Dinosaurs
He determined that dinosaurs fell into two great groups, the Saurischians and the Ornithischians, based on the nature of their pelvic bones and joints. He published his results in 1888, from a lecture he had delivered the previous year. Paleontologists of his time had been dividing the Dinosauria various ways, depending on the structure of their feet and the form of their teeth. Seeley's division, however, has stood the test of time, though the birds have subsequently been found to descend, not from the "bird-hipped" Ornithischia, but from the "lizard-hipped" Saurischia. He found the two groups so distinct that he also argued for separate origins: not until the 1980s did new techniques of cladistic analysis show that both groups of dinosaurs really did have common ancestors in the Triassic. Seeley described and named numerous dinosaurs from their fossils in the course of his career.
His popular book on Pterosaurs, Dragons of the Air (1901) found that birds and pterosaurs are closely parallel. His belief that they had a common origin has been proved, for both are archosaurs, just not as close as he thought. He upset Richard Owen's characterization of the pterosaurs as cold-blooded, sluggish gliders, and recognized them as warm-blooded active fliers.
He was elected a Fellow of the Royal Society in June 1879 for his work on reptiles and dinosaurs. and delivered their Croonian Lecture in 1887.
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