English: A comparison of animals shown rearing on their hindlimbs; including an African elephant (Loxodonta), a Gerenuk (Litocranius), and the sauropoda dinosaurs, Diplodocus, Giraffatitan, Barosaurus, and Opisthocoelicaudia.

Quadrupedal animals that can rear onto their hindlimbs can increase the height they can feed, giving access to food sources otherwise out of reach.^[1] Living animals that are known to rear up into a bipedal posture regularly include the Gerenuk and the African Elephant.^[2][3][1] It has been suggested that sauropod dinosaurs could have potentially reared for food gathering and defence. Sauropods have been depicted rearing in movies, documentaries, and museum exhibits. A famous example is in the movie Jurassic Park (1993), where a Brachiosaurus was shown rearing up to feed near the beginning of the film. The American Museum of Natural History mounted a cast of Barosaurus in a defensive rearing posture.^[1]

Researcher Gregory Paul has argued that sauropods have several adaptations that suggest that rearing was possible and that they were also better adapted for rearing than many modern mammals, such as elephants. For example, sauropods have larger and taller dorsal vertebrae than similarly-sized mammals, suggesting their dorsal columns were stronger. Sauropod necks and torsos are lightened because of an extensive air sac system which, combined with long, muscular, and dense tails, helps shift the centre of mass (COM) backwards, closer to the hip socket in some sauropod species. The forelimbs of most sauropods are more lightly constructed than the hindlimbs; the opposite is true in most mammals. Some sauropods have retroverted pelves which might have allowed the legs to maintain greater functionality when rearing.^[3]

In 1977, Magdalena Borsuk-Bialynicka described Opisthocoelicaudia as having adaptations that might imply that it reared regularly. The anterior part of the tail has strongly opisthocoelous vertebra which suggests flexibility; this was interpreted as the tail being able to serve as a prop when in a tripodal posture. Borsuk-Bialynicka also argued that the hip socket allowed for a large range of motion, more than needed for normal quadrupedal walking. The wide strongly flared pelvis was thought to further aid stability in a tripodal posture. However, other researchers have suggested these are adaptations for a wide-gauge posture that titanosaurs developed.^[4][1]

Biomechanics researcher Heinrich Mallison has built virtual models of Diplodocus and Brachiosaurus brancai (now Giraffatitan brancai) in order to investigate the potential rearing abilities of sauropods. The COM of Diplodocus is estimated to be very close to the hip socket; this makes prolonged rearing possible and does not require much effort to do it. Combined with its long, massive tail acting as a prop, it was also very stable. Mallison found Diplodocus to be better adapted for rearing then an elephant.^[1] Another study looking at locomotion in various sauropods, also found that the COM of Diplodocus was close to the hip socket; with its COM being the closest to the hip out of the several sauropods that were analysed.^[5]

Giraffatitan, on the other hand, was found to have a COM further forward, which is due to a reduced tail and larger forelimbs compared to other sauropods. When posed in a rearing posture, the COM was high above the hip socket; this makes prolonged rearing difficult and unstable. Small movements to the neck would require large correcting motions in the limbs to maintain an upright pose. Any backward movement puts a large amount of stress on the tail. The forelimbs could potentially suffer damage if they were to land too fast. Based on this analysis, it is unlikely that Giraffatitan would rear often.^[1]

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• Diplodocus, Giraffatitan, Barosaurus, and Opisthocoelicaudia silhouettes are based on skeletal reconstructions by Scott Hartman. Used with permission. [1] [2] [3] [4]. The neck of Opisthocoelicaudia is unknown and greyed out in the diagram.

• The white dots represent the approximate centre of mass as estimated in biomechanical studies; their exact positions are approximate due to unknowns in the reconstruction of soft tissues. The dark crosses represent the location of the hip socket.

• Humans scaled to 170 cm and 160 cm respectively.

References