Iberellus

Altaba, Cristian R., 2022, Nomenclature of Helicidae (Gastropoda: Pulmonata) endemic to the Balearics, Nemus 12, pp. 168-186 : 178-179

Iberellus and Allognathus are separate genera

The reasoning behind placing Iberellus within the same genus as Allognathus (e.g., Thiele, 1931; Bank et al., 2001) is that both genus-level taxa are considered too closely related, either anatomically ( Gasull, 1964) or (phylo)genetically ( Chueca et al., 2013, 2015; Neiber et al., 2021). Gasull (1964, 1967, 1971) placed Allognathus as a subgenus of Iberellus, but that was an incorrect priority inversion. The Balearic branch within Allognathini constitutes a well-defined lineage, just as with the Macaronesian one. In the latter, different genera are recognized ( Neiber et al., 2021), with morphological and genetic differentiation equivalent to that existing inside the former; there is a considerable phylogenetic distance between the monospecific Allognathus and the Iberellus clade. So, one may have to choose between synonymizing genera and the alternative —equally arbitrary— of increasing supraspecific taxa, eventually making them all monotypic. Either path would be rather useless.

If the genus rank has to bear any meaning, it is in relation to diverging adaptive fields ( Wood & Collard, 1999; Cela-Conde & Altaba, 2002). Under this criterion, both Allognathus and Iberellus deserve full genus rank, given that their adaptations are clearly different ( Breure & Gittenberger, 1982; Altaba & Ríos Jiménez, 2021; Juárez-Ruiz & Altaba, 2022). They share a rather uniform Bauplan in their reproductive anatomy, but lack of obvious differences therein occurs in other helicoid clades. Moreover, placing too much weight on the genitalia is a monothetic concept of pulmonate systematics, an epistemological error leading to unrealistic classifications (as already pointed out by Hoagland & Davis, 1987). A comparable mistake was that of Westerlund (1902), who erected the monotypic family Allognathidae on the basis of shell and radula, ignoring everything else.