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Sauropoda
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Sauropoda , or the sauropods , are an infraorder or clade of saurischian ("lizard-hipped") dinosaurs. They notable for the enormous sizes attained by some species, and the group includes many of the largest animals to have ever lived on land. Well-known genera include Apatosaurus (formerly known as Brontosaurus), Brachiosaurus and Diplodocus. Sauropods first appeared in the late Triassic Period, where they somewhat resembled the closely related (and possibly ancestral) group Prosauropoda.
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Sauropoda , or the sauropods , are an infraorder or clade of saurischian ("lizard-hipped") dinosaurs. They notable for the enormous sizes attained by some species, and the group includes many of the largest animals to have ever lived on land. Well-known genera include Apatosaurus (formerly known as Brontosaurus), Brachiosaurus and Diplodocus. Sauropods first appeared in the late Triassic Period, where they somewhat resembled the closely related (and possibly ancestral) group Prosauropoda. By the Late Jurassic (150 million years ago), sauropods were widespread (especially the diplodocids and brachiosaurids). By the Late Cretaceous, those groups had mainly been replaced by the titanosaurs, which had a near-global distribution. However, as with all other non-avian dinosaurs, the titanosaurs died out in the Cretaceous-Paleogene extinction event. Fossilized remains of sauropods have been found on every continent except Antarctica.
The name Sauropoda was coined by O.C. Marsh in 1878, and is derived from the Greek for "lizard foot".
Complete fossil sauropod finds are rare. Many species, especially the largest, are known only from isolated and disarticulated bones. Many near-complete specimens lack heads, tail tips and limbs.
Description
Sauropods were herbivorous (plant-eating), usually long-necked quadrupeds (four-legged), with spatulate (spatula-shaped: broad at the base, narrow at the neck) teeth. They had small heads, huge bodies, and tended to have long tails. Their legs were thick, ending in blunt feet with five toes, though only three bore claws.
Size
Sauropods' most defining characteristic was their size. Even the dwarf sauropods (perhaps 5 to 6 metres, or 20 feet long) were counted among the largest animals in their ecosystem. Their only real competitors in terms of size are the rorqual whales, such as the Blue Whale. But unlike whales, sauropods all lived on land. Some, like the diplodocids, probably held their heads low, while others, like Camarasaurus, held them high.
Their body design did not vary as much as other dinosaurs, perhaps due to size constraints, but they still displayed ample variety. Some, like the diplodocids, were extremely long and with tremendously long tails which they may have been able to crack like a whip to make sonic booms. Supersaurus, at 40 metres (130 ft), is probably the longest, but others, like the old record holder, Diplodocus, are still extremely long. Amphicoelias fragillimus, of which only a drawing of a single vertebra survives, at 55 to 60 metres (180 to 200 ft) would have a spine even longer than the blue whale. The longest terrestrial animal alive today, the reticulated python, only reaches lengths of 10 metres (33 ft).
Others, like the brachiosaurids, were extremely tall, with high shoulders and extremely long necks. Sauroposeidon is probably the tallest, reaching about 18 metres (60 ft) high, with the previous record for longest neck being held by Mamenchisaurus. By comparison the giraffe, the tallest of all living animals, is only 4.8 to 5.5 metres (16 to 18 ft) tall.
Some were almost incredibly massive: Argentinosaurus is probably the heaviest at 80 to 100 metric tonnes (90 to 110 tons), though Paralititan, Andesaurus, Antarctosaurus, and Argyrosaurus are of comparable sizes. There is some very poor evidence of an even more massive titanosaurian, Bruhathkayosaurus, which might have weighed between 175 to 220 tonnes (190 to 240 tons). The largest land animal alive today, the Savannah elephant, weighs no more than 10 tonnes (11 tons).
