#some tetrapods seem to have regretted not taking that chance to recolonise the water after the devonian. just look at how many groups of -

[fishyfishyfishtimes]

why did the ray finned fishes win out in terms of speciation and population in the water compared to us lobe finned fishes? did we lobe finners just have a skill issue (or gill issue if you will) and fumble the aquatic niches?

Ooh, what an exciting question! I didn't know the answer myself, but this news article provides an interesting and good (in my opinion) hypothesis for how. The article explains that in the Devonian, the now-extinct class placoderms and lobe-finned fish were the two dominating animal groups of the aquatic world, no doubt closely followed by sharks who also diversified greatly. So, we did in fact thrive and dominate over ray-fins for a while! However, the Hangenberg event, also known as the end-Devonian extinction that took place about 359 million years ago, sadly brought an end to the placoderms as a whole, along with many, many lineages of lobe-finned fishes that used to exist: heck, over 96% of all vertebrate species were lost during this time! With great losses of diversity like this, entire classes of animals gone, well... a power vacuum forms. With only some cartilaginous fishes and ray-finned fishes left, it took them no time to diversify and fill in those previously full niches! Looking at the current diversity of ray-finned fishes, it really doesn't take much to assume that they took this opportunity with stride.

How come sharks didn't quite get as diverse, then? Well... I'm not sure. Perhaps the ray-finned fishes were quicker to reproduce? Maybe bony skeletons are just that much better? Some ponder that a bony skeleton would've been more protective and offered more points of attachment for muscles to allow for more powerful movement. It could also just be that bony fishes can live in freshwater as well as saltwater which allowed them to diversify away from cartilaginous fishes! But these are just guesses.

TL;DR: Us lobe-fins didn't have a gill issue at all — heck, two of of the "fishy" lobe-fin lineages are still alive today after other numerous extinction events! Though, perhaps us tetrapods did in fact develop a literal gill issue soon enough... we were rather quickly on our way to colonise the land and follow in the footsteps of invertebrates during and after the Devonian. In any case, the ray-finned fish just saw an opportunity once a power vacuum emerged, and took it! Super happy for them for taking that chance ngl, clearly it worked out really well for them. Now they're the most diverse group of vertebrates!! Love to see a heartwarming success story of the underdogs winning haha