The Sauropods: Herbivorous Giants
Before the familiar images of Tyrannosaurus, Triceratops, Stego-saurus, and other iconic dinosaurs of the American West were widely known, the most familiar image of a dinosaur was that of Brontosaurus, a sauropod. It was in 1877, under the guidance of American paleontologist Edward Drinker Cope (1840-1897), that the first scientific restoration—or illustration—of a dinosaur skeleton on paper was produced. Cope chose as his subject the large sauropod Camarasaurus. He created a life-sized illustration of the skeleton and presented it at a scientific meeting, but it was never published. In 1883, Othniel Charles Marsh (1831-1899) produced the first widely published illustration of a dinosaur skeleton. Marsh made an appealing choice in selecting the spectacularly huge Bron--tosaurus (now Apatosaurus) as the first of his many discoveries to be drawn scientifically. The image of this long-necked behemoth became the first broadly distributed and accurate drawing of a dinosaur to gain the public's attention. Even the animal's given name had a sensational ring to it: "thunder lizard"—a dinosaur so huge that it shook the ground when it walked by. One might say that Brontosaurus was the first rock star of dinosaurs.
Sauropods were the tallest, heaviest, and longest animals ever to walk the Earth; of all animals that ever existed, only whales got bigger, and they, of course, cannot walk on land. Sauropods were members of the saurischian clade known as Sauropodomorpha ("lizard foot form"). Evolutionary adaptations in body size in a
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THINK ABOUT IT
The scientific study of sauropod dinosaurs has been replete with its own evolution in the depiction of these most giant of all land animals. Cetio-saurus was the first named sauropod, in 1841, but when Sir Richard Owen (1804-1892) first classified it, he had little to go by except five vertebrae, some scattered pieces of limb, and a chunk of rib. He thought it was a giant crocodile, so attempts at illustrating it as a dinosaur did not emerge until much later, when better specimens were available.
The history of illustrating sauropods closely parallels the various debates surrounding their lifestyle. Were sauropods primarily aquatic? Could they lift their heads high and bend their necks with great flexibility? Did they have a robust terrestrial lifestyle, or were they sluggish, sauntering creatures?
The dinosaur bone rush in America during the 1870s uncovered the first remarkably complete specimens of sauropods and inspired the first attempts to create scientifically accurate illustrations of them. As mentioned, Cope was arguably the first paleontologist to reconstruct a dinosaur skeleton on paper, and he did so with great gusto. The life-sized illustration of Camarasaurus that he created in 1877 depicted a creature with a sturdy, upright posture and a robust vertebral column that paralleled the ground from head to tail. This view was remarkably modern by today's standards, but only a year later, in 1878, Cope's view of sauropods had begun to change. On a brown paper bag, Cope sketched a scene of living sauropods that spent their time almost fully submerged in the water and fed on bottom-dwelling plants. Almost 20 years later, in 1897, the first great dinosaur artist, Charles Knight (1874-1953), transformed Cope's sketch of submerged sauropods into a drawing that was reproduced in the Century, a popular magazine of the day.
In 1883, Othniel Marsh completed his influential drawing of a complete skeleton of Brontosaurus, now known as Apatosaurus. Like Cope, Marsh thought that sauropods were sluggish swamp dwellers, and his beautiful illustration featured a droopy neck and a dragging tail.
Around the start of the twentieth century, the debate over sauropod lifestyles began to heat up. Artists and scientists alike began to wonder whether these creatures were better suited for life on land or in the water. Working with paleontologist Henry Fairfield Osborn (1857-1935) of the American Museum of Natural History, Knight began to hedge his bets in his portrayals of Apatosaurus. Rather than depicting sauropods as either aquatic or terrestrial, Knight began to show them as both in the same paintings, one individual firmly entrenched in the water and others standing about on dry land. Knight is, to this day, the most often imitated artist of dinosaurs, and his images that combined aquatic and terrestrial sauropods became the accepted approach to illustrating sauropods for 80 years.
Over time, images of sauropods have undergone some extremely strange transformations. In 1906, believing that sauropods may have had a sprawling posture, model makers Otto Falkenbach (1878-1952) and Charles Falkenbach of the American Museum of Natural History created a scale model of a creeping brontosaur. This image was questioned in 1908 by an American paleontologist, Oliver P. Hay (1846-1930), and a German, Gustav Tornier (1859-1938). Each man took potshots at the Falkenbach
reconstruction. Both argued for an even more extreme sprawling posture that would have required the dinosaurs to drag their bellies. Though this view was renounced by other paleontologists, the view of sauropods as lazy, partly-aquatic creatures remained in vogue for many years. This popular view of water-loving sauropods was reinforced by such world-famous paintings as the one by Rudolf Zallinger (1919-1995) that appeared on the cover of Life magazine in 1953.
