Xenocratenidae Martynov 2020

Family Xenocratenidae Martynov et al. 2020 (Figs 1, 2; Table 4) Martynov et al. 2020: 14. Diagnosis: Body wide. Notal edge completely reduced. Cerata not stalked, in few continuous rows, no more than five anterior rows (fifth row rudimentary, if present). Rhinophores smooth. Anus acleioproctic. Masticatory edges of jaws commonly bear single row of compound denticles. Radula formula 0.1.0. Central radular teeth pectinate, with strong non-compressed cusp and additional denticles. Distal receptaculum seminis present. Vas deferens long, prostate indistinct, narrow tubular. Supplementary gland present, inserts to penis. Massive external permanent penial collar absent. Penis internal, conical, unarmed. Genera included: Xenocratena Odhner, 1940. Remarks: The family Xenocratenidae, aside from being phylogenetically sister to Murmaniidae (Figs 1, 2), otherwise fundamentally differs from it and evidently displays paedomorphic reduced features in the presence of a small, non-massive body with a completely reduced notal edge, cerata organized in a few rows, and an acleioproctic anus (see: Martynov et al. 2020; Synopsis above). The family Xenocratenidae provides an important insight for the current persistent attempts to lump both fine-scale and large-scale diversity into highly heterogeneous and para- and polyphyletic ‘Cuthona’ and ‘Tenellia’ (see below), because since the original description of the genus Xenocratena (Odhner 1940), it was commonly listed either as a doubtful taxon, or as a synonym of ‘Cuthona’ (Miller 1977, Williams and Gosliner 1979). However, after the first rediscovery of that amazing taxon less than 10 years ago, it has been shown that Xenocratena and Xenocratenidae belong to a profoundly different lineage compared to both families Cuthonidae and Trinchesiidae (Martynov et al. 2020; present study) (Figs 1, 2). Furthermore, because Xenocratena not only has a similar name to one of the most enigmatic metazoan Xenoturbella, but inhabits fundamentally same geographic location, depths, and soft substrates, the conclusions of the paedomorphic reduction of Xenocratena due to inhabiting soft marine sediments, were also applied to the potential origin of Xenoturbella (Martynov et al. 2020), and these conclusions were confirmed most recently by a detailed analysis of the Xenoturbella genome (Schiffer et al. 2024).