Sauropoda

Introduction

The largest of all dinosaurs, sauropods were the giants of the animal kingdom for many millions of years and are still the largest animals to have ever lived on land. The sauropods evolved into many varied genera and species during their 100 million years on Earth, and they have been found on every continent, with the exception of Antarctica.

The Jurassic is often considered the pinnacle for the highly successful and widely distributed sauropod herbivores such as Camarasaurus, Apatosaurus, Diplodocus, and Brachiosaurus, most of which were replaced by the armored titanosaurs in the Late Cretaceous. The last of the sauropods went extinct, along with all remaining non-avian dinosaurs, during the mass extinction event between the Cretaceous and the Paleogene periods, some 66 million years ago.

Fast Facts

Pronunciation: Sauropoda (pronounced saw-ROP-o-da), meaning “lizard footed”

Time Period: Late Triassic (235–201 million years ago) to Late Cretaceous (100–66 million years ago)

Size: 5–60 m (16–196 ft) in length

Weight: 10–120 metric tons (22,000–265,000 lbs)

Diet: Herbivorous

Location: All continents except for Antarctica

Lifespan: 50 years or more

Homologous Traits

Even by dinosaur standards, sauropods were gigantic—the largest species to ever roam the earth. All sauropod dinosaurs were quadrupedal herbivores, characterized by their exceptionally long necks and long tails. Although these dinosaurs had the longest necks of any land animal, they were comparatively light, as their bones were thin, and hollow and each vertebrate had a pair of overlapping spines underneath for support. In addition, elongated muscles connected to the central backbone helped keep the neck in the correct position.

Despite their impressive size, most sauropods had relatively small heads and brains and are considered to be at the lower end of dinosaur intelligence. What they may have lacked in intelligence, however, they made up for in other senses. The large nasal openings situated on the top of their heads suggests that sauropods had a strong sense of smell, which was no doubt useful in detecting possible predators and food sources. Their mouths were small and filled with robust spatulate (chisel-shaped) teeth, used to grind the rough fibrous vegetation that made up their diet.

The specialized limbs of sauropods were built to withstand their monumental weight, with each limb ending in wide weight-bearing club-like feet with five toes each (three of which were capped with claws). A number of the largest sauropods are notable for having longer forelimbs than hind limbs, an unusual anatomical trait among dinosaur species and thought to be an ecological adaption to more successfully compete for high-growing foliage.

Another interesting feature of all sauropod dinosaurs was pneumaticity, or air spaces within their bones, particularly evident in their neck vertebrae. This characteristic is thought to have evolved to reduce the overall weight of an animal's skeleton while maintaining strength.

Evolutionary Divergences

Some 125 years ago, British paleontologist Harry Seeley divided all dinosaurs into one of two orders, Ornithischia (or bird-hipped dinosaurs) and Saurischia (or lizard-hipped dinosaurs). All sauropod dinosaurs belong to the Saurischia order and are classified as herbivorous species, typified by their long neck and tails and small heads and brains.

The Saurischia order is comprised of two major groups—the large herbivorous sauropods and the theropods. The first true sauropods appeared in the fossil record as early as the Late Triassic (about 230 million years ago), and by the Middle Jurassic (170–165 million years ago), the sauropod dinosaurs had begun to diversify. Sauropods are an infraorder of Sauropodomorpha, a term originally used to incorporate the sauropods and their ancestors, the prosauropods. Sauropoda are comprised of a number of families, including the diplodocids (“double beams”) such as Diplodocus and Apatosaurus; the brachiosaurids (“arm lizards”) such as Brachiosaurus; the camarasaurids (“chambered lizards”) such as Camarasaurus; the titanosaurids (“titanic lizards”) such as Titanosaurus; the cetiosaurids (whale lizards) such as Cetiosaurus; and the euhelopodids, such as Mamenchisaurus.

The first true sauropods appeared more than 220 million years ago. Although phylogenetic debate continues among paleontologists because of the incomplete fossil record, sauropods are believed to have evolved from basal sauropodomorphs, with the early sauropods not unlike the earlier and smaller prosauropods, such as Anchisaurus and Massospondylus. Interestingly (and unique among dinosaur lineages), sauropods evolved their very large size within only a few million years. By the Middle Jurassic, sauropods had begun to diversify into a wide array of taxa, with some 137 different genera now described.

Generally, sauropods are grouped into species with longer forelimbs than hind limbs and those with longer hind limbs than forelimbs. In addition to this anatomical divergence, sauropods also showed niche divergence amongst themselves, as evidenced by differences in dentition and neck length.

