The Sauropods: An Essay

You’d be hard-pressed to find a dinosaur cartoon that lacks the ‘lizard-footed’ sauropods. The infant sauropod Little Foot starred as the main character in the 1988 film The Land Before Time, and, more recently, Disney’s 2015 The Good Dinosaur showcased an Apatosaurus as the main character. Sauropods have long been a staple of dinosaur lore since their discovery, likely because their immense size boggles the imagination and forces us to grapple with our own feeble frames. As a species we have never encountered anything so nobly titanic. A full-grown giraffe stands up to fifteen feet off the ground; Sauroposeidon could, hypothetically, reach close to four times that height. The reticulated python – the longest land animal known today – reaches just over twenty feet long; Sauroposeidon could reach more than five times that length. Some sauropod footprints can be over three feet long and two feet across and deep enough to hold sixty gallons of water. More than a hundred different sauropod species have been identified, and many of them echo Sauroposeidon’s epic proportions. Yet size alone may not be the only reason sauropods have top-tier placement in the media’s portrayal of dinosaurs: perhaps they’re staple dinosaurs because that’s precisely what they were in the Mesozoic. They sprang up in the Late Triassic and persisted all the way to the end of the Cretaceous. They were the longest-lived of the herbivorous dinosaurs, and they stand side-by-side with the theropods as the ‘true’ kings of the Mesozoic.

~  The Sauropods: A Brief History  ~

Sauropod origins remains hotly debated: some argue for ancestry through the prosauropod line, but others are adamant that sauropods belong in their own clade. Regardless of their origins, sauropods sprouted in the Late Triassic. Evolving alongside various prosauropods, sauropods didn’t yet have the muscle or means to outstrip their main herbivorous competitors. The earliest sauropods were the vulcanodontids, which exhibited a combination of prosauropod and sauropod characteristics. The first ‘true sauropods’ emerged in the Early and Middle Jurassic, and these included the cetiosaurs and the mamenchisaurs. The mamenchisaurs emerged first, and they put the prosauropods ‘on the wing’; the cetiosaurs showed up in the Middle Jurassic and helped kick the prosauropods out the Mesozoic door. Thus by the end of the Middle Jurassic and the beginning of the Late Jurassic, the prosauropods had gone the way of the dodo and the sauropods dominated the ecological niches they left behind.. Living in tandem with stegosaurs, early ornithopods, and ceratosaur predators, sauropods bloomed into the largest land animals the world has ever seen. The dicraeosaurs dominated Gondwana in the Middle Jurassic but were supplanted by the narrow-snouted diplodocids and apatosaurs. These latter sauropods dominated the Late Jurassic, but they were in turn squeezed out by the boxy-headed macronarians at the end of the period. The macronarians – which included Brachiosaurus and Camarasaurus – became the heavyweight sauropods by the tail end of the Jurassic Period. The Late Jurassic has been called the ‘zenith’ of sauropod history because of their prosperity and abundance. Sauropod dominance in the northern hemisphere waned in the Cretaceous as sauropods were edged out by hadrosaurs and ceratopsians. Sauropod fortunes were easier in the south, where sauropods not only grew to larger-than-ever proportions but even started displaying medieval-like armor. The armored ‘titanosaur’ sauropods didn’t make it past the Cretaceous: they were wiped out along with all other non-avian dinosaurs in the Great Dying. Sauropods paraded across our world for 150 million years; if it had not been for the end-Cretaceous extinction, and the 65 million years of fall-out in its wake, sauropods would likely be current fixtures of our planet—and we would still be avoiding their footfalls in our dens and burrows.

~  Morphology and Behavior  ~

a herd of Brachiosaurus

Disney’s 1940 animation masterpiece Fantasia captured the way scientists used to view sauropods: they were big, lumbering, dumb-witted behemoths lurking in swamps and chewing mushy plant matter. Though it’s unlikely that sauropods spent much (if any) time in swamps, Disney was right, at least, about one thing: of all the dinosaurs in the fossil record, sauropods were, by far, the dumbest of the dumb. In all fairness, we can’t measure their IQs; but scientists have found that the ratio between brain mass and body size often factors into an animal’s intelligence. This is called the encephalization quotient, and it places sauropods in the bottom spectrum of dinosaur intelligence.

Sauropods were quadrupedal, and they range from around twenty to one hundred thirty feet long. Though skeletons can tell us how long they were, measuring the weight of any species is problematic; it depends on how you ‘resurrect’ the bones with muscles and sinews based on the sizes and locations of muscle scarring on the bones. The high estimate for Brachiosaurus, for example, is eighty tons; a lower estimate places it at around seventeen tons; and modern estimates swing like a pendulum between the two.

