The genus Pseudostomella is comprosed of 17 species distributed in distinct coastal and shelf areas of the world. This taxon is characterized by a fleshy preoral apparatus, which is unique to the family and easily distinguishes species of Pseudostomella from species of the other 7 genera. The present study has two aims: 1) provide a phylogenetic reconstruction based on morphological characters using all valid species of the genus Pseudostomella; and 2) perform a Brooks Parsimony Analysis (BPA) based on new and published phylogenetic analyses to determine the biogeographical history of the thaumastodermatid lineage. The cladistic analysis conducted here supports the monophyly of Pseudostomella and the internal branchs are also supported by a relative high number of well-defined synapomorphies. Regarding historical biogeography, our results suggest that the thaumastodermatid diversification first occurred in epicontinental seas from the southern margin of Gondwana. These findings for thaumastodermatid gastrotrichs are similar to those of other historical biogeographical studies on marine invertebrates.

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