While some smaller ornithischians probably evolved a warm-blooded, or endothermic, metabolism similar to mammals, large dinosaurs, such as sauropods, may have been more efficient as cold-blooded, or ectothermic, similar to most modern reptiles. Although research has increasingly supported the theory that dinosaurs were active and endothermic rather than sluggish creatures as originally imagined, there is still much debate over the issue, and it remains an active area of study. A new theory emerged in 2024 revealing that warm-blooded dinosaurs first roamed the planet earlier than previously reported—approximately 180 million years ago during the Early Jurassic period—which led to new information about dinosaurs' habits, including their probable distribution across the earth.

Creatures in This Group

Despite continued debate, general phylogenic consensus has determined that the Neosauropoda clade, which consists of the Diplodocoidea (Diplodocidae, Dicraeosauridae, and Rebbachisauridae) and Macronaria (Camarasauridae, Brachiosauridae, and Titanosauria), was derived from the basal (earliest) sauropods. Based on cladistics, the infraorder Sauropoda contains several nodes: Eusauropoda, Neoseuropoda, Camarasauromorpha, Titanosauriformes, Lithostrotia, and Saltasauridae. Differentiation among these nodes often has to do with skull shape and size, length of neck, placement of nares (nostrils or nasal passages), teeth formation, armor, and other, more complex anatomical differences. As new discoveries are made, placement of creatures may move between nodes, or uncertainties may arise about specific relationships.

Eusauropoda is an unranked clade within the infraorder Sauropoda containing larger sauropods with wide, rounded snouts, longer necks, and smaller heads. Eusauropods later developed longer necks and extra cervical vertebrae. The “true sauropods” include Shunosaurus (China), Jobaria (Africa), and Neosauropoda. A division of Eusauropoda, Neosauropoda are characterized by teeth gathered at the front of the snout, nostrils placed higher in the skull, and column-like metacarpals, among other, more complex features. This clade contains the family Diplodocidae (including diplodocids such as North American Apatosaurus, Barosaurus, and Diplodocus from the Late Jurassic and the Argentinean Amargasaurus, Mongolian Nemegtosaurus, and Moroccan Rebbachisaurus from the Cretaceous) and the subdivision Macronaria (including Atlasaurus and Bellusaurus).

Camarasauromorpha is an unranked clade generally considered the common ancestor of Camarasaurus and Saltasaurus and often considered synonymous with Macronaria. This clade contains the families Camarasauridae (including western United States Camarasaurus from the Late Jurassic) and Saltasauridae (including Saltasaurus in Argentina, from the Cretaceous).

Titanosauriformes are an unranked clade consisting of species considered the heaviest sauropods. Titanosauriformes includes the families Brachiosauridae (Brachiosaurus found in Africa, the western United States, and Portugal from the Late Jurassic) and Titanosauridae (Titanosaurus from the Late Cretaceous in India). The unranked clade Lithostrotia contains species having dermal armor and is thought by some to be included in the Titanosauriformes clade.

Ecology

Paleontological evidence from bone beds and trackways has suggested that many sauropods traveled in herds, a behavior often seen today in herbivore species as a means of protecting younger group members. More interesting, however, is that fossil evidence has indicated some sauropod species traveled in segregated herds, with juveniles and adults traveling in their own separate groups. Current resource partitioning theories have suggested that this behavior was in response to adult and juvenile sauropods expressing different feeding strategies.

The worldwide distribution of sauropods, together with their large size and dietary requirements, suggests that these species may have exhibited migratory behavior. As seen in today's largest migrating herbivores, researchers believe that sauropods consumed vast quantities of vegetation to survive and moved to new areas once their habitat was depleted of food resources. Such large animals also very likely influenced and shaped their surrounding environment and would have been instrumental in opening up forested landscapes, as is seen with modern-day elephant species.

Of particular interest and research focus is how a multitude of massive herbivores successfully lived within the same habitat despite competing for the same limited vegetation food sources. Fossilized teeth displaying different wear patterns, as well as neck length and body size differences, have suggested the possibility of resource partitioning, whereby different herbivores targeted different vegetation at different heights.

Paleontology News

Although the first sauropod fossil was discovered more than 170 years ago, as with all dinosaur species, revisions are an ongoing process as new fossil finds increase our understanding of sauropods and their place in the evolutionary tree.

Historically, sauropods were classified into four separate clades, namely the Diplodocidae, Brachiosauridae, Camarasauridae, and Titanosauridae, as well as the polyphyletic Cetiosauridae group. Although sauropod classification is, perhaps, often considered more stable than that of other groups, in-depth phylogenetic analyses have clarified much regarding sauropod structure and their evolutionary relationships, and today sauropods are generally grouped into six node-based clades (as seen above). It is believed this classification more adequately explains the relationships seen between the more derived Neosauropoda such as the diplodocids from the Late Jurassic, the brachiosaurids and camarasaurids from the Cretaceous, and the titanosaurids from the Late Cretaceous.