Apatosaurus

Apatosaurus (meaning "deceptive lizard") was a genus of plant-eating sauropod dinosaur that lived in North America during the Late Jurassic period. Othniel Charles Marsh described and named the first known species, Apatosaurus ajax, in 1877. A second species, Apatosaurus louisae, was discovered and named by William H. Holland in 1916.

Apatosaurus lived about 152 to 151 million years ago, during the late Kimmeridgian to early Tithonian age. Its fossils are now known from the Morrison Formation in Colorado, Oklahoma, New Mexico, Wyoming, and Utah in the United States. On average, it was 21 to 23 metres (69 to 75 feet) long and had a mass of 16.4 to 22.4 tonnes. A few specimens suggest it could grow up to 11 to 30% longer than average, with a mass of about 33 tonnes.

Apatosaurus belonged to the diplodocid family. It differed from Diplodocus mainly in having sturdier cervical vertebrae, the bones of its neck, and much stockier leg bones. These features suggest it was a more robust animal. Its tail was held above the ground when it walked. It had one claw on each front foot and three on each back foot. Its skull was once thought to be like that of Camarasaurus, but it is much more similar to that of Diplodocus. Apatosaurus was a generalized browser that likely held its head raised. To make its bones lighter, it had air sacs that left the bones full of holes inside.

For a long time, the skull of Apatosaurus was confused with those of Camarasaurus and Brachiosaurus. This changed in 1909, when the holotype of A. louisae was found along with a complete skull close to the front of the neck. Later, Henry Fairfield Osborn disagreed with this link. Skeletons were displayed with guessed skull casts until 1970, when McIntosh showed that sturdier skulls once given to Diplodocus were more likely from Apatosaurus.

Apatosaurus is one of the more basal genera in its family, with only Amphicoelias and possibly a new, unnamed genus being more primitive. The subfamily Apatosaurinae was named in 1929, but it was not used validly until a detailed study in 2015. Brontosaurus is the only other member of this subfamily. For a long time, Brontosaurus was considered a junior synonym of Apatosaurus, and a 2015 study concluded that it is a separate genus, though not all scientists agree. In North America during the late Jurassic, Apatosaurus lived alongside dinosaurs such as Allosaurus, Camarasaurus, Diplodocus, and Stegosaurus.

Description

Apatosaurus was a large, long-necked, quadrupedal animal with a long, whip-like tail. Its forelimbs were slightly shorter than its hindlimbs. Most size estimates are based on specimen CM 3018, the type specimen of A. louisae, which reached 21–23 m (69–75 ft) long and 16.4–22.4 t in body mass. A 2015 study compared it with Dreadnoughtus and Giraffatitan. Some A. ajax specimens, such as OMNH 1670, may have been much larger and could have rivaled the largest titanosaurs, but a 2020 study revised that estimate to 30 m (98 ft) long and 33 t. The skull was small compared with the body. Its jaws held chisel-like teeth suited to an herbivorous diet, and its squared snout was like that of other diplodocoids, though only Nigersaurus had a squarer skull. Teeth with exposed enamel showed little wear.

The neck was long and wide, with deep bones and air sacs that made it lighter. The animal had 15 neck bones, 10 dorsals, 5 sacrals, and 82 caudals in the holotype of A. louisae, though the tail count could vary. Its neck bones were stouter than those of other diplodocids, and Charles Whitney Gilmore found them most similar to Camarasaurus. The cervical ribs reached closer to the ground than in diplodocines. The atlas-axis complex was nearly fused in A. louisae. Like its close relative Supersaurus, it had tall spines on its vertebrae. Its tail was slender compared with other diplodocids, and its ribs made its chest unusually deep. The limb bones were also very sturdy. Within Apatosaurinae, the scapula of A. louisae was between those of A. ajax and Brontosaurus excelsus. Its humerus resembled those of Camarasaurus and Brontosaurus. In 1936, Charles Gilmore noted that earlier drawings wrongly showed the radius and ulna crossing, when they would have stayed parallel. Like other sauropods more advanced than Shunosaurus, it had one large claw on each forelimb. Its single manual claw bone (ungual) was slightly curved. The ilia, fused pubes, and fused ischia made up its hip area. Its femora were among the sturdiest of any sauropod.

