What Is the Hip Division? A Split That Named the Groups and Then Wobbled

The hip is built from three bones on each side. One projects upward and anchors the leg muscles, one projects forward and downward, and one projects backward and downward. The traditional division rests on the orientation of the forward-pointing bone, which in one group points forward and down in the arrangement described as lizard-hipped, and in the other points backward alongside the rear bone, in the arrangement described as bird-hipped. That naming is the source of endless confusion, since birds are descended from the lizard-hipped group rather than the bird-hipped one, and the backward-pointing arrangement evolved independently in the two lineages. The functional explanation usually offered is that rotating the bone backwards makes room for a larger gut, which suits a plant diet, and the bird-hipped group was entirely herbivorous.

The traditional division separated animals that are otherwise easy to group by appearance:

• •The lizard-hipped group held the theropods, which were the bipedal predators and the ancestors of birds
• •It also held the sauropods, the very large long-necked plant eaters, which look nothing like theropods
• •The bird-hipped group held the horned dinosaurs, the duck-billed dinosaurs, the armoured forms and the stegosaurs
• •That group was entirely herbivorous and contains most of the familiar ornamented animals
• •The division was proposed in 1887 and became standard, with the two groups understood to have separated very early
• •It implied that theropods and sauropods were more closely related to each other than either was to any bird-hipped form, which few people find intuitive

A study published in 2017 analysed a large set of anatomical characters and recovered a different arrangement, placing the theropods with the bird-hipped group rather than with the sauropods, and proposing a new name for that combination. The reasoning was that a substantial number of features are shared between theropods and the bird-hipped dinosaurs, and that the traditional scheme had been retained partly through familiarity. The proposal implied that the backward-rotated hip evolved once in a common ancestor rather than twice, which is simpler, and it required other features to have evolved more than once instead. It attracted immediate attention because it overturned a framework that had appeared in every textbook for a century, and because such a fundamental revision at that stage of a well-studied field was unexpected.

Behind the classification sits a functional structure that matters in its own right. The hip transfers the entire weight of the animal from the spine into the legs, so the bones are heavily built and the joint between them and the backbone is reinforced by fusing several vertebrae into a solid block. The socket receiving the head of the thigh bone is perforated in dinosaurs, which is unusual, and it combines with an inturned head on the thigh bone to hold the limb vertically under the body rather than sprawling to the side. That arrangement is one of the defining features of the group and is what permitted efficient walking and running at large size. Muscle attachment sites on the upward-projecting bone indicate the size of the muscles that drove the leg, and reconstructions of locomotion depend on measuring them, so the hip carries information about speed as well as about relationships.

The response was substantial and the question is not settled. Reanalyses of the same data by other researchers recovered the traditional arrangement when certain coding decisions were changed, and the disagreement turned on how individual anatomical features were scored, which is where such disputes usually live. Subsequent analyses have gone both ways, with the traditional scheme still favoured by most workers and the alternative retaining support. The episode is instructive for what it shows about the method, since a classification of this kind is a hypothesis derived from a matrix of characters, and it depends on which characters are included, how they are defined and how ambiguous specimens are coded, all of which involve judgement. The general public reaction, that scientists could not decide something so basic, misreads what happened, since revisiting a long-standing arrangement when a reason appears is the method working rather than failing.

The division rests on whether one hip bone points forward or is rotated backward, and the bird-hipped name is misleading because birds come from the other group. A 2017 study placed theropods with the bird-hipped forms instead, reanalyses recovered the traditional arrangement when character scoring changed, and the question remains open. Such disputes turn on how features are defined and coded.