Osteology of the Malaysian phallostethoid fish Ceratostethus bicornisc with a discussion of the evolution of remarkable structural novelties in its jaws and external genitalia

The osteology of the phallostethoid Ccratostethtis bicornis (Regan) is described and figured. Comparative observations on osteology of atherinoids, cyprinodontoids, and other phallostethoids are also given. Phallostethoids apparently originated from atherinids. The most closely related atherinids are Taeniomembrasinae. The osteological observations tend to support the idea that atherinoids and cyprinodontoids are related, as postulated ])y Rosen ( 1964 ) in proposing the order Atheriniformes. Atherinifomis exhil^it a widespread tendency to develop teeth with two and three cusps, especially on the pharyngeal bones. The trend is pronounced in cyprinodontoids, exocoetoids, and scomberesocids, practically absent in atherinoids, and completely absent in phallostethoids and belonids. The Phallostethoidea can be divided into two families, Phallostethidae and Neostethidae. Neostetliidae comprises two subfamilies, Neostethinae and Gulaphallinae. These groups are distinguishable on the basis of morphological differences in the jaws and external genitalia. The highly protractile jaws of Neostethinae ( as exemplified" ])y Ceratostethus and Neostethtis) are remarkalile in having several pairs of bones without homologues in other fishes. These new bones, invohed mainly in protrusion of the jaws, evolved in soft structures already present in the jaws of atherinids. The functional anatomy of the jaws of phallostethoids is briefly discussed. Phallostethidae and Gulaphallinae lack the neomorphic jaw bones found in Neostethinae. Two ctenactinia formed from pelvic fin spines or rays occur only in CeratostetJius. The so-called "second ctenactinium" of Gulaphallus is ^ Published by a grant from tlie Wetmore Colles Fund. Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 an externalized pelvic bone. Phallostetliidae apparently arose from Neostcthus. The toxactinium, the main externalized bony element in the priapium of Phallostethidae, is derived from the inner pulvinular bone, which is the anteriormost internal liony element in tlie priapium of Neostetliidae. The inner pulvinular bone of Neostethus bears a small lateral projection, the pulvinular spine, which may be a rudimentary toxactinium. The structure of the papillary ])one, intimately associated with the genital pore, is relatively simple in Gulaphallinae and Phallostethidae, ])ut in Neostethinae it divides into numerous thin processes each bearing a booklet at its tip. A comprehensive definition is given for the superfamily Phallostethoidea. All taxa used in this paper were proposed l^y previous authors. Oviparous Atheriniformes with internal fertilization have external genitalia far more complicated than the gonopodium of any of the viviparous .\theriniformes. The explanation of this difference is sought in terms of selection pressures for and against the evolution of liighly complicated external genitalia in forms with internal fertilization. Neotenic characters probably played an important role in the origin of phallostethoids from Atherin-