John S McIntosh. Encyclopedia of Dinosaurs. Editor: Philip J Currie & Kevin Padian. Academic Press, 1997.
The term Sauropoda [Greek sauro (lizard) +poda (foot)] was coined by O. C. Marsh in 1878 as a suborder of the Order Dinosauria, but he advanced it in 1882 to the status of an order of the Subclass Dinosauria. The older term Opisthocoelia, introduced by R. Owen in 1860 as a suborder of crocodiles and including the sauropod genus Cetiosaurus and the theropod genus Streptospondylus, was used briefly for this group of dinosaurs around the turn of the century by E. S. Riggs and a few other authors, but Marsh’s term has long since been adopted. In modern cladistic terminology the hierarchy of higher taxa is Archosauria, Dinosauria, Saurischia, Sauropodomorpha, and Sauropoda. Sauropoda may be defined as cetiosaurs, brachiosaurs, diplodocids, titanosaurs, and all sauropodomorphs closer to these than to prosauropods, assuming the latter as a monophyletic group.
Anatomy
The sauropods were, for the most part, very large quadrupedal animals including the largest terrestrial beasts that ever lived. The head was small, the neck long, and the tail long. The articulation between the neck and the skull was a very weak one. Thus, there was a tendency for the skull to snap off and drift downstream in the rivers in which the dead animals had fallen. Consequently, for many years the skulls in most of the otherwise well-established taxa were unknown. That situation is being gradually rectified, but there are still many forms in which the skull is unknown, including all the members of one very large group, the Titanosauridae.
Superficially, the sauropod skulls may be divided into two types. The first (basal) of these have large, spoon-shaped teeth and nares (nostrils) that open outward. Those of the other type are long snouted with peg-like teeth confined to the front part of the jaws and nares that open upward on the top of the skull. Of the five pairs of major openings in the sauropod skull, the nares, subcircular orbits (containing a sclerotic ring), and lateral temporal fenestrae face outward and are the largest. The supratemporal fenestrae are smaller and open upward, whereas the anteriormost antorbital fenestrae are the smallest. Most genera lack mandibular fenestrae except for a small circular opening in the surangular. The palate is highly vaulted.
The primal number of presacral vertebrae in the Sauropodomorpha is 25. There are two primary sacrals to which are often added one or more from each of the dorsal and caudal series. The typical prosauropod Plateosaurus has 10 neck or cervical vertebrae, 15 trunk or dorsals and three sacrals (a caudosacral having been added), and approximately 46 tail or caudal vertebrae. The trend in the sauropods is for the “cervicalization” of the presacral column, i.e., the modification of anterior dorsals into cervicals, with the addition of as many as 4 or 5 additional cervicals in some of the Chinese forms. The presacral openings in the sides of the centra (pleurocoels) are simple in more basal forms but complex in the neck region of many derived forms and very deep in the dorsal region of some. The neural arches and spines, particularly in more derived forms, are greatly reduced to a set of struts and laminae providing maximum strength for minimum weight—a true triumph of engineering. The sacrum in most adult genera consists of five functioning sacrals, including one dorsosacral and two caudosacrals. In the Cretaceous titanosaurids and in very old individuals of an occasional Jurassic form, a second dorsosacral is incorporated. The tail is long, with about 45 vertebrae in basal forms and more than 80 in some of the diplodocids, the last 40 or so being reduced to simple rods of bone that form a whiplash. The scapula has an expanded lower (anterior) end to which, in adults, a large coracoid usually fuses. In several genera, what are supposed to be slender rod-like clavicles have been reported. Recently, one genus, Apatosaurus, has been shown to possess a full set of gastralia. The pelvic bones are large plates, usually not fused to one another. The ilium has developed a large anterior lobe, which tends to differentiate the sauropods from the prosauropods.