Sauropods had columnar limbs that straightened at the elbows, knees, wrists, and ankles. This arrangement echoes that of modern elephants, who support much of their weight directly on their limb bones rather than with their muscles. The ‘cementing’ of the joints, so that they allowed little if any movement, enabled them to support their weight; if their joints were like ours, they would’ve buckled under the strain. Because the limb bones were far more dense than the rest of the skeleton, the sauropods located their weight where it was needed most. Sauropod ‘hands’ were digitigrade, meaning they walked on their fingertips; the fingers were arranged in a horseshoe-shaped semicircle, and the thumbs possessed large claws that would’ve been razor-sharp in real life. Sauropod hind-feet were semi-plantigrade, meaning they supported their weight along the length of their toe bones. Their feet were asymmetrical and tended to have three large claws. Some sauropod track-ways have preserved impressions of a heel pad behind the claws of all four feet. Because their track-ways are narrow, we can deduce that their feet were aligned towards the body’s midline. Because few track-ways preserve tail drag-marks, it seems likely that many (if not all) sauropods carried their tails off the ground. As far as locomotion, these titanic quadrupeds didn’t have much in the way of speed: top speeds of Brachiosaurus, Apatosaurus, and Diplodocus fall in the range of twelve to eighteen miles per hour.

a low-browsing Diplodocus

Despite their bulky appearance, sauropods were masterpieces of engineering. Their sheer sizes seem to run contrary to the laws of physics, but as scientists study more and more specimens, they’re able to see just how they accomplished such a feat, a triumph that has yet to be rivaled in our planet’s history. But why did they grow to such mind-boggling sizes? Was it because, in their environment, size meant more access to food? Sauropods were designed to eat vegetation higher than those enjoyed by the low-grazing stegosaurs of the Jurassic and the litany of ornithopods and armored herbivores of the Cretaceous; perhaps competition between sauropods to dominate this niche prodded them to greater and greater sizes. Did they grow bigger as a defensive mechanism? As a general rule, the larger an animal is, the better it can defend itself. Full-grown African elephants are virtually invulnerable to predation by lions solely because of their size. The Mesozoic saw predators that far outweighed even the swarthiest of African lions, and sauropods dwarf even them. The sheer mass of a Diplodocus meant that a single blow from its thumb claw or whip-like tail could be lethal. We see sauropod ‘bigness’ early on in the Jurassic as they co-existed with carnivores that were also growing bigger; as predators got larger, sauropods would’ve responded with increasing their own size, as well. Because sauropods grew to such larger-than-life proportions, the majority of dinosaur scientists doubt sauropods were endothermic (or warm-blooded); rather, they may well have been homeothermic. If this is the case, then the heat absorbed by their massive bodies, supplemented by the heat produced by their own metabolism, would have been enough to mimic warm-bloodedness.

Why sauropods became big is one thing; how they managed it is quite another, and scientists are beginning to piece together how these titans accomplished such sizes. In most derived sauropods, vertebra of the neck and backbone was hollowed out into a complex of thin-walled struts and processes used to anchor muscles while minimizing overall mass. The backbones were ‘carved’ by specialized bone-eating cells, resulting in air sacks that attached to the lungs. These ‘pleurocels’ may have been components of ‘pneumatic foramina’, a unidirectional style of breathing employed by birds today. The end result is that the backbone was made lighter, enabling the sauropods to go about their lives without being crushed beneath the weight of their bones. Their necks, too, are marvels of engineering. A system of girders and air pockets maximized lightness and strength. Their vertebrae had Y-shaped neural arches that supported an elastic bundle of connective tissue called the nuchal ligament. This ligament ran down the sauropod’s back, giving support to the neck and tail so that the muscles didn’t have to do all the work.

a Camarasaurus family

One of the defining traits of sauropods has to do with the number of vertebrae in their neck and tail. During the evolution of the sauropods, vertebrae from the trunk were converted into neck vertebrae, and up to six new vertebrae supplemented the head and neck. In some sauropod species, the neck vertebrae elongated, reaching up to three feet long. Some of the earliest sauropods had forty-four tail vertebrae; Camarasaurus had fifty-three, and Diplodocus had over eighty tail vertebrae. Forty of these vertebrae had rod-like structures that would’ve formed a whip-lash at the end of the tail. It’s been estimated that the whip-cracking tail of a diplodocid could have broken the sound barrier, resulting in thunderous claps echoing through the Jurassic woodlands. Because of the frailty of these ‘whip-fashioned’ vertebrae, some scientists question whether it could actually be used as a defensive weapon; they argue that if it were ‘whipped’ into an attacker, the force of the blow would damage the vertebrae. In case of fact, broken and healed tail vertebrae are rather common among the diplodocids, indicating they suffered damage on a regular basis. It could be that their primary ‘mode of defense’ was frightening predators with supersonic whiplashes, and if that didn’t work, they could then grit their teeth and suffer the pain of beating predators with their tails.