The evolution of sauropod illustration began to come full circle in 1986, when paleontologist Robert Bakker (b. 1945) published his enthusiastic arguments for fully terrestrial, active sauropods. Bakker's extreme sauropods inspired today's generation of artists. In many ways, however, the current depiction of Apatosaurus with a long neck and tail held aloft and parallel to the ground is remarkably similar to Cope's first spectacular illustration in 1877.
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land animal have never been pushed to the anatomical, physiological, and metabolic extremes that were present in the largest of the sauropodomorphs.
The clade Sauropodomorpha is divided into two groups that had a common ancestor but diverged on two separate lines of large, herbivorous dinosaurs. The earliest group was that of the "Prosaurop-oda," which lived during the Late Triassic and Early Jurassic Epochs before being supplanted by the second group, the Sauropoda. The "prosauropods" are detailed in Dawn of the Dinosaurs, a companion volume of The Prehistoric Earth series. The sauropods had roots in the Late Triassic but did not begin to radiate widely until the Early Jurassic. Their span encompassed the Middle and Late Jurassic
Epochs, and a few members were present until nearly the end of the age of dinosaurs.
This chapter investigates the traits, lifestyles, and members of the sauropods that thrived during the Middle and Late Jurassic, the heyday of the largest herbivorous dinosaurs.
Sauropods evolved separately from the "prosauropods," although both clades are considered part of the Sauropodomorpha and therefore shared a common ancestor. The "prosauropods" did not achieve the enormous sizes seen in the sauropods and maintained a basic body plan that grew to no more than about 35 feet (10.5 m) in the largest species. Most "prosauropods" were extinct by the begin--ning of the Middle Jurassic, the same general time during which sauropods were beginning to diversify and radiate with great suc--cess. It has been presumed that the success of sauropods probably played a role in the demise of the "prosauropods"; sauropods edged them out of the middle to high browsing ranges of available vegetation and competed them out of existence.
Fossil evidence of the earliest sauropods is fragmentary, but even the best known of the earliest sauropods were already geographi--cally widespread by the end of the Early Jurassic. Evidence of basal sauropods has been found in Africa, Asia, Europe, and possibly North America and South America.
The earliest known sauropod is Antetonitrus (Late Triassic, South Africa) dating from 220 million to 215 million years ago. Measuring between 26 and 33 feet (8 to 10 m) long, Antetonitrus was a transitional form between the common ancestor of "prosau--ropods" and sauropods. It had some of the features found in "pro--sauropods," such as a grasping claw on its front feet, and legs and feet that had not yet been optimized like the weight--bearing limbs of later sauropods. The dinosaur is known from a single specimen, probably that of a juvenile, consisting primarily of limb, foot, and vertebral elements. The skull is not yet known. Antetonitrus was
identified and named by British paleontologist Adam Yates and South African paleontologist James Kitching (1922-2003). The name Antetonitrus, given by Kitching, means "before the thunder," acknowledging this sauropod as an ancestor of the so-called earth-shaking giants to follow.
Other noteworthy early sauropods—including Blikanasaurus (Late Triassic, Lesotho); Gongxianosaurus (Early Jurassic, China); Kotasaurus (Early Jurassic, India); Vulcanodon (Early Jurassic, Zimbabwe); and Tazoudasaurus (Early Jurassic, Morocco)—continued the trend in sauropod evolution toward quadrupedal gigantism. Among the anatomical innovations advanced by these early sauro-pods were forelimbs that were becoming longer, an upper-hind-limb bone (femur) that was straighter and longer than the lower leg bone (tibia), four fused sacral vertebrae, and a general trend toward a rounded, more U-shaped snout and jaw with plucking and piercing teeth.
The anatomical features of sauropods that made them a unique clade of dinosaurs revolved around two aspects of their lifestyle: a tendency toward gigantism and their vegetarian diet. Most of the diagnostic traits, or features, of sauropod skeletons are adaptations that improved their ability to grow large and to eat and metabolize huge quantities of plants.
All sauropods had a generally similar body plan. It featured a small head, a long neck, quadrupedal posture, and a long tail. This is not to suggest, however, that all sauropods were basically the same. There was significant variation among different groups of sauropods, which resulted in highly distinguishable anatomical features of the skulls, vertebrae, and limbs.
The most prominent anatomical traits that identify members of the sauropods include the following.
Forelimbs and hind limbs provided strength and mobility. The proportions, bone shapes, flexibility, and joint structures found in the legs of sauropods were significantly different from those of "prosauropods" and theropods. One telltale sign of a sauropod was that the femur was straighter and longer than the tibia—one of the many limb adaptations that enabled the bearing of great weight yet allowed the animals to move about with relative ease. Quadrupedal movement also required that the front legs be nearly the same length as the hind limbs. In one group of sauropods, the brachiosaurs, this trend resulted in forelimbs that were longer than the hind limbs, an advantage that allowed these largest of the sauropods to reach even higher into trees with their long necks.