The limb bones tend to have reduced central cavities. The humeri are longer than the radii and ulnae, and the femur is longer than the tibia and fibula. In most forms the hindlimbs are longer than the forelimbs, but in some brachiosaurids this is reversed. The wrist or carpus is reduced to three elements in basal forms, to two elements in most later forms, and to a single element in at least one genus (Apatosaurus). The manus has five digits with reduced phalanges on the outer four. Usually, only digit I possesses a claw, but there is some evidence that in some of the Cretaceous forms even this claw is absent. The ankle or tarsus is reduced to a large astragalus, strongly interlocking with the tibia, and a much smaller globular calcaneum, which disappears in the diplodocids. The pes has five digits with claws on digits I, II, and III.
Systematics
Six to eight or nine sauropod subtaxa are currently recognized. The most basal, the Vulcanodontidae, is represented by Vulcanodon from the Triassic–Jurassic boundary in Zimbabwe. It is incompletely known and might possibly be an advanced prosauropod. The skull and vertebrae of the neck and trunk are unknown, as is the very diagnostic anterior part of the ilium. The girdle bones resemble those of both the sauropods and prosauropods, whereas the limbs appear to be more nearly sauropod, except that the femur retains an inner trochanter, a feature absent in all later sauropods.
The Cetiosauridae is a generalized group of largely Middle Jurassic sauropods, in most of which the skull is unknown. The teeth are of the broad type. The vertebral spines in the neck and trunk region are simple (uncleft). The tail is of only moderate length and the hindlimbs are only moderately longer than the fore. Typical representatives are Cetiosaurus in England, Patagosaurus in Argentina, Barapasaurus in India, and Bellusaurus in China. Haplocanthosaurus is a survivor from the Upper Jurassic of North America. It retained a large number of trunk vertebrae (14). A primitive Middle Jurassic Chinese form, Shunosaurus, is known from a number of complete skeletons with skulls. Several are mounted in Zigong. This animal possessed a number of small spatulate teeth and retained an opening in the lower jaw. It had 12 vertebrae in the neck, 13 in the trunk, 4 in the sacrum, and 44 in the tail. The middle chevron bones are of the forked type typical of the Diplodocidae to be discussed below. A bony club somewhat reminiscent of that found in the ankylosaurs was present on the end of the tail. A contemporary, somewhat heavier, related Chinese sauropod was Datousaurus. Reference of the two Chinese forms to a separate taxa from the other cetiosaurids is not unlikely but should be delayed until more is known of the latter genera.
A related but more advanced group is the Brachiosauridae. It is characterized by large teeth, greatly elongated neck vertebrae with very long cervical ribs, and deep pleurocoels (side cavities) in the dorsal vertebrae. The tail was relatively short. Most characteristic were the greatly elongated forelimbs. In the typical Upper Jurassic Brachiosaurus from Tanzania and Colorado, the forelimbs were longer than the hindlimbs. The skull of this animal was large, with a vaulted nasal bone. There were 13 vertebrae in the neck, 11 or 12 in the trunk, and 5 in the sacrum. The metacarpals were especially elongated, and the claw on digit I of the manus was greatly reduced. Brachiosaurus was one of the heaviest land animals that ever lived. Colbert estimated its weight to be 86 tons, but D. Russell calculates this as too high. Mounted skeletons are to be found in Berlin and Chicago. A similar animal with slightly shorter forelimbs, found in the Middle Jurassic of Madagascar, has been referred to the ill-defined British genus Bothriospondylus. To this animal may belong a large amount of juvenile material named Lapparentosaurus, also from Madagascar. Pleurocoelus, from the Lower Cretaceous of Maryland and Texas, also has greatly hollowed-out vertebrae, but more slender teeth, and appears to be a smaller member of this group.