Among the smallest sauropods were the primitive Anchisaurus (2.4 m, or 7 ft long) and Ohmdenosaurus (4 m, or 13 ft long), the dwarf titanosaur Magyarosaurus (5.3 m or 17 ft long), and the dwarf brachiosaurid Europasaurus, which was 6.2 meters long as a fully-grown adult. Its small stature was probably the result of insular dwarfism of a herd of sauropods stranded on an island in what is now Germany. Also notable is the diplodocoid sauropod Brachytrachelopan, which was the shortest member of its group thanks to its unusually short neck. Unlike other sauropods, whose necks could grow to up to four times the length of their backs, the neck of Brachytrachelopan was shorter than its backbone.
Limbs and feet
As massive quadrupeds, sauropods developed specialized graviportal (weight-bearing) limbs. The hind feet were broad, and retained three claws in most species. Particularly unusual compared with other animals were the highly modified front feet (manus). The front feet of sauropods were very dissimilar from those of modern large quadrupeds such as elephants. Rather than splaying out to the sides to create a wide foot as in elephants, the manus bones of sauropods were arranged in fully vertical columns, with extremely reduced finger bones (though the most primitive sauropods, such as Vulcanodon and Barapasaurus, retained splayed and finger-bearing forefeet). The front feet were so modified in eusauropods that individual digits would not have been visible in life, with the whole manus modified into a hoof-like structure.
The arrangement of the forefoot bone (metacarpal) columns in eusauropods was semi-circular, so sauropod forefoot prints are horseshoe-shaped. Unlike elephants, print evidence shows that sauropods lacked any fleshy padding to back the front feet, making them concave. The only claw visible in most sauropods was the distinctive thumb claw (associated with digit I). Almost all sauropods had such a claw, though what purpose it served is unknown. The claw was largest (as well as tall and laterally flattened) in diplodocids, and very small in brachiosaurids, some of which seem to have lost the claw entirely based on trackway evidence.
Titanosaurs also lost the thumb claw completely (with the exception of early forms such as Janenschia). Titanosaurs were most unusual among sauropods, as in addition to the external claw, they completely lost the digits of the front foot. Advanced titanosaurs had no digits or digit bones, and walked only on horseshoe-shaped "stumps" made up of the columnar metacarpal bones.
Print evidence from Portugal shows that in at least some sauropods (probably brachiosaurids), the bottom and sides of the forefoot column was likely covered in small, spiny scales, which left score marks in the prints. In titanoaurs, the ends of the metacarpal bones that contacted the ground were unusually broad and squared-off, and some specimens preserve the remains of soft tissue covering this area, suggesting that the front feet were rimmed with some kind of padding in these species.
Armor
Some sauropods had armor. There were genera with spined backs, such as the Agustinia, and some has small clubs on their tails, like Shunosaurus. Several titanosaurs, such as Saltasaurus and Ampelosaurus, had small bony osteoderms covering portions of their bodies.
Palaeobiology
Posture
From early on there has been speculation by Osborn and others that sauropods could reach up on hind legs, using their tail as the third 'leg' of a tripod (somewhat like kangaroos), and a famous restoration of a Barosaurus rearing up on hind legs in the American Museum of Natural History illustrates this hypothesis well. One study postulated that if sauropods had adopted a bipedal posture at times, there would be evidence of stress fractures in the forelimb 'hands'. However, none were found after examining a large number of sauropod skeletons.
If a sauropod stood in the tripod posture, there would be a heavy weight load on the haemal spines on part of the tail. As the sauropod got heavier as it grew, when it reared, these haemal spines would have to carry more and more load, until some of them would break due to stress fracture, and that would make rearing painful and the sauropod would have to stay on four feet after that. That may have evolved as a safety measure to prevent rearing when it got too heavy for rearing to be safe. There are reports of such haemal spine fractures being found in sauropod tail vertebrae.
Air sacs
Like other saurischian dinosaurs (such as birds and other theropods), sauropods had a system of air sacs evidenced by indentations and hollow cavities in most of their vertebrae. Such openings in the back, neck, and tail bones are referred to as pneumaticity, and pneumatic, hollow bones are a characteristic feature of all sauropods.
The bird-like hollowing of sauropod bones was recognized early in the study of these animals, and in fact at least one sauropod specimen found in the 19th Century (Ornithopsis) was originally misidentified as a flying pterosaur because of this.
