Brachiosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Brachiosauridae

Genus: Brachiosaurus

Species: Brachiosaurus altithorax

Introduction

Noted for its gigantic anatomical features, particularly its long body and neck, Brachiosaurus is one of the most recognizable of dinosaurs.

During the Late Jurassic, long after the separation of Pangaea into the supercontinents, Laurasia and Gondwana, and their subsequent separation into the five large regions of East, Central, and West Laurasia, and East and West Gondwana, Brachiosaurus roamed the earth, feeding on the gymnosperms (non-flowering, seed-bearing plants) present before the emergence of angiosperms in the Cretaceous period.

Although it was assumed for many years that Brachiosaurus was an aquatic animal predominantly, with the water helping to support its tremendous weight, it is now believed to have been terrestrial. Elmer S. Riggs, who first described Brachiosaurus, espoused this point of view of an aquatic dinosaur in 1904, but only recently has evidence further indicated that Brachiosaurus's limbs and feet were not suitable for aquatic life and would have been unable to support such an animal in muddy-bottomed environments. Although the position of the nostrils also seemed to indicate an aquatic lifestyle, further research has shown that their narrow chests were not suited to breathing underwater.

Classification

Brachiosaurus falls under the Chordata phylum because of basic body characteristics including spinal chord, tail, and notochord (stiff rod of cartilage that runs the length of its body). The dinosaur also falls under the Saurischia order, as it is “lizard-hipped,” and under the Brachosauridae family, as it is a four-legged, long-necked, herbivore.

Cladistics is the second classification system used and is favored by many paleontologists. First introduced in the 1950s, cladistics aims to highlight relationships between organisms in regards to kinship and ancestry. In this way, cladistics classifies organisms into a “clade” based on the premise that organisms with common characteristics also have common ancestors. Referring to a cladistic analysis, Brachiosaurus falls under the larger Macronaria (meaning “large nostrils”) clade, and is further divided into the four-legged, long-necked, herbivorous Brachiosauridae family.

The first true sauropods, large, four-legged herbivore dinosaurs, appeared in the fossil record as early as the Late Triassic, more than 230 million years ago. By the Late Jurassic, the sauropod dinosaurs had begun to diversify, including the evolution of the macronarian (large nostrils) sauropod dinosaurs, such as Brachiosaurus altithorax.

Anatomy

Brachiosaurus was a quadrupedal dinosaur, with each of its very strong legs ending in thick club-like feet with five toes. Research indicates that the front feet of the dinosaur had at least one claw, while the back feet had as many as three. It is also distinguished by having longer forelimbs than hind limbs, an anatomical trait that is considered quite unusual among dinosaur species. It is thought that this physical difference was an adaption to better compete for tall vegetation. Some researchers have also suggested that Brachiosaurus may have been able to stand up on its hind legs to further extend its reach. Other paleontologists, however, consider this improbable as it would have caused stress fractures in their hind limbs, and, to date, no such evidence has been seen in the fossils of these species. The tail of Brachiosaurus was relatively thick and short; its skin was leathery.

With its gigantic anatomical features, particularly its 28 meter (92 foot) long body and 10 meter (32 foot) long neck, Brachiosaurusaltithorax was one of the largest and tallest known creatures to exist on earth. Yet despite its impressive length, height, and massive weight, Brachiosaurus had a relatively small head and brain. Large nasal openings situated on the top of its head suggest that this species had a strong sense of smell. Brachiosaurus may have had a fleshy nose at the front of its nasal opening, much as modern crocodylians, birds, and amniotes do. Its small mouth was filled with more than 50 large, robust spatulate (chisel-shaped) teeth, used to grind rough fibrous vegetation.

Intelligence

Despite being one of the largest species, the Brachiosaurus's relatively small head has led to the assumption that its brain was also very small. With an EQ of 0.17, Brachiosaurus is thought to be one of the least intelligent of dinosaur species to have ever lived. This also supports the theory that most herbivorous dinosaurs were less intelligent than were their carnivorous cousins.

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 Brachiosaurus 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.

It is considered likely that Brachiosaurus was oviparous, or egg-laying, as were other sauropods. Fossil evidence has indicated that sauropod eggs were not laid in nests but along the ground in a linear pattern. This suggests that eggs were laid while the female was walking and indicates that it is unlikely that Brachiosaurus incubated the eggs and/or looked after their young.

Although paleontological theory suggests female dinosaurs may have possessed a wider, more-robust pelvic anatomy to allow for the passage of eggs, precise determinants of sex are usually destroyed during the fossilization process, making it difficult to measure sexual dimorphism. Unlike birds, however, it is unlikely that this very large dinosaur practiced contact incubation due to the likelihood of crushing the eggs.

Because of the animals' extreme size, some researchers have hypothesized that sauropod mating must have taken place in water. Others counter, however, that sauropods like Brachiosaurus were too buoyant and must have mated on land. The biomechanics involved have also been a source of debate.

