A recent paper by Cuezzo et al. provides a revision of Argentinian species. Their abstract reads in full:
“Background: Land gastropods of the Dry Chaco merit special attention because they comprise a highly diverse but barely studied group. Clessinia Doering, 1875 are typical inhabitants of this ecoregion. The inclusion of their distribution areas into Spixia range, their shell shape similarities, and a former molecular study raised doubts on the monophyly of this genus. The present study review the species of Clessinia, under a morphological, geometric morphometrics, and molecular combined approach.
Methods: Adults were collected, photographed, measured, and dissected for anatomical studies. Shell ultrastructure was studied with scanning electron microscope. Geometric morphometric analyses on shells were performed testing if they gave complementary information to anatomy. Two mitochondrial genes, and a nuclear region were studied. Phylogenetic reconstructions to explore the relationships of DNA sequences here obtained to those of Clessinia and Spixia species from GenBank were performed.
Results: Species description on shell, periostracal ornamentation and anatomy is provided. We raised former Clessinia cordovana striata to species rank, naming it as Clessinia tulumbensis sp. nov. The periostracum, consisting of hairs and lamellae, has taxonomic importance for species identification. Shell morphometric analyses, inner sculpture of penis and proportion of the epiphallus and penis, were useful tools to species identification. Nuclear markers do not exhibit enough genetic variation to determine species relationships. Based on the mitochondrial markers, genetic distances among Clessinia species were greater than 10%, and while C. cordovana, C. nattkemperi, and C. pagoda were recognized as distinct evolutionary genetic species, the distinction between C. stelzneri and C. tulumbensis sp. nov. was not evident. Clessinia and Spixia were paraphyletic in the molecular phylogenetic analyses. Species of Clessinia here treated have narrow distributional areas and are endemic to the Chaco Serrano subecoregion, restricted to small patches within the Dry Chaco. Clessinia and Spixia are synonymous, and the valid name of the taxon should be Clessinia Doering, 1875 which has priority over Spixia Pilsbry & Vanatta, 1894.
Discussion: Our results support the composition of C. cordovana complex by three species, C. cordovana, C. stelzneri, and C. tulumbensis sp. nov. The low genetic divergence between C. stelzneri and C. tulumbensis sp. nov. suggests that they have evolved relatively recently. The former Spixia and Clessinia are externally distinguished because Clessinia has a detached aperture from the body whorl forming a cornet, periostracal microsculpture extended over dorsal portion of the peristome, five inner teeth on the shell aperture instead of three–four found in Spixia. Morphological similarities exists between both genera in shell shape, type of periostracum microsculpture, reproductive anatomy, besides the overlap in geographic ranges”.
This is an interesting paper for me, as more than 6 years ago I did research on type material in the London museum and found the species of the two ‘genera’ difficult to entangle, the more when phylogenetic results proved a paraphyletic relationship (Breure & Romero, 2012). This study comes to the same phylogenetic outcome as shown in the figure below. And for clarity: the Clessinia specimens used in our 2012 study were identified and supplied to us by one of the current authors and another Argentinian malacologist; they had more expertise and resources available.
The conclusion of the study by Cuezzo et al. is that Clessinia and Spixia are synonyms, with the older name (Clessinia) taking precedence. As such this is correct, but at the same time they conclude that the two ‘genera’ are morphological distinguishable (see the Discussion in their abstract). As taxonomists we have a solution for this: the subgenus…, which is treated in MolluscaBase as ‘alternate representation’. So instead of complete synonymisation, I would say there seems enough reason to distinguish the two as subgenera despite not being strictly monophyletic. The nomenclature then becomes:
Clessinia Doering, 1875
Clessinia (Clessinia) Doering, 1875 – type species Bulimus (Clessinia) stelzneri Doering, 1875.
Clessinia (Spixia) Pilsbry & Vanatta, 1898 – type species Clausilia striata Spix in Wagner, 1827.
See Cowie et al. (2004) for details on the names of Spix and Wagner.
In this paper also one new species is described. The authors say “The new species, Clessinia tulumbensis sp. nov. include Clessinia cordovana striata (Parodiz, 1939). The name striata has not been used here to avoid homonymy with Pupa striata Spix, 1827 [= Clausilia striata Spix in Wagner, 1827], the type species of Spixia, since in the present study the genera Clessinia and Spixia are proposed as synonymous. The new species with its own holotype and paratypes is defined based on live-collected material from which DNA sequences were obtained and the anatomy described. In this sense, although the Parodiz name is preoccupied, we are not replacing the name proposed by him in 1939 but creating a new species with its own type series”.
With Parodiz’ name mentioned by the authors as a full synonym, I fail to see the reason to introduce the name tulumbensis as a species novum. Although it is correct to replace the name of Parodiz to avoid homonymy, it is nonsense to say that you can introduce a replacement name with its own type series. The name tulumbensis is thus not a ‘sp.nov.’ but a nomen novum. And the holotype of Clessinia cordovana striata Parodiz, 1939 (MACN-In 9127) becomes automatically the holotype of Clessinia tulumbensis! The “holotype IBN 883” and the paratype material mentioned in this paper has no status other than being vouchers for this study.
Breure, A.S.H. & Romero, P.D., 2012. Support and surprises: a new molecular phylogeny of the land snail superfamily Orthalicoidea (Gastropoda, Stylommatophora) using a multi-locus gene analysis. – Archiv für Molluskenkunde, 141: 1–20.
Cowie, R.H. et al., 2004. The South American Mollusca of Johann Baptist Ritter von Spix and their publication by Johann Andreas Wagner. – The Nautilus, 118: 71-87.
Cuezzo, M.G. et al., 2018. From morphology to molecules: a combined source approach to untangle the taxonomy of Clessinia (Gastropoda, Odontostomidae), endemic land snails from the Dry Chaco ecoregion. – PeerJ, 6: e5986 (54 pp.).