The Urodermals in Lanternfish Family Myctophidae (Pisces: Myctophiformes)

Abstract

Urodermal bones are found in the species of the subfamily Ia.mpanyctinae of the Myctophidae, although some species of Diaphus, Lampanyctus and Taaningichthys lack them. One urodermal occurs in the tendon of the epaxial caudal muscle which inserts on the ninth caudal-fin ray. The occurrence of urodermals in the tendon raises the question of their homology. They may be homologous with the rhombic scales of the Jurassic teleosts in which case they have lost their enamel layer, or they may be newly formed structures in the tendon of the epaxial caudal muscle. Further search for the distribution of urodermals will be needed to clarify the homology. A series of paired elongated bones on the dorsal surface of the last several caudal centra of primitive teleosts were named as uroneurals by Regan (1910). Nybelin (1963) showed that the most posterior elements of the series may extend further on the epaxial caudal-fin rays in Upper Jurassic teleosts, and proposed the name urodermals for the bones. He considered the urodermals to be homologous with the rhombic scales found in Acipenser, Polyodon, Birgeria and Tarassius. Monod (1968) accepted Nybelin's proposal and interpreted as urodermals all the series of paired bones on the dorsal surface of the last several caudal centra and epaxial caudal-fin rays in teleosts. Patterson (1968) examined the caudal skeleton in the lower Liassic Pholidophoridae including three genera, Pholidophorus, Pholidolepis and Pholidophoropsis. In comparison of the caudal skeletons of these genera with those of primitive teleosts, he demonstrated convincingly that the uroneurals of teleosts are modified neural arches which are