What Is a Sauropod Neck? The Structure Nothing Since Has Matched
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Some sauropods carried necks over fifteen metres long, six times the length of a giraffe's and far beyond anything alive. Explaining how an animal could support that, pump blood to the top of it and breathe through a windpipe that long has taken decades and is not entirely finished.
How the weight problem was solved
A long neck of solid bone and muscle would be impossibly heavy, and the solution was to make the bone mostly air. Sauropod vertebrae are riddled with internal cavities connected to the respiratory system, a condition called pneumaticity that is shared with birds and with other theropods, and in the most extreme examples the bone is a thin scaffold of struts around large air spaces, reducing density dramatically while retaining strength in the directions that matter. The same system meant the neck was supported partly by tension rather than compression, with long tendons and ligaments running along the top of the vertebrae acting like the cable of a suspension bridge, anchored over the shoulders, so that holding the neck out required far less muscular effort than it appears. Vertebrae were also elongated individually and increased in number, with some species carrying nineteen neck vertebrae against the seven that nearly all mammals have including the giraffe, which spreads the bending across many joints.
The physiological problems
Length created difficulties beyond structure, and each has a proposed answer that is not fully settled:
- •Blood pressure, since raising blood to a head many metres above the heart requires pressures far above anything measured in a living animal, which is the main argument used against habitually vertical postures
- •Dead space in the airway, since a very long windpipe holds air that is rebreathed, which the bird-like air sac system plausibly addresses by moving air in one direction through the lungs
- •Feeding rate, since a head that small on a body that large must gather enormous quantities, which is why sauropods appear to have swallowed food without chewing and processed it in a large fermenting gut
- •Heat, since a large body generates heat that must be shed, and an extensive air sac system may have assisted
- •Support at the joints, requiring cartilage and articulation capable of withstanding sustained load
- •Growth, since these animals hatched from small eggs and grew at rates among the fastest ever inferred, which the bone microstructure records directly
What the neck was for
Two main explanations compete and both have support. Vertical browsing would allow access to foliage no other herbivore could reach, which is the giraffe model and which suits species whose shoulder structure and vertebral angles suggest a raised posture. Horizontal sweeping would allow an animal to stand still and harvest an enormous area of low vegetation by swinging the neck, which saves the considerable energy that moving a body of that mass costs, and this suits species with more horizontally oriented necks. The evidence is mixed and probably differs between groups, since sauropods were not one animal and their necks vary in proportion, vertebral shape and articulation in ways that suggest different feeding strategies. Sexual selection has also been proposed, on the analogy of giraffes using necks in combat, and it is generally regarded as a contributory rather than a primary explanation since both sexes had them. The honest summary is that the question is open and the answer is likely to be several answers.
Why nothing has matched it
No animal since has approached this, which invites the question of what was special. Several factors combine. The air-filled skeleton inherited from their ancestors provided a lightweight structural solution unavailable to mammals, whose bones are solid. The bird-like respiratory system supplied efficient gas exchange at large size. Laying many small eggs rather than bearing live young removed the constraint that limits maximum size in mammals, since a mother does not have to carry a proportionally huge offspring. High growth rates allowed animals to pass quickly through the vulnerable small sizes. Abundant low-quality vegetation suited a strategy of eating enormous quantities without chewing, and the absence of chewing removed the head-size constraint that limits large mammalian herbivores, since a skull needs no room for a grinding apparatus. Each of those is individually present in some living animals and the combination is not, which is the most satisfying available account of a body plan that ran for well over a hundred million years and has not recurred.
The takeaway
Vertebrae filled with air spaces connected to the lungs cut the weight dramatically, and tendons running over the shoulders held the neck in tension like a bridge cable. Some species carried nineteen neck vertebrae against the seven in every mammal. Blood pressure requirements are the main argument against habitually vertical posture. Not chewing removed the head size limit that constrains large mammal herbivores.