Final example that illustrates several themes in the origin of novelty. Seems like a good design because the joint between them makes it possible to point the fangs outward when striking at prey, but inward when trying to hold and swallow prey. So you could go from one bone to two simply by altering timing a bit. Stopping ossification before it gets to its last stage. Evolving a functional joint may take a little more time, but perhaps not.
Vertebrates have genes and developmental programs for forming joints.
One of the things that turns on the program is having two bones rubbing next to each other, and that's what the two new bones do. So perhaps the joint formed nearly automatically. But development and internal constraints may not be the whole answer. External selective constraints may be important too. It suggests that the absence of competitors may have been important.