Temporal range: Tithonian, 152–151 Ma
|Mounted A. louisae holotype (specimen CM 3018), Carnegie Museum of Natural History|
Apatosaurus (meaning "deceptive reptile" //) is a genus of extinct sauropod dinosaurs that lived in North America during the Late Jurassic period. It was originally described in 1877, when Othniel Charles Marsh named the first-known species, Apatosaurus ajax. A second species, Apatosaurus louisae, was discovered in 1916. They range in age from 152 to 151 million years ago (mya), during the early Tithonian ages of the Morrison Formation in modern-day Colorado, Oklahoma, and Utah, in the United States. Apatosaurus had an average length of 22.8 m (75 ft), and an average mass of at least 16.4 metric tons (18.1 short tons). A few specimens indicate a maximum length up to 30% greater than average and a mass of 33–73 t (32–72 long tons; 36–80 short tons).
Brontosaurus has long been considered a junior synonym of Apatosaurus; its only species was reclassified as A. excelsus in 1903. However, an extensive study published in 2015 concluded that Brontosaurus was a valid genus of sauropod distinct from Apatosaurus.
The cervical vertebrae of Apatosaurus were less elongated and more heavily constructed than those of Diplodocus and the bones of the leg were much stockier despite being longer, implying that Apatosaurus was a more robust animal. The tail was held above the ground during normal locomotion. Like all sauropods, 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; it may have grown 520 kilograms (1,150 pounds) per year until it reached 70 years of age. To lighten its bones, Apatosaurus had air sacs that made the bones internally sponge-like and full of holes. Like that of other diplodocids, its tail may have been used as a whip to create loud noises.
Apatosaurus is a genus in the family Diplodocidae. It is one of the more basal genera, with only a potentially unnamed genus and Amphicoelias more primitive. While the subfamily Apatosaurinae was named a while ago, the group was not used validly until the extensive 2015 study. Only Brontosaurus is also in the subfamily, with the other genera being considered as synonyms or reclassified as a diplodocine.
Apatosaurus was a large, long-necked, quadrupedal animal with a long, whip-like tail. Its fore limbs were slightly shorter than its hind limbs. 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 t (34 long tons; 39 short tons). Some specimens of A. ajax (like OMNH 1670) represent individuals 11–30% longer, suggesting masses twice that of CM 3018 or 33–73 t (32–72 long tons; 36–80 short tons), potentially rivalling the largest titanosaurs.
The skull was small in comparison with the size of the animal. The jaws were 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. The bones are articulated and their fusion indicates the bones are mature. The braincase was placed in a phylogenetic analysis; its morphology was found to be very similar to that 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 were deeply bifurcated; they carried paired spines, resulting in a wide, deep neck. The vertebral formula was 15 cervicals, 10 dorsals, 5 sacrals, and 82 caudals. The caudal vertebra number was said to vary, even within species. The cervical vertebrae were stouter than those of other diplodocids, and were found to be most similar to Camarasaurus by Charles Whitney Gilmore. 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. It had 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 spines, which make up more than half the height of the individual bones, on 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 last portion of the tail possessed a whip-like structure.
The limb bones were 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; the humerus resembles that of Camarasaurus, and those of B. excelsus and A. ajax are nearly identical. In 1936, Charles Gilmore said previous reconstructions 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; 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. By 1936, it was recognized that no sauropod had more than one hand claw preserved; this single claw is now accepted as the maximum number throughout the group. 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 A. louisae are the stoutest of any member of Sauropoda. Roughened surface textures (rugosities) are present on both ends of the femur; the top edge of this bone protrudes above the great trochanter. 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. In an individual of A. louisae, the astragalus was not found fused to the tibia. The stoutest metatarsal is in digit 1; the third metatarsal is the second stoutest.
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 Eobrontosaurus. 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. In 1903, Elmer S. Riggs said the name Sauropoda would be a junior synonym of earlier names; he grouped Apatosaurus within Opisthocoelia. Most authors still use Sauropoda as the group name. In 2011, John Whitlock published a study that placed Apatosaurus as the most basal diplodocid, a placement that is often supported.
