The sauropod group Neosauropoda is subdivided into two major clades: the Diplodocoidea (comprising the Rebbachisauridae, Di-craeosauridae, and Diplodocidae) and the Macronaria (composed mainly of the Camarasauridae, Brachiosauridae, and Titanosauria). Of these, only the Macronaria had taxa that survived into the Early and Late Cretaceous Epochs as members of the brachiosaurs and titanosaurs. The Macronaria were distinguished from other sauro-pods by skull features that included a nasal opening that was larger than the eye opening. In contrast to the Diplodocoidea, macronar-ians were generally larger, taller, and bulkier sauropods. They were also typically stockier and heavier than the slender, whip-t ailed diplodocids and their kin.
Brachiosaurs were distinguished by extremely long necks and by forelimbs that were longer than their hind limbs—a body plan different from that of other kinds of sauropods. This anatomy gave brachiosaurs great height, and some members of this clade are considered the tallest of all dinosaurs. Only a few brachiosaur genera are currently recognized. The most famous is Brachiosau-rus itself, which dates from the Late Jurassic of Tanzania and the western United States and is known from some magnificent specimens, including a composite skeleton mounted in the Museum für Naturkunde in Berlin, Germany. It is the largest real mounted dinosaur skeleton in the world. Two other recently described genera of brachiosaurs lived in the Early Cretaceous Epoch and were discovered in the United States.
Cedarosaurus (Early Cretaceous, Utah) is known from a partial postcranial skeleton—a specimen lacking the head. Described in
1999, the original specimen of Cedarosaurus is also notable for having been associated with a small cluster of 115 so-called gastroliths, or "stomach stones." The rounded stones were discovered within the body cavity of Cedarosaurus and were originally interpreted as being part of a gastric mill in a birdlike gizzard of the sauropod, an interpretation that has since been refuted. Current research comparing sauropods to other tetrapods concludes that the largest of the dinosaurs probably relied on a simple bacterial fermentation process and a slow digestive process to extract nutrients from their food. The stones found with the specimen of Cedarosaurus may have simply been swallowed accidentally by the dinosaur.
Sauroposeidon lived during the Early Cretaceous Epoch and was found in Oklahoma. First described in 2000, the specimen consists only of four neck vertebrae, but these are so closely similar to those of other brachiosaurs that the Sauroposeidons classification as a member of this clade can be claimed with near certainty. A key difference in the neck vertebrae of Sauroposeidon is that they are elongated by about 25 percent more than comparable vertebrae in Brachiosaurus; this indicates that the last of the known brachiosaurs had continued the trend towards longer and longer necks. Assuming that Sauroposeidon was much like Brachiosaurus in all other respects, a hypothetical reconstruction on paper would make Sau-roposeidon about 60 feet (18 m) tall, which is about 25 percent taller than the most complete specimen of Brachiosaurus in the Museum für Naturkunde in Berlin. The long neck of Sauroposeidon is also a record holder among dinosaurs. Measuring about 40 feet (12 m) long, Sauroposeidons neck was longer than those of all other known sauropods. There are a few sauropods that may have had a neck nearly as long—for example, large specimens of Mamenchisaurus and the newly named Patagonian titanosaurs Futalognkosaurus and Puertasaurus.
Not all paleontologists agree that brachiosaurs raised their heads to such heights. For those who believe in this, however, the presumed upstretched posture and extreme height of brachiosaurs raises some interesting questions about the physiological challenges of pumping
blood to the elevated head. This posture is unlike most quadrupedal terrestrial vertebrates, whose heads are normally carried at about the same level as the heart—the giraffe being an informative exception. In the giraffe, the heart is much larger than that in mammals of similar mass, enabling the animal to pump blood upwards to its elevated head. Special valves in the neck open and close to prevent blood from rushing up and down the neck as the giraffe raises and lowers its head.
By comparison, if a brachiosaur had a similar circulatory system, its heart would have been massive in order to maintain the pressure needed to pump blood to the brain. British paleontologist David Norman (b. 1941) further explained that the brachiosaur heart was probably divided into two parts: one for generating the high pressure needed to pump blood to the head, and the other, requiring much lower pressure, to pump blood into the lungs. A two-part heart such as this is also found in mammals (including the giraffe)
and birds, but is only imperfectly present in extant reptiles; this further suggests that dinosaurs had closer affinities with the physiology of mammals and birds than with cold-blooded reptiles.
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