Geol 006 Dinosaurs and their Mesozoic World

J Bret Bennington

Sauropods

Sauropods were quadrupedal animals ranging in size from about 7 meters (21 feet) to as much as 40 meters (120 feet) in length and weighing perhaps as much as 70 tons! In spite of their large size, they have the smallest skulls relative to body mass of any other dinosaur, as well as the smallest EQ (encephalization quotient) which is the estimated ratio of brain mass to body mass.

All sauropods have long necks, long tails, and massive legs supported by blunt feet with five toes. Sauropods first appear in the Early Jurassic and they are present in different parts of the world until the end of the Cretaceous. The peak of sauropod evolution appears to have come in the Late Jurassic, which has the largest number and variety of known species. Originally, sauropods were thought to have dwindled greatly into the Cretaceous, but it is now recognized that this is only true of Laurasia. In other parts of the world such as South American and Africa sauropods appear to have been common in Cretaceous ecosystems.

How to make a Sauropod

What did evolution have to do to transform a prosauropod-like-animal into a sauropod? Basically, the animal had to get bigger - a lot bigger. The body became more massive and the neck much longer. These changes in body form required major changes in the skeletal anatomy seen in all sauropods:

• incorporation of additional vertebra into the sacrum to provide stronger support for the pelvis. The primitive dinosaurian condition is two, to which sauropods add two more from the tail. In the Late Jurassic most forms have a fifth sacral vertebra added in front from the trunk. Finally, Late Cretaceous titanosaurs add a sixth, also from the trunk.
• lengthening of the forelimb to support the chest in a quadrupedal stance
• broadening of the upper pelvic bone (ilium) to support massive leg muscles - particularly the ones the hold up the body during walking
• straightening and strengthening of the femur and humerus
• alteration of the manus and pes to become stubby and padded
• formation of cavities in the vertebrae (pleurocoels) to reduce weight
• formation of additional articulating projections on the vertebrae to stiffen the back

Saurpods walked with their toes splayed outward around a central, fleshy pad, much like that seen in the feet of elephants. A similar pad also supported the phlanges of the hand, which in all sauropods are nearly vertical so that the animal is essentially walking on its finger tips.

The oldest fossils of sauropods also appear to be the most primitive, showing some, but not all, of these important modifications.

Vulcanodon - a partial skeleton of this sauropod is known from the Early Jurassic of Zimbabwe. This animal is definately a sauropod, but it lacks the broad pelvis and pleurocoels characteristic of all other sauropods.

Barapasaurus - from the Early Jurassic of India, is known from partial skeletons that show depressions on the sides of the vertebrae, but not true hollow cavities (pleurocoels).

No skull has been found for either of these primitive sauropods, so there is not much we can say about what they ate or how they lived.

Cetiosaurids - these relatively small sauropods are known from the Early and Middle Jurassic. The most complete skeletons come from China and show a lack of pleurocoels. Interestingly, one Chinese Cetiosaur - Shunosaurus - had a bony club at the end of its tail.

By the Late Jurassic, three important groups of sauropods had evolved:

Diplodocidae - these sauropods had long, slender skulls with elongate muzzles. The jaws contained peg-like teeth confined to the front of the mouth. The nostrils are at the top of the skull in front and above the orbits. Diplodocids were, in general, long, slender animals - probably the longest dinosaurs to ever live, but not nearly the heaviest. The necks of diplodocids were extremely long - in the primitive diplodocid Mamenchisaurus 42 feet of a 68 foot animal was neck! Other well-known diplodocids include Apatosaurus, Barosaurus, and Diplodocus.

The tails of diplodocids are extremely long and slender, and composed of up to 80 vertebrae. The last 30 to 40 of these vertebrae are elongated rods that form a whiplash-like structure. It has been suggested by several researchers that diplodocids could have used their tails as a means of defense, either as a powerful whip, or as a mechanisms to generate loud, whip-crack noises to frighten away predators.

Camarasauridae - these sauropods, best known from the genus Camarasaurus, have short, blunt skulls with large, spatulate teeth, and nostrils located on the side of the skull, just in front of the eyes. Camarasaurids were neither exceptionally long, or exceptionally heavy. They were, however, exceptionally common. Camarasaurus is the most common dinosaur found in the Late Jurassic deposits of the Morrison Formation in the western US.

Brachiosauridae - Brachiosaurs are similar in skull morphology to camarasaurs, but the rest of the body is quite different. Brachiosaurs have elongated forelimbs that are longer than the hindlimbs, giving the animal a giraffe-like form. Although there are only 13 neck vertebrae, they are elongated, which, combined with the forelimbs, gave brachiosaurs a very elevated reach.

Titanosaurs - this group of sauropods is mostly known from the Cretaceous, particularly in the southern hemisphere. There is one titanosaur that lived in North America until the end of the Cretaceous - Alamosaurus. One titanosaur species called Saltasaurus had small, bony plates embedded in the skin along its back. This suggests that perhaps other, if not all, titanosaurids had bony body armor.