Why sauropods had long necks; and why giraffes have short necks

Abstract

The necks of the sauropod dinosaurs reached 15 m in length: six times longer than that of the world record giraffe and five times longer than those of all other terrestrial animals. Several anatomical features enabled this extreme elongation, including: absolutely large body size and quadrupedal stance providing a stable platform for a long neck; a small, light head that did not orally process food; cervical vertebrae that were both numerous and individually elongate; an efficient air-sac-based respiratory system; and distinctive cervical architecture. Relevant features of sauropod cervical vertebrae include: pneumatic chambers that enabled the bone to be positioned in a mechanically efficient way within the envelope; and muscular attachments of varying importance to the neural spines, epipophyses and cervical ribs. Other long-necked tetrapods lacked important features of sauropods, preventing the evolution of longer necks: for example, giraffes have relatively small torsos and large, heavy heads, share the usual mammalian constraint of only seven cervical vertebrae, and lack an air-sac system and pneumatic bones. Among non-sauropods, their saurischian relatives the theropod dinosaurs seem to have been best placed to evolve long necks, and indeed their necks probably surpassed those of giraffes. But 150 million years of evolution did not suffice for them to exceed a relatively modest 2.5 m.

Air Space Proportion in Pterosaur Limb Bones Using Computed Tomography and Its Implications for Previous Estimates of Pneumaticity

“…The ASP values also appear to be within the range of cervical vertebrae of sauropod dinosaurs with high ASP, although they do not exhibit the wide range seen in sauropods [20]. However, these ASP values do exhibit a higher percentage of air in the bones than do bird wing bones [21]–[23] (Table 3).…”

Inter-Vertebral Flexibility of the Ostrich Neck: Implications for Estimating Sauropod Neck Flexibility

“…Whilst heart size and output [16], [17], the structure of the respiratory system [18], [19], risk of predation, and intraspecific niche partitioning [13] are all affected by neck function, there are also major implications for sauropod diet and ecology [11]. Whilst neck posture and flexibility in most species has relatively little effect on their ecology due to their relatively short necks, sauropod necks can reach up to 15 m in length [20], meaning small differences in the angle at which the neck is held can lead to differing head heights of a metre or more. Sauropods display a wide array of body sizes and neck morphologies, but broadly speaking if they were to have a gentle downward curve, the heads of many sauropods would reach heights of 2–4 m, whilst an extreme vertical ‘swan-like’ posture would lead to some species with head heights of 16–20 m [11], [21], [22].…”

Neck biomechanics indicate that giant Transylvanian azhdarchid pterosaurs were short-necked arch predators

“…However, published attempts to estimate the length of giant azhdarchid necks are rare and presently limited to isometric scaling of Quetzalcoatlus bones to the same linear proportions as the Arambourgiania holotype (Frey & Martill, 1996; Steel et al, 1997). Subsequent discussions of neck length in giant azhdarchids (e.g., Martill, 1997; Taylor & Wedel, 2013) have relied on these figures. However, some pterosaur necks, like those of virtually all long-necked tetrapods, are known to scale with positive allometry against body size (Wellnhofer, 1970): this calls into question the assumption that azhdarchid necks scaled isometrically and the accuracy of these predicted values.…”

“…That the giant azhdarchids had the same long necks as their smaller relatives has been verified by the discovery of several gigantic vertebrae, including University of Jordan, Department of Geology (UJA) specimen VF1: the 620 mm long holotype cervical V of A. philadelphiae . This specimen is argued by some authors to pertain to an animal with a c. 3 m long neck (Frey & Martill, 1996; Martill et al, 1998), a dimension which would make large azhdarchids among the longest-necked animals outside of Sauropoda (Taylor & Wedel, 2013) and Plesiosauria, despite their necks being formed of only nine vertebrae (Bennett, 2014). …”