Reinvestigation of the osteology of the miniature African freshwater fishesCromeria andGrasseichthys (Teleostei, Gonorynchiformes, Kneriidae), with comments on kneriid relationships

Abstract

The osteology of the three miniaturized African freshwater gonorynchiforms Cromeria nilotica, C. occidentalis and Grasseichthys gabonensis is investigated. We show that the two species of Cromeria, long considered to be subspecies, differ significantly in a number of characters. This has some consequences because all recent studies of the relationships of Cromeria were based only on C. occidentalis. We review previous osteological accounts of Cromeria and Grasseichthys and discuss the differences that we encountered in our material. In addition we re-evaluate the characters considered of phylogenetic importance for the placement of Cromeria and Grasseichthys among kneriids. We demonstrate that a surprisingly large number of these was based on misinterpretations of anatomical characters or was applied at the wrong level of inclusiveness. Cromeria nilotica, C. occidentalis, and Grasseichthys gabonensis share numerous developmental truncations, but no derived progressive characters, which makes it difficult to place them phylogenetically among kneriid gonorynchiforms. Grasseichthys is the most developmentally truncated of the three miniature species. In addition to bones that are absent in the two species of Cromeria (suprapreopercle, symplectic, ectopterygoid, hypural 6, scales), Grasseichthys lacks the premaxilla, coronomeckelian, urohyal, posttemporal, cranial rib, epineural and epipleural intermuscular bones, and the anterior most branchiostegal ray. We discuss characters that we think will be of significance in future studies on kneriid intrarelationships. Finally, we address the general issue that taxa that have undergone extreme reduction of body size, as the three miniature kneriids, frequently show severe developmental truncations that render it difficult to determine their phylogenetic position with confidence. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)