Merfolk are one of the major species in my book. However, unlike in most depictions, they're fully mammalian and lack gills. Because of this, they need to breathe air, but I still wanted them to be able to open their mouths underwater. I know that dolphins' lungs aren't connected to their mouths, which allows them to hunt for prey without filling their lungs with water. Would it be possible for a hominid to develop a system like this if it became fully aquatic?
dolphins' lungs aren't connected to their mouths, which allows them to hunt for prey without filling their lungs with water.
Are you sure that's the reason? I don't know about you, but I am capable of both eating and drinking underwater without drowning, and I don't think I'm anatomically unusual. Blowholes seem like an adaptation to breathing whilst swimming, rather than hunting underwater (though I'm happy to be corrected on this.. I Am Not A Cetologist). Competetive fast swimmers need to do awkward head-twisting, arm-stroke-synchronising breaths . Dolphins can just poke the top of their heads out of the water, maintaining more speed and (I think) greater hydrodynamic efficiency.
Moreover, being a member of an intelligent and tool using species, I can also hunt underwater (spearfishing, specifically) and, if I were so inclined, I could conceivably have some really, really fresh sashimi. In practise, I know too much about fish tapeworms and my breathholding isn't quite long enough to do such a thing, but for serious apnoeists I'm sure it would be straightfoward. Your merpeeps are presumably also capable of making and using tools (gotta justify those unhydrodynamic forelimbs somehow) and traditional depictions of merfolk often include things that look a lot like fishing spears, so this sort of thing would be well within their remit.
Also, given that merfolk traditionally have, y'know, hands and arms, they don't actually need to open their mouths underwater, unlike most marine predators, and they can just go back to the surface to chow down.
Would it be possible for a hominid to develop a system like this if it became fully aquatic?
This would require some fairly substantial replumbing of the upper part of the neck, with a whole new pharynx. It would also prevent human-like speech and other vocalisations, which prevents the classic mermaid singing. You may or may not care about that. They could always make dolphin-like vocalisations intead, I suppose.
Additionally, without the ability to carefully blow air from pursed lips, making fire becomes necessarily more challenging. Again, maybe you don't care, and you're happy to have stone and bone tools, and merfolks filled with giant fish tapeworms (Diphyllobothrium latum can grow to be 10m long!) caught from uncooked fish and marine predators. It'll help keep them looking svelte, at least.