Recognising and defining a new crown clade within Stromboidea Rafinesque, 1815 (Mollusca, Gastropoda).

This paper defines a new crown clade Neostromboidea to separate the Strombidae, Rostellariidae, and Seraphsidae from their sister families Struthiolariidae and Aporrhaidae. There is significant value to understanding evolutionary processes within Stromboidea to recognise the universal similarity in the position of the eye on the end of peduncles and a diminished cephalic tentacle that arises from the middle to the end on that peduncle. This is in contrast to other members of the Stromboidea where the eye is located at the base of the cephalic tentacle. These physiological differences represent two set of organisms with divergent and independent evolutionary life histories and therefore these differences need to be identifiable within the nomenclature to bring meaning to the way we name things.

Introduction

Current Rafinesque, 1815 systematics has suffered from the effects of taxonomic inflation that has destroyed the evolutionary contextualisation that was once found within the historical nomenclature (Abbott 1960). This paper brings back that evolutionary contextualisation with the recognition of a new clade. There is a long history of morphologically based division with the . Early studies classified the Linné, 1758 in terms of gross anatomy, with the radula being the dominating feature in some classifications (Troschel 1856–1863; Mӧrch 1866; Cooke 1895; Thiele 1931), while other classifications were based on the structure and positioning of the mantle cavity and the buccal mass, or movement of the sole of the foot (MacDonald 1857; Cooke 1885, 1927). The historically recognised recent members of the Rafinesque, 1815 (s. l.) included the now separated Gabb, 1868 and Jung, 1974, both of which share a universal similarity in the positioning of the eye on the end of the peduncle, and a diminished cephalic tentacle that arises from the middle to the end on the peduncle. This contrasts with other members of the , the outgroups Gabb, 1868 and Gray, 1850, where the eye is located at the base of the cephalic tentacle, which is not reduced (Figure 1).

Figure 1.

The anatomy of representatives of the five families with Rafinesque, 1815 indicating the eye (e) eye stalk (es) and the tentacle (t) A Gabb, 1868 – (Gmelin, 1791) (Simone 2005, fig. 254) B Gray, 1850 – (Beck, 1836) (Simone 2005, fig. 297) C Jung, 1974 – (Linné, 1758) (Simone 2005, fig. 231) D Gabb, 1868 – (Röding, 1798) (Simone 2005, fig. 249) E Rafinesque, 1815 – Linné, 1758 (Simone 2005, fig. 164).

This study argues for the division of the crown clade based on shared morphological synapomorphies between families within this clade, which indicate a level of divergent and independent evolutionary life histories. This separation is needed to assist in resolving the higher order systematics of the to enable a more focused approach to understanding relationships and ancestral morphological states and patterns. There is a requirement for a name-bearing reference point that brings together the historically recognised members of the that have now been divided into three separate families, and to distinguish those families from the other taxa, to achieve basal resolution of the crown clade through a clear definition and diagnosis enabling the separation from sister taxa, thus enabling an evolutionary meaning to be brought to the nomenclature of the clade.

The aim of this paper is to divide the superfamily by morphological evidence into two distinctive evolutionary crown clades. Crown clades are clades that are defined based on living taxa. The crown clade focussed upon here contains the families , , and .

Systematic part

Linné, 1758

Cuvier, 1797

Ponder & Lindberg, 1987

Rafinesque, 1815

new clade

78b4c4ac-f813-5a28-b7db-9b382b6f5acd

Type.

The genus Linné, 1758.

Definition: The clade is nested within , with the characteristics outlined in the diagnosis, and contains taxa more closely related to Linné, 1758 () (Linné, 1758) () and (Linné,1758) () than Gabb, 1868 and Gray, 1850.

Diagnosis: The animal possesses eyes on the end of the peduncles. The cephalic tentacle is also located on the peduncle towards the distal end. The radula has a central rachidian tooth with three lateral teeth either side. The foot is laterally compressed, with a defined propodium and a metapodium. The shell form changes upon maturation with the development of an outer lip structure.

Remarks: is well supported in previous revisions and studies on the phylogeny of (Figure 2; Latiolais et al. 2006; Simone 2005). Simone (2005) marked this clade as “node 9” and noted that it was monophyletic within the . Latiolais et al. (2006) used as the outgroup in their analysis, which demonstrated a significant genetic distance between the taxa and . brings a higher level of resolution to the nomenclature by restoring the cladistic understanding and evolutionary meaning that had been lost as a consequence of taxonomic inflation (Abbott 1960; Simone 2005; MolluscaBase 2019).

Figure 2.

The new phylogeny of A and alternatives found from previous revisions B molecular analysis after Latiolais et al. (2006: 440, fig. 2) C anatomical analysis after Simone 2005: 261, fig. 388 D cladogram based on the nomenclature after MolluscaBase (2019).

Discussion

The falls within the clade which belongs to the highly variable invertebrate (). The ancestral resolution of is extremely unstable, with many conflicting views on the exact ancestors that provide a division between and the two immediate outgroups and (Gabb 1869; Roy 1994; Kiel and Bandel 2002; Bandel 2007). It is postulated that Gabb, 1868 and d’Orbigny, 1843 form the shared common ancestor of the new clade and the and based on gross morphology and the similarities with known stromboidal ancestors of .

The Cretaceous clade Kiel & Bandel, 1999, which is considered an ancestor of (Wenz 1938; Sohl 1960), is somewhat fluid in its content. However, recent revisions have clarified the taxonomic position of , which is now considered to be a descendant of the (Kiel and Bandel 1999). This position is based on the structure of the protoconch, the low height of the teleoconch, the lack of ornamentation typical of , the presence of a posterior rostrum with a groove, and, importantly, the extension of the callus from the inner lip, which covers a greater portion of the teleoconch (Popenoe 1983; Kiel and Bandel 1999).

Morphologically, recent members of the and differ from in having a broad rather flattened foot, as well as eyes on the base of the tentacles rather than on peduncles as with the (Gardner 1875). These recent members also differ for the most part in their feeding processes, whereby the animal lies buried and extends its proboscis to ‘grasp’ at potential food items, or they are filter feeders (Purchon 1977; Savazzi 1988, 1991). However, the buried grasping feeding habit is not a significant distinguishing characteristic separating and from the (Jung and Abbott 1967). Given the general instability of the aporrhaid group, it is not within the scope of this study to argue inclusiveness or provide a definition for that complex.

Conclusions

The incorporates those taxa that developed a basal sinus on the shell outer lip in conjunction with eyes placed on peduncles. The co-evolution of this shell structure and morphological trait allowed the eyestalk to protrude whilst the animal remained aperture face down on the substrate, protecting the soft parts from exposure. Furthermore, the movement of the cephalic tentacle towards the distal end of the eyestalk, thus protruding out from the basal sinus, enables the animal to achieve sensory awareness without any of the soft parts being exposed. There is much greater resolution within the with the recognition and naming of this clade, enabling researchers to focus on the evolution of either of the two divergent evolutionary trajectories of that clade’s members.