Discovery and species

Fossils of Apatosaurus were first found in the spring of 1877 near Morrison, a town in the eastern foothills of the Rocky Mountains in Jefferson County, Colorado. A local miner, Arthur Lakes, and his friend Henry C. Beckwith made the discovery. Lakes wrote about it to Othniel Charles Marsh, a Paleontology professor at Yale University, and to Edward Drinker Cope, a paleontologist in Philadelphia. One well-preserved partial skeleton, YPM VP 1860, was named Apatosaurus ajax in November 1877. The name comes from two Greek words meaning "deception" and "lizard," so it is loosely translated as "deceptive lizard." Marsh chose this name because the chevron bones looked more like those of mosasaurs, most likely the species Mosasaurus, than like those of other dinosaurs. Marsh also separated the new genus from Atlantosaurus. In 1879 he named the larger Como Bluff specimen Brontosaurus excelsus. All specimens now considered Apatosaurus came from the Morrison Formation.

After the Bone Wars, big museums in the eastern United States wanted their own dinosaur collections. The American Museum of Natural History, the Carnegie Museum of Natural History, and the Field Museum of Natural History all sent expeditions west. In 1898, Walter Granger found a nearly complete skeleton north of Medicine Bow, Wyoming. Its skull was a guess, and Henry Fairfield Osborn labeled the museum's Apatosaurus mount Brontosaurus, a name that became famous. In 1903, Elmer Riggs studied a skeleton from the Grand River Valley near Fruita, Colorado and argued that Brontosaurus should be a junior synonym of Apatosaurus. In 1909, Earl Douglass found a skull at what became the Carnegie Quarry in Dinosaur National Monument. The species A. louisae was named after Louise Carnegie, wife of Andrew Carnegie. Later, John Stanton McIntosh and David Berman supported the idea that Apatosaurus had a skull like that of Diplodocus. In 1998, a skull once linked to Apatosaurus was suggested to belong to Brachiosaurus, and a 2020 study supported this with brachiosaurid material.

New finds continued. In 2011, a specimen called CMC VP 7180 was described as the first Apatosaurus with its skull still attached to its neck bones. Brigham Young University specimen 17096 is another well-preserved skull and skeleton. Most scientists agreed with Riggs, but the rules of the ICZN give the name Apatosaurus priority because it was published first. In the 1990s, Robert T. Bakker argued that A. excelsus deserved its own genus. In 2015, Emanuel Tschopp, Octávio Mateus, and Roger Benson proposed separating Brontosaurus again, using features such as the acromial edge of the shoulder blade and the ratio measured on the astragalus bone. Scientist Donald Prothero criticized the strong media reaction to this study as premature. Many Apatosaurus species were named from little material, and in a 2005 study, a polytomy of A. ajax, A. parvus, and A. excelsus was found. A. ajax was named after Ajax, a hero from Greek myths, and its holotype comes from the late Kimmeridgian to early Tithonian ages. A cladogram from the 2015 analysis is shown below. In 2012, Mike P. Taylor and Matt J. Wedel described A. minimus as hard to place, and it was linked to Saltasaurus in one analysis. A. alenquerensis was named by Albert-Félix de Lapparent and later became Lourinhasaurus. A. yahnahpin was moved to the new genus Eobrontosaurus and later became Brontosaurus yahnahpin.

Classification

Apatosaurus belongs to the family Diplodocidae, a clade of gigantic sauropod dinosaurs. This family includes some of the longest creatures ever to walk the earth, such as Diplodocus, Supersaurus, and Barosaurus. Apatosaurus is sometimes placed in the subfamily Apatosaurinae, which may also include Suuwassea, Supersaurus, and Brontosaurus. Othniel Charles Marsh first described Apatosaurus as related to Atlantosaurus within a group called Atlantosauridae, which is no longer used.