Four or more sacral vertebrae. Sauropods had four or more sacral vertebrae, the fused backbones that anchored the pelvic structure
that supported the hind limbs. "Prosauropods" before them only had three sacral vertebrae. Sacral vertebrae provided a rigid connection to strengthen the pelvic and hind limb structure and improved the ability of the skeleton to support great weight.
Strong, weight-bearing feet. The treelike limbs of sauropods were supported by feet with large, robust toes and claws. The first, or inside, digit of each foot—in the same position as the human big toe—was weight bearing and reinforced by an especially large and deep claw on the ungual. There was a marked reduction in the size of the unguals from the inside to outside toes, making the limbs into veritable columns: compact yet sturdily built weight-bearing structures. The ankle bones of sauropods were not fused or ossified, providing flexion while the animals walked.
Elongation of the neck. The gradual elongation of the neck marked one of the hallmarks of sauropod evolution. With longer necks, these giants increased the reach of their heads to the sides in some species and to greater heights in others, presumably to improve the efficiency of gathering food.
Skulls, nostrils, and teeth. The so-called "business end" of the sauropod was the head, where it took in vegetation to nourish its enormous body. Generally speaking, sauropods had small heads in comparison to the sizes of their bodies. The jaws had a U-shaped curvature at the snout instead of the sharper, nearly pointed snouts of theropods and most "prosauropods." Sauropod teeth were adapted for plucking and stripping vegetation from branches rather than chewing it. The skulls of sauropods conformed to three basic shapes: long and slender with elongate snouts, short and broad with abrupt snouts, and a version that was somewhat in between these two extremes. There was a gradual shift in the position of nostrils from the front of the snout, as in most other animals, to a position higher up on the top of the skull. In later sauropods, such as the brachiosaurids and diplodocids, the nostrils had migrated well up on the skull, either in front of or between the eyes on top of the skull. All of these features of the sauropod skull are diagnostic at various taxonomic levels and will be discussed in greater detail below.
The fossil record of sauropods is generous, but their enormous size produced a bias in the fossil record against the preservation of complete specimens. More than 90 genera of sauropod dinosaurs have been validated by scientific scrutiny, and another 30 or more genera have been proposed but not generally accepted due to the lack of enough diagnostic fossil material. Of the 90 valid genera, only about 22 have been based on the remains of the skull as well as other skeletal parts of the body. The biggest and bulkiest bones of sauropods are typically represented largely by their thick and robust limb bones, which are more commonly found in the fossil record than other, smaller and more fragile body parts. It is fair to say that, given a choice, paleontologists would gladly trade the effort needed to recover several large limb bones for one good skull or jaw. Such is the nature of digging up the bones of sauropods.
Sauropods maintained a remarkably consistent body plan and often tended toward gigantism throughout their history, from the earliest members of the clade in the Late Triassic to the relatively few surviving lines at the end of the days of the dinosaurs. That sauropods existed in one form or another for a span of about 163 million years is astounding in itself. That they lasted for so long with relatively few major changes in their overall body plan is noteworthy for such a large group of animals. Despite the superficial resemblance among taxa, however, sauropods exhibited great diversity at the level of individual genera.
The fragmentary nature of the sauropod fossil record for many years made it difficult to ascertain these creatures' closest evolutionary relationships. A spate of discoveries in the past 20 years and a dedicated new generation of researchers have done much to correct this. British paleontologist Paul Upchurch and American paleontologist Jeffrey Wilson have each recently conducted detailed cladistic analysis of sauropods. The result of their work has been a better breakdown of groups within the sauropods, based on shared traits within a given clade. The sauropod groups described in the text that follows are based on their work as of 2004.
Sauropods are divided into three smaller groups as shown in the figure "Dinosaur Clades and Relationships: Sauropoda," which depicts the evolutionary relationships of the Sauropodomorpha.
The most primitive members of this clade are known as basal sauropoda and include such dinosaurs as Blikanasaurus and Vul--canodon, which lived during the Late Triassic and Early Jurassic. These animals are described in detail in another book in this series, Dawn of the Dinosaur Age.
Eusauropoda. This is a group of somewhat primitive sauropods whose traits still resembled those of basal sauropods more than
Dinosaur Clades and Relationships: Sauropoda
later, more advanced taxa. The eusauropods lived from the Early Jurassic to Late Jurassic. Notable adaptations shared by taxa in this clade include a broad, rounded snout; the placement of the nostrils more toward the top of the skull—a trait also found in neosauro-pods and macronarians; an increased number of cervical vertebrae, sometimes resulting in an extremely long neck; a trend toward lighter, more sculpted vertebrae; and weight-supporting modifications to the feet, limbs, pelvis, and ankles. Eusauropods are known from nine genera for which there is adequate diagnostic fossil material. Some of the best-known eusauropods are Shunosaurus (Middle Jurassic, China); Omeisaurus (Middle Jurassic, China); Patagosau--rus (Middle Jurassic, Argentina); and the very long-necked Mamen--chisaurus (Late Jurassic, China).
