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A survey of 55 osteological and myological features provided the basis for a phylogenetic analysis of the genera of the loricariid subfamily Hypoptopomatinae. Eleven genera are recognized. Aceslridium, Hypoptopoma, Microlepidogaster, Otocinclus (sensu stricto), Otothyris, Oxyropsis, Par otocinclus, Pseudotocinclus, Schizolecis and an unnamed new genus from Venezuela are each hypothesized as monophyletic on the basis of at least one uniquely derived, unreversed character stale. Pseudotothyris is diagnosed by one character state representing evolutionary convergence. Otocinclus, as presently defined (Isbriicker, 1980), is paraphyletic. Species related to Otocinclus vestitus, the type species, are more closely related to other Hypoptopomatinae than to other Otocinclus species. The hypothesis of generic relationships described herein is based on the consensus among three equally parsimonious trees of length 89 steps (c = 0.652). Distribution of character states on that tree and character‐state evolution are discussed in terms of support for each lineage. Thirty‐three trees were found one step longer (length = 90, c = 0.644) than the shortest, most parsimonious trees. The consensus among these 33 trees revealed two additional unresolved polychotomies involving members of the tribe Otothyrini and a clade composed of Hypoptopoma, Oxyropsis and Aceslridium. A preliminary framework for the analysis of historical biogeography of the Hypoptopomatinae is discussed.

The "decoupling hypothesis" has been proposed as a mechanistic basis for the evolution of novel structure and function. Decoupling derives from the release of functional constraints via loss of linkages and/or repetition of individual elements as redundant design components, followed by specialization of one or more elements. Examples of apomorphic decoupling have been suggested for several groups of organisms, however there have been few empirical tests of explicit statements concerning functional and morphological consequences of decoupling. Using the loricarioid catfishes, we tested one particular consequence of decoupling, the prediction that clades possessing decoupled systems having increased biomechanical complexity will exhibit greater morphological variability of associated structures than outgroups having no such decoupled systems. Morphometric procedures based on interlandmark distances were used to quantify morphological variance at three levels of design at successive nodes in the loricarioid cladogram. Additional landmark-based procedures were used to localize major patterns of shape change between clades. We report significantly greater within-group morphometric variance at all three morphological levels in those lineages associated with decoupling events, confirming our predictions under the decoupling hypothesis. Two of 12 comparisons, however, yielded significant variance effects where none were predicted. Localization of the major patterns of shape change suggests that disassociation between morphological and functional evolution may contribute to the lack of fit between variance predictions and decoupling in these two comparisons.

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Morphology of the postotic laterosensory canal was surveyed across loricarioid and outgroup catfishes in order to resolve conflicting statements regarding homology and phylogenetic significance of intrinsic character variation. A pterotic branch is widespread among catfishes and has been identified as a synapomorphy for siluriforms, but its presence in loricarioid catfishes has been disputed. In contrast to previous statements that absence of a pterotic branch is synapomorphic for loricarioids, we confirm the presence of a pterotic branch in Nematogenys inermis and other trichomycterids, callichthyids, and loricariids. The pterotic branch is secondarily absent in scoloplacids and astroblepids. We present criteria for establishing homology of the pterotic branch and review character state optimization schemes on the currently accepted phylogeny. The postotic region of loricariids is further specialized in having an expanded swimbladder capsule that incorporates the trunk lateral line canal and has a lateral opening covered by a greatly expanded pterotic complex. The trunk lateral line enters the swimbladder capsule mesial to the pterotic lateral wall and passes anteromedially as a fleshy tube before forming the postotic canal in the pterotic, a morphology reported previously for a single loricariid representative. Variation in the relative extent and topographic position of postotic canal branches and other morphologies is diagnostic of certain loricariid taxa, suggesting a rich character complex of potential utility in phylogeny reconstruction. J. Morphol. 246:212–227, 2000. © 2000 Wiley‐Liss, Inc.

Aspects of the morphology of the opercular region of siluroids (catfishes) were examined to determine the homologies of the opercular bones of loricarioid catfishes, a diverse monophyletic neotropical group. Homology of the component structures was inferred by examination of three criteria: (1) functional relations to the jaw abduction mechanism of primitive halecostome fishes; (2) positional relations to the bones of the suspensorium and the path of the preopercular laterosensory canal; and (3) pattern of development of the preopercular canal and lateral cheek plates of loricarioid catfishes. The majority of siluroids have lost one or more of the preopercular canal exit branches to the skin surface present in most primitive siluroids. A number of specializations of the canal pathway and components of the opercular series have occurred in loricariids. Trichomycterids have the preopercular canal reduced to a short branch from the pterotic. Callichthyids share the presence of a preopercular canal with primitive siluroids, but have lost the communication of the canal between the preopercle and pterotic bones. Callichthyids and scoloplacids share one additional loss of a preopercle exit. The second of two subopercular elements of astroblepids is homologous with the interhyal. Astroblepids and loricariids have lost one additional preopercle exit, the interoperculo‐mandibular ligament and the corresponding biomechanical couple for lower jaw abduction. Loricariids have probably lost the interopercle as well. These derived features of loricariids and astroblepids are viewed as specializations for a particular feeding behaviour: scraping algae and detritus from the substratum.

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