Barosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Diplodocidae

Genus: Barosaurus

Species: Barosaurus lentus

Introduction

Perhaps due to its relative lack of fossil representation, Barosaurus remained one of the lesser known sauropods, a group of large herbivorous dinosaurs that walked on all fours and had long necks and tails and small heads and which included more recognizable species such as Diplodocus and Brachiosaurus. However, this changed in 1991 when a prominent exhibit at the American Museum of Natural History in New York depicted the large, plant-eating dinosaur rising on its hind legs to protect its offspring from a threatening Allosaurus. Thanks to the massive, life-size construction of a female Barosaurus, the giant dinosaur is now arguably more famous than some of its dinosaur cousins.

Towering 15 meters (49 ft) high, the installation caused a great deal of discussion within the scientific community after its unveiling because of the dinosaur's two-legged posture. Fossil tracks clearly show that dinosaurs such as Barosaurus were quadrupeds, walking on all four legs not unlike a modern-day elephant. Curators at the American Museum of Natural History have defended the animal's posture, however, stating that fossil evidence cannot conclusively demonstrate that Barosaurus would have been unable to temporarily balance on just its two powerful hind legs if necessary. (Another debatable aspect of the installation is whether an Allosaurus, a bipedal predator, would have preyed on a Barosaurus.)

Classification

Paleontologists use two major systems for classifying organisms. Linnaean taxonomy uses overall physical similarity to place organisms into different groups. By contrast, cladistics seeks to create groups of organisms based on evolutionary relationships; these groups are called “clades.” Cladistic analysis focuses on important key traits and attempts to trace the inheritance of a trait among descendants of a common ancestor.

Under the Linnaean classification system, Barosaurus was part of the Saurischia, or lizard-hipped, order. Like most other members of the saurischian order, Barosaurus had a large body with short legs, a whip-like tail, and a long, slender neck topped with a very small head. The dinosaur's greatly elongated neck is also a defining characteristic of the Sauropoda infraorder, and members of this infraorder are commonly termed sauropods. Barosaurus is then classified as a member of the Diplodocidae family, which also includes dinosaur species such as Apatosaurus, Brontosaurus, Diplodocus, and Supersaurus. Diplodocids are typically characterized by long tails (with more than 70 vertebrae) and shorter forelimbs than most other sauropod dinosaurs.

Under the principles of cladistics, Barosaurus is grouped into the clade Diplodocidae, which, similar to Linnaean classification, also includes the North Americans dinosaurs Apatosaurus and Diplodocus. These creatures are generally acknowledged to have shared major physical characteristics, particularly those relating to the spine and body stance. Close relatives Barosaurus and Diplodocus, for example, shared a bulky body structure, proportionately short legs, and a slim head, but differed slightly in terms of relative neck and tail length, with Barosaurus having a lighter, longer skeleton with longer cervical structures. Another distinguishing characteristic of the clade is that the teeth were restricted to the front of mouth.

Classification remains an inexact science, however, as shown in the debate over the placement of Barosaurus africanus and Barosaurus gracilis. When remains of the former creature were discovered in the African country of Tanzania in 1908, paleontologist Eberhard Fraas first classified them as Gigantosaurus africanus and Gigantosaurus robustus. However, these names were already in use. Later, German paleontologist Werner Janensch instead assigned the two species to the genus Barosaurus, but this classification remained controversial as fossil evidence supporting this assignment is scarce. Some scientists today believe that the Barosaurus africanus species should instead be classified as Tornieria africanus, and the Barosaurus robustus as Janenschia robusta.

Anatomy

Perhaps the most striking physical characteristic of Barosaurus, aside from its immense size, was its very long neck. Extending as much as 9 meters (30 feet) from its shoulders, this slim neck was counterbalanced by a long, fairly thick tail that tapered to a whiplash end, much like sister dinosaur Diplodocus. These greatly extended features made Barosaurus, measuring up to 29 meters (95 feet), one of the longest known dinosaurs. Also similar to Diplodocus, which Barosaurus resembles in numerous ways, are the presence of chevrons, or bones that line the underside of the animal's large tail. They were forked and featured a prominent forward spike. Barosaurus's tail was believed to be noticeably shorter than that of Diplodocus, however.

Barosaurus's vertebrae structure distinguished it from other, similar sauropods. The dinosaur's cervical bones were half as long as those of Diplodocus in the upper part of the neck, for example, and it had fewer and longer bones along the back of its spine. Some scientists have proposed that the creature had eight total hearts throughout its body, with three pairs of hearts throughout its long neck fed with blood by larger hearts in the chest. Other scientists have dismissed this claim, however, arguing instead that arterial valves and special muscles throughout the neck allowed blood to reach the brain without running back down to the heart between pumps. The related dispute over blood pressure has fed into the discussion of how Barosaurus held its neck—out and downward, or straight up—and what it ate.

Intelligence

Researchers use a measurement known as the encephalization quotient (EQ) to standardize their discussions and comparisons of dinosaur intelligence. The EQ figure for an individual animal is calculated by comparing the relative size of its brain and body, resulting in a value of up to 5.8 for dinosaurs (humans have an EQ of 7.44 and dolphins measure a 5.31). The higher the number, the bigger the brain in relation to the body, and the higher the level of intelligence.

Barosaurus—like its fellow gigantic sauropods—ranks among the least intelligent of all dinosaurs; scientists have estimated that the brains of sauropods were only about the size of those of modern-day house cats. However, Barosaurus had an enlarged lobe at the front of its braincase that is believed to indicate a keen sense of smell. In addition, according to scientists, the positioning of its eyeballs on its skull suggests that the dinosaur had sharp vision with good peripheral sight. These advantages would have helped Barosaurus identify and avoid potential predators.

