Sauropod neck posture

Sauropod neck posture is a subject occasionally debated among scientists, with some favoring postures closer to horizontal whilst others a more upright posture. Research has looked at various avenues of evidence and analysis including: attempting to reconstruct the neutral posture of their necks and estimating range of motion by studying the bones; attempting to reconstruct sauropod metabolism and the energy requirements of sustaining incredibly long necks in various postures; and comparing sauropod neck anatomy to those of living animals.

Biomechanics The biomechanics of sauropod skeletons and necks can help determine at what angle the neck was positioned.

Flexibility In 2013, a study led by Matthew J. Cobley and published in PLOS ONE focused on the flexibility of the necks of sauropods. They compared the necks of ostriches with sauropod genera to find out how flexible the necks really were. The study noted that previous biomechanics studies found the necks to have been positioned between the extremes of a vertical, and a downward slanted neck. In conclusion, the study found that sauropod neck flexibility should not be based on osteology alone, and if it is, the results should be used with caution. Even though there is a lack of preserved muscle tissue that would determine flexibility, sauropod necks were probably less flexible than previously thought. In 2014, Mike P. Taylor analysed the flexibility in the necks of Apatosaurus and Diplodocus. He found that Cobley et al. was incorrect in the fact that vertebrae imply the neck is less flexible than in actuality. Cobley et al. found necks to be much less flexible than in reality when cartilage was added. It was found that the cartilage between the joints would have allowed for the neck to flex far past 90°. However, Taylor noted that while the neck could flex above the vertical, the osteological neutral pose would have been around horizontal, with the habitual pose having the head held upwards in an alert pose.

Muscling Sauropod necks were probably highly muscled to suit their feeding level. Brachiosaurus brancai (now Giraffititan) was probably a high browser, so it would have been more muscled along the neck than other sauropods like Diplodocus and Dicraeosaurus interpreted as low browsers. The tail and limb length of B. brancai would also need to be greater, to balance out the inclined neck. However, the question of whether sauropods were endothermic or ectothermic plays a major part in how sauropods were muscled, as endotherms have particularly more intestines and stomach than ectotherms. The amount of gut needed could determine how much food was eaten by sauropods, and therefore at what elevation their heads were held.

Heart and metabolic stress The upright posture of sauropod necks is seen by some as requiring implausibly high blood pressure and heart strength. A 2000 study conducted by Roger Seymour and Harvey Lillywhite found that the blood pressure needed to reach the head with an upright neck would be 700 millimetres of mercury (28 inHg), interpreted as fatal to an endotherm, or highly dangerous to an ectotherm, even with adequate heart musculature. A later study by Seymour concluded that it would have required half the animal's energy intake to pump the blood to the head. This would disfavor sauropods being high browsers, and instead having lower necks while feeding than commonly portrayed. The above work summarily dismisses the hypothesis of secondary hearts in the neck as evolutionarily implausible, assuming arterial valves could have no role without associated musculature.

Hypotheses

A few hypotheses have been generated to solve the dispute over how sauropods held their necks.

Horizontal pose Kent Stevens and Michael Parrish have been the two main supporters of a horizontal neck posture. In 1999, they studied the genera Apatosaurus and Diplodocus, finding the habitual pose of the genera to be slightly declined. They claimed that both sauropods had necks much less flexible than previously thought, with the neck vertebrae of Diplodocus being more inflexible than Apatosaurus. Those two poses would suggest that the sauropods were ground feeders, instead of browsing off taller flora. Later, in 2005, Stevens and Parrish studied the biomechanics of sauropod necks on a wider variety of sauropods, from the Jurassic: Apatosaurus, Diplodocus, Camarasaurus, Brachiosaurus, Dicraeosaurus, Cetiosaurus, and Euhelopus. All were stated to have a horizontal, or even declining neck. However, in 2009 multiple flaws were found with this argument. Michael P. Taylor et al. compared the neck posture of sauropods to that of extant reptiles and other tetrapods, finding these animals' habitual poses to be entirely different from the assumptions of Stevens and Parrish. The latters' errors come mainly from their preconceptions about animals' habitual pose in life, which they simply assumed would naturally match the Osteological Neutral Pose (or ONP). Taylor et al. find the ONP to be, not the actual habitual pose of any examined animal, but an arbitrarily chosen midpoint between the two structural extremes of bone placement. ONP, then, is merely one place in the range of physically possible motion.

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