Sauropods

Sauropods are a group of dinosaurs that include the largest land animals that ever existed. Together with the theropods, or carnivorous dinosaurs, they form a group known as the Saurischia.

Principal Terms

Cretaceous: a period of time that lasted from about 146 to 65 million years ago gastroliths: polished pebbles that may have facilitated dinosaur digestion gigantothermy: a form of metabolism in which internal temperature is maintained by the large mass of the animal Jurassic: a period of geological time that lasted from about 208 to 146 million years ago theropods: the carnivorous dinosaurs and the closest relatives of the sauropods Triassic: a period of geological time that lasted from about 245 to 208 million years ago

Sauropods are a suborder of the Dinosauria, a vast group of reptiles that dominated terrestrial environments during the Mesozoic (245 to 66 million years ago). Dinosaurs ("terrible reptiles") are characterized by erect limbs and a pelvis that incorporates at least five vertebrae, characteristics that are related to their active lifestyle. Dinosaurs are divided into two major groups based on the structure of the pelvis. In the Saurischia ("lizard-hipped"), the pubis points forward, while in the Ornithischia ("bird-hipped"), the pubis has swung backward to lie parallel to the ischium. The Saurischia includes the carnivorous dinosaurs, or theropods, as well as the herbivorous sauropods ("lizard-foot"), the largest land animals that have ever existed.

Systematics and Anatomy

The earliest dinosaurs are known from the late Middle Triassic (220 million years ago) of South America, but the earliest sauropods do not appear until somewhat later, in the Early Jurassic. Sauropods are most closely related to the prosauropods, a group that originated in the Late Triassic and consisted of large, heavily built herbivores that reverted to the quarupedal locomotion of their dinosaur ancestors. Although it has been suggested that prosauropods were carnivores or omnivores, their teeth are coarsely serrate and numerous and were clearly adapted for shredding coarse vegetation. They were probably adapted as the first high browsers, capable of reaching up to high foliage to feed.

The earliest sauropods occur in the Early Jurassic of Africa and Asia, and from then on are a feature of dinosaurian faunas worldwide until the end of the Cretaceous, when the dinosaurs became extinct. All sauropods were quadrupedal and most were very large. The head was small and lightly built and contained peglike or spoon-shaped teeth that were confined to the front of the mouth, and nostrils that were placed on the top of the head. The neck was long and consisted of up to fifteen vertebrae, which although large were penetrated by many openings, so that they were constructed of struts and laminae providing maximum strength for minimum weight. The tail is also very long, perhaps up to seventy feet in Seismosaurus, and contained up to eighty vertebrae, of which the last forty were reduced to simple rods of bone that formed a whiplash. The limbs and girdles were massively constructed to support the enormous weight of these animals and the feet were broad and elephant-like, with the animal standing on the tips of its toes like all dinosaurs.

Several main groups are recognized—the diplodocids, the camarasaurids, and the brachio-saurids—but there are other, more poorly known sauropods, particularly at the beginning of sauro-pod evolution, that do not fit into these groups. The earliest sauropods come from Zimbabwe and are very similar to the prosauropods. Following these are Chinese forms from the Early Jurassic, in which weight-bearing features, such as shortening of the fingers and toes, become more apparent. In addition, teeth become restricted to the front portion of the jaws only. Sauropods had become diverse and widespread by the Middle Jurassic, but it is not until the Late Jurassic that they became common in North America. At this point, the three main groups are well represented and they remain abundant into the early part of the Cretaceous.

One of the best known diplodocids is Diplodocus, which is represented by a number of skeletons collected from the western United States. A related form is Apatosaurus, which for a long time

Sauropod Facts

Classification:

Kingdom: Animalia Subkingdom: Bilateria Phylum: Chordata Subphylum: Vertebrata Class: Reptilia Order: Saurischia Suborder: Sauropoda

Geographical location: Every continent except

Antarctica Habitat: Terrestrial habitats Gestational period: Unknown Life span: Estimated at fifty years Special anatomy: Very large quadrupedal dinosaurs in which the head was small, the neck and tail long was incorrectly known as Brontosaurus due to confusion caused by initial descriptions based on incomplete material. These animals were characterized by a protruding muzzle and small, pencillike teeth restricted to the front of the jaws. The tail was very long, with more than eighty vertebrae present in some species. The fore limbs are considerably shorter than the hind limbs, so that the hip was high compared to the shoulder. Brachiosaurs and camarosaurs were contemporaries of the diplodocids. Brachiosaurus ("arm-lizard") is particularly well known from material collected from Tendaguru in Tanzania by German expeditions between 1908 and 1912. The very fine articulated skeleton mounted in the Humboldt Museum in Berlin stands 12 meters tall and is almost 22.5 meters long, and shows the relatively short tail and very long front limbs that are characteristic of this group. In addition, the head of brachiosaurs is more compact than that of diplodocids and the jaws are lined with large, chisel-shaped teeth. The camarosaurs, such as Camarosaurus ("chambered reptile"), were similar in many ways to the brachi-osaurs, and like them had compact skulls, with large, chisel-shaped teeth in the jaws, although the muzzle was shorter. Camarosaurus had a much shorter neck, however, and the front limbs were shorter than the hind limbs, as in the diplodocids, so that the hips and shoulders were at about the same level.

