The first point to make is that this is not an example of a transitional fossil. …
Rather, the new fossil has been hailed as illustrating an important evolutionary transition: detachment of the middle ear bones from the mandible. It is therefore better described as a fossil claimed to have a transitional structure associated with ear bones.
You get that? It’s a fossil of a transitional form, not a transitional fossil.
Here’s the thing. Practically every fossil is transitional in some sense. Every fossil had ancestors and most represents a population which left descendants. Fossils like Tiktaalik or Yanoconodon are important not because the are located at some transition between a specific, known ancestor and a specific, known descendant, but because they possess structures which are transitional between a form characteristic of one major group of organisms and another. They show how an evolutionary pathway happened by retaining major anatomical features from that transition.
I don’t know whether Yanoconodon has any living modern descendants. Odds are against that. Most species go extinct, which means that most descendants of an ancient species are probably extinct. But Yanoconodon is a recent descendant of one of the first individuals to possess a particular arrangement of its jaw bones, and retains that arrangement.
Modern mammals have a single bone in their lower jaw, while reptiles have 4. In one of our reptilian ancestors, the bones had gotten re-arranged, the actual hinge of the jaw moved to the bone with all the teeth, and freed up the other bones. In time, the other three got attached to the ear canal, but were still attached to the main jaw bone, making them both ear bones and jaw bones.
In Yanoconodon and species like it, those three bones are somewhat separated from the main bone of the jaw, and are becoming exclusively ear bones. By successive modifications after the time of Yanoconodon, those bones became the smallest in the human body, the malleus, incus and stapes – the three bones of the ear.
When we look at this 5 inch-long fossil, We don’t know if we are looking at our direct ancestor. Chances are it is a close relative of our ancestors, a different branch off of the tree of life. What makes it transitional is the unique arrangement of its ear bones. What makes it fascinating is that it is a peek into a part of life’s great history. It stands on its own four legs as a sign of what life can do.