6. Biomechanical function of the vertebral column in sauropods
Most sauropods, those with short spinous processes, were water-dwellers
In comparison to the ornithischian dinosaurs discussed above, in saurischians the vertebral column is marked by a different design with conspicuously short spinous processes. This fact cannot be harmonized with the idea of a terrestrial lifestyle of all dinosaurs. More than one hundred years ago palaeontologists such as Cope, Marsh, and Owen have already considered that sauropods could have largely lived in the water (Coombs 1975). There were reasonable arguments for this supposition, and it was later emphasized by other workers such as Bird (1944), Erben (1975), or Ishigaki (1989). The reasons were
- the enormous overall size,
- relatively weak vertebral columns in relation to the strong limb bones and to the animal`s total weight,
- the pin-shaped teeth which were adapted to eat soft plants,
- the high positioned narial openings.
In modern mammals the latter trait can be found only in water-dwellers such as whales, dolphins, or in hippopotamuses.
Remainders of sauropods are found in sediments that were deposited in lakes, swamps and rivers, even near the coast. This fact is not a matter of chance, as maintained by palaeontologists, rather sauropods inhabited such environments.
Fig. 6. Camarasaurus as an example of a sauropod, redrawn from Carroll (1993). Note the comparatively short spinous processes of dorsal and cervical vertebrae. This dinosaur would have had some trouble to carry neck and head on land continuously. Camarasaurus does not appear to be adapted to a mere terrestrial lifestyle.
In fact, the relatively short spinous processes of most of these forms, for example Camarasaurus outlined in Fig.6, do not yield unambiguous indications of an adaptation to a terrestrial lifestyle, contrary to statements in the literature, e.g. Sander (1994). Generally, the spinous processes of the dorsal vertebrae show a remarkably uniform length. Therefore, they do not well correspond to the piers of a bridge, at least much less than the vertebrae in the shoulder region of ungulate mammals. The spinous processes, although arranged in a double row, appear too short for the suspension of strong muscles and, consequently, for the transmission of strong vertical forces to the ground. The absence of ossified tendons (Coombs 1975) suggests that in contrast to ornithischians a strong dorsal musculature and a continuous stiffening of the backbone were not required. On land these dinosaurs surely would have had some trouble in carrying their long tail and in particular the head and the long neck with its short spinous processes. But there are only very few traces of a dragged tail, if at all, although the tail design and the saurischian pelvis suggest that it could not be carried high above the ground (Abel 1912). The weight reduction in the vertebrae of most sauropods does not appear as a feature of an unusual and sophisticated light-weight design of land-dwelling vertebrates, rather it indicates that generally the load on the spinal column was low, and the weight reduction could help to keep the animal buoyant, above all its long neck and the head. Nature aims at high efficiency, but not at extreme designs if there is no urgent need. The remarkable contrast between light vertebrae and relatively massive limb bones was interpreted by Matthew (1915) as a means to keep the animal erect while wading. The long flexible tail was suitable for an eel-like propulsion by undulation, although the attainable speed was very low. But the animals were not lost after losing ground contact of the feet.
Position of narial openings in various sauropods. In the course of evolution the position has changed from the Triassic Plateosaurus to the Jurassic forms
A shift of the nasal openings on top of the head would only make sense if lightweight head and neck usually floated near the water surface and were supported by their hydrostatic buoyancy. The structure of the cervical vertebrae with short processes and, therefore, short lever-arms suggests that the head had not to be raised (Fig. 8), respectively that it was normally held in a low position or, as I presume as the likely reason, floated at the water surface. A recent digital reconstruction of two diplodocoid sauropods by Stevens & Parrish (1999) suggests that the necks of these forms were preferably held in a straight horizontal posture. An estimation of musculature available versus that one required made Alexander (1985) arrive at the presumption that the sauropod Diplodocus might barely have been able to raise its neck to a vertical position.
Furthermore, this attitude of neck and head appears very reasonable because it would have allowed the blood pressure to stay within the limits known from other reptiles (Ostrom 1980), whereas raising the head, in large forms up to more than 9 m above the heart level, would have required an entirely unrealistic systemic systolic pressure of more than 500 mm Hg for supplying the brain (Hohnke 1973) and a heart of almost incredible size and pump performance (Norman 1991). Such a high blood pressure cannot be found in other animals. It exceeds the normal human one by a factor of roughly four and would require a compression strength of the blood-vessels comparable to a bicycle tyre. It is well known that, for comparison, the atmospheric standard pressure at sea level equals 760 mm Hg. Since the blood pressure needed for safely supplying the brain during erection of the head above the heart level is proportional to the product of height times blood density, the pressure must rise by approximately 76 mm Hg per metre. On the other hand, the only benefit of such an enormous modification would have been the ability to erect head and neck, similar to a giraffe. This comparison is, however, inappropriate for the absolute size differences and for the resulting physical reasons. There are no indications that the heart of dinosaurs might have been more efficient than that of other reptiles or mammals. Therefore, it does not appear allowable just to postulate superior properties. It is a peculiar method of certain authors, e.g. Paul (1991), to claim that postulated advanced capabilities of dinosaurs close to those of modern mammals have not been falsified. But such capabilities have not been confirmed as well. What else can be done if arguments such as those of Hohnke (1973) are not appreciated? Ideas not in strict accordance with physical laws can never be correct. An amphibian lifestyle did not require such capabilities. The necessity of a horizontally carried neck appears as an inevitable consequence of the available anatomical and physical evidence.