Discovery and species
Marsh, a Professor of Paleontology at Yale University, described and named an incomplete, juvenile skeleton of Apatosaurus ajax in 1877. The composite term Apatosaurus comes from the Greek words apate (ἀπάτη)/apatelos (ἀπατηλός) meaning "deception"/"deceptive", and sauros (σαῦρος) meaning "lizard"; thus, "deceptive lizard". Marsh gave it this name based on the chevron bones, which were dissimilar to those of other dinosaurs; the chevron bones of Apatosaurus showed similarities with mosasaur chevrons. Two years later, Marsh announced the discovery of a larger and more complete specimen at Como Bluff, Wyoming—which, because of discrepancies including the size difference, he identified as belonging to an entirely new genus and species. He named the new species Brontosaurus excelsus, meaning "thunder lizard", from the Greek brontē/βροντη meaning "thunder" and sauros/σαυρος meaning "lizard", and from the Latin excelsus, meaning "noble" or "high". All Apatosaurus specimens are from the Morrison Formation. In 1877, this formation became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope. Because of their rivalry, Marsh and Cope hurried to publish and describe the taxa.
In the 1903 edition of Geological Series of the Field Columbian Museum, Elmer Riggs argued that Brontosaurus was not different enough from Apatosaurus to warrant its own genus; he created the combination Apatosaurus excelsus. Riggs said, "In view of these facts the two genera may be regarded as synonymous. As the term 'Apatosaurus' has priority, 'Brontosaurus' will be regarded as a synonym". Nonetheless, before the mounting of the American Museum of Natural History Apatosaurus, Henry Fairfield Osborn chose to label the skeleton "Brontosaurus", even though he opposed Marsh and his taxa.
The AMNH specimen, sometimes assigned to A. ajax, is very complete; only the head, feet, and sections of the tail are missing, and it was the first sauropod skeleton mounted. 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. Henry Fairfield Osborn said in a publication that the skull was "largely conjectural and based on that of Morosaurus" (now Camarasaurus).
In 1903, Riggs also described a new specimen of Apatosaurus, which was placed in the collections of the Field Columbian Museum. Found near the Grand River Valley of Fruita, Colorado, the last cervical vertebrae were exposed from the hillside. Those cervicals were articulated with the thoracic vertebrae, which were excavated from the slope. 10 ft (3.0 m) downhill, the distal end of a femur could be seen in the matrix. The rocks were so hard that drills and dynamite had to be used to extract the specimen. Nearly the entire rear of the skeleton, measuring 5.5 m (18 ft), was uncovered. Apparently when it was first preserved, the entire skeleton was present, but the cervical vertebrae and forelimbs were eroded away. Now this specimen is considered to either be a basal diplodocine or a a derived apatosaurine.
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 Andrew Carnegie's wife Louise. The skull was designated CM 11162; it was very similar to the skull of Diplodocus. It 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 was placed on the mount.
In 1931 at the Yale Peabody Museum, a skeleton was mounted with a skull different from the others. While at the time most museums were using Camarasaurus casts, the Peabody Museum sculpted a completely different skull. They based the lower jaw on a Camarasaurus mandible, with the cranium resembling Marsh's 1891 illustration. The skull also included forward-pointing nasals—something truly 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, and the specimen was found to have a majority of features related to those of A. ajax.
Almost all modern paleontologists agreed with Riggs that the two species 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 is considered a junior synonym and has therefore been discarded from formal use. Despite this, at least one paleontologist—Robert T. Bakker—argued in the 1990s that A. ajax and A. excelsus are in fact sufficiently distinct that the latter continues to merit a separate genus. In 2015 Emanuel Tschopp, Octávio Mateus, and Roger Benson released a paper that 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 dinosaurs. Among the many notable differences were the wider - and presumably stronger - neck of other Apatosaurus species compared to A. excelsus. Other species previously assigned to Apatosaurus, such as Elosaurus parvus and Eobrontosaurus yahnahpin were also reclassified as Brontosaurus. However, some are sceptical of the large number of sauropod taxa in the Morrison, instead grouping taxa like Brontosaurus and Apatosaurus together.