Reproduction and Population

Due to the rarity of fossils, it is often difficult for paleontologists to determine or measure past population numbers of prehistoric animal behavior accurately. Fossils can provide clues, however. For example, although some hypotheses state that animals as gigantic as Barosaurus could have had exceptionally long life spans (up to 100 years or more), no research to date has been able to provide credible evidence that this species lived any longer than about 50 years.

Although paleontologists have speculated over the years that sauropods such as Barosaurus may have given birth to live young due to their large pelvic canals, most scientists accept that the dinosaur was oviparous, or egg-laying, as were other sauropods. It is believed that Barosaurus laid roughly spherical eggs about 13 to 25 centimeters (5 to 10 inches) in diameter. Because Barosaurus was among the larger sauropods, its eggs were probably at the top end of this scale.

Based on fossil evidence of other sauropods, Barosaurus probably laid as many as 20 eggs in a clutch, either in rows or in a semi-circle. Females protected their eggs, and family groups seem to have traveled together after the relatively small baby dinosaurs hatched. Because these juveniles were perhaps only 122 to 182 centimeters (4 to 6 feet) long at birth, they faced much greater danger from predators than did an adult Barosaurus.

Diet

An herbivorous, or plant-eating, dinosaur, Barosaurus consumed perhaps as much as a ton of plant matter (such as ferns, mosses, and cycads) every day. Some scientists, however, believe that the large creature may have been cold-blooded with a slow metabolism, thus reducing its daily needs to about 225 kilograms (500 lbs) of food. Although no complete Barosaurus skull has been discovered, paleontologists believe that the dinosaur's teeth, jaw, and skull shape were similar to those of Diplodocus. Barosaurus thus probably had peg-like teeth used for stripping plant matter from tree branches. Because of the creature's long neck, some scientists believe that Barosaurus browsed extensively on high tree tops, much like modern-day giraffes. However, others think that its body shape would have made it better suited to eating low-lying, water-logged plant matter. Researchers who examined Barosaurus vertebrae concluded it had greater lateral (side-to-side) flexibility than vertical flexibility, making it better suited to sweeping motion; this bolsters the hypothesis that it ate low-lying vegetation.

Because Barosaurus's teeth were more suited to ripping than to chewing, scientists believe that it swallowed its food in large pieces, crushing them internally with stomach stones in much the same way as a modern-day bird uses its gizzard.

Behavior

Like other sauropods, Barosaurus seem to have traveled in fairly large groups. A 1934 American Museum of Natural History team discovered a collection of fossils believed to belong to nearly 20 members of either Barosaurus or the similarly built Diplodocus genus. Because all of these fossils appeared to be of relatively young creatures, scientists believe that this discovery suggests that the herd would have remained together throughout their lives. Scientists have further speculated that younger, more vulnerable dinosaurs traveled at the center of a pack, while the largest adults took the most dangerous positions at the front and rear, although little evidence supports this conjecture.

Despite its tremendous size, scientists believe Barosaurus—and other large sauropods such as Diplodocus—was probably a gentle animal without many aggressive tendencies. If it were to be attacked by a predator, some speculate it may be likely that it used its long, powerful tail as a kind of whip to defend itself.

Habitat and Other Life Forms

During the Late Jurassic, the present-day North American landmass was part of a vast continent called Laurasia, which also included Eurasia. Roaming an area of what is now the Western United States that included parts of Utah and South Dakota, Barosaurus shared its territory with many other creatures. This area did not feature the dry, mountainous terrain of today, but instead was swampy and hot, with volcanoes and mountains lying to the northwest and west. Gymnosperms such as ferns and cycads flourished throughout the Jurassic period, and tall coniferous trees offered the large herbivorous dinosaurs of the area such as Diplodocus, Apatosaurus, Brachiosaurus, Camarasaurus, Stegosaurus, and Barosaurus ample food. (The Late Jurassic was, however, a time before the emergence of angiosperms.)

Barosaurus's greatest defense against potential predators, such as carnivorous dinosaurs like Allosaurus and Torvosaurus, was its immense size and weight.

Research

Paleontologist Othniel Charles Marsh first published a description of Barosaurus in 1890 based on discoveries he had made in South Dakota the previous year. Nearly a decade passed before an associate of Marsh's returned to the site to collect additional bones from the American species of Barosaurus; these fossils led to the reclassification of Barosaurus from the Atlantosauridae to the Diplodocidae family. During the mid-1910s, additional vertebrae finds helped paleontologists determine the immense length of the Barosaurus neck. Although scientists originally believed that Barosaurus was a relatively rare creature, later twentieth-century discoveries refuted this idea, and many now think that Barosaurus was in fact a very common dinosaur.

In 1908, Fraas discovered the series of fossils sometimes associated with Barosaurus in Tanzania, Africa. However, most scientists reject this classification or at least consider this possible African Barosaurus to be significantly different than the North American type.

In 2007 the Royal Ontario Museum rediscovered the most complete Barosaurus skeleton to date—among its collection. The skeleton had been excavated from the Carnegie Quarry in the early twentieth century, and included a rare skin imprint on a limb bone.

In the 2000s some paleontologists hypothesized the differences between Barosaurus and Diplodocus are really due to sexual dimorphism, with the former being larger because of being male. Another possible explanation is a difference in evolutionary age, but the ages are difficult to discern for specimens from the Morrison Formation. A potential challenge to the sexual dimorphism idea is that Barosaurus not only had more and elongated vertebrae than Diplodocus, but its vertebrae were shown to have "wings" that Diplodocus lacked. Another study examining maturation in diplodocines found juvenile Barosaurus had short, rounded neural spines, whereas adults had bifurcated ones.

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