The last surviving group of sauropods were titanosaurs, and their distribution was nearly worldwide. Titanosaurs had roots in the Late Jurassic of Tanzania, Thailand, and western North America. Although specimens have been found all over the world, most titanosaur remains are fragmentary and lack skull elements. The most complete specimens have been found in Argentina and Madagascar, where the recent discoveries of two exquisite skulls have enabled paleontologists to better understand the evolution of these sauropods.
Anatomical traits found in all titanosaurs included a sacrum consisting of at least six fused vertebrae; hind limbs that were spread more widely than those of other sauropods; relatively small feet; and enlarged breastbones. Their front limbs were reduced in length compared to those of other sauropods. Unique among sauropods, many titanosaurs had ball-and-socket joints between their tailbones; that is, a "ball" of bone on one vertebra fit into a "socket" on the next. The tail was usually short and the neck of medium length.
The skulls of titanosaurs have rarely been found, although a few fine examples have recently been unearthed. In Argentina, paleontologist Rubén Martínez recovered an isolated titanosaur skull and neck from the earliest Late Cretaceous badlands of Chubut Province in central Patagonia. Around the same time, in 2001, American paleontologists Kristina Curry Rogers and Catherine A. Forster described a remarkably complete skeleton and skulls of a medium-sized titanosaur from the Late Cretaceous of Madagascar that they named Rapetosaurus ("mischievous giant lizard"), after a figure from Malagasy folklore. The best-known specimen of Rapetosaurus is a juvenile that measured about 30 feet (9 m) long. The skull of Rapetosaurus and the skull of the as yet-unnamed Argentinean specimen found by Martínez are most similar to those skulls
of the diplodocids, with nostrils positioned high on the skull and a U-shaped jaw equipped with peglike teeth for plucking vegetation positioned in the front of the mouth. In contrast, the as yet-unnamed Argentinean titanosaur found by Martínez most closely resembles that of Brachiosaurus and in many ways seems to be an intermediate between brachiosaurids and later titanosaurs.
Titanosaurs are also known for representing the opposite ends of the size scale for sauropods. Magyarosaurus (Late Cretaceous, Romania) is among the smallest known adult specimens of a sau-ropod. It was a dwarf species whose maximum length was about 20 feet (6.2 m). In contrast, the largest of all known dinosaurs was also a titanosaur. Argentinosaurus (Late Cretaceous, Argentina) is known from only a partial skeleton, but it appears to have been the most massive of dinosaurs, weighing about 82 tons (70 metric tons) and measuring about 120 feet (36 m) long.
Other well-known members of the titanosaurs have been found in widely distant geographic locations and include:
Alamosaurus (Late Cretaceous, New Mexico, Utah, Texas) Europe
Lirainosaurus (Late Cretaceous, Spain) Ampelosaurus (Late Cretaceous, France)
Huabeisaurus (Late Cretaceous, China) Nemegtosaurus (Late Cretaceous, Mongolia) Opisthocoelicaudia (Late Cretaceous, Mongolia) Quaesitosaurus (Late Cretaceous, Mongolia) Phuwiangosaurus (Early Cretaceous, Thailand)
Isisaurus (Late Cretaceous, India) Jainosaurus (Late Cretaceous, India)
Chubutisaurus (Early Cretaceous, Argentina) Ligabuesaurus (Early Cretaceous, Argentina) Aeolosaurus (Late Cretaceous, Argentina) Andesaurus (Late Cretaceous, Argentina)
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THINK ABOUT IT
The title of largest—or, more appropriately, most massive—dinosaur is currently held by Argentinosaurus, an enormous titanosaur from the Late Cretaceous of Argentina that is known from only a very incomplete skeleton. Conservative estimates place its weight at about 82 tons (70 metric tons) and its length at about 120 feet (36 m) or more. It was neither the tallest nor the longest dinosaur—records that are currently held by Sau-roposeidon and Supersaurus, respectively—but it was the heaviest and most massive based on reasonably good fossil evidence.
Dinosaur paleontology has its share of legends and myths, and none is more intriguing than the mystery of the "one that got away": the remains of a gigantic sauropod discovered in the nineteenth century, that, if ever confirmed, would dwarf even Argentinosaurus.