How sauropods carried their long necks is a matter of debate. Although older reconstructions portray sauropods with their necks raised high in the air like supersized giraffes, this was likely physiologically impossible for the majority of sauropods. A giraffe-like posture would’ve strained the heart and lungs, making the sauropods black out. Giraffe necks of the tallest specimens reach up to eighteen feet off the ground; Sauroposeidon could hypothetically stretch its neck up to sixty feet in the air. The physical ramifications of elevating such a neck are hard to fathom. It’s far more likely that sauropods tended to keep their necks held horizontally off the ground, feeding on hip- to shoulder-height vegetation. Ornithischian dinosaurs dominated this niche in the northern Cretaceous ecosystems, and their superior dental abilities may have been a factor in edging the northern sauropods to the sidelines. A minority of scientists argue that sauropods could hold their necks in a giraffe-like posture; indeed, Brachiosaurus seems especially designed to do this. Because Brachiosaurus’ neck stretched twenty-five feet higher than its heart, it would’ve needed a monstrous heart capable of pumping blood that far against gravity (recent estimates put the required heart on the order of eight hundred pounds). Some scientists have proposed that sauropods had a four-chambered double-pump heart, with one pump for oxygenated blood and the other for deoxygenated blood. Others have gone even further and speculated that sauropods had multiple hearts, creating an ‘assembly line’ of pumped blood to the brain. Because heart tissue rarely fossilizes, we are left to guess at the nature of the Brachiosaurus heart. How the tiny blood vessels in the brain would’ve withstood the pressure of such a heart is unknown. New research implies that some sauropods, such as Diplodocus, may have reared back on their hind legs and used their tails as a third leg. This tripodal posture would’ve been helpful in reaching higher, out-of-reach vegetation. Never minding that such a stance would still require a titanic heart to prevent blackouts, these sauropods seem built for employing such a technique. By anatomically shifting their center of gravity rearward, they reduced the torque effects of rearing up.

sauropods take necking to a whole
new level

The reason sauropods evolved such ridiculously long necks is debated (and, let’s be honest, they are quite ridiculous; what can you do with a sixty-foot neck that you can’t do with a twenty-foot one?). It is clear that evolution favored sauropods with long necks; what we don’t know is why. Of the likeliest theories, the first focuses on function and the second on reproduction. If the long necks of sauropods evolved for reaching higher, out-of-reach foliage, then evolution would have favored those sauropods with longer and longer necks. However, long necks pose all sort of mechanical problems, especially if they’re elevated (as they would need to be to reach higher foliage). Since it’s likely that most sauropods kept their necks horizontal to the ground to feed on lower foliage, a long neck wouldn’t necessarily be advantageous; indeed, all the extra bells and whistles needed to keep it functioning seem a bit over-the-top. However, the ‘driving force’ behind long necks may not have been functional at all; the long necks could be a result of sexual selection. Natural selection focuses on those traits that are most beneficial; sexual selection focuses on those traits that are most desirable. Because genes are passed down sexually, what a mate thinks about a partner is just as critical as to how that partner functions. A possible mate may be the fittest of his class, but if he’s an ugly duckling, no one’s going to be jostling for him right out of the gate. Many of the bizarre attributes seen in modern and prehistoric animals are designed to attract mates. In the realm of the Dinosauria, the ornamentation of ceratopsian skulls and the outlandish horns of the hadrosaurs may have evolved because those forms were preferred by possible mates. It’s thus possible that sauropod necks grew longer and longer not because they functioned better but simply because they looked better. It may simply be that long necks were sexy.

Because the necks were so long, sauropods needed to have small, lightweight heads that could be amply supported by the neck. Because the first neck vertebra (called the atlas) had a weak connection to the skull, sauropod heads tended to detach after death. Most sauropod skeletons are found sans heads, but the heads we do find tend to fall into two distinct categories: short skulls and blunt snouts (seen in the camarasaurs and brachiosaurs) or slender, elongated skulls with peg-like teeth reserved for the front of the jaws (seen in the diplodocids and a number of titanosaurs). Diplodocids and titanosaurs often had nostrils retracted to a position atop the skull; scientists have puzzled out different theories as to why the nostrils were positioned this way. Were the nostrils on the top of the skull for underwater breathing? Or was it designed to make noises? Of the three theories, nose flutes make the most sense. Some hadrosaurs utilized nose flutes, and we have reason to believe some sauropods—like Camarasaurus and Brachiosaurus—may have done this, too. An odd theory was that the retracted nares opened up to an elephant-like trunk, but in 2006 a study found that there was no evidence for a trunk. In animals with trunks, such as elephants, the facial nerve is grossly enlarged; but in diplodocids, it is very small. Thus this ‘trunk’ theory has fallen out of vogue.

