I have just come to the end of a Vertebrate Paleobiology module, so I thought I would share something that I have learned during its study – the evolution of gigantism and neck elongation in Sauropod Dinosaurs.
Sauropods are regarded as the largest terrestrial animals to have ever lived, with body lengths greater than 40 m, heights of more than 17 m, and estimated body masses of 50 to 80 tonnes1 (see above). Their gigantic size is thought to have been a definitive factor in their success, allowing them to dominate the ecosystem for more than 100 million years, from the Middle Jurassic to the end of the Cretaceous1. Along with gigantism, they were also able to evolve extremely long necks, three to four times the length of their bodies. But how and why did a group of dinosaurs derived from small, bipedal omnivores such as Panphagia2, achieve such gigantism and neck elongation?
Sauropod gigantism is largely thought to have evolved due to predation pressures3, although high C/N ratios available in plant foods during the Mesozoic is also likely to have been a factor4. Gigantism in sauropods was made possible by a number of key biological evolutionary innovations, such as a high body mass ratio (BMR), an avian-style respiratory system and the absence of food mastication (see figure below). The evolution of a long neck is thought to have allowed more efficient energy uptake and therefore greater BMR, by making food accessible that was out of reach for other herbivores2. The absence of mastication added to this through unrestricted food uptake rates2.
The avian-style respiratory system includes cross-current gas exchange and a voluminous, highly heterogeneous lung, both with great implications for gigantism in sauropods5. Extensive pneumaticity of the axial skeleton contributed to the evolution of both gigantism and neck elongation, allowing a lowered cost of breathing, reduced specific gravity, and the removal of excess body heat2. Cervical pneumaticity was, therefore, an important prerequisite for neck enlargement, and vertebral pneumaticity in other parts of the body is expected to have had a similar role in enabling gigantism6.
Long necks were able to evolve due to the small head, absence of mastication and pneumatic axial skeleton2. Despite this knowledge, there is continuing debate over the function and posture of these long necks. The conventional view on sauropod neck length states that high browsing was necessary to meet high energy demands1. This theory assumes that the long neck allowed sauropods to reach more nutritious foliage that other herbivores could not. There is evidence for high browsing in a number of species, including Euhelopus zdanskyi, suggesting that high browsing was worthwhile despite an increased metabolic rate7.
Despite this, some studies state that the evolution of neck elongation was driven by runaway sexual selection, and not over competition for foliage. This suggests that the sauropod neck was primarily a display organ1, although there is a lack of evidence for this and only a few studies have discussed such a function15.
There is much debate over whether sauropods held their giant necks horizontally or vertically. This is largely due to the physiological problems associated with long necks. For example, the hypertension required for vertical neck posture would have involved the sauropod expending half of its energy intake just to circulate blood8. Digital reconstructions by articulating the vertebrae with the zygapophyses in maximum contact show an inflexible neck9, yet these studies did not account for the presence of soft tissue, cartilage or intervertebral spacing. However, reconstructions of sauropod neck posture inferred from extant animals, like ostriches, show that there would have been enough flexibility at the neck base and neck-head junction to vertically extend and hold the neck10,11. Evidence strongly suggests that habitual neck posture varied between sauropod species, implying that feeding strategy varied too12. For example, Brachiosaurus has been reconstructed to have giraffe-like necks13, whereas Diplodocus is thought to have had a camel-like neck14.
The above evidence suggests that it was a combination of biological factors that led to the evolution of gigantism in sauropods, including an avian-style respiratory system and an elongated neck. I believe that the evolution of large body size and long necks were central to each other, with the long neck providing access to a greater food source with very little whole body movement required, and the large body size providing protection from predation for the vulnerable long neck. Understanding neck function has major implications on our understanding of sauropod foraging strategies, ecology and biomechanics. The current literature strongly suggests a foraging related function. Although sauropods were capable of vertical neck extension and flexibility, neck postures and foraging strategies varied interspecifically and intraspecifically.
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12 Christian, A. et al. in Biology of the Sauropod Dinosaurs: Understanding the Life of Giants (Klein, N. et al.) 251-259 (2011).
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