Apatosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Diplodocidae

Genus: Apatosaurus

Species: Apatosaurus ajax

Introduction

Apatosaurus was a sauropod dinosaur that inhabited the western United States during the Late Jurassic period roughly 150 million years ago. Sauropods were large, plant-eating quadrupeds with small heads, long necks, long tails, and columnar legs. Although Apatosaurus was not the biggest sauropod in existence, it grew to 25 meters (82 feet) in length from head to tail, stood about 9 meters (30 feet) high, and weighed up to 30 tons.

In 1879, two years after American paleontologist Othniel C. Marsh identified and named the first Apatosaurus specimen, he received the remains of a dinosaur similar in appearance to Apatosaurus, which he called Brontosaurus excelsus. Before long, the large, herbivorous dinosaur known as Brontosaurus excelsus was popularized in the media and was one of four dinosaurs featured on a U.S. postal stamp. After a specimen resembling both genera was unearthed, Brontosaurus was deemed simply a misnomer of Apatosaurusin 1903. Since the name Apatosaurus was published first, the scientific community required that B. excelsus be renamed A. excelsus.

Classification

Scientists use two primary methods to classify organisms. Linnaean taxonomy groups organisms in a hierarchical system according to overall physical similarity, while an alternate method called “cladistics” attempts to create categories that better reflect the evolutionary relationships between species.

According to Linnaean taxonomy, Apatosaurus diverges from all other dinosaurs at the order level Saurischia. It belongs to the infraorder Sauropoda, which is defined by small heads, long necks and tails, large legs, peg-like teeth, and quadrupedal locomotion. A member of the Diplodocidae (“double beams”) family (along with Diplodocus and Barosaurus), Apatosaurus had attributes common to all diplodocids, including nares near the top of the skull between the eye sockets, a long, horse-like skull, and teeth located only in the front of the jaws.

Cladistics, the phylogenetic system of classification, groups organisms together based on their evolutionary relationships, which are determined by shared physical traits. Each grouping, or clade, consists of an original ancestor and all of its descendents, the original ancestor also being the most recent common ancestor of its descendents in the grouping. Apatosaurus belongs to the Diplodocoidea clade, a sauropod clade which also includes Rebbachisaurus, Dicraeosaurus, Barosaurus and Diplodocus. It is defined by a reduced number of teeth, a long whip-like tail, and nares between the eye sockets.

Anatomy

Easily identifiable as a sauropod, with its small head, extremely long neck and tail, large body, and columnar legs, Apatosaurus grew to 25 meters (82 feet) in length and stood up to 9 meters high (30 feet). Apatosaurus’ horse-like skull had nares positioned high up between the eye sockets. Its peg-shaped teeth, located in the front of its jaws, were suitable only for tearing vegetation, not for chewing. Its legs descended from its body in vertical columns similar to an elephant, and its ankles and feet were compactly boned.

Apatosaurus carried its neck at a downward-sloping angle to the ground. Its fifteen neck vertebrae fit together in a way that limited side-to-side and upward movement. Apatosaurus could lower its head several feet below ground level, but it may have been unable to hold its neck erect (like a giraffe does). Its tail was comprised of eighty-two vertebrae that tapered whip-like from a very large diameter close to its body to a very small diameter at the tip. Apatosaurus vertebrae were light and thin-walled and some contained air sacs, empty spaces that filled with air during respiration.

Intelligence

According to the encephalization quotient, sauropods were among the least intelligent of all dinosaurs. Apatosaurus, whose mouse-sized brain was disproportionately small for an animal of its mass, had an EQ of roughly 0.2. The most intelligent dinosaurs, the carnivorous dromaeosaurids and troodontids, had EQs of 5.8. The size of Apatosaurus, however, would have protected it from most predators.

Reproduction and Population

For a long time, it was unclear whether Apatosaurus and all other sauropods were viviparous, meaning they gave birth to live young, or laid eggs. The confusion was laid to rest when fossilized sauropod eggs were discovered. It is uncertain, though, what kind of care Apatosaurus extended to its eggs and hatchlings, if any.

There is a lack of information about juvenile Apatosaurus because few specimens have been collected. Several studies of the bone composition of adults have determined that juveniles grew quickly, achieving near-adult size and sexual maturity in anywhere from ten to twenty-five years. (One study that examined bone tissue, however, concluded that it took Apatosaurus 70 years to reach full maturity at a maximum annual growth rate of 520 kilograms, or 1,146 pounds). Rapid growth may have been advantageous to survival because large body mass deters predators.

