 His early work was also notable for reclassifying and naming the English theropod dinosaurs "Eustreptospondylus" and "Metriacanthosaurus".In the late 1960s Walker studied the origin of crocodilians and of birds, which became controversial in 1972 with his publication of a paper in "Nature" arguing for a close relationship between sphenosuchian crocodylomorphs and birds.He later accepted that this hypothesis might be incorrect in a 1985 paper on "Archaeopteryx".