The functional morphology and inferred biology of the enigmatic South African 'quadrivalve' bivalve Clistoconcha insignis Smith, 1910 (Thracioidea: Clistoconchidae fam. nov.): another anomalodesmatan evolutionary eccentric

摘要

The commarginally-ribbed shell of Clistoconcha insignis is anteriorly globose, posteriorly short, tapered and characterised by dorsal cracks, which arise from just posterior to the juvenile shell and extend to the ventral margin of each valve. In effect, thereby, a unique quadrivalve structure is formed. Shell morphology suggests that following a short planktonic larval phase, an approximately isomyarian, crawling, juvenile is formed. Upon choice and adoption of a place suitable for adult occupation, which is believed to be crevices and pits on the undersides of large stones, situated within shallow subtidal sands, the adult shell is formed. With growth, the shell becomes distorted and, finally, assumes a form in which the valves are marginally united, closing the pedal gape, except posteriorly (to allow for extension of the siphons) and mid-dorso-ventrally surrounded by a calcareous callus that is formed from secretions produced by glands of the outer epithelium of the mantle. Dissection, sectioning and examination of a single preserved specimen have confirmed that Clistoconcha insignis is a member of the Anomalodesmata and is assigned to a new family, Clistoconchidae, with affinities to the Thraciidae (Thracioidea). It is, however, structurally and functionally, and thus convergently, most similar to the endobenthic representatives of the Laternulidae and Periplomatidae, which also possess, albeit only partial, dorso-ventral shell cracks. The secretion of a callus, probably to sustain the shell's integrity following fracturing as a result of adduction stresses and damage, is reminiscent of the similarly anomalodesmatan representatives of the Clavagelloidea but is produced in a uniquely different and analagous manner. The sunken and posteriorly oriented ligament of C. insignis concludes a trend in the Thracioidea towards posterior shell and ligament foreshortening to create a unique form that dramatically broadens our understanding of not only anomalodesmatan, but also bivalve, adaptive radiation.