Sauropods: Triumph of the Titans
Sauropoda or the sauropods (“lizard-footed”), are an infraorder of saurischian (“lizard-hipped”) dinosaurs. They had very long necks, long tails, small heads (relative to the rest of their body), and four thick, pillar-like legs. They are notable for the enormous sizes attained by some species, and the group includes the largest animals to have ever lived on land. Well-known genera include Brachiosaurus, Diplodocus, Apatosaurus and Brontosaurus.
Sauropods first appeared in the late Triassic Period, where they somewhat resembled the closely related (and possibly ancestral) group “Prosauropoda”. By the Late Jurassic (150 million years ago), sauropods had become widespread (especially the diplodocids and brachiosaurids). By the Late Cretaceous, those groups had mainly been replaced by the titanosaurs, which had a near-global distribution. However, as with all other non-avian dinosaurs alive at the time, the titanosaurs died out in the Cretaceous–Paleogene extinction event. Fossilised remains of sauropods have been found on every continent, including Antarctica.
The name Sauropoda was coined by O.C. Marsh in 1878, and is derived from Greek, meaning “lizard foot”. Sauropods are one of the most recognizable groups of dinosaurs, and have become a fixture in popular culture due to their large sizes.
Complete sauropod fossil finds are rare. Many species, especially the largest, are known only from isolated and disarticulated bones. Many near-complete specimens lack heads, tail tips and limbs.
As with any broad definition, though, there are some important “buts” and “howevers.” Not all sauropods had long necks (witness the oddly truncated Brachytrachelopan), and not all were the size of houses (one recently discovered genus, Europasaurus, seems to have only been about the size of a large ox). On the whole, though, most of the classical sauropods–familiar beasts like Diplodocus and Apatosaurus (the dinosaur previously known as Brontosaurus)–followed the sauropod body plan to the Mesozoic letter.
Sauropods were herbivorous (plant-eating), usually quite long-necked quadrupeds (four-legged), often with spatulate (spatula-shaped: broad at the tip, narrow at the neck) teeth. They had tiny heads, massive bodies, and most had long tails. Their hind legs were thick, straight, and powerful, ending in club-like feet with five toes, though only the inner three (or in some cases four) bore claws. Their forelimbs were rather more slender and ended in pillar-like hands built for supporting weight; only the thumb bore a claw. Many illustrations of sauropods in the flesh miss these facts, inaccurately depicting sauropods with hooves capping the claw-less digits of the feet, or multiple claws or hooves on the hands. The proximal caudal vertebrae are extremely diagnostic for sauropods.
Sauropod necks have been found at over 50 feet in length, a full six times longer than the world record giraffe. Enabling this were a number of essential physiological features. The dinosaurs’ overall large body size and quadrupedal stance provided a stable base to support the neck, and the head was evolved to be very small and light, losing the ability to orally process food. By reducing their heads to simple harvesting tools that got the plants into the body, the sauropods needed less power to lift their heads, and thus were able to develop necks with less dense muscle and connective tissue. This drastically reduced the overall mass of the neck, enabling further elongation.
Sauropods also had a great number of adaptations in their skeletal structure. Some sauropods had as many as 19 cervical vertebrae, whereas almost all mammals are limited to only seven. Additionally, each vertebra was extremely long and had a number of empty spaces in them which would have been filled only with air. An air-sac system connected to the spaces not only lightened the long necks, but effectively increased the airflow through the trachea, helping the creatures to breathe in enough air. By evolving vertebrae consisting of 60% air, the sauropods were able to minimize the amount of dense, heavy bone without sacrificing the ability to take sufficiently large breaths to fuel the entire body with oxygen. According to Kent Stevens, computer-modeled reconstructions of the skeletons made from the vertebrae indicate that sauropod necks were capable of sweeping out large feeding areas without needing to move their bodies, but were unable to be retracted to a position much above the shoulders for exploring the area or reaching higher.
When sauropods were first discovered, their immense size led many scientists to compare them with modern-day whales. Most studies in the 19th and early 20th centuries concluded that sauropods were too large to have supported their weight on land, and therefore that they must have been mainly aquatic. Most life restorations of sauropods in art through the first three quarters of the 20th century depicted them fully or partially immersed in water. This early notion was cast in doubt beginning in the 1950s, when a study by Kermack (1951) demonstrated that, if the animal were submerged in several metres of water, the pressure would be enough to fatally collapse the lungs and airway. However, this and other early studies of sauropod ecology were flawed in that they ignored a substantial body of evidence that the bodies of sauropods were heavily permeated with air sacs. In 1878, paleontologist E.D. Cope had even referred to these structures as “floats”.
In a study published in Plos One on October 30, 2013, by Bill Sellers, Rodolfo Coria, Lee Margetts et al., Argentinosaurus was digitally reconstructed to test its locomotion for the first time. Before the study, the most common way of estimating speed was through studying bone histology and ichnology. Commonly, studies about sauropod bone histology and speed focus on the postcranial skeleton, which holds many unique features, such as an enlarged process on the ulna, a wide lobe on the ilia, an inward-slanting top third of the femur, and an extremely ovoid femur shaft. Those features are useful when attempting to explain trackway patterns of graviportal animals. When studying ichnology to calculate sauropod speed, there are a few problems, such as only providing estimates for certain gaits because of preservation bias, and being subject to many more accuracy problems.
To estimate the gait and speed of Argentinosaurus, the study performed a musculoskeletal analysis. The only previous musculoskeletal analysises were conducted on homonoids, terror birds, and other dinosaurs. Before they could conduct the analysis, the team had to create a digital skeleton of the animal in question, show where there would be muscle layering, locate the muscles and joints, and finally find the muscle properties before finding the gait and speed. The results of the biomechanics study revealed that Argentinosaurus was mechanically competent at a top speed of 2 m/s (5 mph) given the great weight of the animal and the strain that its joints were capable of bearing. The results further revealed that much larger terrestrial vertebrates might be possible, but would require significant body remodeling and possible sufficient behavioral change to prevent joint collapse.
Sauropods are gigantic, and descendants of surprisingly small ancestors. Basal dinosauriformes, such as Pseudolagosuchu sand Marasuchus from the Middle Triassic of Argentina, weighed approximately 1 kg (2.2 lb) or, in most cases, less. At the evolutionary point named Saurischia, a rapid increase of bauplan size appeared, although more primitive members like Eoraptor, Panphagia, Pantydraco, Saturnalia and Guaibasaurus still retain a moderate size, possibly even less than 10 kg (22 lb). Even with these small, primitive forms, there is a notable size growth in sauropodomorphs, although scanty remains of this period of sauropod evolution make assumptions necessary as the size is difficult to interpret. There is one definite example of a derived sauropodomorph being small however, and that is Anchisaurus, which reached under 50 kg (110 lb), even though it is closer to the sauropods than Plateosaurus and Riojasaurus, which were upwards of 1 t (0.98 long tons; 1.1 short tons) in weight.
Compared to even derived sauropodomorphs, sauropods were huge. Their even larger size probably resulted because of an increased growth rate, which appears to have been linked with tachymetabolic endothermy, a condition that evolved in sauropodomorphs. Once branched into sauropods, sauropodomorphs continued steadily to grow larger, with smaller sauropods, like the Early Jurassic Barapasaurus and Kotasaurus, evolving into even larger forms like the Middle Jurassic Mamenchisaurus and Patagosaurus. Following the size change of sauropods, theropods continued to grow even larger, shown by an Allosaurus-sized coelophysoid from Germany. As one possible explanation for the increased body size is less risk of predation, the size evolution of both sauropods and theropods are probably linked.