The Camarasauridae resembles the Brachiosauridae in that the neck and trunk vertebrae were very hollow and the cervical ribs very long. On the other hand, these vertebrae were more specialized in that the vertebral spines in much of the column possessed a deep U-shaped cleft. The hindlimbs were longer than the forelimbs. Camarasaurus was the most common North American sauropod. It was found in the Upper Jurassic of Colorado, Wyoming, Utah, and other Morrison Formation locales. Its skull was short with a “bulldog-like” muzzle and large spoonshaped teeth. There were 12 vertebrae in both the neck and trunk, 5 in the sacrum, and 53 in the tail. Mounted skeletons are found in New Haven, Pittsburgh (where there is a 5-m-long, nearly complete juvenile skeleton), Washington, DC, Drumheller, Alberta, Canada, and Albuquerque. The European representative of the group is the very similar Aragosaurus. Euhelopus, from the Upper Jurassic of China, had a skull resembling that of Camarasaurus, with similar teeth. The vertebral spines are only slightly cleft, and the neck and trunk vertebrae have increased to 17 and 14, respectively. This has led some experts to refer it to the Camarasauridae and others to the Chinese group Mamenchisauridae, which will be discussed below. A partial skeleton is mounted in Uppsala, Sweden. Another controversial form is Opisthocoelicaudia from the Upper Cretaceous of Mongolia. This animal possessed the deeply U-shaped cleft spines but had a very short tail with vertebrae unlike those of any other sauropod. The centra articulated with one another with a ball on the front and a cup on the back (opisthocoelous condition). The all-important head and neck are unknown. Until these parts are recovered, Opisthocoelicaudia may be referred to the Camarasauridae with a query.
The Diplodocidae have been found almost exclusively in rocks of Upper Jurassic age. These include two of the best known dinosaurs, the North American Diplodocus and Apatosaurus (also known by its junior synonym, Brontosaurus). These animals had skulls with protruding muzzles and nostrils that opened upward on the top of the head. The very characteristic teeth were small and peg shaped and were confined to the front of the jaws. The neck was long (15 vertebrae), the trunk short (10 vertebrae), and the tail very long with 80+ vertebrae ending in a whiplash. The vertebral spines in much of the neck and trunk possessed a deep V-shaped cleft. An important character of the family involves the shape of the chevron bones in the mid-tail region. Instead of being directed downward, these elements developed fore and aft extensions so as to lie almost horizontally, a feature most highly developed in Diplodocus itself. The forelimbs were the shortest of those in any sauropod family. Apatosaurus differed from Diplodocus in being much more robust. Adults of both genera could exceed 23 m (75 ft) in length, but Colbert has estimated their weights at 30-35 and 11 or 12 tons, respectively. A contemporary of these animals in North America, also found in East Africa, was Barosaurus. This animal closely resembled Diplodocus but had a longer neck and shorter trunk. Incomplete remains of three truly gigantic animals from the same beds in North America have been named Supersaurus, Seismosaurus, and Amphicoelias. Adults may have attained lengths up to or exceeding 38 m (125 ft). All three are of the very slender type and have not been absolutely distinguished from Diplodocus and Barosaurus. A basal diplodocid, Cetiosauriscus, is known from a partial skeleton found in England. Mounted skeletons of Diplodocus are found in Pittsburgh, Washington, DC, Denver, Houston, and Frankfurt, Germany. Casts of the Pittsburgh specimen, Diplodocus carnegii, were sent to London, Paris, Berlin, Madrid, Bologna, Vienna, St. Petersburg, Mexico City, and La Plata, Argentina. Mounts of Apatosaurus are found in New Haven, New York, Pittsburgh, Chicago, and Laramie, Wyoming. A skeleton of Barosaurus has recently been mounted in New York in a controversial pose—rearing up on its hindlegs to protect a juvenile from a marauding Allosaurus. Finally, the partial skeleton of Cetiosauriscus is mounted in London.
An offshoot of the typical diplodocids is a subtaxon, the Dicraeosaurinae. (Bonaparte and collaborators consider these animals to belong to a separate taxon, Dicraeosauridae.) Dicraeosaurus from the Upper Jurassic of Tanzania has a diplodocid-like skull, limb, and girdle bones. The 12 neck and 12 trunk vertebrae are conservative in number but are distinguished by the extraordinary development of the vertebral spines in the neck and trunk. These were very high and deeply cleft. This trend reached its maximum development in Amargasaurus, from the Lower Cretaceous of Argentina. In this animal the spines made up three-quarters of the total height of some vertebrae. Mounted skeletons of Dicraeosaurus are found in Berlin and of Amargasaurus in Buenos Aires.
