February 18, 1839 (a Monday)

On this date, the English paleontologist Harry Govier Seeley was born. A man of humble origins, Seeley attended Cambridge University but quit before earning a degree. In 1859, he began working as an assistant to Adam Sedgwick at the Woodwardian Museum. For many years Seeley worked on his own, only much later in life accepting a position with King’s College.

Prior to Seeley, in the early 1840s, Richard Owen had established the order Dinosauria. Between 1866 and 1883, various authorities on dinosaurs, including Huxley, Cope, and Marsh, had produced classification schemes that attempted to bring order to the great variety of dinosaur specimens that had been and were being discovered. Marsh, in particular, had proposed to divide dinosaurs into four orders: sauropods, theropods, stegosaurs, and ornithopods; Cope had offered a different scheme. Most of the systems of classifying dinosaurs were based on the structure of their feet and the form of their teeth.

Ornithischia (above) and Saurischia (below).

However, in a paper delivered in 1887 and published in 1888, Seeley pointed out that the term dinosaur was being used for two rather different kinds of reptiles. There were those, like Marsh’s theropods and sauropods, that had a pelvis with a forward protruding pubic bone and those, like the stegosaurs and ornithopods, that had a divided pubic bone with one branch extending backwards along the ischium. Since the backward-protruding pubis is characteristic of modern birds, he called this group Ornithischia, the bird-hipped dinosaurs, and gave it the status of an order. He put the sauropods and theropods in another order he called Saurischia, or lizard-hipped dinosaurs. In the line drawing which Seeley printed with the paper, the top two figures represent the order Ornithischia, and the bottom two the order Saurischia.

Seeley not only argued for separate groups among dinosaurs, he even argued for separate origins, writing:

I see no ground for associating these two orders in one group, unless that group includes Birds, Crocodiles, Anomodonts, and Ornithosaurs; for differences of pelvic structure have been as persistently inherited as any condition of the vertebrate skeleton.

Saurischian hip (left side)

Even though Marsh had identified many characteristics common to all dinosaurs, Seeley’s interpretation held sway into the late twentieth century. In the 1980s, however, new techniques of cladistic analysis revealed that both groups of dinosaurs really did have common ancestors in the Triassic. Still, Seeley’s classification of saurischian and ornithischian dinosaurs remains intact, though, ironically, the birds have subsequently been found to descend, not from the “bird-hipped” Ornithischia, but from the “lizard-hipped” Saurischia.

Ornithischian hip (left side)

References:

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