Diet

Brachiosaurus was among the largest terrestrial herbivores to have ever existed and lived during the Late Jurassic period, some 150 to 145 million years ago. One of the perplexing questions regarding the diet of this incredibly large herbivore is how it was able to consume enough vegetation to power its huge body before the arrival of energy-efficient angiosperms, or flowering plants.

Research on large extant mammals, such as elephants, has demonstrated that the larger a species, the longer the retention time of food in the digestive tract. This means that an animal has more time to extract necessary nutrients from low-value food such as browse (edible vegetation such as twigs and leaves). Paleontologists believe the same long ingestion retention processes also occurred for sauropods such as Brachiosaurus, allowing it to survive on pre-angiosperm vegetation. Research has concluded that plants such as Angiopteris (ferns), Equisetum (horsetails), and Ginkgo would have been staples of the dinosaur's diet. In particular, it is to be expected that the globally distributed and tall Araucaria (evergreen conifers) species would have been targeted by high-browsing species such as Brachiosaurus, which would have used its long neck and large, wide, flat teeth to feed.

Behavior

Current paleontological evidence from bone-beds and trackways has suggested that certain herbivore species, particularly sauropods, traveled in herds. Such behavior is often seen in extant herbivore species and is thought to be a form of defense and protection of offspring against predator attack. While an adult Brachiosaurus was unlikely to have any natural predators due to its size, immature individuals of smaller size may have benefited from herding behavior.

Interestingly, however, fossil remains of several sauropod dinosaurs have indicated that while these species traveled in herds, it appears the herds were segregated by age, with juveniles and adults traveling in their own separate herds. The explanation for this is unclear, although one current theory suggests that resource partitioning occurred as a result of different feeding strategies among adult and juvenile sauropod dinosaurs. While herding evidence is poor in regards to Brachiosaurus, many paleontologists believe that such a species also traveled and lived in herds.

Habitat and Other Life Forms

Brachiosaurus roamed the earth during the Late Jurassic Period, which is often considered the zenith for the sauropod herbivores such as Camarasaurus, Apatosaurus, Diplodocus, and Brachiosaurus.

While the boundary between the Jurassic and Cretaceous was marked by a period of cooling, most of the Jurassic was characterized by a warm and moist climate. While some arid areas were still present, a significant portion of the terrestrial area was covered in lush green vegetation. It was, however, a time before the emergence of angiosperms. Instead, gymnosperms such as bennettitaleans, cycad, ferns, Ginkgoes, and conifers flourished across the terrestrial landscape. Close relatives of these plants include the modern-day Chinese Ginkgo, redwoods, cypresses, pines, and yews.

Research

American paleontologist Elmer S. Riggs first discovered Brachiosaurus altithorax in 1900 in Colorado. Brachiosaurus species are some of the largest dinosaurs to have ever lived; their gargantuan size is only dwarfed by a few other species. Although the type species is the North American Brachiosaurus altithorax, many of the depictions of Brachiosaurus are actually based on its African cousin, Brachiosaurus brancai (although there is significant debate as to whether this species should have its own genus, with some researchers renaming it Giraffatitan brancai).

The great success of these species has been one of the eternal questions of dinosaurs: How did the species grow so large? Particularly perplexing is the question of how different massive herbivores successfully lived within the same habitat areas despite competing for the same limited vegetation food sources. Research on dental wear, however, suggests the possibility of resource partitioning between Brachiosaurus and other large herbivores such as Diplodocus, Apatosaurus, and Camarasaurus. Different dentition (teeth development and arrangement) and body shapes, particularly the long necks of Brachiosaurus, indicate that these large species targeted vegetation at different heights and of different species.

Sauropod gigantism in general might be attributable to a number of metabolic and anatomical features, such as not needing to chew food, consuming large amounts of vegetation with minimal movement, having many small offspring, lacking a gastric mill (a grinding apparatus in the stomach), having a bird-like lung, and having high metabolism. While some of those features are widely accepted, the metabolism rate remains controversial among paleontologists. Isotopic evidence has suggested Brachiosaurus was warm-blooded and its metabolism slowed at maturity for better thermoregulation.

Additional discoveries and reassessments continue to inform scientists' understanding of Brachiosaurus. A juvenile sauropod from the Morrison Formation in Wyoming, described in 2007 and reassessed in 2012, might have been a brachiosaur but too little was known about Brachiosaurus specifically to identify it positively. The earliest brachiosaurid sauropod found to date, Vouivria, lived 160 million years ago during the Late Jurassic; discovered in southern France in 1934, it was described in the late 2010s and shed light on the evolution of Brachiosaurus and its titanosaur relatives. In 2020 the most complete Brachiosaurus humerus to date—a two-meter-long right limb—was excavated from the Morrison Formation in Utah.

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