Historical Transformation of Functional Design: Evolutionary Morphology of Feeding Mechanisms in Loricarioid Catfishes

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Abstract
The current hypothesis of relationship among the Loricarioidea (a diverse group of Neotropical catfishes) is used as the basis for a case study of historical transformation in the feeding mechanism in this monophyletic clade. The historical approach to functional morphology is discussed and is used in this analysis as a means of examining the evolutionary sequence of structure-function transformation. The distribution of structural novelties in the feeding mechanism and their functional correlates are mapped onto the current hypothesis of loricarioid phylogeny. Three major steps in the transformation of the feeding mechanism are identified. The first was the acquisition of highly mobile premaxillae in the lineage leading to the Callichthyidae, Astroblepidae, and Loricariidae. The second step occurred in the lineage leading to the Astroblepidae and Loricariidae: both families possess highly mobile premaxillae with new muscular insertions and movements that are independent of maxillary mobility. Another specialization at this level was the loss of the interoperculo-mandibular ligament and the acquisition of independent bilateral lower jaw mobility via loss of the tight connection between right and left sides of the lower jaws. The third major step occurred in the lineage leading to the Loricariidae. Further subdivision of the jaw adductor muscle resulted in an additional mesial subdivision with a direct muscle insertion onto the premaxillae. A neomorphic structure, the intermandibular cartilage plug (also shared by astroblepids) is specialized in loricariids with a direct attachment to the hyoid, resulting in an additional biomechanical linkage between the hyoid and mandible. A major trend in the evolution of the loricarioid feeding mechanism is the increase in the number of biomechanical linkages. This is hypothesized to be causally related t o morphological and trophic diversity. Two explicit predictions are made testing the hypothesis that decoupling of primitively constrained biomechanical elements is related to increased morphological and functional diversity in descendant species.