Revisiting Calohypsibiidae and Microhypsibiidae: Fractonotus Pilato, 1998 and its phylogenetic position within Isohypsibiidae (Eutardigrada: Parachela)

Abstract

Tardigrades constituting the order Parachela are characterised by a generally uniform, worm-like external morphology. Taxa with larger, well-pronounced dorsal cuticular protuberances, tubercles and spines are found in seven genera representing various evolutionary lineages within Hypsibioidea and Isohypsibioidea:

Deceptive conservatism of claws: distinct phyletic lineages concealed within Isohypsibioidea (Eutardigrada) revealed by molecular and morphological evidence

“…in the newly erected Dianea, Ursulinius and Grevenius gen. nov., respectively; see Appendix). However, as recently suggested by Gąsiorek et al (2019), some Isohypsibius spp. appear more closely related to Fractonotus than to Isohypsibius s.s., which could explain the current paraphyletic character of Isohypsibius with respect to Fractonotus ( fig.…”

“…3A, 6B) as well as densely arranged smooth tubercles that cover the entire dorsum and limbs (figs. 3A, 6A; Gąsiorek et al, 2019). Isohypsibioid gibbosities can be generally divided into two types: small, weakly demarcated (almost flat in LM) and pointy gibbosities present in Dianea gen. nov. (figs.…”

“…The problematic systematic position of calohypsibiid genera and species (order Hypsibioidea) has been a subject of long debate Guil et al, 2013;Bertolani et al, 2014a;Gąsiorek et al, 2019) et al, 2016). Interestingly, a similar buccal apparatus morphotype is also present in two doryphoribiid genera: Apodibius and Doryphoribius (although with two macroplacoids in some species), but absent in the remaining doryphoribiid genera (Pseudobiotus, Thulinius and Grevenius gen. nov.).…”

“…In fact, this group was erected relatively late mainly because for decades various taxa with Isohypsibius and Hypsibius type claws were traditionally grouped into a single order Hypsibioidea Guil et al, 2019. Although the erection of Isohypsibioidea clarified the taxonomy of the Eutardigrada, the few efforts to resolve phylogenetic relationships within the order suffered from insufficient sampling and resulted in prevailing polytomies Gąsiorek et al, 2019). So far, only a single study identified a monophyletic lineage within Isohypsibioidea.…”

“…We uncovered four distinct phylogenetic lineages, corresponding to two previously identified and two new families. The largest tardigrade genus, Isohypsibius Thulin, 1928, as indicated earlier Cesari et al, 2016;Gąsiorek et al, 2019), is artificial and comprises at least five distinct evolutionary lineages. For three of these, we propose new formal taxonomic ranks and we demonstrate, for the first time, that even slight qualitative differences in claw anatomy, the number of macroplacoids in the pharynx, and the presence of cuticular gibbosities can be of high taxonomic importance in delineating isohypsibioid genera.…”