Were the largest herbivores ever to live and the largest land animals ever; by the end of the Late
Triassic had surpassed all previous land living animals in size, and kept on going.
The most primitive known sauropodomorph is Saturnalia of the Late Triassic of
Basal sauropodomorphs are often called "prosauropods", however, this is a paraphyletic grade leading to the Sauropoda. In addition to Saturnalia, it has at its base small bipedal forms such as Thecodontosaurus,
Efraasia, and Plateosauravus, followed by larger-bodied (3-10 m) long facultative
bipeds such as the well-known Massospondylus long considered to be able to switch between bipedality and quadrupedality.
Recent studies of preserved embryos show that as hatchlings, these creatures were distinctly quadrupedal.
Basal sauropodomorphs are characterized by:
Basal sauropodomorphs were the most common herbivorous dinosaurs from the Late Triassic to the Early Jurassic, but no basal sauropodomorph survived into the Middle Jurassic. They were the first large-bodied dinosaurs. Their long necks would allow them to browse higher in trees than any contemporaneous herbivores. Also, larger size would give them bigger guts to digest more plants and defense against predators.
- Originally small size (1.5-2 m long), but eventually reaching 10 m or more
- Small heads
- Long necks
- Femur longer than tibia, even in small forms (Graviportal or cursorial?)
Sauropoda (Late Triassic - Cretaceous)These three selective seem to have led to the evolution of the true Sauropoda. Primitive sauropods are known from the Late Triassic, but sauropods do not become common outside of the
southern continents until the Middle Jurassic.
Sauropods are characterized by:
- Extremely large size: all sauropods were at least elephant-sized as adults, and
many much, much larger
- Elongated forelimbs (60% or more hindlimb length)
- Obligate quadrupedality
- Bony nostrils placed at least as high dorsally as the eye sockets. Sauropods had no secondary palate. Arguably the displacement of the bony nostril served the same function as in phytosaurs - an adaptation to breathing while eating.
- Tooth-to-tooth occlusion for precise bites (Note: Teeth seem to be for cropping vegetation, not for processing it. Indeed, sauropods are often found with polished gastroliths - stomach stones used for grinding food inside the digestive tract.)
- Extra cervical vertebrae
- Columnar forelimbs with reduced fingers
Shunosaurus of the Middle Jurassic of China is a good example of an early sauropod.
The giant size of sauropods would allow them to feed even higher in trees, digest more plants, and serve as defense against ever-larger predators. (Some sauropods developed additional
defenses: Shunosaurus, for instance, had a tail club).
Evolutionary mechanism: Given what we know about the development of primitive sauropodomorphs, do sauropods, in their general body plan (not their size) resemble adults or juveniles of their ancestors? What general pattern of heterochrony seems to have been active int eh evolution of sauropods?
Sauropod diversity: From this general body plan, there developed a truly remarkable diversity and range of adaptations.
The derived sauropods of the Late Jurassic and the Cretaceous are characterized by:
Most derived sauropods fall into two distinct groups: Diplodocoidea and Macronaria.
Diplodocoids: (Middle Jurassic to early Late Cretaceous)
Additionally, some diplodocoids have:
- a battery of self-replacing teeth in the front of the mouth. A good example is the recently described Nigersaurus (a very small sauropod, no bigger than an African elephant.)
- Tails ending in a whip-lash of very narrow and long caudal vertebrae. Probably a weapon.
Macronarians ("big noses") (Middle Jurassic - end of Cretaceous) are characterized by:
Jobaria of the Early Cretaceous of Africa is a
typical primitive macronarian. The more advanced macronarians,
and are characterized by expanded snouts and nares placed on top of their
skulls. Within Macronaria, we see such specializations as:
- forelimbs longer than hindlimbs, i.e. "built uphill" to reach high into trees without having to rear up. (An extreme example is the well-known Brachiosaurus.
Locomotion: Since their discovery in the 19th century, the biomechanical interpretation of sauropods has been trouble. Just as paleontologists wondered how "reptilian" pterosaurs could fly, they wondered how gigantic "reptilian" sauropods could walk:
- Early restorations in the late 19th century showed the animals with elephantine, erect limbs.
- During the early 20th century, this interpretation was challenged by revisionists who argued that, as cold-blooded reptiles, they would have been unable to support their weight this way and would have to be sprawlers.
- This created obvious problems. This plus the discovery of narrow-guage sauropod trackways made it clear that they definitely walked with an erect stance, but the nagging question remained - how could a bradymetabolic cold-blooded animal marshall the energy to support its weight in that way.
- By mid twentieth century, the conventional wisdom was that sauropods were mostly aquatic, using bouyancy to support their bodies. (Cf. the dinosaur books of my childhood.)
