Stromboidea

Stromboidea View in CoL

Varices occur widely in the Stromboidea . Most are enlarged rounded ribs, but some are adorned with spines, as in the aporrhaids Spiniloma and Spinigeropsis (Spinilomatidae) of the Early Jurassic, Pietteia (Dimorphosomatinae) of the Middle Jurassic, and Diempterus ( Aporrhainae ) of the Late Jurassic ( Kollmann, 2009; Gründel, Nützel & Schulbert, 2009). In most aporrhaids, varices occur at half-whorl intervals, but in some aporrhaids and most strombids, varices are more closely spaced and confined to the spire whorls ( Fig. 2H View Figure 2 ).

Largely based on Kollmann’s (2009) fossil-based evolutionary scenario, we infer three independent origins of varices in the Stromboidea . No varices occur in the oldest member of the group, the genus Dicroloma ( Aporrhainae ) of the Early Jurassic (Sinemurian) ( Kollmann, 2009; Gründel et al., 2009). Varices evolved first in Spiniloma (Spinilomatidae) during the Sinemurian, separately in Pietteia (Dimorphosomatinae) in the Middle Jurassic, and a third time in a lineage of Aporrhainae including the Late Jurassic genera Dicroloma and Diempterus . According to Kollmann’s (2009) scenario, in the Aporrhaidae , the varices of the Early Cretaceous (Barremian) to Recent Arrhoginae (observed in the genera Arrhoges , Graciliala , Latiala , and Mexopus ) were inherited from dimorphosomatine ancestors, as were varices in some members of the Anchurinae ( Drepanochilus and Helicaulax ), which originated in the Barremian. The varicate Rostellariidae (Campanian Late Cretaceous to Recent) are diphyletic with separate origins in the varicate Arrhoginae ( Graciliala and Latiala ). The Strombidae of the Cenozoic are derived via the Rimellinae from the Calyptraphorus group of Rostellariidae ( Kronenberg & Burger, 2002; Kollmann, 2009).

Finally, varices occur in the Late Mesozoic family Tylostomatidae (genera Pterodonta and Tylostoma ). These varices are visible on the internal moulds (steinkerns), with generally two per whorl ( Squires & Saul, 2004). Although Bandel (2007) considers them stromboids, Kollmann (2009) rejects this hypothesis on the grounds that a rostrum and an expanded outer lip, both characteristic of Stromboidea , are absent. Squires & Saul (2004) place this family as Caenogastropoda incertae sedis. The phylogenetic placement of Tylostomatidae remains doubtful, but we maintain its placement in the Stromboidea for now and consider their varices to have arisen separately.

Manystromboideancladeslackvarices.InKollmann’s (2009) scenario, some of these clades are derived from nonvaricate Aporrhainae . These include the subfamilies Pterocerellinae and Harpagodinae . A loss of varices apparently occurred in some Dimorphosomatinae and in its derived clade Pugnellinae , as well as groups within the subfamilies Anchurinae and Arrhoginae , and separately in the temperate southern-hemisphere Struthiopterinae , which Kollmann (2009) derives from varicate Anchurinae . Within Strombidae , true varices have been lost in such genera as Euprotomus , Harpago , Lambis , Lobatus (some species), Mirabilistrombus , and Tricornis , but how often the loss of varices occurred in this group or in the Aporrhaidae remains unclear. The living southern-hemisphere family Struthiolariidae and the Paleocene to Recent tropical Seraphsidae likewise lack varices, but the absence of a phylogenetic hypothesis prevents conclusions about whether this absence is primary or secondary.