Multiple species of Apatosaurus and its synonyms 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. Their analysis was revised and expanded with many additional diplodocid specimens in 2015, though this larger study found that only two species could be referred to Apatosaurus.
- Apatosaurus ajax was named by Marsh in 1877 after Ajax, a hero from Greek mythology. Marsh designated the fairly complete skeleton YPM 1860 as its holotype. The species remained largely unknown; it was overshadowed by A. excelsus and A. louisae. 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, AMNH 460 was found to potentially be Brontosaurus, and in the phylogeny only the holotype was assigned to A. ajax. 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 in honor of Mrs. Louise Carnegie, wife of Andrew Carnegie who funded field research to find complete dinosaur skeletons in the American West. Apatosaurus louisae was 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. A. louisae is one of the most distinct species, yet it was recovered as the most primitive species in the Upchurch phylogeny. 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.
- Apatosaurus grandis was named in 1877 by Marsh in the article that described A. ajax. It was briefly described, figured, and diagnosed. Marsh said it was only provisionally assigned to the genus when he reassigned it to his new genus Morosaurus in 1878. It is now the oldest species of Camarasaurus after being reassigned to that 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.
- 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.
- Apatosaurus alenquerensis was named in 1957 by Albert-Félix de Lapparent and Georges Zbyweski. It was based on postcrania from Portugal. In 1990, this was referred to Camarasaurus, but in 1998 it was given its own genus, Lourinhasaurus.
- 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 reclassified it as Brontosaurus yahnahpin in 2015.
It was historically believed 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. 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 in Apatosaurus suggests it was a ground-height, nonselective browser. It may have eaten ferns, cycadeoids, seed ferns, horsetails, and algae. Kent Stevens and Michael Parrish (1999 and 2005) said 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 graze below the level of the body, leading some scientists to speculate that these sauropods grazed from riverbanks on submerged water plants.
Matthew Cobley et al. (2013) disputed 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 et al. were disputed by Mike Taylor, who analyzed the amount and positioning of intervertebral cartilage to determine the flexibility of the neck of Apatosaurus and Diplodocus. He found the neck of Apatosaurus was very flexible.
In 1987, James Farlow calculated that an Apatosaurus-sized dinosaur about 35 t (34 long tons; 39 short tons) would have possessed 5.7 t (5.6 long tons; 6.3 short tons) of fermentation contents. Assuming Apatosaurus had an avian respiratory system and a reptilian resting-metabolism, Frank Paladino et al. (1997) estimate the animal would need to consume only about 262 litres (58 imp gal; 69 US gal) of water per day.
Posture and locomotion
Diplodocids like Apatosaurus are often portrayed with their necks held high up in the air, allowing them to browse on tall trees. Some studies say diplodocid necks were less flexible than previously believed because the structure of the neck vertebrae would not have allowed the neck to bend far upwards, and that sauropods like Apatosaurus were adapted to low browsing or ground feeding. Other studies by Taylor say 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 upwards with the head pointing downwards in a resting posture.
In 2008, footprints of a juvenile Apatosaurus were reported from Quarry Five in Morrison, Colorado. Discovered in 2006 by Matthew Mossbrucker, these footprints show that juveniles could run on their hind legs in a manner similar to that of the modern basilisk lizard. Trackways of sauropods like Apatosaurus show their average range was around 20–40 km (12–25 mi) per day, and they could potentially reach a top speed of 20–30 km (12–19 mi) per hour. The slow locomotion of sauropods may be due to the minimal muscling or recoil after strides.
