Adapted from notes for HONR259C by Tom Holtz

Dinosauria: The most recent common ancestor of Megalosaurus and Iguanadon, the first known dinosaurs, and all of its descendants. General info:

Before we can approach this, we need some context.

Sauropodomorpha: (Late Triassic - Cretaceous)

  • Characterized by:
  • Herbivores (early forms maybe omnivores)
  • Primitive forms are facultative or obligate bipeds; later forms were so large they were obligate quadrupeds
  • 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 Brazil.

    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.

    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:

    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:

    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:

    Biomechanical issues:

    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:

    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?

    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: A typical sauropod lifting its head to a high elevation would require: 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?

    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.