For many years one of the arguments against birds being dinosaurs was that birds have a furcula and dinosaurs do not. Now clearly the avian furcula (or wishbone) is a highly specialised bone and it is formed by the fusion of the clavicles (the collar bones) but the absence of a furcual in theropods hardly rules out theropod ancestry. After all, basal theropods don’t have feathers, or beaks, or could fly. One rather key concept of evolution is that things change over time, so an absence of a furcula hardly rules them out of contention – nothing else had a furcula either so unless birds somehow sprang out of the ground fully formed, then the furcula had to have a genesis in one lineage or another.
However, despite this obvious flaw in the argument, there is a rather better argument – the presence of a furcula in numerous theropods. A recent survey of the literature and specimens shows that they are now known in coelophysoids, spinosauroids, allosauroids, tyrannosauroids, compsognathids, oviraptorosaurs, torrdontids and dromaeosaurs. Outside of the theropods, there’s clavicles known in a prosauropod and several in some ornithischians. Dinosaurs have clavicles and furculae.
And just to show off what they look like, here is the furcula of Sinornithosaurus. It’s the boomerang-shaped element in the centre of the picture and is quite typical in morphology, though many of them also have a short spar coming out of the middle.