Complex sentence: The anatomy of the "O. cuvieri" closely resembled a species of trout which is now found in Lake Titicaca, a similarity which has led many researchers to hypothesize that competition between the two groups was the reason for the extinction of the "O. cuvieri".
Split into simpler sentences: The anatomy of the "O. cuvieri" closely resembled a particular species of trout which is now found in Lake Titicaca. The similarity between the features of the trout and the "Orestias cuvieri" has led many researchers to hypothesize that competition between the two groups was the ultimate reason for the extinction of the "O. cuvieri".