Genus Fractonotus Pilato, 1998

Fractonotus Pilato, 1998: 132 .

AMENDED DIAGNOSIS. — Small isohypsibiid (rarely exceeding 200 μm, Fig. 1). Cephalic elliptical organs present (Fig. 7A). Dorsum and limbs covered with densely arranged, blunt protuberances. Six peribuccal lobes present (Fig. 6A). Apophyses for the insertion of stylet muscles (AISM) asymmetrical with respect to the frontal plane – the dorsal apophysis subdivided into two portions: the anterior portion in the shape of a slightly convex longitudinal thickening (and the posterior portion as weakly developed blunt hook); the ventral apophysis in the shape of a mild and long ridge (Fig. 9A). Very large pharyngeal apophyses and placoids in the muscle pharynx. Stylet furcae of the Fractonotus - type, i.e. with broad, trapezoid base, thin arms and rounded apices (Figs 8D, 10A). Claws of the modified Isohypsibius - type, with triangular bases and strongly curved claw branches (Fig. 12A, B). Accessory points symmetrical or occasionally asymmetrical. Smooth eggs laid in exuviae.

DIFFERENTIAL DIAGNOSIS. — Fractonotus shares pronounced cuticular sculpturing with some species of six other parachelan genera: Calohypsibius Thulin, 1928, some Ramazzottius Binda & Pilato, 1986, Hypsibius Ehrenberg, 1848, Pilatobius Bertolani, Guidetti, Marchioro, Altiero, Rebecchi, Cesari, 2014, Doryphoribius Pilato, 1969 and Isohypsibius Thulin, 1928, but it can be readily distinguished from these genera by the morphology of the stylet furcae (square/trapezoid in Fractonotus vs narrower and more rectangular in the latter genera; compare Figs 7B, D; 10 A-C). Furthermore, Fractonotus differs from Ramazzottius, Hypsibius and Pilatobius by having Isohypsibius -like claws (claws of the latter genera are of the Hypsibius or of the modified Hypsibius- type). Moreover, the genus differs specifically from:

– Calohypsibius Thulin, 1928 ( Hypsibioidea: Calohypsibiidae), by having a different type of cuticular sculpture (roundish or oval tubercles covering the entire dorsum and limbs with smooth dorsal pebble-shaped plaques in Fractonotus, Fig. 5 A-D vs multangular or star-like tubercles and occasional spines arranged less densely in Calohypsibius, Fig. 5E, F), different structures surrounding the mouth opening (six soft and large peribuccal lobes in Fractonotus, Fig. 6A vs six small well defined papulae in Calohypsibius, Fig. 6B), a reversed morphology of the dorsal apophysis for the insertion of stylet muscles (an anterior thickening and a tiny posterior hook in Fractonotus, Fig. 7 E-G vs an anterior large blunt hook and a slight posterior thickening in Calohypsibius, Fig. 9A, B), and by claw morphology (modified Isohypsibius - type claws with pseudolunulae, triangular bases, and elongated, strongly curved branches with conspicuous accessory points in Fractonotus, Figs 11 A-D; 12A, B vs very small, rigid, with the base width equal to the sum of the primary and secondary branch widths, with the vertical septum between the two branches, and without pseudolunulae in Calohypsibius, Figs 11E; 12C, D).

– Doryphoribius Pilato, 1969 ( Isohypsibioidea: Isohypsibiidae), by the presence of elliptical organs on the head (absent in Doryphoribius), and by the absence of the ventral lamina on the buccal tube (ventral lamina present in Doryphoribius).

– Isohypsibius Thulin, 1928 ( Isohypsibioidea: Isohypsibiidae), by the presence of elliptical organs on the head (absent in Isohypsibius), a different shape of AISM (asymmetrical with respect to the frontal plane in Fractonotus, Fig. 7E vs ridges symmetrical with respect to the frontal plane Isohypsibius, Figs 7H, I; 9A, C), and by the claw morphology (modified Isohypsibius - type claws with triangular bases, especially well-marked on the fourth pair of legs, in Fractonotus vs Isohypsibius - type claws with stalk-like bases in Isohypsibius, Figs 11H, I; 12E, F).

COMPOSITION AND REMARKS

Currently only three species, Fractonotus caelatus (the nominal taxon), F. verrucosus n. comb. and F. gilvus n. comb., are assigned to the genus. The three species are placed in the single genus because they share a number of taxonomically important traits: AISM shape, the presence of elliptical cephalic organs, two granular macroplacoids in the pharynx, and the type of cuticular sculpturing. On the other hand, Pilato (1998) described the claws of F. caelatus as of the Microhypsibius type, whereas claws in F. verrucosus n. comb. and F. gilvus n. comb. are closer to Isohypsibius type claws. Therefore, given the differences in claw morphology, there is a possibility that F. verrucosus n. comb. and F. gilvus n. comb. belong to a new isohypsibioid genus, and are only delusively similar to Fractonotus due to convergent evolution in the remaining traits. Nevertheless, the majority of traits suggest that all three species should be placed in Fractonotus . Biserov (1986) misinterpreted the AISM of F.gilvus n. comb. (Fig. 3 therein) as Isohypsibius - type AISM, but our observations of the type material confirm that the species has the AISM of the Fractonotus - type. However, there are more Isohypsibius and Hypsibius species, that exhibit cuticular sculpturing similar to that of Fractonotus . Thus, they may in fact belong to Fractonotus rather than Isohypsibius or Hypsibius . Nevertheless, we refrained from enacting more transfers, as a careful examination of individuals is needed to confirm whether these species, in addition to cuticular sculpturing, also exhibit other characteristics of Fractonotus .