The phylogenetic placement and circumscription of the genus Synotaxus (Araneae: Synotaxidae), a new species from Guyana, and notes on theridioid phylogeny

The genus Synotaxus Simon, 1895 is reviewed. Modern systematic work has challenged the classical placement of Synotaxus as an argyrodine theridiid. Its placement is evaluated in a phylogenetic analysis containing a wide selection of theridiids, including all major argyrodine genera. For the analysis, two published matrices are fused to produce a dataset containing 83 orbicularian taxa and 302 morphological characters. Although the two matrices share only 10 taxa and 33 characters, a single most parsimonious tree is obtained. The results are congruent with results from each independent matrix, and unambiguously corroborate the placement of Synotaxus outside Theridiidae. Some superficial similarities, such as the elongate abdomen extending far beyond the spinnerets, are clearly convergent in the two taxa. Synotaxus, furthermore, lacks the suite of synapomorphies defining Theridiidae. Thus, its transfer out of Theridiidae is corroborated, and a sister relationship with Chileotaxus Platnick, 1990 is proposed, based on similarities in web and somatic morphology. A synapomorphy-based circumscription of the genus is given. Synotaxus waiwai, sp. nov. is described, Synotaxus monoceros (Caporiacco, 1947), previously only known from males, is redescribed and synonymised with S. pupularum, Exline & Levi, 1965, syn. nov., previously only known from females. A description of the webs of both species is given, as well as that of Chileotaxus sans Platnick, 1990. Many further Synotaxus species remain to be described.