SAUROPODOMORPHA

     The sauropods and prosauropods were large herbivores which lived from the Upper Triassic to Late Cretaceous evolved into at least 75 genera and perhaps 100 species.  The rivalry between the dinosaur excavations of Cope and Marsh in the late 1800s produced 5 genera of sauropods including Diplodocus and Apatosaurus.  They are known from every continent including Antarctica (given the find of the sauropod Antarctosaurus).

     Unfortunately, specimens are typically incomplete, often missing extremities such as the head, tail, and feet.  This is not unexpected with animals of this size.  If rapid burial is a key to fossilization, it is obviously difficult to rapidly bury the largest of all terrestrial animals.

     Sauropodomorphs lived in a diverse array of habitats, including lakes and river environments and floodplains (their footprints have been found in these areas).  Some of these areas may have experienced dry seasons and sauropods might have migrated as a result. There have been 5-6 species found in the same general locality suggesting that there was  probably some habitat specialization (Fastovsky, 1996)..

     Were sauropods aquatic?  Sir Richard Owen (the first to see their remains and name them) once thought that Cetiosaurus was a big crocodile and later concluded that sauropods were “probably marine.”  Cope and Marsh both felt that they were aquatic.  Early drawings of them pictured them walking on deep lake bottoms with only their heads above the water.  Were sauropods aquatic dinosaurs?  Although the orientation of their nostrils was thought to be evidence of their aquatic nature, similar nostril positions are seen in a variety of animals including some which are fully aquatic, amphibious, and fully terrestrial. Although these nostrils would have allowed them to be submerged with only their heads above water, their lungs would not have been able to function at such depths.   Given the heights of the largest sauropods, the lungs of a completely submerged animal would have experienced twice the atmospheric pressure and this would have forced air out of the lungs. 

     Sauropod legs were very sturdy with no evidence of adaptations for aquatic life,  indicating that they were adapted for terrestrial walking.  While some of the bones of sauropods (such as the vertebrae) were hollow, the leg bones were solid, lacking the cavities that most animals have in their leg bones).  They obviously did spend some time in water, given that their remains are common in areas which were wet lowland and floodplain areas.  Their trackways indicate that these dinosaurs were walking in muddy ground (the tracks can be 12-18 inches deep) and one trackway consists only of forefeet and was made by an animal which was floating the rear part of its body (see picture below).

In conclusion, although sauropods were not aquatic dinosaurs, they certainly did spend time in aquatic environments (Coombs, 1975).

     The biological requirements for such enormous animals are clearly outside the range of any animal alive today.  Most mammals have a systolic blood pressure reading of 110 to 150 millimeters of mercury and large mammals must generate pressures greater than this (giraffes have a systolic pressure of 320).  Sauropods would have needed an estimated systolic pressure of about 620 and the heart of a 30 ton sauropod would have weighed about 150 kg while that of an 80 ton sauropod would have weighed about 400 kg (Weishampel).

      Prosaruopods could possess as few as 10 elongated neck vertebrae.  Sauropods evolved additional cervical vertebrae, increasing their number from 10 to at least 12 (Serreno, 1999).  Some had clefts in neck vertebrae (bifid neural spines) for a strong supporting ligament.  Perhaps this supported the neck when it was held horizontally or perhaps this allowed the neck to remain elevated without continued muscle contraction.

     Vertebral differences are often critical in identifying sauropod fossils since the heads are often missing.  In sauropods, vertebrae joined at ball-and socket joints. Various terms are used to distinguish between types of intervertebral joints: if the cup (ball) faced anteriorly (forward) it is referred to as procoelus, if it faced posteriorly it is the opisthocoelus condition, and if there were 2 cavities on either side of the vertebrae it is the amphicoelous condition.  The vertebral centra in many species had deep cavities called pleurocoels.  These cavities lightened the vertebrae without reducing their strength.  The tail vertebrae had bones called chevrons which attached to the centrum from beneath and protected blood vessels (Coombs, 1975; Fastovsky, 1996; Lucas, 2004).

Although nostrils placed at the top of the head could not have been used for breathing if the animal were deep underwater, they nevertheless might have helped a wading animal whose neck was held horizontally in relatively shallow water. The peglike teeth and higher nostrils of the titanosaurs may have been an adaptation for aquatic vegetation while the more robust teeth and nostril position of the camarosaurs may have better adapted them for browsing (Czerkas, 1990).

There was some variation in the number of teeth held in the maxillary (upper jaw) and dentary (lower jaw) bones in different species of sauropod (Weishampel).

