Dinosaur Division is All in the Hips

Time has not been very kind to classic dinosaur science. As new discoveries have piled up and different theoretical frameworks have taken hold, dinosaurs as we know them today are vastly different from the creatures envisioned by paleontologists who worked during the 19th and 20th centuries. The idea that some hadrosaurs used their crests as air-supply tanks and the notion that the most spectacular of dinosaurs became so big and spiny that they doomed themselves to extinction are among the ideas that have been tossed. But not all early research has met such a fate. One anatomical division proposed by British paleontologist Harry Govier Seeley in 1888 remains one of the most important organizing concepts for understanding dinosaurs.

Early dinosaur finds were scrappy. Extremely so. Bits of jaw, spinal columns and limbs were often all that remained, and some dinosaurs, such as Megalosaurus, were reassembled from isolated parts of different animals found in the same strata. By the 1880s, however, paleontologists had uncovered more complete material. The American Bone Rush and a Belgian coal mine full of complete Iguanodon skeletons spurred a major image change. Dinosaurs went from the strange, pseudo-mammalian creatures envisioned by Richard Owen to roughly bird-like animals which were closer in form to dinosaurs as we know them today.

The influx of new dinosaur varieties during the late 19th century required a classification system to organize all the strange beings. There were more kinds of dinosaur than anyone expected. Seeley reviewed three previously proposed arrangements in an 1888 presentation before the Royal Society of London. Edward Drinker Cope had used aspects of the hip and leg to divide dinosaurs into groups he called the Orthopoda, the Goniopoda and the Symphopoda. Thomas Henry Huxley differed and used a broader suite of characteristics to establish the Megalosauridae, Scelidosauridae and Iguanodontidae, while separating out little Compsognathus—the most bird-like of all dinosaurs then known—in a category he called the Ornithoscelida (roughly, “bird legs”). Othniel Charles Marsh disagreed with both—he suggested that dinosaurs could be shuffled into the Sauropoda, Stegosauria, Ornithopoda and Theropoda. (Some of these names are still in use today for particular dinosaur groups, even if applied differently than these scientists originally suggested.)

Seeley had something different in mind. Each of the systems was based on different anatomical points, and some of these were not particularly informative. Huxley, for example, used the presence of bony armor as part of his definition for the Scelidosauridae, but armor plating of a greater or lesser degree had also been found in other dinosaur groups. What Seeley aimed to do was find a simple and unambiguous way of dividing dinosaur groups. He found that in the anatomy of dinosaur hips, which he believed should be “the prime element in classification.”

The dinosaur hip is primarily divided into three parts. There is the ilium (the large, upper flange of the hips), the ischium (a smaller spine that runs below and behind) and the pubis (another slender extension of the lower hip that can be found in various orientations from front to back). The orientation of this latter bone seemed to divide dinosaurs into two easily distinguishable groups. While sauropods like Camarasaurus and theropods such as Allosaurus had a pubis directed forward, various other dinosaurs such as Stegosaurus and Iguanodon had a pubis directed backward, often in close contact with the ischium. (The diagram above, modified from Seeley’s paper, shows the two different types.)

Seeley used the resemblance of dinosaur hips to those in other animals to name the two major groups. The hips of dinosaurs with forward-oriented pubic bones approximated the hips of lizards, so Seeley called them the saurischians (“lizard-hipped”). The hips of dinosaurs with backward-oriented pubic bones, on the other hand, looked like those of birds, and these animals were cast as ornithischians (“bird-hipped”). In place of the various orders other workers had proposed, Seeley advocated these two, hip-based denominations.

Paleontologists still use Seeley’s division today. Pick up almost any book about dinosaurs, textbook or otherwise, and you will probably find an early section on the difference between saurischian and ornithischian dinosaurs. But the usefulness of Seeley’s suggestion does not mean that everything about his proposed classification was correct. Seeley believed that the hips of dinosaurs were so different that the saurischians and ornithischians did not belong to a single, natural group. He thought that the resemblances between the dinosaur groups was a result of independent descent from similar ancestors rather than a close relationship. We now know this to be incorrect. Both ornithischian and saurischian dinosaurs are united by a suite of subtle anatomical characteristics and both lineages descended from a common, early dinosaur ancestor (though exactly what this animal looked like is as yet unknown).

There is also an irony in Seeley’s scheme. Heaps of evidence have confirmed that birds are dinosaurs, yet the “bird-hipped” dinosaurs had nothing to do with avian ancestry. The ornithischian dinosaurs—from hadrosaurs to ankylosaurus and horned dinosaurs—were about as distantly related to birds as possible while still being dinosaurs. Birds are highly specialized saurischian dinosaurs, and saurischian dinosaurs such as Deinonychus, Anchiornis and others show how the pubis bone of the hip was oriented backwards to create the avian condition. If you want to begin to understand dinosaur differences, you have to start with the hips. Just don’t be misled by the names Seeley chose.

References:

Seeley, H.G. (1888). “On the classification of the fossil animals commonly named Dinosauria.” Proceedings of the Royal Society of London, 43: 165-171.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.