For unknown reasons the sauropods almost perished at the end of the Jurassic, and there are only a few species known from the Cretaceous, although they were present throughout the period. The last survivors are members of a group known as the titanosaurids, which are widespread but known only from incomplete material. One of the interesting features of this group is that they bore bony armor that consisted of large bony plates scattered across the back with masses of small nodules in the hide between them. No other sauropods are known to have possessed armor.

Lifestyle

Sauropods were originally thought to have been aquatic animals, as it was considered that their large size would have precluded a terrestrial existence and that the long neck and nostrils on top of the head would have enabled them to reach the surface to breathe. However, it is now considered that sauropods lived a terrestrial lifestyle similar to that of modern elephants. Several features of the skeleton support this view. In particular, weight-reducing features, such as the openings developed in the vertebrae of the neck and tail, are adaptations associated with living on land where bodily support under the effects of gravity is critical in large animals. In addition, the presence of clefts in the top of neural spines on the neck vertebrae provides evidence of the presence of thick, ropelike ligaments that ran from the shoulders along the neck and allowed it to be held clear of the ground effortlessly. A similar feature is seen in the tail, indicating that it also was held horizontally, and this is supported by the evidence of tracks which only rarely show a tail-drag mark.

Trackways provide evidence that sauropods probably moved around in small groups, possibly up to twenty individuals. Although adults were probably big enough to have been immune to attack by large theropods, juveniles might have needed the protection that herding behavior would have given them. Studies of trackways have also shown that they would have moved slowly, perhaps at no more than four miles per hour, and that it would not have been possible for them to run. In all probability, they moved slowly from one stand of conifers to another, browsing high up in the trees.

As they had only small, weak teeth at the front of the mouth and no chewing teeth, it was originally thought that they fed on soft aquatic vegetation. However, stomach contents show that they fed on resistant conifers, and it is now thought that they stripped vegetation from branches and then passed it down to a gizzard, probably lined with stones, that would have processed the vegetation. The dwindling of conifers through the Cretaceous has been suggested as a cause of the decline of the sauropods during the same period.

The Size of Sauropods

Although sauropods were clearly the largest land animals that ever existed, it has been difficult to estimate the length and mass of particular species because this has to be based on reconstructions of skeletons and soft tissue estimates. The tallest sauropod seems to have been Brachiosaurus,asthe head of the Berlin specimen is twelve meters above ground level. Other sauropods have a different build, and the neck was probably oriented more horizontally. Calculations of masses of sauropods are particularly difficult, as the amount of soft tissue probably varied for individuals depending on age, health, and even from season to season. Apatosaurus may have weighed between thirty and forty tons and Brachiosaurus thirty to ninety tons; however, some poorly known sauro-

Gastroliths

Collections of rounded pebbles are found in the rib cage of some sauropod skeletons and these have been named gastroliths, or "stomach stones." The best-documented case is the sauropod Seismosaurus, in which more than 240 stones were found in two groups, one in the pelvic region and another near the base of the neck. It is clear that these stones were held within the animal during life, as they are unknown in the surrounding sediment. A number of modern animals swallow stones and grit; it seems that crocodiles and turtles do this for buoyancy control, but in birds it is done to aid digestion. By analogy with birds, it has been proposed that in sauro-pods the gastroliths were held in a crop and a gizzard and were involved in the pulping of vegetation during digestion. In this scenario, the pencillike teeth at the front of the mouth would have been used to strip vegetation, which would then have been passed down to the crop for initial processing before being passed on to the stomach for chemical digestion, and then to the gizzard for more mechanical breakdown before moving on to the intestine. However, a rival hypothesis is that the gastroliths were there to stir the digestive juices rather than mechanically abrade the food.

pods, such as Supersaurus and Ultrasaurus, may have been heavier. The record for the longest sauropod was long held by a specimen of Diplodocus, at 87 feet, but extrapolations of length from partial material suggest that Seismosaurus may have been as much as 150 feet long. Great size may have been a metabolic advantage for sauro-pods. As very large animals lose heat slowly, due to their relatively small surface area, they proba bly operated as gigantotherms, requiring only a small food intake to maintain their internal temperature.

See also: Allosaurus; Apatosaurus; Archaeopteryx; Dinosaurs; Evolution: Animal life; Extinction; Fossils; Hadrosaurs; Ichthyosaurs; Paleoecology; Paleontology; Prehistoric animals; Pterosaurs; Stego-saurs; Triceratops; Tyrannosaurus; Velociraptors.

Bibliography

Alexander, R. McNeill. Dynamics of Dinosaurs and Other Extinct Giants. New York: Columbia University Press, 1989. Informative coverage of how large animals such as sauropods operated.

Benton, Michael J. Vertebrate Paleontology. 2d ed. London: Chapman and Hall, 1997. General vertebrate paleontology text that devotes one chapter to the sauropods.

Currie, Philip J., and Kevin Padian. Encyclopedia of Dinosaurs. San Diego, Calif.: Academic Press, 1997. Excellent coverage of all aspects of dinosaur biology.

Gillette, David D. "Seismosaurus," the Earth Shaker. New York: Columbia University Press, 1994. Reader-friendly discussion of the collection and description of the largest known dinosaur.

Norman, David. The Illustrated Encyclopedia of Dinosaurs. New York: Crescent, 1985. This older book has wonderful illustrations and an excellent text with extensive coverage of sauropods.

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