Neosauropoda. These sauropods make up the largest number of taxa and certainly the most famous. Among them are the longest (Diplodocus), tallest (Brachiosaurus), and heaviest (Argentinosau--rus) of the sauropodomorphs, as well as the most complete specimen (Camarasaurus) and the animal with the longest neck—40 feet (12 m)—of any known vertebrate (Sauroposeidon). Living from the Middle Jurassic to Early Cretaceous, neosauropods continued to evolve traits to improve their weight-bearing capacity and size; these traits included the lengthening of the neck by the inclusion of 12 or more cervical vertebrae; pillarlike limbs; modified pelvic, ankle, and foot bones; and continued development of less bulky vertebrae. The Neosauropoda are further divided into two groups, the Diplodocoidea and the Macronaria, each of which includes smaller clades:
Diplodocoidea. This clade includes lengthy, somewhat slender animals with a long, narrow skull, a boxlike snout, and peglike teeth. The nostrils had migrated to the top of the skull and were positioned just in front of the eyes. The Diplodocoidea had whiplike tails, and their limbs showed a reduction in ankle and foot bones. Members of the Diplodocoidea also had elaborately carved and hollowed vertebrae that lightened their weight while making space for vital blood vessels and possibly for air sacs, as in birds, to be used in respiration and cooling.
Rebbachisauridae. Several genera are known in this clade. The rebbachisaurids ranged in what was the Southern Hemisphere of the Mesozoic Era, and at least one taxon is known from the Northern Hemisphere (Spain). These sauropods lacked the V-shaped neural spines on their vertebrae and the forked chevrons that are found in other members of the Diplodocoi--dea. Some taxa had spines on their backs. Rebbachisaurids include Nigersaurus (Early Cretaceous, North Africa); Rayo-saurus (Early to Late Cretaceous, Argentina); Rebbachisaurus (Early Cretaceous, Morocco); Limaysaurus (Early Cretaceous, Argentina); Cathartesaura (Late Cretaceous, Argentina); and possibly Amazonsaurus (Early Cretaceous, Brazil).
Dicraeosauridae. This branch of the Diplodocoidea includes three genera. Dicraeosaurids had shorter necks and long spines on their vertebrae that may have supported skin--covered fins or sails. Dicraeosaurids are known from partial skeletons includ--ing some skull material. The two genera are Dicraeosaurus (Late Jurassic, Tanzania) and Amargasaurus (Early Cretaceous, Argentina).
Diplodocidae. This clade comprises eight genera and is well known from some exquisite specimens, particularly those of
Diplodocus. These large dinosaurs had forked chevrons on their spines. Their skulls were somewhat flat on top, with their nostrils facing skyward. They had 70 to 80 tail vertebrae. Current thinking is that the necks of this taxon were not flexible enough to be lifted high for treetop browsing, making these dinosaurs grazers and browsers of low- to middle-height flora, extending their necks over a horizontal plane about level with the shoulders. The known genera of diplodocids include Diplodocus (Late Jurassic, Utah, Wyoming, New Mexico, and Colorado); Apatosaurus (Late Jurassic, Colorado, Wyoming, Oklahoma, and Utah); Barosaurus (Late Jurassic, South Dakota and Utah); Australodocus (Late Jurassic, Tanzania); Dinheirosaurus (Late Jurassic, Portugal); Cetiosauriscus (Middle to Late Jurassic, England); Supersaurus (Late Jurassic, Colorado); and Eobronto--saurus (Late Jurassic, Colorado and Wyoming). A large specimen of Diplodocus, and previously identified as Seismosaurus, is known from a pelvis and partial vertebral column and is thought to have been the longest dinosaur, at a possible 110 feet (33 m) long.
Macronaria. This subgroup of the Neosauropoda is distinguished by skull features that include a nasal opening that was larger than
the eye opening. In contrast to the Diplodocoidea, macronarians were generally larger, taller, and bulkier but shorter sauropods.
Camarasauridae. This clade comprises one genus. These bulky sauropods had a boxy skull with broadly spoon-shaped teeth and nostrils elevated high on the top of the skull that faced outward rather than upward. Members of this taxon include Camarasaurus (Late Jurassic, Colorado, Wyoming, and Utah).
Brachiosauridae. Three genera make up this clade. The tallest and some of the bulkiest sauropods belong to the Brachiosauridae. They were distinguished by forelimbs that were longer than their hind limbs, and by long necks. These spectacular animals included Brachiosaurus (Late Jurassic, Tanzania and Colorado); Cedarosaurus (Early Cretaceous, Utah); and Sauroposeidon (Early Cretaceous, Oklahoma).
