Two artists’ renderings of “Proavis”, a hypothetical bird ancestor. The lizard-limbed would-be creature is certainly a product of its time, as birds were once thought to be descendants of the clade Pseudosuchia (the name Crurotarsi is more commonly used today and generally includes the same taxa). Traditionally, Pseudosuchia was considered a suborder of the now-obsolete order Thecodontia.
Gerhard Heilmann laid out the hypothesis of a pseudosuchian ancestry of birds in his highly influential 1926 book The Origin of Birds, which was considered the last word on the subject of bird evolution for several decades after its publication. Heilmann acknowledged a close relationship between non-avian theropod dinosaurs and birds but rejected the notion that the former directly gave rise to the latter. One example of his reasoning was that birds have a furcula (wishbone), a feature which non-avian theropods did not appear to him to possess (though we now know otherwise), yet ancient reptilian fossils that predated dinosaurs did show clear evidence of a furcula-like structure. Though the evidence to which we now have access overwhelmingly supports the hypothesis that birds are indeed the descendants of dinosaurs rather than the crocodile-like “thecodonts”, much of the book’s research remains of interest.