some sauropods were high-browsers; others were low-browsers

No sauropods are suspected of being carnivorous, and no one expects that to change (but since when did that amount to anything?). Sauropods seem to be dyed-in-the-wool herbivores. Their teeth weren’t inset, unlike their ornithischian counterparts. Sauropod teeth had simple crowns and tended to fall into two design types. The first design employed chunky, spoon-shaped teeth lining most of the jaws. These teeth were built for chewing, and the wear on camarasaur teeth suggests a diet of course vegetation. The second design – seen most vividly in the diplodocids – uses slim, pencil-shaped teeth restricted to the front end of the jaws. Using this design, Diplodocus would slice vegetation and swallow it without chewing. The ‘chewing’ would’ve been accomplished with gastric mills. Sauropod ‘guts’ probably had large fermentation chambers filled with bacteria that helped break down tough plant material. Some may have utilized gastroliths – stones that were swallowed to ‘chew up’ food in the gastric mills – but scientists aren’t sure; while there’s some evidence of the use of gastroliths in some genera, one would expect a lot more to be found, given the propensity of these large dinosaurs for fossilization.

Turn-of-the-century sauropod paintings depicted them wading through the swamp lands shoulder-deep and feasting on soft aquatic vegetation, and it was assumed that sauropods were semi-aquatic. It was thought that sauropods spent most of their time in the water, coming out on dry ground only to migrate or lay eggs. Now we know that sauropods flourished in every sort of terrestrial ecosystem, from deserts to coastlines to river floodplains. The consensus of sauropod habitats shifted away from water to solid ground. Some scientists went so far as to argue that sauropods avoided deep water simply because the weight of the water against their lungs would be suffocating. For a long while the consensus was that sauropods would suffocate if they were fully submerged (unless they had muscles strong enough to open the lungs against the weight of the surrounding water), but in 2004 scientists argued that sauropods couldn’t submerge even if they wanted to, because the air sacs carved into their bones would’ve made them buoyant. Many sauropod track-ways preserve only the manus (forefeet) and no tail drag marks; perhaps this is evidence of floating sauropods using their fore-limbs to propel themselves forward? Sauropod track-ways are common among ancient coastlines, and their fossils have been mingled with marine organisms. Though sauropods may have been able to float in deep water, they would have been ungainly and awkward; it’s far more likely that sauropods lived a terrestrial life with marine excursions happening only in the most dire of circumstances (to cross a river or, perhaps, to flee a predator).

a herd of Brachiosaurus

Sauropods seem to have been gregarious in nature, socializing in large herds or family groups. Evidence for this is found in the numerous ‘sauropod graveyards.’ Large groups of sauropods likely wreaked havoc on local ecosystems, either by eating all the vegetation they could or by trampling it to mush; thus these herds were likely migratory, cutting wide swathes across the landscape in seasonal patterns that followed the vegetation. Some sauropods that aren’t as numerous in the fossil record may have lived a more solitary existence, and they would’ve been able to live in a single ecosystem for most of their lives. Most likely some sauropods traveled in massive ‘clan-like’ herds whilst others traveled in smaller ‘family herds.’ Some bone-beds from the Middle Jurassic of Argentina contain herds composed of individuals of various age groups, from juveniles to adults; however, other fossil sites and trackways indicate age-segregated herds in which juveniles – perhaps all from a cluster of nests – lived and traveled and died together up into old age. Such segregated herding strategies have been found in species of Alamosaurus, Bellusaurus, and numerous diplodocids. Such age-segregation may be explained by the fact that, according to studies of microscopic tooth wear, juveniles had diets that differed from adults; herding together would not have been as productive as herding separately.

a Diplodocus uses its tail as a whip against a pack of theropods

The biggest defense sauropods had against predators was their size. Even though theropod predators grew larger in the ‘arms race’ with their sauropod counterparts, sauropods were still fifty to three hundred percent larger than their contemporary predators. By living in herds, healthy sauropods were likely nearly invincible. If a healthy, adult sauropod was caught ‘out in the open,’ away from the protection of the herd, it could use its large thumb claws and hind claws to thrash and stab, and its tail could be used as a whip-like battering ram. Later sauropods, particularly of the titanosaur variety of the Cretaceous, had a pavement of osteoderms on their backs that could serve as an extra layer of defense (though it’s unlikely that their primary purpose was defensive in nature). Many sauropods had whip-like tails; in the diplodocids, for example, the final ten feet of the tail had slender bony rods in the core; when these sauropods swung their muscled tails, the whiplash effect could inflict crippling wounds on an unwary predator (though, as has been aforementioned, such actions could damage the tail vertebrae). Other sauropods developed particular defensive strategies: Agustinia evolved a spined back, and some genera like Shunosaurus had small clubs on their tails. Some of the ‘armored’ titanosaurs, such as Saltasaurus, had small bony protrusions on portions of their bodies that would protect against bites and claw stretches. Nevertheless, the sauropod’s greatest asset in a fight was its behemoth size. Predators likely hunted the aged, young, sick, and wounded animals unable to defend themselves – and there would be delicious windfalls as sauropods died from accident, disease, or old age, and their corpses became a feast for scavengers. It’s likely that the majority of sauropods were killed by predators before reaching adulthood; even with rapid growth rates, sauropods emerged as small hatchlings and had to survive ten to twenty years before reaching the behemoth sizes of adulthood. Only then could they breathe and enjoy a stroll through the Mesozoic ecosystem without having to pay much mind to prowling theropods.

