A fractured homecoming: Fractonotus reinstated in Calohypsibiidae (Eutardigrada: Hypsibioidea)
Year: 2026
Type: article
Abstract: Relationships among two eutardigrade families Calohypsibiidae and Microhypsibiidae remain unclear, chiefly due to the lack of molecular data. A particularly entangled history of taxonomic reshuffling concerns the genus Fractonotus Pilato, 1998, a minute sculptured eutardigrade with unclear affinities, transferred several times between different families and superfamilies, and most recently classified again as a microhypsibiid. Here, we summarise the history of systematic arrangement of calohypsibiids and microhypsibiids over decades and – thanks to rich material collected in Greenland, India, and New Zealand – we deliver novel morphological and genetic data for Fractonotus, Calohypsibius Thulin, 1928, and Microhypsibius Thulin, 1928. These data allowed us to conclusively re-transfer Fractonotus to Calohypsibiidae, and pinpoint its phyletic position as a sister genus of Calohypsibius. Both calohypsibiids and microhypsibiids are most likely closely related to acutuncids within Hypsibioidea. Moreover, Calohypsibius verrucosus (Richters, 1900) stat. rev. is redefined based on the original description, instead of on an erroneous depiction by Thulin later repeated in literature (and which, in fact, represents an isohypsibiid). Finally, Calohypsibius is divided into two groups, the ornatus and verrucosus assemblages; the latter having claws of the morphotype intermediate between the C. ornatus and the Fractonotus type claws.
Language: English
Domain: Life Sciences
Open Access status: hybrid
Estimated APC paid: $3,541
OpenAlex ID: W7204793918