Vexillum, VS.

Fedosov, Alexander, Puillandre, Nicolas, Kantor, Yuri & Bouchet, Philippe, 2015, Phylogeny and systematics of mitriform gastropods (Mollusca: Gastropoda: Neogastropoda), Zoological Journal of the Linnean Society 175 (2), pp. 336-359 : 354-356

VEXILLUM VS. ‘BASAL’ LINEAGES

The family Costellariidae is commonly perceived as consisting of the large and extremely diverse genus Vexillum and a few, sometimes bizarre, small genera like Zierliana or Visaya . Our analysis evidences a series of previously unrecognized costellariid lineages, essentially consisting of still-unnamed taxa. The four costellariid lineages C-I–C-IV ( Fig. 3) correspond to four to-be-established genera with numerous undescribed new species, which will be named elsewhere.

Although Vexillum species with multicuspidate rachidians occupy the most derived position in our phylogenetic trees, costellariids other than Vexillum , which are characterized by tricuspidate rachidians, form deep lineages. This radula morphology, which can be regarded as plesiomorphic, is characteristic also for Ceratoxancus , Latiromitra , and Exilia , as well for some other lineages of the superfamily Muricoidea ( Fedosov & Kantor, 2010) . The difference in radula morphology between Vexillum and the ‘basal Costellariidae’ parallels differences in the structure of the foregut glands. Earlier studies on the anatomy of the digestive system of costellariids revealed a large, bulky gland of Leiblein in the only studied species referable to ‘basal Costellaridae’ ( Fedosov & Kantor, 2010), and a similar gland of Leiblein is present in Latiromitra ( Bouchet & Kantor, 2000) , Ceratoxancus ( Kantor & Bouchet, 1997) , and Exilia ( Harasewych, 1987; Kantor et al., 2001). By contrast, many species of Vexillum are characterized by the reduction of the gland of Leiblein and the presence of a long, highly convoluted, glandular tube, originating from the stripped-off folds of the midoesophagus, and resembling the venom gland of the Conoidea . An intermediate morphology, with a still noticeable gland of Leiblein and already a rather short tubular duct, was found in Vexillum cf. salisburyi Cernohorsky, 1976 , and V. rubrum ( Fedosov & Kantor, 2010) , the latter species falling into the Vexillum s.l. group. The origin and evolution of the genus Vexillum thus appear to have been linked to the appearance of a multicuspidate rachidian and a rearrangement of the foregut that possibly allowed these snails to develop a new feeding strategy(ies), permitting their explosive radiation. It is however at present unclear whether the multicuspidate rachidian appeared only once in the evolutionary history of the Costellariidae . It could be a synapomorphy of the Vexillum s.l. clade (as supported by our MCPV-4G data set), or it could have evolved independently in different Costellariidae lineages. The presence of tricuspidate rachidians in some species of Vexillum (Pusia) complicates the picture, and the affinities of these species need to be further examined. Moreover, the radula of ‘ V. ’ balutense of the ‘basal’ clade C-IV, with its rachidian bearing two rudimentary cusps flanking the central cusp, may be considered intermediate between the simple tricuspidate and true multicuspidate.

The discovery of multiple costellariid taxa with tricuspidate rachidians and a well-developed, nontubular gland of Leiblein renders the morphological difference between the Costellariidae and traditional Ptychatractidae quite subtle. In fact, the foregut anatomy of ‘basal Costellariidae’ is much closer to the one found in Ptychatractidae than it is to the one found in Vexillum . Although all costellariids as currently construed lack an operculum, and an operculum is still present in Ceratoxancus and Latiromitra , we nevertheless suggest the transfer of Ceratoxancus and Latiromitra from the Ptychatractidae to the Costellariidae . Although this rearrangement extends the boundaries of the Costellariidae , it remains a monophyletic group. Altogether, the transfer of Ceratoxancus and Latiromitra to Costellariidae , the revised status of Zierliana and Protoelongata , and the future erection of new genera for the newly identified deep-water lineages will significantly impact the generic composition of the family.