Short Communication |
Corresponding author: Stephen J. Maxwell ( stephen.maxwell@my.jcu.edu.au ) Academic editor: Thierry Backeljau
© 2019 Stephen J. Maxwell, Aart M. Dekkers, Tasmin L. Rymer, Bradley C. Congdon.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Maxwell SJ, Dekkers AM, Rymer TL, Congdon Bradley C (2019) Recognising and defining a new crown clade within Stromboidea Rafinesque, 1815 (Mollusca, Gastropoda). ZooKeys 867: 1-7. https://doi.org/10.3897/zookeys.867.34381
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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.
Aporrhaidae, Rostellariidae, Seraphsidae, Strombidae, Struthiolariidae
Current Stromboidea 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 (
The anatomy of representatives of the five families with Stromboidea Rafinesque, 1815 indicating the eye (e) eye stalk (es) and the tentacle (t) A Strutholariidae Gabb, 1868 – Tylospira scutulata (Gmelin, 1791) (
This study argues for the division of the crown clade Stromboidea 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 Stromboidea 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 Strombidae that have now been divided into three separate families, and to distinguish those families from the other Stromboidea 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 Stromboidea 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 Seraphsidae, Strombidae, and Rostellariidae.
The genus Strombus Linné, 1758.
Definition: The clade is nested within Stromboidea, with the characteristics outlined in the diagnosis, and contains taxa more closely related to Strombus pugilis Linné, 1758 (Strombidae) Terebellum terebellum (Linné, 1758) (Seraphsidae) and Tibia fusus (Linné,1758) (Rostellariidae) than Struthiolariidae Gabb, 1868 and Aporrhaidae 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: Neostromboidea is well supported in previous revisions and studies on the phylogeny of Stromboidea (Figure
The Neostromboidea falls within the clade Stromboidea which belongs to the highly variable invertebrate Gastropoda (Mollusca). The ancestral resolution of Neostromboidea is extremely unstable, with many conflicting views on the exact ancestors that provide a division between Neostromboidea and the two immediate outgroups Struthiolariidae and Aporrhaidae (
The Cretaceous clade Pugnellidae Kiel & Bandel, 1999, which is considered an ancestor of Strombidae (
Morphologically, recent members of the Struthiolariidae and Aporrhaidae differ from Neostromboidea in having a broad rather flattened foot, as well as eyes on the base of the tentacles rather than on peduncles as with the Neostromboidea (
The Neostromboidea 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 Stromboidea 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.