Off course, for egg deposition the animals had to go on land, but presumably not far away from the water, maybe the behaviour resembled in a way turtles
Skull of Diplodocus, which shows that no teeth for chewing plants were present, furthermore, the nasal opening high on the head is visible. (Senckenberg museum Frankfurt)
The problem of sauropods: swallowing food
Because of their long necks of up to 9 m length the type of food must have been restricted. It is hardly imaginable that sauropods could raise their head almost upright, break off branches and leaves from trees, and swallow this relatively dry food, as it is considered by certain palaeontologists. It would have stuck in their deep throat. Since they were unable to chew plants this food must have been so slippery that it could pass the long throat without problems. Otherwise, such a problem would have been lethal. Obviously, sauropods had no difficulty with their food. This is another argument for water plants.
A semiaquatic lifestyle does not necessarily mean that sauropods fed on plants in the water only and never went on land. However, in general their vertebral column does not present indications of a special adaptation to a life on the dry land. Hence, they cannot be regarded as land-dwellers. Sauropodian remainders are not restricted to fresh water deposits (Dodson 1990: 402). They occur also in marine deposits, as well as do sauropod tracks (Meyer 1990), preferably in intertidal to supratidal shallow water sediments (Meyer 1998).
Track of a swimming sauropod that touched the ground only with its forefeet with the exception of one hindfootprint where the track changes its direction, after R. Bird (1944).
This idea is also supported by the well-known track published by Bird (1944) which consists of forelimb footprints only, with the exception of a single footprint of a hindlimb where the track changes its direction. Apparently, this track was generated by a sauropod swimming in shallow water. The track described by Bird (1944) demonstrates that sauropods obviously were buoyant and, therefore, could not walk on the ground of deep water. Thus a sometimes alledged snorkel function is obsolete. The incomplete ossification of the limb joints (Coombs 1975, 1978) can serve as an additional argument for an amphibious lifestyle.
Some later sauropods became land-dwellers
Just as in all other vertebrates appears the length of the spinous processes in sauropods as a reliable indicator of preferred lifestyle and environment. However, there are certain length differences between different forms. Although generally fairly short, in Dicraeosaurus they show an elongation which also encompasses the cervical vertebrae. Yet, an adaptation to a terrestrial lifestyle or to very shallow water can clearly be recognized only in one single species, namely in the unique Amargasaurus cazaui from Argentina (Salgado & Bonaparte 1991), outlined in Fig. 7. This moderately large form differs remarkably from all other sauropods by its strongly elongated spinous processes. Apparently, an elongation of the spinous processes in the area between pelvis and shoulder as in Ouranosaurus was not sufficient in Amargasaurus, rather the elongation was extended to the cervical vertebrae, probably in order to enable the animal to raise its head on land. But this modification must not lead to the conclusion that this form was specialized to browse on trees. The considerably shortened neck contradicts such an idea. The differences are simply a documentation of skeletal modifications necessary for the transition from life in water to a predominantly terrestrial lifestyle. Bailey (1997) is of the opinion that these animals lowered their head for browsing. It appears possible that such modifications indicate a shallowing of the water level of the preferred milieu which did no more allow the head and the body being continuously supported by its buoyancy. Nevertheless, presumably the environment was still characterized by vegetation in shallow water.
Fig. 7. Dicraeosaurus with markedly elongated spinous processes. The elongated vertebral processes indicate that this medium-sized dinosaur was adapted to a terrestrial mode of life.
Fig. 8. Schematic relationship between a lengthening of the spinous processes in Amargasaurus, present the longest known vertebral processes known up to now in a sauropod. and the size of the muscular force required to generate a moment of definite magnitude M.
The elongation of the cervical spinous processes in Amargasaurus and consequently the increasing distance between acting muscular force and axis of the vertebral column brought about a decrease in the size of muscular forces for equal efficiency, respectively the efficiency could considerably be enhanced with the original muscular force. This context follows from Fig. 8. Amargasaurus cazaui is an excellent example for demonstrating the skeletal differences due to physical requirements between forms living mainly in the water and those living on the dry land.
Reconstruction of Diplodocus from the Upper Jurassic by O. Abel, overall length approx. 22 m. This reconstruction appears still today plausible to me, with its horizontal neck posture and an environment which is dominated by water.