the titanosaur Agustinia took body armor to a whole new level

Sauropods, like all dinosaurs, reproduced by laying eggs (though it was believed by many, up until the late 1990s, that sauropods gave birth to live young!). The first sauropod ‘nesting ground’ was discovered in Patagonia in 1997. This nesting ground covered a square kilometer, and tens of thousands of large, un-hatched eggs were preserved. The eggs were uncovered and organized into clusters of fifteen to thirty-four, and they were layered four deep. The eggs were linearly-paired, and some eggs contained fossilized embryos with impressions of embryonic skin. No adult specimens were found, which may mean that the eggs were left abandoned; however, more likely the adults guarded the eggs along the periphery. It would be difficult for such large creatures to navigate the interior of the nesting site without crushing eggs underfoot. More nesting sites have been discovered in France, Mongolia, and India. Bone studies indicate that most sauropods reached maturity at twenty years, in contrast to the old view that maturity was reached around sixty years with healthy adults living up to two- to three-hundred years (most scientists now believe the healthiest sauropods lived to about a century before succumbing to the diseases of old age). A 2014 study suggested that the time between the laying of the egg and the hatching of the egg was sixty-five to eighty-two days.

a Saltasaurus hatchling takes its first screeching breath

~  Taxonomy  ~

The sauropod ‘family tree’ below is based off the theory that the sauropods didn’t evolve from the prosauropods but were a ‘sister taxon’ to the prosauropods. A cursory glance reveals that sauropods emerged in the Early Jurassic (though some postulate that they emerged in the Late Triassic), diversified like wildfire during the Middle and Late Jurassic, survived ‘in strength’ into the Cretaceous, but started to wane by the Middle Cretaceous. The last branch of sauropods to survive up to the Cretaceous-Tertiary Extinction were the Titanosaurs, which emerged either in the Late Jurassic or Early Cretaceous (depending upon whom you talk to). An analysis of the different clades and family groups is given below. Clades are given in bold all-caps; family groups are given in bold italics.

The clade GRAVISAURIA includes the clade EUSAUROPODA and the family ground of Vulcanodontidae. The Vulcanodontidae are some of the earliest sauropods, clearly distinguished from earlier sauropodomorphs but still ‘primitive’ in that they lacked more derived sauropod characteristics. Examples include Vulcanodon and Tazoudasaurus. The EUSAUROPODA are the ‘true sauropods,’ and this includes all sauropods except the vulcanodontidae and some outliers. The ‘true sauropods’ emerged in the Early Jurassic and survived until the Late Cretaceous; the last sauropod family, the Titanosaurs, were the last off-shoots of the ‘true sauropods.’ The eusauropods are distinguished by anatomical characterists not present in the vulcanodontidae and some outliers. In eusauropoda we see a movement towards ‘bulk-browsing’ feeding in the developmental of lateral plates of tooth-bearing bones. These plates were used to strip foliage. The eusauropod’s ‘U-shaped’ jaws created a wide bite, and their loss of ‘fleshy cheeks’ - seen in more basal sauropodomorphs – increased their gape. The crowns of eusauropod teeth also have wrinkled textures on the enamel. Eusauropoda subdivides into the family groups Mamenchisauridae and Cetiosauridae and to the NEOSAUROPODA clade.

Mamenchisauridae includes several extremely long-necked ‘true sauropods,’ such as Mamenchisaurus, Datousaurus, and Omeisaurus. They emerged in the Early Jurassic and, along with the cetiosaurs, were essential in squeezing out the prosauropods. Though the mamenchisaurs lasted until the Early Cretaceous, their heyday was in the Early Jurassic; throughout the rest of the Jurassic and the Cretaceous, they suffered the same fate they’d thrust upon the prosauropods. Their Cetiosauridae cousins emerged slightly later, in the Middle Jurassic, and they generally replaced the prosauropods already put to flight by the mamenchisaurs. The cetiosaurs include the sauropods Cetiosaurus, Patagosaurus, and Barapasaurus. An off-shooting cetiosaur branch includes some basal sauropods reminiscent of later diplodocids, and Shunosaurus may belong to an off-shoot of this family group.