History of discovery
The first scrappy fossil remains now recognized as sauropods all came from England and were originally interpreted in a variety of different ways. Their relationship to other dinosaurs was not recognized until well after their initial discovery.
The first sauropod fossil to be scientifically described was a single tooth known by the non-Linnaean descriptor Rutellum implicatum. This fossil was described by Edward Lhuyd in 1699, but was not recognized as a giant prehistoric reptile at the time. Dinosaurs would not be recognized as a group until over a century later.
Richard Owen published the first modern scientific description of sauropods in 1841, in his paper naming Cetiosaurus and Cardiodon. Cardiodon was known only from a two unusual, heart-shaped teeth (from which it got its name), which could not be identified beyond the fact that they came from a previously unknown large reptile. Cetiosaurus was known from slightly better, but still scrappy remains. Owen thought at the time that Cetiosaurus was a giant marine reptile related to modern crocodiles, hence its name, which means "whale lizard." A year later, when Owen coined the name Dinosauria, he did not include Cetiosaurus and Cardiodon in that group.
In 1850, Gideon Mantell recognized the dinosaurian nature of several bones assigned to Cetiosaurus by Owen. Mantell noticed that the leg bones contained a medullary cavity, a characteristic of land animals. He assigned these specimens to the new genus Pelorosaurus, and grouped it together with the dinosaurs. However, Mantell still did not recognize the relationship to Cetiosaurus.
The next sauropod find to be described and misidentified as something other than a dinosaur were a set of hip vertebrae described by Harry Seeley in 1870. Seeley found that the vertebrae were very lightly constructed for their size and contained openings for air sacs (pneumatization). Such air sacs were at the time known only in birds and pterosaurs, and Seeley considered the vertebrae to come from a pterosaur. He named the new genus Ornithopsis, or "bird face" because of this.
When more complete specimens of Cetiosaurus were described by Phillips in 1871, he finally recognized the animal as a dinosaur related to Pelorosaurus. However, it was not until the description of new, nearly complete sauropod skeletons from the United States (representing Apatosaurus and Camarasaurus) later that year that a complete picture of sauropods emerged. An approximate reconstruction of a complete sauropod skeleton was produced by John A. Ryder, based on the remains of Camarasaurus, though many features were still inaccurate or incomplete according to later finds and biomechanical studies. Also in 1877, Richard Lydekker named another relative of Cetiosaurus, Titanosaurus, based on an isolated vertebra.
In 1878, the most complete sauropod yet was found and described by Othniel Charles Marsh, who named it Diplodocus. With this find, Marsh also created a new group to contain Diplodocus, Cetiosaurus, and their increasing roster of relatives to differentiate them from the other major groups of dinosaurs. Marsh named this group Sauropoda, or "lizard feet."
Taxonomy
Classification of the sauropods has largely stabilised in recent years, though there are still some uncertainties, such as the position of Euhelopus, Haplocanthosaurus, Jobaria and Nemegtosauridae. The following are two alternative recent classifications (showing supra-generic clades only in the second example). These are by no means an exhaustive list of recent sauropod classification schemes. In some cases, families like Vulcanodontidae, Cetiosauridae and Omeisauridae are not included because they are considered paraphyletic, or even (in the case of Camarasauridae) polyphyletic.
Classification
This taxonomy follows Wilson & Sereno 1998, Yates 2003, Galton 2001, and Wilson 2002, with ranks after Benton, 2004.
Phylogeny
Cladogram simplified after Wilson, 2002.
Further reading
- Bob Strauss, 2008, , The New York Times
- Kristina Curry Rogers and Jeffrey A. Wilson, 2005, The Sauropods: Evolution and Paleobiology, University of California Press, Berkeley, ISBN 0-520-24623-3
- Upchurch, P., Barrett, P.M. and Dodson, P. 2004. Sauropoda. In The Dinosauria, 2nd edition. D. Weishampel, P. Dodson, and H. Osmólska (eds.). University of California Press, Berkeley. Pp. 259-322.
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