In 1878, the fossil-hunting team of American paleontologist Edward Drinker Cope (1840-1897) was unearthing plentiful remains of sauropod dinosaurs in Colorado. Many of the specimens represented were quite complete, providing the first good look at the anatomy of sauropods. In one case, however, Cope's crew uncovered only a single vertebra from an enormous backbone. Nothing else of this animal was found, but the vertebra resembled that of smaller, known sauropods. Cope remarked that it was the "largest saurian" he had yet seen, and he published a brief account of the find along with a drawing of the bone. He believed that the vertebra was from a larger specimen of a sauropod he had previously described, Amphicoelias, whose name means "biconcave," in reference to the concave shape of its vertebrae.
The vertebra was incomplete but clearly that of the back area of a true giant among giants. Cope's sketch, in which he outlined the missing elements of the vertebra, showed that it would have measured about 7.9 feet (2.7 m) tall when complete. By comparison, a similar vertebra from the 30-ton sauropod Apatosaurus is only about half as tall at 4 feet (1.2 m). Even in those early days of dinosaur science, Cope knew that he had found a remarkable creature. He wrote that in "the extreme tenuity of all its parts, this vertebra exceeds those of this type already
a ) Amphicoelias b) Apatosaurus
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By comparing the vertebrae of Amphicoelias to Apatosaurus, it appears that this lost specimen may have been the largest of all dinosaurs.
described. . . . The dimensions of its vertebra much exceed those of any known land animal."
Using as a basis for comparison the kinds of diplodocid sauropods that this vertebra most closely resembled, estimates for the size and bulk of Amphicoelias suggest that it could have weighed up to 135 tons (122 metric tons) and could have been up to 191 feet long (58 m). This would make Amphicoelias more than 50 tons (45 metric tons) heavier than Argentinosaurus and at least 50 feet (15 m) longer than Supersau-rus. Unfortunately, the vertebra of Amphicoelias apparently crumbled to pieces while in transport and storage and no longer exists, making the case for this record holder impossible to verify.
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Antarctosaurus (Late Cretaceous, Argentina) Bonitasaura (Late Cretaceous, Argentina) Epachthosaurus (Late Cretaceous, Argentina) Futalognkosaurus (Late Cretaceous, Argentina) Mendozasaurus (Late Cretaceous, Argentina) Muyelensaurus (Late Cretaceous, Argentina) Puertasaurus (Late Cretaceous, Argentina) Rinconsaurus (Late Cretaceous, Argentina) Baurutitan (Late Cretaceous, Brazil) Trigonosaurus (Late Cretaceous, Brazil)
Malawisaurus (Early Cretaceous, Malawi) Janenschia (Late Jurassic, Tanzania) Paralititan (Late Cretaceous, Egypt) Aegyptosaurus (Late Cretaceous, Egypt)
The renaissance in knowledge of titanosaurs experienced another spike in 1998, when a joint team of American and Argentinean paleontologists discovered a rich deposit of titanosaur eggs and nests in northwestern Argentina. As many as 195 egg clusters were found, as well as the complete skeletons of embryonic titanosaurs, some isolated remains of adult titanosaurs, and specimens of theropods that probably preyed on the sauropods and their young. Each cluster of eggs contained a half-dozen or more eggs. Each egg was only about 5 or 6 inches in diameter and nearly round. The egg nests were layered in such a way that it became clear that titanosaurs returned to the site on a seasonal basis to lay their eggs.
Including four genera found to date, the Saltasaurinae are considered to be the most derived titanosaurs. Saltasaurs have been thought of as the armored sauropods because of evidence that their backs and sides were sparsely studded with a matrix of protective, bony knobs. This protection was not extensive enough to have offered much protection, so the function of such bony nodules may have merely been for visual display.
Other distinguishing features of saltasaurs are found in the morphology of the animals' vertebrae and forelimbs. Saltasaurs had short tails, small feet, six or sometimes seven sacral vertebrae, and a wide, deep division along the tops of the vertebrae between the neural spines. Saltasaurs were moderately long, measuring between 20 and 35 feet (6 and 11 m). Saltasaurines are known with certainty only from the Late Cretaceous of Argentina and include Saltasau-rus, Neuquensaurus, Rocasaurus, and Bonatitan (Late Cretaceous, Argentina); and Opisthocoelicaudia (Late Cretaceous, Mongolia).
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