the Late Jurassic Mamenchisaurus

The NEOSAUROPODA clade were the ‘new sauropods’ that lived from the Early Jurassic to the Late Cretaceous. This clade contains most of the sauropod genera. They diverged from Eusauropoda in the Early Jurassic and quickly became the dominant sauropod lineage. All neosauropods shared at least thirteen derived characteristics. Two examples will suffice: first, all neosauropods have a large opening in the skull, known as the preantorbital fenestrae; this opening is differently shaped among various neosauropod genera, and it is practically closed up in adult camarasaurs, but otherwise it is ubiquitous among this clade. Second, all neosauropods lack denticles on most of their teeth (though in the camarasaurs and brachiosaurs, they are retained on the most posterior teeth). Neosauropoda has a few outlying genera, and then it subdivides into DIPLODOCOIDEA and MACRONARIA. During the Jurassic Period, most neosauropods belonged to the diplodocids and brachiosaurs (the latter of which is a subgroup of Macronaria); but by the Cretaceous, they were outdone by the emergent titanosaurs. By the Late Cretaceous, titanosaurs were the dominant group of neosauropods, particularly on the southern continents. In North America and Asia, their role as dominant herbivores was supplanted by hadrosaurs and ceratopsians, though they remained in smaller numbers all the way to the Cretaceous-Tertiary Extinction.

the Late Jurassic Supersaurus

Our examination of neosauropoda will begin with the DIPLODOCOIDEA. Members of this clade lived from the Middle Jurassic to the Late Cretaceous. The apatosaurs lived in the Late Jurassic; the diplodocids lived from the Late Jurassic to the Early Cretaceous; and the rebbachisaurids lived during the Late Cretaceous. Diplodocoidea includes some of the longest animals of all time, including slender giants like Supersaurus, Diplodocus, Apatosaurus, and Amphicoelias. Most diplodocoideans had very long necks and long, whip-like tails (though one family, the dicraeosaurids, are the only sauropods known to have re-evolved a short neck, ostensibly as an adaption for low browsing). A study of snout shape and dental war in diplodocoideans showed that the square snouts, large proportion of pits, and fine sub-parallel scratches in Apatosaurus, Diplodocus, Nigersaurus, and Rebbachisaurus indicate ground-height nonselective browsing; in contrast, the narrow snouts of Dicraeosaurus, Suuwassea, and Tornieria – not to mention the coarse scratches and gouges on the teeth of Dicraeosaurus – indicate mid-height selective browsing. Diplodocoideans had the highest tooth replacement of any vertebrates. This clade subdivides into two families and one clade; the families are Rebbachisauridae and Dicraeosauridae, and the clade is DIPLODOCIDAE (though it should be mentioned that the dicraeosaurs and diplodocids are subdivisions of Flagellicaudata, a clade formed by the Dicraeosauridae family and the Diplodocidae clade as separate from the Rebbachisauridae family).

the Middle Cretaceous Nigersaurus

The Rebbachisauridae outlived the rest of the Diplodocoidea, with fossils dating only from the Late Cretaceous (though we can surmise that they were around during the Jurassic; we just haven’t found them yet). The rebbachisaurids lacked the divided cervical neural spines that characterize the diplodocids and dicraeosaurids, and thus they’re considered more primitive. They may or may not have had the distinctive whip-like tail of the other diplodocoideans. Their distinctive teeth had low angles with internal wear facets and asymmetrical enamel. Some rebbachisaurids – such as Nigersaurus – had tooth batteries similar to those of hadrosaur and ceratopsians dinosaurs, a feeding adaptation that evolved independently in all three lineages. Other rebbachisaurids include Zapalasaurus and Limaysaurus.

a herd of the Early Cretaceous Amargasaurus

The Dicraeosauridae family lived from the Middle Jurassic to the Late Cretaceous, though only a few genera – such as Amargasaurus – survived into the Cretaceous. This sister group to the diplodocidae clade emerged from the diplodocoideans sometime in the Middle Jurassic, as evidenced by the diversity of dicraeosaurids in both South America and East Africa when Gondwana was still united by land. Dicraeosaurs have been found in Africa, South America, and North America, though the distribution of genera is primarily Gondwanan (Suuwassea is unique to North America). The dicraeosaurs are united by thirteen characteristics, such as relatively small body size, short necks, vertebrae without pleurocels (hollowed-out air cavities), and long neural spines. They were likely low browsers. Dicraeosaur examples include Dicraeosaurus, Amargasaurus, and Brachytrachelopan.