Long considered the largest and tallest sauropods, the Brachio-sauridae were named for Brachiosaurus, a partial first specimen discovered in Colorado in 1909 by Elmer S. Riggs (18691963). The dinosaur became much better understood after the unearthing of five partial skeletons in Tanzania between 1909 and 1912. The African expeditionary force was supervised by German Werner Janensch (1878-1969) for the Humboldt
Brachiosaurus forelimbs and torso, showing muscles
Brachiosaurus forelimbs and torso, showing muscles
Museum of Natural History in Berlin and to this date ranks as one of the most elaborate dinosaur digs ever mounted, based on the number of people engaged in the work (400 to 500 African workers per season); the geographic area of the dig (2 square miles/5 square km); the quantity of fossils (250 tons/225 metric tons); and the number of specimens (almost 100 articulated skeletons and hundreds of separate bones). Among the prizes were enough partial skeletons of Brachiosaurus to mount an extraordinary composite individual that still serves as the centerpiece of the main exhibit hall of the Humboldt Museum.
This specimen is the largest mounted dinosaur skeleton in the world.
Brachiosaurus could reach upward of 53 feet (16 m), making it one of the tallest of dinosaurs. The heavyweight title for the bulkiest of dinosaurs now goes to another group of macronarians, the titanosaurs, a clade of mostly Cretaceous sauropods described in Last of the Dinosaurs.
Titanosauria. This large clade, predominantly of the Southern Hemisphere, comprises 29 reliably known genera. The last of the sauropods were titanosaurs. Their largest members included the heaviest land animals ever to grace the Earth. The sacrum consisted of six vertebrae. The hind limbs of titanosaurs were spread more widely than those of other sauropods. The front limbs were reduced in length but supported by an oversized scapula. Titanosaur feet were small. The spines of the backbone were divided by a deep cleft. The tail was short, and
Cross-section of Brachiosaurus gut in which food slowly fermented
Cross-section of Brachiosaurus gut in which food slowly fermented the neck was robust and directed upward. Some titanosaurs had extremely long necks, as evidenced by the recently discovered Erketu (Early Cretaceous, Mongolia), whose neck was nearly 30 feet (9 m) long.
Titanosaurs are also known from opposite ends of the size scale. A diminutive species discovered in Germany and named in 2006 has become the smallest known adult specimen of a sauropod: Europasaurus (Late Jurassic, Germany) was a dwarf species whose maximum length was about 20 feet (6.2 m). In contrast, the heaviest of all dinosaurs was also a titanosaur:
Argentinosaurus (Early Cretaceous, Argentina) is known from only a partial skeleton but appears to have been the mast massive of dinosaurs, weighing upward of 99 tons (90 metric tons) and measuring about 100 feet (30 m) long. Other members of the titanosaurs have been found in widely distant geographic locations and include Chubutisaurus (Early Cretaceous, Argentina); Huabeisaurus (Late Cretaceous, China); Janenschia (Late Jurassic, Tanzania); Phuwiangosaurus (Early Cretaceous, Thailand); Alamosaurus (Late Cretaceous, New Mexico, Utah, and Texas); Antarctosaurus (Late Cretaceous, Argentina, Chile, and Uruguay); Malawisaurus (Early Cretaceous, Malawi); Paraliti-tan (Late Cretaceous, Egypt); Rapetosaurus (Late Cretaceous, Madagascar); and Isisaurus (Late Cretaceous, India). Nemegto-saurus (Late Cretaceous, Mongolia) and Quaesitosaurus (Late Cretaceous, Mongolia), once thought to be diplodocids, are now considered titanosaurs that exhibited some convergent evolution with diplodocids.
Fragmentary remains of the sauropod Cetiosaurus (Middle Jurassic, England) were on hand when British paleontologist Sir Richard Owen coined the name "Dinosauria" in 1842; however, Owen thought the bones were from a huge, extinct marine crocodile, so they were not a component in his original conception of a "dinosaur." Spare parts of sauropods continued to be discovered for the next 30 years, but none revealed the true size and spectacular nature of these giants. The dearth of knowledge about sauropods changed in a flash when the search for dinosaurs picked up in the American West in the 1870s. A spectacular series of Late Jurassic dinosaur discoveries in Colorado and Wyoming revealed a hitherto unknown dinosaur ecosystem and some of the most complete specimens of sauropod giants ever discovered.
The rush to discover dinosaurs in the American West was led by two competing paleontologists from the Northeast: Edward Drinker Cope of Philadelphia and Othniel Charles Marsh of New
Haven, Connecticut. Among the puzzles they sought to unravel was the lifestyle of the giant herbivorous sauropods.
