INTRODUCTION

Small size and sparse number of specimens raise difficulties for detailed examination of the skeletal anatomy of several groups of fossil and extant fishes. Traditional methods such as maceration or skeletal mounting are not ideal because both increase the risk of losing skeletal elements, and positional relationships are lost or compromised. Comparative morphologic and phylogenetic research on these groups is thus hindered. To address these problems, we used microfocus X-ray computed tomography (MicroCT) to resolve the osteological details needed to conduct comparative anatomical and systematic studies of very small fishes.

Here we report the results of our first case study, an analysis of the rare and diminutive trichomycterid catfish Sarcoglanis simplex (Figure 1). Sarcoglanis simplex belongs to the trichomycterid subfamily Sarcoglanidinae. This is a small clade of six genera and eight species, all of which are miniature, sand-dwelling catfishes found in rapidly flowing upland streams and rivers. All species of sarcoglanidines have been discovered during the last 40 years (Myers and Weitzman 1966; de Pinna 1989; de Pinna and Starnes 1990; Costa and Bockmann 1994; de Pinna and Winemiller 2000; Schaefer 2003; Costa et al. 2004). The distinctive characteristics marking this clade and its relationships among trichomycterids were treated by de Pinna (1989), de Pinna and Starnes (1990), Costa (1994), and Costa and Bockmann (1994). The osteology of the nominal genus Sarcoglanis, however, was not previously described. The detailed anatomical data presented here are thus critical to testing the phylogeny of the Sarcoglanidinae.

Until recently, Sarcoglanis was known only from the holotype specimen collected in the San Gabriel Rapids of the Rio Negro, Brazil (Myers and Weitzman 1966). Expeditions led by researches at the Academy of Natural Sciences in Philadelphia (ANSP) to the Essequibo River, Guyana in 2002-2003 and Rio Ventuari, Venezuela in 2004, yielded several additional specimens of Sarcoglanis. Representatives of these were made available by M. Sabaj and were identified as S. simplex by J. N. Baskin (personal commun., 2004). These newly collected specimens offer us, for the first time, the opportunity to describe the bony skeleton of Sarcoglanis and to identify diagnostic characters useful for resolving its phylogenetic affinity.