a pair of Late Jurassic Apatosaurus

The DIPLODOCIDAE clade is united by at least ten distinguishing features, and it is further subdivided into the Diplodocinae and Apatosaurinae families. The diplodocids were some of the largest creatures to ever walk the earth. Some, such as Supersaurus, reached over a hundred feet in length. These sauropods were extremely long but relatively slender, especially when compared to macronarians. They had short legs, and their rear legs were longer than their forelimbs, resulting in a distinctive downward slope towards the neck. Though their necks were very long, they may not have been able to raise them high into the air like other sauropods such as Brachiosaurus. It’s postulated that their necks were kept horizontal to the ground, where they browsed low vegetation by swinging their necks in a crescent-shaped pattern; they may stood on the shores of marshes and reached their necks out over the water to eat plant growth. They may have been able to raise their necks just high enough to feed on conifers. Their heads were tiny with the nasal openings on top of the head, though in life the nostrils would’ve been close to the tip of the snout. Their long, whip-like tails were thick at the base but tapered off to be very thin at the end. Computer simulations have revealed that diplodocids could have snapped their tails like a bullwhip, and doing so could generate a sonic boom over two hundred decibels. They may have done this in mating displays, to communicate with other animals in the herd, or to ward off predators. Many diplodocids have been discovered with fused or damaged tail vertebrae; the damage may have been incurred by repeated whip-like snapping or by using their tails as whips against attacking predators. Diplodocid teeth were present only at the front of the mouth, and they looked like pencils or pegs. The tooth crowns are long and slender, and elliptical in cross-section, while the apex forms a blunt, triangular point. They likely used their teeth to crop off food, without chewing, and relied on gizzard stones or enzyme vats in gastric mills to break down tough plant fibers. Some diplodocid fossils have preserved portions of the skin from around the tip of the tail; the skin preserves a sequence of conical spines, and larger spines have been found scattered around larger tail vertebrae. In life the spines were likely oriented in a single row along the mid-line of the tail, and this row may have continued over a diplodocids’ entire back and neck. The Diplodocinae family included Diplodocus, Supersaurus, and Barosaurus, and genera of this family emerged in the Late Triassic and survived into the Early Cretaceous; their remains have been found in Gondwana (South America) and northern Laurasia (North America and Europe). The Apatosaurinae emerged in the Late Jurassic but didn’t make it into the Cretaceous; Apatosaurs are distinguished from the more derived diplodocids by their massive limbs and shorter vertebrae. Only two apatosaurs are known, Apatosaurus from North America and another unnamed apatosaur from the same region.

the Late Cretaceous Argentinosaurus

The other subgroup of Neosauropoda is MACRONARIA. The Macronarians split from Diplodocoidea sometime in the Middle to Late Jurassic, and they thrived in the Late Jurassic and survived until the Cretaceous-Tertiary Extinction. They had a global distribution and were the dominant sauropods of the Cretaceous. Their name comes from the large diameter of their skull’s nasal opening, called the external naris, which was larger than the orbit (where the eye was located); hence the name ‘large nose.’ Macronarians had several derived characteristics, including robust, spoon-like (spatulate), and broad-crowned teeth; head crests formed by a large protruding nose; and their forelimbs were relatively long in comparison to the hind limbs (this trait is exaggerated by the brachiosaurs). Macronarians weren’t exclusively large-bodied, however; this group shows a wide divergence in body size: Argentinosaurus reached up to fifty tons while Saltasaurus only reached up to three tons. Fossil trackways and bone-beds indicate that macronarians preferred dry, terrestrial environments located near waterways such as rivers and lakes. Unlike the diplodocoideans, the macronarians tended towards upward-oriented necks for browsing trees and taller plants. Macronaria is subdivided into the Camarasauridae family and the TITANOSAURIFORMES clade. The titanosauriformes are themselves subdivided into the Brachiosauridae family and the SOMPHOSPONDYLI clade. The somphospondyli clade is subdivided into the Euhelopodidae family and the TITANOSAURIA clade.

the Late Jurassic Camarasaurus

Camarasauridae was the sister group to the titanosauriformes clade. These sauropods had limited distribution in Late Jurassic northern Laurasia (North America and Europe); a possible camarasaur has been identified in North Africa of northeastern Gondwana, but the jury is out if it’s legitimate. The camarasaurs were small- to medium-sized with relatively short necks. They had short skulls with large nares, blunt and bulldog-like muzzles, and broad, spoon-shaped teeth set in thick jaws. They likely moved their necks in a vertical rather than horizontal sweeping motion, in contrast to most diplodocoideans. The main staple of the camarasaurs is their namesake Camarasaurus, which was the most common sauropod of the Morrison Formation of Late Jurassic North America.

