How did heart valves evolve?
I'm studying the heart right now in school, and I'm fascinated by how the heart evolved a septum to separate the ventricles and prevents mixing oxygenated and deoxygenated blood, solving a limiting factor suffered by animals like frogs.
I can't find much on heart valves and chordae tendineae, though. I see frogs have spiral valves, but how did get from having no valves (or even spiral valves) to having chordae tendineae mounted in the ventricles anchoring the tricuspid and mitral valves to prevent blood back-flow into the atria?
What do we know and what can we guess about how these wonderful structures came to be?