British paleontologist John Phillips (1800-1874) preceded Cope and Marsh in speculating about the lifestyles of sauropods. In 1871, after studying a well-preserved partial specimen of Cetiosaurus, Phillips concluded that the sturdy limbs of the dinosaur strongly suggested that it was a fully terrestrial beast. By 1878, after Cope and Marsh had discovered several astoundingly complete sauropod specimens—including Camarasaurus and Apatosaurus—the two Americans came to the same conclusion as Phillips. As they contin--ued to scrape away at the blocks of fossils to reveal the finer details of these dinosaurs, however, certain anatomical features persuaded Cope and Marsh that a terrestrial lifestyle may not have been possible for these heaviest of animals. Among the fossil clues that changed their minds were the placement of nostrils on the tops of the skulls, the long necks, and nonchewing teeth that the men perceived as "weak." Cope and Marsh concluded that sauropods were probably semiaquatic animals that lived in the water to support their great weight. The long neck and nostrils on top of the skull were viewed as the dinosaurian equivalent of a snorkel, to be used for breathing air while the animal's body was almost fully submerged. The teeth seemed capable of grasping only soft-bodied plants.
By 1878, Cope had sketched an illustration of several Camarasaurus fully submerged in the water, eating plants from the bottom and lifting their heads to get a gulp of air. Marsh agreed with this interpretation. He wrote in 1895 that "In habits Brontosaurus was more or less amphibious, and its food was probably aquatic plants or other succulent vegetation."
Once established, the view of sauropods as semiaquatic creatures was difficult to refute. It stuck for nearly 100 years, despite several reasoned arguments to the contrary. The case for aquatic sauropods began to crumble for good in 1951, when British mammal paleontologist Kenneth Kermack demonstrated that the idea of deep-water sauropods defied the laws of physics. A sauropod
Edward Drinker Cope
Edward Drinker Cope submerged up to its head in water would find it impossible to take a breath due to the effects of extreme water pressure on the lungs and the base of the neck, where the muscles for respiration are located in vertebrates.
Despite Kermack's assertions to the contrary, the concept of amphibious sauropods persisted into the 1960s, in academic textbooks as well as the popular press. The case for terrestrial sauropods finally began to gain widespread acceptance due to the innovative paleontological work of American Robert Bakker. In 1971—
precisely 100 years after Phillips first suggested that sauropods were most likely terrestrial—Bakker published a provocative study that provided more than enough reasons to release sauropods from the water once and for all. His reasons included the following.
• Nostrils placed high on the skull were not a compelling argument for a snorkel-like function. Many extant lizards have large nostrils high on the skull, and those nostrils are not used as snorkels.
• Aquatic vertebrates usually have flattened tails to aid in swimming; in contrast, the tails of some sauropods were long, whiplike, and useless for propelling a sauropod through the water.
• Sauropod teeth show tooth wear and abrasion that would not be present if the animals ate only soft aquatic plants.
• Similarly, a lack of flat, grinding tooth surfaces in the sauropod mouth was not a convincing argument for a diet of soft aquatic plants. Sauropods could ingest tough vegetation and digest it in the large fermenting chambers of the gut.
• Sauropods had a deep body shape similar to that found in extant terrestrial quadrupeds such as the elephant. Modern amphibious animals such as the hippopotamus have round body forms not found in the sauropods.
• The vertebral column of a sauropod was well adapted to support the animal's large body weight on land. In particular, the shapes of sauropod vertebrae provide many sculpted surfaces and spines for the attachment of muscles.
• The rigid and straight legs of sauropods are characteristic of animals adapted for walking on land.
• Sauropods had closely spaced toes to provide sure footing, in contrast to the widely spread toes of aquatic animals that tread in areas where gaining a foothold is less necessary.
• Tall animals tend to be tree browsers. While not all sauropods were adapted for high browsing, many were.
Although somewhat speculative, Bakker's compelling arguments nonetheless withstood the critical review of other paleontologists and turned the tide in favor of a terrestrial lifestyle for the sauropods. This is not to say that sauropods never ventured into the water, but that they were predominantly adapted for a life on dry land.
The success of Bakker's study of sauropods sparked a renaissance in the study of dinosaur behavior, encouraging paleontologists to broaden their investigation of dinosaurs beyond the dimensions of fossil bones.
Bakker made a convincing case for a terrestrial lifestyle of the sauropods. One aspect of his circumstantial evidence supporting sauropod life out of water was his assessment of where sauropod fossils have been found. Sauropod bones have been discovered all over the globe and often, as in the rich Morrison Formation fossil beds of Colorado and Wyoming, in deposits that were once part of vast floodplains, "not lakes and swamps," as Bakker reminds us. A floodplain is an expanse of open land surrounding a river that experiences periodic and sudden seasonal flooding. In the Morrison
Formation, these floodplains were dry and often arid environments, home to the kinds of vigorous evergreen plants that were widespread in Jurassic times. Given a terrestrial lifestyle, the next vital link in understanding just how sauropods lived is to examine their adaptations for eating conifers and other kinds of plants that were abundant in their ecosystem.