- That also entailed problems. The big one: sauropod remains are more common in relatively dry upland depositional environments.
- The Dinosaur Renaissance of the 1970s and 1980s reframed the argument completely as the notion of dinosaur endothermy came into play. Were sauropods actually warm-blooded? Indeed, what did that term even imply for an animal that big? One product of the debate was the identification of gigantothermy as a composite thermoregulatory strategy. Indeed, it was noted that as ectothermic and endothermic animals scale up, their actual rates of metabolism converge, so that a leatherback turtle has roughtly 86% of the energy output off an elephant. Perhaps, for an animal as large as Brachiosaurus the debate was meaningless.
Feeding: One thing for sure. For sauropods to have the energy to move about like elephants, they would need to be eating constantly. How did they feed?
- This much is certain, they used their teeth to crop vegetation, but not to "chew" or process it. That apparently occurred in the gut, with the aid of gastroliths.
- Early reconstructions had them eating aquatic vegetation. As it became clear that they were really land animals, two hypotheses arose:
- Dinosaur giraffes: They used their long necks to reach foliage growing high in trees. The ability of primitive sauropodomorphs to do this during the Late Triassic when all other herbivores were stuck feeding close to the ground (E.G. aetosaurs) might explain their great success compared to other dinosaurs, who remained rare and small during the Triassic.
- Dinosaur vacuum-cleaners: They used their long necks to extend their reach to foliage growing near the ground, and could "hoover-up" ground plants without having to walk to them.
- The two major groups of derived sauropods definitely seemed to have had different feeding specializations:
- Macronarians were built "going uphill" so that their heads would naturally be elevated significantly, and their teeth could crop vegetation from the sides and front of the mouth - appropriate for working among tree branches.
- Diplodocoids were built so that their heads, in a neutral posture, would be close to the ground. Indeed, recent biomechanical modelling has shown that they couldn't elevate their heads significantly. Also, their teeth were crowded toward the front of the mouth - a useful adaptation for cropping plants growing low to the ground. The newly described Nigersaurus, it seems, must have done this habitually. Even the position of its horizontal semicircular canal indicates that its "alert posture" was nose-down.
- But wait! There is another area of significant anatomical difference between diplodocoids and macronarians - the sacrum:
- Macronarian sacral are "normal" in their proportions.
- Diplodocoid sacral are have extremely elongate neural spines.
What's up? The inevitable suggestion: Those spines gave the axial muscles leverage to enable diplodocoids to reach the tree tops by rearing up. Artists love this dramatic image, but it is possible that the tall vertebral spines are simply part of a mechanism for suspending long necks and tails from a rather short torso in the manner of a suspension bridge.
Blood pressure: If they reared up, sauropods somehow overcame a major biomechanical constraint - getting blood to their brains. This is a difficult task for creatures that routinely hold their heads significantly above their hearts. Consider the giraffe:
This creature uses a number of special adaptations including:
- Systolic blood pressure = 350 mm Hg (compare to 120 for a human)
- Heart weight = 11.3 kg
A typical sauropod lifting its head to a high elevation would require:
- tight skin to prevent pooling of blood in the neck when the head is elevated
- a specialized circulatory network at the base of the skull to prevent burst blood vessels when the head is depressed.
This constraint leasd some researchers to speculate that the prominent cavities and excavations in the sauropod neck vertebrae were occupied by accessory hearts. Of course, there is neither direct fossil evidence for this nor a modern analog. It is a type three inference of the extant phylogenetic bracket. What else might have filled those cavities?
- Systolic blood pressure = 880 mm Hg
- Heart weight = 1.6 tons
Breathing: We've noted the enlargement and retraction of the bony nostrils toward the top of the skull. Even something as simple as this stimulates endless speculation about the configuration of the fleshy nostril:
In considering the internal respiratory system, things get quite strange. This much seems clear: sauropods had an extensive system of internal air-sacs that invaded the axial skeleton.
This is taken to indicate the presence of an avian-style respiratory system in which air flows through a system of thoracic and abdominal air sacs and through a unidirectional flow lung. A type II inference of the extant phylogenetic bracket, but well supported by skeletal evidence.
The presence of an extensive air sac system explains one sauropod enigma: the cross-sectional area of sauropod limb bones seems to scale with strong negative allometry when we base mass-estimates on mammals. This would suggest that their limbs bones should be too weak to support their weight. If much of their internal volume is occupied by air, however, the problem is diminished.
So, could they rear up and get their heads into the tree-tops without passing out? We don't yet know. That doesn't keep us from projecting our desires and fantasies onto these inoffensive animals.