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On rearing in sauropods

Here's an abstract from the Darwin-Bernissart meeting (February 9 -- 13 this year, Brussels) that is relevant to the current discussion on sauropod neck mobility.


Heinrich Mallison: Rearing for food? Kinetic/dynamic modeling of bipedal/tripodal poses in sauropod dinosaurs, p. 63 in Pascal Godefroit & Olivier Lambert (eds): Tribute to Charles Darwin and Bernissart Iguanodons: New Perspectives on Vertebrate Evolution and Early Cretaceous Ecosystems

Popular art and movies often portray sauropods as able to rear into a bipedal or tripodal (with tail support) pose for feeding or inter- and intraspecific combat. Such suggestions can also be found in the scientific literature (e.g. Borsuk-Bialynicka, 1977; Bakker, 1986), and Alexander (1985) attempted to estimate rearing ability in diplodocids based on the position of their Center of Mass (CoM). Rearing would give sauropod[s] an enormous height range from which to select their food, although the long necks already give quadrupedal sauropods great reach. Although Stevens & Parrish (2005) argued that sauropod necks were fairly stiff, Christian & Dzemski (2007) concluded that their mobility was high. in either case, a bipedal pose would have greatly increased the maximum feeding height for non-brachiosaurid sauropods and open[ed] access to food sources beyond the reach of other animals.
To date, no investigation of the biomechanics of sauropod rearing has been published. I used kinetic/dynamic computer modeling to assess the mechanics of attaining a bipedal pose, and the torques required to sustain them for a prolonged time in *Diplodocus* and *Brachiosaurus*. The two genera form the extremes of the wide range of sauropod _bauplans_, with short forelimbs in *Diplodocus* and the longest known forelimbs of all sauropods in *Brachiosaurus*. The retention of an upright pose was investigated using a quasi-static approach, and compared to extant elephants [which sometimes rear in the wild].
Several osteological characters of titanosauriforms, some of which are also present in brachiosaurids, are supposedly connected to rearing ability: flared ilia, a wide range of motion of the femur in the acetabulum, and opisthocoelus [argh! He makes mistakes like a native speaker!] anterior caudals as well as a fused pubic symphysis (Borsuk-Bialynicka, 1977). Wilson & Carrano (1999) mentioned the wide gauge of titanosaurs as helpful for rearing. The modeling results confirm that a wide gauge indeed increases stability, but suggest that none of the others is connected to rearing. To the contrary, higher mobility in the post-cervical vertebral column may be detrimental, because it makes higher muscle forces in the axial musculature necessary. Aside from the extremely posterior position of the CoM, *Diplodocus* shows further adaptations that combine with upright feeding: the most mobile neck in Sauropoda, a proportionally large pelvic girdle for large muscles, and chevrons with a highly derived shape that allows supporting weight on them in the area of the tail that touches the ground when the animal is in an upright pose.
Modeling of the rearing motion showed that *Brachiosaurus* would have had serious stability problems, due to the large moment arm of its CoM around the hip joint. In *Diplodocus* the CoM is located close to the hips, and slight imbalances do not result in large accelerations that threaten to topple the animal. The modeling results also indicate that diplodocids could sustain a bipedal/tripodal pose with significantly less effort than elephants, while *Brachiosaurus* would have to produce extreme torques in the limb joints. Sufficient muscle mass to produce these can not [sic] be placed on the bones, even assuming that dinosaurs were much more muscular than mammals. It therefore appears likely that diplodocids were the only sauropods capable of prolonged upright feeding.


When asked about the blood pressure problem, the author said that some kind of simple adaptation must have been present anyway just to let the animal lower the head to the ground and back to a horizontal neck posture.

Outside of his talk, he also talked a lot about the rest of his recently finished PhD thesis. We can look forward to a lot of very interesting and very, very, very, very well argued papers.