Robert Denison, 73, a retired paleontologist, was curator of fossil fishes from 1958 to 1970 at Field Museum of Natural History. He also was a prolific writer on the subject of primitive fish.

You’re considering new fish for your home aquarium, but all you see in your local pet store are the usual guppies, tetras, and small cichlids. You want a more unusual fish that stands out from the ...