Various uses for the single claw on the forelimb of sauropods have been proposed. It has been suggested they were for defense, but their shape and size makes this unlikely. It was also suggested they were for feeding, but the most probable use for the claw was grasping objects such as tree trunks when rearing.
Given the large body mass and long neck of 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 about 0.184 m3 (184 l) for a 30 t (30 long tons; 33 short tons) specimen. Paladino calculated its tidal volume (the amount of air moved in or out during a single breath) to be 0.904 m3 (904 l) with an avian respiratory system, 0.225 m3 (225 l) if mammalian, and 0.019 m3 (19 l) 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 0.60 m3 (600 l) compared with a mammalian requirement of 2.95 m3 (2,950 l), which would exceed the space available. The overall thoracic volume of Apatosaurus has been estimated at 1.7 m3 (1,700 l), allowing for a 0.50 m3 (500 l), four-chambered heart and a 0.90 m3 (900 l) lung capacity. That would allow about 0.30 m3 (300 l) 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) say the large body size of Apatosaurus and other sauropods would have made them unable to maintain high metabolic rates because they would not be able to release enough heat. They assumed Apatosaurus had a reptilian respiratory system. Wedel said an avian system would have allowed it to dump more heat. Some scientists have said the heart would have had trouble sustaining sufficient blood pressure to oxygenate the brain. Others say 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.
A 1999 microscopic study of Apatosaurus bones concluded the animals grew rapidly when young and reached near-adult sizes in about 10 years. 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 attained a mass of 25 t (25 long tons; 28 short tons) in 15 years. This would imply sauropods had a growth rate of 5,000 kg (11,000 lb) per year. An alternative method, using limb length and body mass, found Apatosaurus grew 520 kg (1,150 lb) per year until it was about 70 years old. These estimates have been called unreliable because the calculation methods are not sound; old growth lines would have been obliterated by bone remodelling. One of the first identified growth factors of Apatosaurus was the number of sacral vertebrae, which increased to five by the time of the creatures' 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 et al. (2013) examined long bone histological data and concluded the Apatosaurus sp. SMA 0014 weighed 20,206 kg (22.3 short tons), reached sexual maturity at 21 years, and died aged 28. The same growth model indicated Apatosaurus sp. BYU 601–17328 weighed 18,178 kg (20.0 short tons), reached sexual maturity at 19 years, and died aged 31.
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. The juvenile material is from at least two different age groups, and is likely to have come from more than three individuals. The specimens exhibit distinguishing features of Apatosaurus, justifying their referral. The bones are proportionately slightly different from all adult sauropod specimens.
An article that appeared in the November 1997 issue of Discover Magazine reported research into the mechanics of Apatosaurus tails by Nathan Myhrvold, a computer scientist from Microsoft. Myhrvold carried out a computer simulation of the tail, which in diplodocids like Apatosaurus was a very long, tapering structure resembling a bullwhip. This computer modeling suggested sauropods were capable of producing a whip-like 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.
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 146.8 mya 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 Lourinha Formation in Portugal and the Tendaguru Formation in Tanzania.
Apatosaurus was the second-commonest sauropod in the Morrison Formation ecosystem, after Camarasaurus. Apatosaurus may have been more solitary than other Morrison Formation dinosaurs. Supersaurus has a greater total length and is the greatest of all sauropods from the Morrison Formation. Apatosaurus fossils have only been found in the upper levels of the formation. Fossils of Apatosaurus ajax are known exclusively from the upper portion of the formation (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 151 mya. Additional Apatosaurus remains are known from younger rocks, but they have not been identified as any particular species.
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 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 Lourinha Formation—mainly Allosaurus, Ceratosaurus, and Torvosaurus—or have a close counterpart—Brachiosaurus and Lusotitan, Camptosaurus and Draconyx, Apatosaurus and Dinheirosaurus. Other vertebrates that shared this paleo-environment include ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans, and several species of pterosaur. Shells of bivalves and aquatic snails are also common. The flora of the period has been eveidenced in fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns and ferns (gallery forests), to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.
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