the Late Jurassic Giraffatitan

Brachiosauridae was a subdivision of the titanosauriformes clade and sister taxon to the somphospondyli clade. Brachiosaurs lived from the Late Jurassic to the Early Cretaceous. They were widely successful with worldwide distribution. Their global distribution at the end of the Jurassic indicates they diversified prior to the severing of the northern land bridges between Laurasia and Gondwana. Brachiosaurs flourished in the Late Jurassic but winnowed down in the Early Cretaceous before disappearing altogether. Brachiosaurs had long necks that enabled them to access the leaves of tall trees that low-browsing sauropods couldn’t reach. Their spoon-shaped teeth helped them consume tough plants more efficiently than their pencil-toothed contemporaries. Their spatulate-shaped teeth could slice through food rather than simply stripping it off branches. The characteristic long necks of brachiosaurs are distinct from other sauropods: their narrow necks were composed of twelve to thirteen extremely long cervical vertebrae that were flexible so as to enable sauropods to angle their necks up and lift their heads towards the trees. The tallest could reach up to a height of forty-five feet off the ground. Brachiosaur forelimbs – like those of all macronarians – were long relative to the forelimbs, but they took this trait to the next level. Their slender fore-limb bones combined with elongated metacarpals gave brachiosaurs an awkward gait. Scientists believe that they had the ability to rear up on their hind-limbs – in which case the largest brachiosaurs could reach foliage sixty feet high! – but their leg structure made it impossible for them to row. It’s been estimated that their walking pace was just twelve to twenty-four miles a day, though they could hustle up to eighteen miles per hour when needed. Brachiosaur examples include Brachiosaurus, Giraffatitan, and Cedarsaurus. Europasaurus was a midget brachiosaur that reached only twenty feet long snout-to-tail; as it lived on an island in modern Germany, this was likely due to insular dwarfism.

the Late Cretaceous Alamosaurus

The Euhelopodidae was a subdivision of somphospondyli and a sister taxon to the Titanosauria clade. All Euhelopodids lived in East Asia of southern Laurasia and are represented by Euhelopus, Erketu, and Daxiatitan. The other subdivision of somphospondyli, TITANOSAURIA, was wildly diverse with global distribution. Titanosaurs have been discovered in Africa, Asia, South America, North America, Europe, Australia, and Antarctica. These sauropods emerged during the Cretaceous (or super late in the Late Jurassic, depending on who you talk to) and were common in Gondwana, though at least one titanosaur, Alamosaurus, made up north into North America. The titanosaurs are named after the Titans, the Greek deities who preceded the Twelve Olympians, and they’ve earned this title. They were the last surviving group of sauropods, with several genera thriving at the Cretaceous-Tertiary Extinction Event. The titanosaurs include the largest land animals known to have ever existed, such as Patagotitan, estimated at 121 feet long and a weight of seventy-six tons. 

the Early Cretaceous Patagotitan compared to an average-sized man

Titanosaurs have long been a poorly-known group, and the relationships between titanosaur genera are poorly understood. Because of this, taxonomic relationships are hotly debated, and new cladograms for the clade come out every six months or so. Titanosaurs had the wide skulls of macronarians, but their skulls were smaller when compared to other sauropods. Skull variation was common: Sarmientosaurus had a skull like the brachiosaurids; Rapetosaurus had a head that resembled the diplodocids; and Antarctosaurus had a head similar to the rebacchisaurid Nigersaurus.  Titanosaurs had the large nostrils and head crests of the macronarians, but their teeth had wide variation: some retained the spoon-like macronarian teeth, whereas others had pencil-like teeth like the diplodocids; in either case, titanosaur teeth were small. Titanosaurs also varied in size: while many were of average sauropod size, others were enormous – like the aforementioned Patagotitan – and others were small, such as the island-dweller Magyarsaurus at twenty feet long snout-to-tail. Titanosaurs had smaller pelvises than most sauropods, but their chest area was much wider, giving them a uniquely ‘wide-legged’ stance. They had stocky forelimbs that were longer than their hind limbs. Strangely enough, some titanosaurs had no fingers or toes on their feet, so that they walked on horseshoe-shaped ‘stumps’ made up of the columnar metacarpal bones. Their vertebrae weren’t hollowed-out by pleurocels, which may be a reversion to more basal saurischians characteristics. Their spinal column was relatively flexible, making them more agile than other sauropods and likely able to rear up onto their hind legs. A distinguish trait was the ball-and-socket articulations between the vertebral centra in their proceolous caudal vertebrae. Titanosaur examples include Aegyptosaurus, Kaijutitan, Dreadnoughtus, and Saltasaurus. 

the armored titanosaur Saltasaurus

Some of the later titanosaurs had peculiar skin armored with a mosaic of small, bead-like scales surrounded by larger scales. The precise arrangement of osteoderms on a titanosaur’s body is a matter of guesswork, but most paleontologists believe the osteoderms were arranged in two parallel rows along the sauropod’s back. It’s likely that different species had different arrangements. This armor was nowhere near as heavy-duty as that found in the ankylosaurs, and the armor didn’t completely cover the sauropod’s back in scutes. Because of this sparse arrangement, it’s unlikely they were meant for defense. More likely they were used for sexual display, species recognition, or perhaps for storing nutrients for the sauropods that lived in highly seasonal climates or for females storing nutrients for when they laid eggs. These ‘armored sauropods’ were the last of the sauropod lineage, for they died out 65 million years ago during the Cretaceous-Tertiary Extinction – and creatures of their size have not been seen since.