Diet

Paleontologists have determined that Apatosaurus was strictly herbivorous due to its physiology; its teeth were designed to pull leaves and branches off of plants and trees, not to tear or chew meat. Its large size meant that it needed to consume enormous amounts of vegetation, more than a ton a day some estimate. It most likely ate whatever vegetation was available, such as gymnosperms like cycads, ferns, and horsetails, as it could not afford the energy expenditure of being selective. To aid digestion, Apatosaurus may have swallowed gastroliths, or stones that helped break down fibrous plant material, although there is no direct evidence of this. It probably also had a very long digestive tract that allowed plant material time to ferment and break down even further.

It is debatable whether Apatosaurus could reach the tops of very tall trees since its neck movement was limited. One theory is that it was able to rear up for short periods of time on its hind legs. Several factors support this theory: Apatosaurus had a center of gravity near its hips, the front of its body was lighter than the back, and its front legs were shorter than its hind legs.

Behavior

Footprints preserved in rock provide evidence that Apatosaurus may have been a social animal that travelled in herds with other sauropods. Apatosaurus had a slow gait of 3–5 kilometers (1.9–3 miles) an hour. However, unusual tracks from a juvenile Apatosaurus suggest that not only could a juvenile run, but it could run elevated on its hind legs. The location of Apatosaurus and Jurassic plant remains within the Morrison Formation in North America imply that Apatosaurus may have migrated in search of food. It likely did so in herds that offered extra protection against predators. Some paleontologists speculate that it used its whip-like tail for defense. Other researchers think it may have been used to generate a loud noise to communicate, intimidate rivals, or attract mates.

Habitat and Other Life Forms

Apatosaurus specimens have been found in Colorado, Oklahoma, Wyoming, and Utah; these states, along with New Mexico, Montana, and South Dakota, comprise the outcrop of the Morrison Formation. Rich in plant and animal fossils, the Morrison Formation is a 1.5 million square kilometer (579,153 square mile) area of Late Jurassic rock. During the Late Jurassic period, the southern region of the formation was hot and arid, while the wetter northern region contained swamps and lakes.

Dinosaurs that co-existed with Apatosaurus in the Morrison Formation during the Late Jurassic include Camptosaurus , Camarasaurus, Allosaurus, Diplodocus, Stegosaurus , and Haplocanthosaurus. Aquatic and semi-aquatic animals include fish, frogs, and crocodiles. Pterosaurs (flying reptiles) and rat-sized mammals were present also. Cycads, conifers, and sequoias probably grew in the dryer areas, while ferns, tree ferns, horsetails, and ginkgoes grew near water.

Research

Until the 1960s, Apatosaurus was believed to have been semi-aquatic. There are several reasons for this conclusion. First, it had nares near the top of its skull. Nineteenth and early-to-mid twentieth-century paleontologists assumed that the location of the nares allowed it to breathe while partially submerged in water. Second, it grew to an enormous size, which, they postulated, could only have happened in a supportive medium such as water.

Now, the general consensus among paleontologists is that Apatosaurus was terrestrial, meaning that it lived on dry land. Research indicates that it would have been impossible for Apatosaurus to be submerged up to its head in water because the water pressure against its chest would have prevented it from breathing. The location of its remains also supports the theory that it was terrestrial. Apatosaurus fossils and tracks have been found in sandstone that came from dry regions, places it would have been unable to access if it was semi-aquatic. Finally, paleontologists have determined that it was terrestrial by simply comparing it to living elephants, terrestrial animals that have similar leg structure.

The first Apatosaurus ajax snout was discovered in 2003 and identified in 2011. Examination of the specimen showed that the antorbital fenestra, or "window," in the skull was distinctively large in A. ajax as compared to other long-necked dinosaurs.

A. ajax is the holotype of the species, and there is one other recognized Apatosaurus species, A. louisae (1915). Five other named species were initially thought to belong to the genus but later reclassified: Camarasaurus grandis (1877), Brontosaurus excelsus (1879), , Apatosaurus minimus (1917), Lourinhasaurus alenquerensis (1957), and Brontosaurus yahnahpin (1994).

Notably, in 2015 Brontosaurus was proposed once more as a distinct grouping from Apatosaurus, based on phylogenetic analysis of 477 traits across forty-one diplodocid specimens. Apatosaurus was found to have a thicker neck than Brontosaurus and have differently shaped shoulder and ankle bones. However, some paleontologists remained unconvinced, saying more data were needed to truly determine whether Brontosaurus is legitimately a separate genus.

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