Abstract
Seven species of four cletodid genera previously described or reported from Mexico (Cletodes, Enhydrosoma, Geehydrosoma, and Stylicletodes) and an undescribed genus presumably related to Sphingothrix and Triathrix, were found in sediment samples from a coastal system in north-western Mexico. The Bayesian analysis of 95 morphological characters of thirty-four cletodid genera revealed that the new genus, Chalarosthrix gen. nov., is closely related to Sphingothrix. The clade Sphingothrix-Chalarosthrix bears a sister group relationship with Triathrix. These three genera constitute a monophyletic group defined by the presence of a posterior chitinous extension of the cephalothorax with four sensillum-bearing socles, and the presence of a median tube-pore on the anal operculum. Other synapomorphies for Sphingothrix and Triathrix proposed by earlier researchers are re-evaluated upon the latest definition of the Cletodidae. Our analysis supports previous studies in that Sphingothrix can be objectively defined by i) the reduction of the distal outer seta of the last exopodal segment of the first swimming leg being at most as long as the subdistal outer spine, and ii) the distal inner seta of the same segment inclined outwards intersecting the distal outer element at its base. On the other hand, no (aut)apomorphies were detected for Triathrix. The new genus, Chalarosthrix gen. nov., is defined here by the apomorphic secondary loss of the inner element of the second endopodal segment of the first swimming leg. Some comments on the relationships of the new genus are given.
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