The climate, terrain, and aridity of the sauropod habitat were similar to what the African elephant experiences today. It was a warm world with seasonal dry spells interrupted by shorter periods of torrential rain. Trackway evidence for herding sauropods suggests that the animals migrated, possibly to follow food supplies, because a large herd of sauropods could deplete available vegetation quickly. A herd would need to move continually to find more food and to give the just-vacated herbivory a chance to recover. Sauropods may even have returned to familiar eating sites, perhaps as part of a cyclical migratory route to and from nesting grounds.
The size and body plan of the sauropods strongly suggest that they ranged from being low grazers to medium- and high-browsing herbivores, feeding from ground cover in some species and from tree branches in others. The kinds of plants they ate included conifers (evergreens), tree ferns, ginkgo trees, and taller cycads—all examples of hearty, tough vegetation. The grazing and low-browsing diplodocoids almost certainly also fed on shrubby ferns, cycads, and horsetails, not just trees. Conifers are cone-bearing, woody seed plants. They generally have straight trunks and can reach heights of 330 feet (100 m) in the most robust species. Today's conifers typically have long, needlelike leaves. Some Jurassic varieties, resembling the modern Norfolk Island pine and "monkey puzzle" tree, had long, drooping branches from which close-knit, scalelike leaves grew directly from the limbs, without branches. Ginkgo trees, which also survive to this day, are another form of tree with branches and broad, fan-shaped leaves. These examples of probable sauropod food were hardly moist and succulent. Obtaining nutrition from them would have required some specialized adaptations for gathering the leaves and extracting the maximum amount of nutrition from them.
The teeth and skulls of sauropods varied little over the long course of their evolution. Even the earliest sauropods had a tendency for large bodies and developed jaws and teeth for plucking and stripping leaves from vegetation. The significance of such teeth becomes dramatically apparent when they are compared to the teeth of mammals, even humans. The mammal mouth is heterodont: It has teeth of many different shapes, with each shape adapted for a particular job during the process of eating. Some teeth are good at stabbing; others at plucking or shredding food; still others are better for chewing food. What most mammals have in common is a set of ridged or bumpy grinding teeth that form a relatively continuous surface, plus a jaw that allows the lower jaw to move in directions other than just up and down for grinding food between the upper and lower jaws. Chewing or grinding food in the mouth mechanically breaks it down and begins the process of releasing nutrients from food cells. The chewing of food in the mouth makes the food easier to digest when it reaches the bacteria and enzyme bath of the stomach.
Sauropods never evolved grinding teeth. This may seem unimaginable for animals that were the largest plant eaters of all time. The reason for this may lie in their tremendous size. Chewing food in the mouth takes time that may have been better spent obtaining more food to nourish such giants. Instead, sauropods achieved the task of eating through an innovative yet elegantly simple process. They plucked or stripped food, such as pine needles and ginkgo leaves, from tree branches with their teeth and swallowed bunches of it whole. From the mouth, food was swallowed down the long esophagus and fed into a huge fermentation chamber in the gut that slowly extracted nutrients from ingested food. One paleontologist who has devoted much time to understanding the digestive process of sauropods is American James Farlow. He pictured a large fermentation chamber beyond the stomach where gastric juices consisting of helpful bacteria did the work of breaking down plant cells and releasing nutrients into the blood stream of the dinosaur.
In retrospect, the simple eating process of sauropods seems to have been ideally suited to their enormous energy requirements. Having such a fermentation chamber in a large animal makes good sense because plant matter sits in the gut longer, allowing more nutrients to be extracted. Sauropods did not have to eat voraciously to get enough nutrients. In comparison to smaller animals, just being big enabled sauropods to get more out of the food they ate. Considering their size and the type of food they ate, sauropods probably processed food slowly along a long gut in order to maximize nutrient extraction.
Sauropod skulls were small in comparison with their bodies. The jaws of most known sauropods were specialized for shearing, plucking, or stripping vegetation from branches. There was a trend in sauropod skulls for the external nostril opening to be on the top, or dorsal, surface of the skull, in front of and above the eyes. The reason for a dorsally positioned nostril is not entirely understood; it has become a sauropod puzzle in need of a solution. Whales have blowholes on the tops of their skulls so that they can inhale and exhale while partially submerged. Sauropods were not waterdwelling creatures, however, so the topside position of the nostrils cannot be attributed to a snorkel-like purpose.
