The idea was proposed by Harry Govier Seeley. But note the reaction he received. Not much has changed in that respect.
"In the early 1840s, Richard Owen established the Order of Dinosauria. Over the years, T.H. Huxley, E.D. Cope and O.C. Marsh attempted to further classify dinosaurs. But the lasting classification, not to mention challenge to Owen's order, came from H.G. Seeley.A man of humble origins, Harry Govier Seeley attended Cambridge University, but quit before earning a degree. He began working as an assistant to Adam Sedgwick at the Woodwardian Museum in 1859 and was later offered positions with both the British Museum and the Geological Survey of Britain. Rather than accept either, he worked on his own, and only accepted a position with King's College much later in life.
In 1887, Seeley swept aside Owen's classification of the dinosaurs and argued that the ruling reptiles actually fell into two distinct groups: Ornithischia and Saurischia. The classification was based on the structure of the pelvis. The saurischians had lizard-like pelvic structures, and ornithischians had pelvic structures resembling those of birds. (The resemblance to birds was superficial; paleontologists now [incorrectly] generally regard saurischian dinosaurs as the ancestors of modern birds.)
While Seeley's new classification certainly didn't keep the public from thinking of dinosaurs as a group, it was enormously influential among paleontologists. It was also problematic. 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." Even though O.C. Marsh had identified many characteristics common to all dinosaurs, Seeley's interpretation held sway into the late 20th century. In the 1980s, however, as paleontologists reexamined evolutionary relationships in a method known as cladistics, Seeley's interpretation of separate origins was overturned. Paleontologists now recognize that dinosaurs evolved from a common ancestor. Still, Seeley's classification of saurischian and ornithischian lines remains intact.
Seeley was also an authority of pterosaurs, and in 1901 published a popular book on the subject, Dragons of the Air. In it, he gave an overview of animal flight, reptiles, the discovery of pterosaurs and pterosaur skeletal structure. He initially believed that birds descended from pterosaurs, but under intense criticism from his peers, backed off this assertion and argued that they shared common ancestry. "It would therefore appear from the vital community of structures with Birds, that Pterodactyles and Birds are two parallel groups, which may be regarded as ancient divergent forks of the same branch of animal life," he wrote. Like separate origins of "bird-hipped" and "lizard-hipped" dinosaurs, this interpretation has been overthrown — birds are now understood to be the descendants of dinosaurs. But once again, he had challenged Richard Owen, and Seeley had been right. Owen had considered pterosaurs as cold-blooded, poor flyers. Seeley opposed him on both counts."
Note the point that "birds are now understood to be the descendants of dinosaurs." But they are not descendants of dinosaurs and Seeley was correct. It is a shame he was hounded away from that idea.
Ornithodesmus (meaning "bird link") is a genus of small, deinonychosauriandinosaur from the Isle of Wight in England, dating to about 125 million years ago. The name was originally assigned to a bird-like sacrum (a series of vertebrae fused to the hip bones), initially believed to come from a pterosaur. More complete pterosaur remains were later assigned to Ornithodesmus, until recently a detailed analysis determined that the original specimen in fact came from a small theropod, specifically a dromaeosaur. All pterosaurian material previously assigned to this genus has been renamed Istiodactylus
Ornithodesmus cluniculus was first described by Harry Govier Seeley in 1887, based on a set of six fused vertebrae from the hip (sacrum), specimen number BMNH R187, found by William D. Fox in the Wessex Formation ofBrook Bay. Seeley thought the bones came from a primitive bird, and gave it a name meaning "bird link", from Greek ὄρνις (ornis), "bird", en δεσμός (desmos), "link". The specific name cluniculus means "little buttock" in Latin, a reference to the small thighs indicated by the size of the specimen.
Later that year, John Hulke (in an anonymous paper) suggested the remains actually belonged to a pterosaur. Seeley himself later changed his opinion when he described the complete skeleton (specimen number BMNH R176) of a new pterosaur species he believed was closely related to O. cluniculus. He named this new species Ornithodesmus latidens in 1901. Although he now considered it a pterosaur, Seeley at the time still considered Ornithodesmus close to the origin of birds, and suggested the (now defunct) theory that birds and pterosaurs shared a close common ancestry. For over a century following this, the pterosaur O. latidens was used as the standard example ofOrnithodesmus, and the fragmentary type specimen was largely ignored. In 1913, Reginald Walter Hooley named a new family to distinguish Ornithodesmus from other large pterosaurs known at the time, Ornithodesmidae.
In 1993, Stafford C. Howse and Andrew Milner re-examined the type specimen of O. cluniculus and determined that Seeley had incorrectly referred the pterosaur species to this genus. They identified O. cluniculus as a theropod dinosaur. Specifically, they suggested it was a troodontid, based on its similarity to the supposed troodontid specimen BMNH R4463. However, later study by Peter Makovicky and Mark Norell showed this specimen to be a dromaeosaurid; because of this mis-identification, they suggested Ornithodesmus was likely a dromaeosaurid as well. Darren Naishand colleagues in 2001 argued against a dromaeosaurid identity for Ornithodesmus, suggesting instead it was related to the ceratosaurs or coelophysids. However, those scientists later changed their opinions, publishing a paper in 2007 that agreed with previous studies and classifying Ornithodesmus as a dromaeosaurid.