Some Strange Teleost Skulls and Their Derivation from Normal Forms

Abstract

T HE student of the evolution of the fish skull at present has access to three sources of evidence which may throw light on the steps by which the most highly specialized skulls have been produced: (1) by comparing the less specialized with the more specialized in different groups he may work out a tentative hypothesis as to the sequence of stages; (2) he may then examine the palaeontological record of fish life, which extends from early Palaeozoic to modern times; (3) for the answer to certain morphological questions he may appeal to embryology. In the future experimental biology may also contribute its quota to the evidence already amassed by comparative anatomy, taxonomy, palaeontology and embryology. On the palaeontological side the student of evolution of the fish skull has access to a record that extends from early Palaeozoic times to the present day. Fortunately survivors of many of the earlier groups still persist along with their more highly evolved relatives. The comparison of these fossils with their ancient and modern relatives has already often enabled the palaeichthyologist and taxonomist to work out the general sequence of types from the oldest ganoids of the Old Red Standstone to the most specialized forms of living teleosts. The typical fish skull is a structure of great complexity, which has doubtless been diversely modelled by many intrinsic and extrinsic factors. It comprises two general systems, the neurocranium and the branchiocranium.