inferring neck posture from modern vertebrate behavior

This skeleton of a kiwi (Apteryx australis) is mounted with its neck vertebrae in the undeflected state, or Osteologically Neutral Pose (ONP), which often corresponds to a default posture of the animal when "just hanging around" vigilant or alert state of rest. There are some notable exceptions to ONP being one of the default alert poses of an animal, and animals generally feed by lowering the head below the height defined by the ONP (as does the kiwi when foraging, where it scrunches down close to the ground seeking insects).

Photographed at the Cambridge University Museum of Zoology in collaboration with John Martin.

Modern vertebrates help us infer how dinosaurs held their necks in life.

Animals today exhibit one characteristic pose for feeding (CFP), another for locomotion (CLP) and another when just hanging around but remaining alert and vigilant (CVP). Vertebrates spend most of their waking time in a few such behavioral routines, each with its characteristic pose. With the vertebrae aligned and all intervertebral joints in an undeflected state, the vertebral column assumes a characteristic curve (a pronounced sigmoid curve in the neck of the kiwi above yet a nearly straight line in the anteater below). This Osteologically Neutral Pose (ONP) reflects the intrinsic curvature of the vertebral column due to the shape of the bones, without its owner flexing the intervertebral joints at any point along the length of the column.

Recently, ONP was falsely claimed to be merely the midpoint in the dorsoventral range of motion (which, if true, would imply all joints have equal dorsal and ventral flexibility). In fact, ONP is a geometrically-defined undeflected stated, relative to which dorsal and ventral flexion limits are naturally, and universally, measured.

ONP is a useful reference state for describing behaviors. In ONP the head is naturally held above the shoulders in most large vertebrates (exceptions including rhino, buffalo, hippo, ground sloth, and this anteater (Myrmecophaga tridactyla). Most vertebrates spend much of their day in ONP when not feeding (CLP = CVP = ONP). Moreover, most lower their head when feeding (CFP < ONP). Again, there are exceptions, e.g., bunnies and ostriches hold their heads extra high when vigilant and not eating (i.e., CVP >> ONP).

Photographed at the Cambridge University Museum of Zoology in collaboration with John Martin.

But if you still wish to imagine sauropods with necks rising above ONP with (little or) no visible means of support, stay tuned. In the meantime, here are our two talks from the 2010 SVPCA, held in Cambridge.

The reconstruction of sauropod neck posture is critical to the interpretation of their feeding behavior. While the undeflected, osteologically neutral pose (ONP), has been proposed as the characteristic pose (CP) for the sauropod neck, the proposal that sauropods habitually held their necks in ONP has been questioned, since some extant vertebrates habitually hold the head higher than ONP. Rather than consider but one generic CP, we distinguish characteristic poses for locomotion (CLP), feeding (CFP), and vigilance (CVP), and examine these poses for a broad range of extant taxa (22 birds in 20 orders; 10 mammals; two non-avian archosaurs; and three other reptiles). CFP is generally below ONP, CLP is close to ONP, and CVP is close to, or above ONP. Exceptions where ONP is significantly lower than CLP and CVP (as noted in the ostrich) are consistent with other environmental requirements, not the least of which is the need for vigilance when not browsing at ground level. As the only pose that can also be configured with certainty in fossils, we conclude that it has high value for hypothesizing about sauropods, their having have no exact biomechanical analogue among extant tetrapods.

A representation for the space of possible poses of a vertebrate neck is introduced and applied.  For a neck with K cervical vertebrae, and intervertebral joints with 2 degrees of freedom (mediolateral and dorsoventral flexion), the full configuration space has a computationally-unwieldy 2*K dimensions. But regarding the neck as permitting the head to reach to a specific location from a specific direction of approach, the configuration space reduces to a 6-space (3 positional, 3 directional). As the neck extends to reach increasingly far from the body, the range of approach directions (at the atlas) diminishes. In the limit, the “reachability envelope” (RE) is a surface defined by the points of greatest radial distance reachable without stepping. Vertebrate necks vary considerably in RE area, and the directional flexibility within the volume bounded by the RE (the swan, e.g., can preen points on its own neck from various directions). A computational model explores how neck flexibility (both RE and directional flexibility) is affected by factors including vertebral count and the distribution of centrum length and intervertebral flexibility along the neck. The flexibility observed in representative vertebrates is replicated, and implications for sauropod dinosaurs are derived.