in about 10years. In 2008, a study on the growth rates of sauropods was published by Thomas Lehman and Holly Woodward. They said that by using growth lines and length-to-mass ratios, ''Apatosaurus'' would have grown to 25t (25 long tons; 28 short tons) in 15years, with growth peaking at in a single year. An alternative method, using limb length and body mass, found ''Apatosaurus'' grew per year, and reached its full mass before it was about 70years old. These estimates have been called unreliable because the calculation methods are not sound; old growth lines would have been obliterated by bone remodeling. One of the first identified growth factors of ''Apatosaurus'' was the number of sacral vertebrae, which increased to five by the time of the creature's maturity. This was first noted in 1903 and again in 1936.

Long-bone histology enables researchers to estimate the age that a specific individual reached. A study by Eva Griebeler etal. (2013) examined long-bone histological data and concluded the ''Apatosaurus'' sp.SMA0014 weighed , reached sexual maturity at 21years, and died aged 28. The same growth model indicated ''Apatosaurus'' sp.BYU 601–17328 weighed , reached sexual maturity at 19years, and died aged 31.

Juveniles

Compared with most sauropods, a relatively large amount of juvenile material is known from ''Apatosaurus''. Multiple specimens in the OMNH are from juveniles of an undetermined species of ''Apatosaurus''; this material includes partial shoulder and pelvic girdles, some vertebrae, and limb bones. OMNH juvenile material is from at least two different age groups and based on overlapping bones likely comes from more than three individuals. The specimens exhibit features that distinguish ''Apatosaurus'' from its relatives, and thus likely belong to the genus. Juvenile sauropods tend to have proportionally shorter necks and tails, and a more pronounced forelimb-hindlimb disparity than found in adult sauropods.

Tail

The estimated tail length of ''Apatosaurus'' is approximately 54% of the total body length, with the tail sometimes hypothesized to be capable of functioning like a very long, tapering bullwhip. An article published in 1997 reported research of the mechanics of ''Apatosaurus'' tails through computer simulation by Nathan Myhrvold and paleontologist Philip J. Currie. This computer modeling suggested diplodocids were capable of producing a whiplike cracking sound of over 200 decibels, comparable to the volume of a cannon being fired.

A pathology has been identified on the tail of ''Apatosaurus'', caused by a growth defect. Two caudal vertebrae are seamlessly fused along the entire articulating surface of the bone, including the arches of the neural spines. This defect might have been caused by the lack or inhibition of the substance that forms intervertebral disks or joints. It has been proposed that the whips could have been used in combat and defense, but the tails of diplodocids were quite light and narrow compared to ''Shunosaurus'' and mamenchisaurids, and thus to injure another animal with the tail would severely injure the tail itself. More recently, Baron (2020) considers the use of the tail as a bullwhip unlikely because of the potentially catastrophic muscle and skeletal damage such speeds could cause on the large and heavy tail. Instead, he proposes that the tails might have been used as a tactile organ to keep in touch with the individuals behind and on the sides in a group while migrating, which could have augmented cohesion and allowed communication among individuals while limiting more energetically demanding activities like stopping to search for dispersed individuals, turning to visually check on individuals behind, or communicating vocally.

Paleoecology

The Morrison Formation is a sequence of shallow marine and alluvial sediments which, according to radiometric dating, dates from between 156.3 mya at its base, and 146.8mya at the top, placing it in the late Oxfordian, Kimmeridgian, and early Tithonian stages of the Late Jurassic period. This formation is interpreted as originating in a locally semiarid environment with distinct wet and dry seasons. The Morrison Basin, where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan; it was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels, and floodplains. This formation is similar in age to the Lourinhã Formation in Portugal and the Tendaguru Formation in Tanzania.

''Apatosaurus'' was the second most common sauropod in the Morrison Formation ecosystem, after ''Camarasaurus''. ''Apatosaurus'' may have been more solitary than other Morrison Formation dinosaurs. Fossils of the genus have only been found in the upper levels of the formation. Those of ''Apatosaurus ajax'' are known exclusively from the upper Brushy Basin Member, about 152–151 mya. ''A.louisae'' fossils are rare, known only from one site in the upper Brushy Basin Member; they date to the late Kimmeridgian stage, about 151mya. Additional ''Apatosaurus'' remains are known from similarly aged or slightly younger rocks, but they have not been identified as any particular species, and thus may instead belong to ''Brontosaurus''.

The Morrison Formation records a time when the local environment was dominated by gigantic sauropod dinosaurs. Dinosaurs known from the Morrison Formation include the theropods ''Allosaurus'', ''Ceratosaurus'', ''Ornitholestes'', and ''Torvosaurus''; the sauropods ''Brontosaurus'', ''Brachiosaurus'', ''Camarasaurus'', and ''Diplodocus''; and the ornithischians ''Camptosaurus'', ''Dryosaurus'', and ''Stegosaurus''. ''Apatosaurus'' is commonly found at the same sites as ''Allosaurus'', ''Camarasaurus'', ''Diplodocus'', and ''Stegosaurus''. ''Allosaurus'' accounted for 70–75% of theropod specimens and was at the top trophic level of the Morrison food web. Many of the dinosaurs of the Morrison Formation are of the same genera as those seen in Portuguese rocks of the Lourinhã Formationmainly ''Allosaurus'', ''Ceratosaurus'', and ''Torvosaurus''or have a close counterpart''Brachiosaurus'' and ''Lusotitan'', ''Camptosaurus'' and ''Draconyx'', and ''Apatosaurus'' and ''Dinheirosaurus''. Other vertebrates that are known to have shared this paleo-environment include ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphs, and several species of pterosaur. Shells of bivalves and aquatic snails are also common. The flora of the period has been evidenced in fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns with fern understory (gallery forests), to fern savannas with occasional trees such as the ''Araucaria''-like conifer ''Brachyphyllum''.

References

External links

*
* Batuman, Elif
Brontosaurus Rising
(April 2015), ''The New Yorker''
* Krystek, Lee
"Whatever Happened to the Brontosaurus?"
UnMuseum (Museum of Unnatural Mystery), 2002.
* Taylor, Mike
"Why is 'Brontosaurus' now called ''Apatosaurus''?"
MikeTaylor.org.uk, June 28, 2004.

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Diplodocidae
Dinosaur genera
Kimmeridgian dinosaurs
Morrison Formation
Dinosaurs of the United States
Fossil taxa described in 1877
Taxa named by Othniel Charles Marsh