Development of the head and pectoral skeleton of Polypterus with a note on scales (Pisces: Actinopterygii)

Comparative study of the bony head and pectoral girdle skeletons of fishes requires the establishment of sound and acceptable homologies. In this paper, the bony elements seen in a number of species of Polypterus (and Erpeloichlhys ) are described in terms of several developmental stages and the mature condition. Variations in these bones are considered in a developmental sense. As a part of the problem of bone development, a summary account of lateral-line development is included.
A number of particular problems of homology are discussed in some detail. The most radical differences of view involve the endocranial bones. The ideas of Patterson (1975) are examined and an alternative interpretation offered. The main bone in the posterolateral angle of the endo-cranium is viewed as the pterotic-epiotic. The epiotic of the teleost is seen not as the homologue of the epioccipital of the pholidophorid but a part of the so-called "pterotic" of those fishes. Other endocranial problems appear to be of a similar (interpretative) nature. The suspensorium of Polypterus is peculiar in lacking a symplectic and an interhyal. The branchial skeleton is quite like that of a "palaeoniscoid". The pectoral girdle presents a number of problems such as the form of the posttemporal, the lack of a sensory canal on the supracleithrum and the lack of an interclavicle.
The subject matter of this paper, and other papers by this author on the development of bones in fishes, lead to several developmental concepts. These are summarized in 18 statements. A table (Table II) summarizes some of the anatomical information useful in taxonomy in terms of being derived, primitive or unique.