The Sauropods: Evolution and Paleobiology

6 • Digital Reconstructions of Sauropod DInosaurs and Implications for Feeding

178 n recent years, sauropods HAVE been interpreted primarily as quadrupedal herbivores, with sympatric taxa differentiated in their feeding behavior presumably according to their dentition and feeding height in a quadrupedal stance (e.g., Fiorillo 1998; Upchurch and Barrett 2000). In order to generate detailed hypotheses concerning sauropod paleoecology , it is essential to start with as accurate a reconstruction of their body plans as can be afforded from their fossils. An accurate rendering of the life posture of a sauropod is necessary in order to determine the feeding envelope for each taxon in its conventional quadrupedal stance. We review here the body plan of several major sauropod groups, emphasizing the use of whole-body reconstructions to determine the approximate head height when the animal was standing quadrupedally, supporting its weight symmetrically from left to right, and holding the axial skeleton in an undeflected state. The undeflected state is termed the “neutral pose,” defined geometrically, and analyzed on the basis of osteological determinants in extant vertebrates as a guide to their reconstruction for sauropods. Neutral position head height is one key point for analyzing variation in feeding behaviors across sauropods, another being variation in dentition; both are set against the backdrop of available fodder. What was the relationship between a sauropod ’s preferred feeding height and the height at which its head was held when the neck was undeflected? A bridging assumption is necessary to relate these two parameters for an extinct species. The habitual feeding posture of a terrestrial herbivore can relate to the neutral position of its neck in three ways: (1) the herbivore can deflect its neck ventrally relative to the neutral position for browsing, or “browse by ventriflexion” (BV); it can raise its neck relative to the neutral position, or “browse by dorsiflexion ” (BD); or it can feed at or near the neutral position of the neck, or “browse neutrally”(BN). As browsing behavior is not directly preserved in the fossil record, it is necessary to consider the phylogenetic and functional distribution of these three feeding models among extant tetrapods. The form of browsing (BV, BD, or BN) is, in principle, independent of the neutral pose head height (relative to shoulder height) of a given SIX Digital Reconstructions of Sauropod Dinosaurs and Implications for Feeding Kent A. Stevens and J. Michael Parrish I herbivore. Many extant low browsers and grazers , such as Thomson’s gazelle (e.g., Leuthold 1977:table 2), deer, and horses, engage in BV feeding primarily, but not exclusively. In a neutral pose, their heads are held in a high position , presumably for vigilance during periods of inactivity (Walther 1969). Muscular effort is expended to lower the head to feed, increasing the tension on the epaxial nuchal ligaments. One argument posed against low browsing in sauropods, even in diplodocids, is that it would leave them vulnerable to attack (Paul 2000). While the high head heights that gazelles and other fleet-footed herbivores maintain when inactive allow them to detect and flee approaching predators, flight from predation was not a practical option for sauropods, the speeds of which are generally estimated as being much slower than those of their most likely predators (e.g., Alexander 1989; Thulborn 1990). Today, large BN to BV megaherbivores respond to the presence of carnivores by charging (e.g., rhinos, hippos, elephants [Owen-Smith 1988]) or indifference (e.g., elephants , hippos [Owen-Smith 1988]). The giraffe is of particular importance to this chapter, as it has been cited as an extant model for those sauropods, including brachiosaurids , euhelopids, and camarasaurids, that are sometimes reconstructed as giraffe-like (e.g., Paul 1987:figs. 16, 17; Currie 1987:figs. 2, 3; Christian and Heinrich 1998), effortlessly feeding while in a cervical neutral position (BN browsing) with the head held high above the shoulders. We review the osteological basis for the elevation of the giraffe neck, then examine the validity of proposing such a posture for any sauropod. It should first be noted that, perhaps surprisingly, giraffes frequently browse by ventriflexion , with the head at or below shoulder height (Leuthold and Leuthold 1972; Pellew 1984; Young and Isbell 1991; Woolnough and du Toit 2001). The elongate neck of the giraffe is not a simple consequence of vertical niche partitioning (Simmons and Scheepers 1996). Pincher (1949) proposes that predation provided the selection pressure for limb elongation in giraffes, and that neck elongation secondarily provided the ability to drink and reach low fodder. There are many other modern examples of herbivores with neutral head height...