     Sauropods performed virtually no chewing–the jaw joint would have permitted only  moderate amount of movement, there were no cheeks to hold food while chewing (at least no cheeks as significant as seen in ornithischians), there was no “dental battery” of grinding cheek teeth, and jaw muscles very small.  As a result, sauropodomorphs swallowed food with little modification.  Because they were so tall, they could be more selective and choose less woody parts of plants (including fruit-like structures).  Sauropods could feed at heights of 3.5 to 15 meters although their hearts would have had to work much harder if their necks were continually held upright.  The first prosauropods and the South American prosauropod Riojasaurus possessed a short neck (Serreno, 1999).  Because they were so large, they probably relied on the soft leaves of ferns which once covered vast fern prairies for the majority of their diet (Weishampel).  They swallowed stones (called gastroliths) that help grind their food, as do a number of animals alive today.  Diplodocus specimens have been found with several pounds of gastroliths.  A few specimens are known with preserved food in their guts; one find suggests that diet included fibrous material.  Because they were so large, food would have stayed in the digestive system longer, allowing more time for nutrients to be absorbed.

      Some sauropods could hold an estimated half-ton of vegetation in their stomachs.  Feces would have been large; 1 coprolite (fossilized feces) measured 10 square feet and is assumed to be that of a sauropod.  It is stimulated that a 29 ton sauropod would have to eat 50 kg of food per day if it were “cold blooded” or ectothermic.  Endothermic (or “warm blooded”) sauropods would have required significantly more food.  As we will discuss later, very large ectothermic animals might actually be endothermic because their bodies would be so resistant to heat loss (Fastovsky, 1996).

     The earliest prosauropods were fully bipedal.  Later prosauropods could make use of both bipedal and quadrupedal locomotion (although they would have been among the slowest bipedal dinosaurs).  Even early sauropods had shorter forelimbs than hindlimbs, evidence of their descent from bipedal ancestors.  Sauropods could achieve perhaps 20-30 km/hr at top speed and tracks suggest that many walked 20-40 km/day.  Very few tracks have any sign of tail marks providing evidence that sauropods kept their tails horizontal.  Footprints show that they walked on their toes and had large pads on their heels. Because of several mass accumulations of skeletons of both sauropods (such as the Morrison formation in the U.S.) and prosauropods, it is thought that they traveled in herds (Weishampel).  In the illustrations below, the hand and foot of a prosauropod (Plateosaurus) are compared to that of a sauropod (Apatosaurus).

     Sauropods had only a few bones comprising the digits of their hands: -- the phalangeal count in the Shunosaurus hand was 2-2-2-2-1 while that of Brachiosaurus and Camarasaurus was 2-1-1-1-1 (although sometimes vestigial digit II-2 was present (Weishampel)

   How did sauropods defend themselves?  Certainly large animals can inflict injury by nature of their size alone. Prosauropods possessed large thumb claws that could be used for defense and sauropods also had claws on their thumbs (refer to illustrations above) (Serreno, 1999).   Perhaps herding served as a defense function, especially for younger animals.  The long tail (especially in Diplodocus) would have made a fast-moving and dangerous whip.  It probably could not be used often without injury (although fractures in tail vertebrae suggest that it might have been used as such).  Perhaps the sound of a whipping tail alone would have been a deterrent against attack. A few sauropods had armor plating and  Shunosaurus had a club at the end of its tail.  The armor and tail clubs are found on smaller sauropods; perhaps this underscores how important the large size of the other sauropods was for personal defense. 

    Sauropods include the largest terrestrial animals ever.  Diplodocus could reach lengths of more than 80 feet long. The shoulder and neck of Supersaurus suggest that it might have reached 120 feet long.  Argentinosaurus and Paralititan probably measured about 100 feet or more.  In 1878, Edward Cope described part of a vertebrae that was 5'; the full vertebrae (of the individual named Amphicoelias) would have been 8'6".   Unfortunately, the fossil has since been lost.  Estimates suggest that the full animal might have reached 200 feet long and 150 tons.

     One set of sauropod eggs are known and they were covered with vegetation suggesting some degree of nesting behavior.  At one French site, sauropod eggs were found in a straight line without a nest, perhaps indicating that the female laid them while walking.  The tracks of small sauropods are rare and they have never been found with adults, suggesting that the young matured apart from the adults.  One of the smallest dinosaur fossils known is that of a prosauropod hatchling named Mussasaurus (obviously, this may be the hatchling of another species which has already been described but no conclusions can be drawn from this immature animal).  Sauropod eggs have been found whose embryos are in such good condition that their skin can be studied (Fastovsky, 1996).