Among extant terrestrial animals, both the elephant and the tapir have nostrils on top of the skull as well as a trunk—a muscular extension of the lips that the animal uses like a fifth limb to grasp food. Sketches of sauropods with trunks are more fantasy, however; sauropods, like other reptiles, very likely did not possess the kinds of facial muscles needed to operate a trunk. What can be said about sauropod nostrils, however, is that they were overly big. In addition, the sauropod nostril may have been surrounded by a fleshy, bulbous chamber, providing a resonator through which these dinosaurs could have projected nose sounds. Bakker has lovingly called the nostrils of sauropods "nose flutes," and this idea
Aucasaurus attacking titanosaur nests
Aucasaurus attacking titanosaur nests
cannot be entirely dismissed. Another reason to have a large nasal cavity would have been to improve the animals' sense of smell—a highly valuable advantage for sauropods, which shared their world with extra-large predators equipped to bring them down. Being able to smell an approaching predator would have given sauropods a chance to flee or take a defensive stance, whichever they may have preferred. A highly placed nostril may also have given sauropods an additional advantage while eating, keeping the nose out of the way of the animal's snout as it repeatedly and systematically reached into trees to strip vegetation.
Different sauropod clades evolved several variations on the shapes of the skull and jaws. Most sauropods had four premaxil-lary teeth. Early eusauropods, such as Shunosaurus, exhibited the most primitive sauropod jaw morphology. Their snouts were long and deep, and their teeth were positioned in long rows of 21 teeth in the upper and lower jaws. Evidence of extensive tooth wear in eusauropods shows that their bite was scissorlike and could shear vegetation from branches.
Among the more advanced neosauropods were several variations on the sauropod skull. Basal macronarians, such as Camarasaurus, had taller skulls with enormous nostrils and large, spoon-shaped teeth. The bite was more muscular than that of other neosauropods, but the eating strategy was similar in that the teeth could pull, pluck, and shear leaves from tree branches.
Brachiosaurs had longer snouts than Camarasaurus and enlarged nasal openings atop their skulls, in front of and between the eyes. Brachiosaur teeth were long and less spoonlike than those of cama-rasaurs. The crown of the tooth was rounded but usually showed flattening due to tooth wear. The sides of brachiosaur teeth had long, rounded ridges. Brachiosaurs probably used a plucking and stripping action to pull food into the mouth.
Arguably the most advanced and efficient sauropod skull and jaw morphologies were those of the diplodocids. The teeth of these sauropods were clustered at the fronts of their long snouts and were distinctively peglike and unadorned. When closed, the jaws allowed the teeth to overlap in such a way as to form an effective rake for stripping leaves from branches. One can picture a diplodocids eating by placing its jaws over the middle of a branch and then raking leaves into its mouth by pulling its head away from the tree. The best-known skulls of titanosaurs, the last of the great line of sauropods, also have peglike teeth but a taller and possibly more muscular jaw. In most respects, the titanosaurs probably had eating habits more like those of diplodocids than like those of other lines of sauropods. There are some as-yet-unpublished skull specimens of titanosaurs, however, that have brachiosaur/camarasaur-style teeth. Other titanosaurs, such as Malawisaurus, have teeth that are somewhat in between the peglike and spoonlike shapes of other sauropod teeth. All titanosaurs cannot be painted with the same brush; there appear to have been different feeding styles within the group.
The forelimbs and necks of sauropods also figured importantly in their eating habits. It is presumed that their great size made all sauropods middle and high browsers, encouraging comparisons to the giraffe. Only the brachiosaurs, however, with forelimbs that were longer than their hind limbs, come close to the giraffe model. Brachiosaurus was indeed a towering browser and the tallest of all dinosaurs, as its limb and neck anatomy attest. Other taxa of sauropods, however, had shorter forelimbs and neck anatomy that was not always suited for flexing vertically.
The anatomy of sauropod necks provides many clues to the flexibility and reach of the animals' heads. In 1999, paleontologists Kent Stevens and J. Michael Parrish used three-dimensional computer modeling to simulate the maximum vertical and horizontal head movement possible in Diplodocus and Apatosaurus, two dinosaurs known from excellent specimens. Stevens and Parrish concluded that diplodocids were able to move their heads in wide horizontal sweeps somewhat low to the ground, but that raising their heads high was virtually impossible due to the inflexibility of their neck vertebrae for vertical movement. A less flexible neck, such as that of the diplodocids, is also presumed for their close relatives, the dicraeosaurids, which also happen to have dorsally placed spines on the base of their necks to add to the inflexibility.
Not all sauropod experts agree with the computer model. Paul Upchurch notes that the four vertebrae at the base of the neck of Diplodocus have a ball-and-socket morphology that would have added increased vertical flexibility. Even so, other kinds of evidence regarding diplodocids, including jaw mechanics and tooth shape, strongly hint at a low- to medium-browsing animal, so there is general agreement that diplodocids probably ate at somewhat lower levels of the flora than did other sauropods. Types of sauropods that may have had better vertical reach included some eusauropods, camarasaurs, brachiosaurs, and titanosaurs.
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