  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 they probably surpassed those
of giraffes. But 150 million years of evolution did not suffice for them to
exceed a relatively modest 2.5 m.
