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I looked up "Early Triassic" to get a sense of what that meant and I ended up on this image of marine life at the time. I noticed some disk shaped fish (#8 in the picture: Bobasatrania (durophagous actinopterygian)) that looked, to me, like Permit or a Sunny.

I wondered, what ecological niche are disk shaped fish exploiting? Why is this particular body type repeating itself 250 million years later?

EDIT per answers: I view them as same thing. I don't mean to suggest an animal can only exploit one niche, nor that a given niche can only be exploited by one animal. For example, birds with short, thick beaks like parrots eat seeds that are tough to crack. They give up the ability, perhaps, to get at some more delicate flower nectars. Brightly colored poisonous animals are easier to spot, but they are poisonous. So, over time, they are, as a specie, less likely to get eaten. What do disk shaped fish give up by being disk shaped, and what do they gain?

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I believe what you're referring to are fish with the compressiform body shape (fish with a narrow flattened body). The advantage to this body shape is that it allows the fish to swim very quickly for short periods of time. This would, I assume, be an advantage both as predator and prey in that it can use a quick burst to escape from something else trying to eat it and also allow it to use a quick burst to catch food for itself.

--added--

I just re-read your question and caught the "ecological niche" part, which is different from evolutionary advantage. I'm not 100% sure regarding this (I'd say 80% sure though), but I don't think that an animal has to fill a specific ecological niche in order to survive - it just has to...survive. Surely animals that fill specific niches can gain an evolutionary advantage by filling a niche, but it is not (to my knowledge) a requirement.

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  • $\begingroup$ I view them as same thing. I don't mean to suggest an animal can only exploit one niche, nor that a given niche can only be exploited by one animal. For example, birds with short, thick beaks like parrots eat seeds that are tough to crack. They give up the ability, perhaps, to get at some more delicate flower nectars. Brightly colored poisonous animals are easier to spot, but they are poisonous. So, over time, they are, as a specie, less likely to get eaten. What do disk shaped fish give up by being disk shaped, and what do they gain? $\endgroup$ – dwstein Feb 1 at 19:32
  • $\begingroup$ I'm not an expert on fish, but I definitely understand the process of evolution and how evolution plays a part in the survival (and death) of animals. So the following is a well educated guess as to the advantage and disadvantage to such a body shape: The advantage is what I stated in the answer. As to disadvantages, an obvious one is that they are extremely easy to see by predators. Another possible disadvantage (and this is pure speculation) is that having a shape that allows for short bursts of very quick speed may cause them to move more slowly at normal speed (kind of a trade off). $\endgroup$ – jdfinch3 Feb 1 at 21:28

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