A more distantly related taxon is the Mamenchisauridae from the Jurassic of China. The Upper Jurassic Mamenchisaurus was a truly bizarre creature with an enormously long neck of 19 elongated cervical vertebrae. The cleft of some of the spines in the shoulder region and the typical forked diplodocid chevrons led to its early inclusion in the Diplodocidae. However, the recent discovery of a partial skull with broad camarasaur-like teeth and an associated neck that appears to belong to Mamenchisaurus indicates that this animal apparently diverged from the diplodocid line before the development of the slender peg teeth. The Middle Jurassic Chinese genus Omeisaurus also had an inordinately long neck with 17 vertebrae and may be related to Mamenchisaurus. It had broad teeth and lacked the cleft vertebral spines, but it had diplodocid forked chevrons. Mounted skeletons of Mamenchisaurus and Omeisaurus are found in Beijing and in Zigong and Chongqing, respectively.
Finally, two isolated skulls from the Upper Cretaceous of Mongolia appear to be of the general diplodocid type. They have been named Nemegtosaurus and Quaesitosaurus. Their true relationships must await future skeletal discoveries.
The Titanosauridae were the last survivors of the Sauropoda. Numerous remains of this group are found throughout the world in rocks of Cretaceous, especially Upper Cretaceous, age. However, it remains the most enigmatic taxon because no even partly complete skull or articulated skeleton has ever been found. The presence of very small slender teeth led to the association of the titanosaurids with the Diplodocidae for some years, but it is now virtually certain that this feature evolved independently in the two taxa. The spines of the vertebrae of the neck and back were simple, not bifid. A second dorsal vertebra had been incorporated into the sacrum to give a total of six. Perhaps the most characteristic anatomical feature of this family is seen in the tail vertebrae. The bodies (centra) of these vertebrae articulated by having a deep cup on the front and a large ball on the back (procoelous condition). The chevrons were simple, not forked. Another very significant feature was the development of several types of dermal armor ranging from small globular ossicles to moderatesized plates. The titanosaurids ranged in size from relatively small to truly gigantic, rivaling the huge diplodocids mentioned previously. Fragmentary and disarticulated remains are found in abundance in India, France, Romania, parts of Africa, and especially in South America, where numerous genera have been named. Among the best known are the moderately sized Saltasaurus, Titanosaurus, and Neuquenosaurus; the large Antarctosaurus and Argyrosaurus; and some very large unnamed animals. The earliest member of the taxon is often taken to be Janenschia (incorrectly referred to as Tornieria) from Tanzania. The recently discovered and not yet fully described Malawisaurus from the Lower Cretaceous of Malawi promises to throw much light on the origins of the group. The titanosaurids died out in the upper part of the Lower Cretaceous period in North America but were apparently reintroduced from South America late in the Upper Cretaceous. However, they only got as far north as Utah before the final dinosaurian extinction. Ironically, some of the best articulated titanosaur material is that of the large North American genus Alamosaurus. Composite mounted skeletons of titanosaurids are found in several museums in Argentina, namely, in Buenos Aires (Saltasaurus) and La Plata (Neuquenosaurus).
Based on vertebral differences, Bonaparte and collaborators have separated two Lower Cretaceous Argentinian sauropods, Andesaurus and Argentinosaurus, from the other titanosaurs as a distinct subtaxon Andesaurinae. Recently, they have raised it to a separate taxon. Full understanding of these relationships must await the discovery of more material and fuller phylogenetic analyses. Significant at this time is the gigantic size of Argentinosaurus.
Paleobiology
At one time the sauropods were thought to be semiaquatic animals dwelling in shallow seas and lakes, but most experts now agree that they were essentially terrestrial, venturing occasionally into rivers. As clearly shown by the teeth, they were herbivorous. The recent discovery of gastroliths in the gut of Seismosaurus confirms, as some have suspected for years, that these stones were used to aid in digestion. The animals walked in an upright pose, not (as Hay and Tornier had earlier supposed) in a crocodile-like sprawl. Bakker has argued that they were endothermic (warm blooded), but most writers regard them as homeothermic, i.e., their huge bulk permitted heat to be retained so that the practical benefits of endothermy could be enjoyed without the presence of a “biological thermostat.” From a study of the footprints, Alexander has estimated their normal walking speed at about 1 m per second.