In 1878, Marsh placed his family into a suborder that included Apatosaurus and other sauropods, and he grouped them within Sauropoda, a name he had created in the same study. In 1903, Elmer S. Riggs placed Apatosaurus within Opisthocoelia, but Sauropoda is still used as the group name. In 2011, John Whitlock studied the animal and placed it as a more basal diplodocid, meaning it branched off earlier in the family tree, and sometimes less basal than Supersaurus.

Cladogram of the Diplodocidae after Tschopp, Mateus, and Benson (2015).

Paleobiology

For a long time, scientists thought that sauropods like Apatosaurus were too heavy to support their own weight on dry land, so they may have lived partly underwater in swamps. Newer findings suggest they were fully land animals. Apatosaurus had a square snout and teeth that suggest it fed on plants near the ground. It may have eaten cycadeoids, seed ferns, horsetails, and algae, and it may have fed from riverbanks on underwater plants. A 2015 study of its neck suggested the neck may have been used for intraspecific combat. The single claw on each forelimb was most likely used for grasping objects such as tree trunks when rearing. Trackways suggest these sauropods could travel about 25 to 40 km (16 to 25 miles) per day and may have reached speeds of 20 to 30 km (12 to 19 miles) per hour.

Further information: Sauropod neck posture

Apatosaurus is often shown with its neck held high to reach tall trees, but some studies say its neck was less flexible and that it was better suited to feeding low down. Taylor argues that tetrapods hold their necks at the highest possible position when alert, and that the neck of Apatosaurus was very flexible. Cobley and colleagues argue that large muscles and cartilage limited neck movement, so the animal may have moved its whole body to reach food. Scientists have also had trouble working out how Apatosaurus breathed, given its large size and long neck. Some researchers suggest it had a bird-like system, with parabronchi and air sacs like those in avian lungs. A 1997 computer study by Nathan Myhrvold and paleontologist Philip J. Currie suggested that diplodocids could make a whip-like crack of over 200 decibels. Baron (2020) finds the use of the tail as a bullwhip unlikely, and proposes instead that the tail may have helped animals stay in touch with others in their group while migrating. Compared with Shunosaurus and mamenchisaurids, the tails of diplodocids were light and narrow, so using them in combat would likely have injured the tail itself.

Further information: Physiology of dinosaurs

Paleoecology

The Morrison Formation is a layer of rock made of sediments from shallow seas and rivers. Using radiometric dating, scientists found that it formed between 156.3 mya and 146.8 mya, during the late Oxfordian, Kimmeridgian, and early Tithonian stages of the Late Jurassic period. It is thought to have formed in a semiarid environment with wet and dry seasons. The Morrison Basin, where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan. It formed as the early Front Range of the Rocky Mountains began to rise to the west. Streams and rivers carried sediment into swampy lowlands, lakes, river channels, and floodplains. The 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, after Camarasaurus. It may have lived more alone than other Morrison dinosaurs. Its fossils have only been found in the upper levels of the formation, including those of Apatosaurus ajax from the upper Brushy Basin Member, about 152–151 mya.

The Morrison Formation shows a time when giant sauropod dinosaurs dominated the land. Other dinosaurs found there include the theropods Allosaurus, Ceratosaurus, Ornitholestes, and Torvosaurus; the sauropods Brontosaurus, Brachiosaurus, Camarasaurus, and Diplodocus; and the ornithischians Camptosaurus, Dryosaurus, and Stegosaurus. Allosaurus was at the top trophic level of the Morrison food web. Many of these dinosaurs are closely related to those found in the Lourinhã Formation, such as Lusotitan, Draconyx, and Dinheirosaurus. Other animals that lived there included ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, crocodylomorphs, and pterosaur species. Plants included green algae, fungi, mosses, horsetails, cycads, ginkgoes, and conifers. Forests of tree ferns with ferns growing underneath lined the rivers (gallery forests), while fern savannas with a few trees, such as the Araucaria-like conifer Brachyphyllum, covered other areas.