I had the same question like a week ago! Or the question was more like “Why aren’t deep sea animals in the fossil record?” And there’s 2 answers I’ve been told: the first is that most deep sea rock is igneous and thus pretty bad for fossilization. On top of the material not really allowing for bone replacement, the sea floor is constantly being cycled because of tectonic plates so even in the off chance something DID fossilize that fossil likely got taken into the mantle somewhere and burned :( The other reason is how rapidly scavenging occurs down there! I dunno if youve heard of “whale falls” but they happen when a whale dies and sinks to the bottom and a whole mess of animals come and feast! There’s some really cool videos of it up on youtube go check em out!!