Parsimony analysis suggests derivation of the Bivalvia from monoplacophorans rather than from rostroconchs, and additionally indicates that a phylogenetic classification of the Bivalvia can be achieved by erecting the superorder Nuculaniformii nov. and the order Nuculanoida nov. for the superfamily Nuculanoidea; relegating all other palaeotaxodonts to the superorder Nuculiformii; restricting the order Nuculoida to the families Nuculidae and Pristiglomidae; expanding the order Solemyoida to include ctenodontid genera as basal plesions; restricting the superorder Heteroconchia to palaeoheterodonts and heterodonts, exclusive of the Modiomorphidae; relegating the new family Evyanidae, the Colpomyidae, Matheriidae and Modiolodontidae to near-basal plesion status within the superorder Pteriomorphia; restricting the Mytiloida to the superfamily Mytiloidea, inclusive of modiolopsid genera as basal plesions; placing Ortonella as a basal plesion within the Cyrtodontoida; expanding the order Pectinoida to include the Myodakryotidae and the suborders Limina and Pectinina; and expanding the superfamily Arcoidea to include the frejid genera and Catamarcaia as basal plesions, and the family Glyptarcidae. Modiomorphid anomalodesmatans appear to be more closely related to the Pteriomorphia than to the Heteroconchia, and Evyana lies close to the common ancestry of modiomorphids and colpomyid pteriomorphians. Arcoids may have evolved from left-right symmetrical but otherwise rhombopteriid-like ancestors, rather than from actinodontoids or directly from cyrtodontids. The new family Eodonidae is proposed to distinguish the nacreous genus Eodon from the non-nacreous Astartidae within the superfamily Crassatelloidea.
The microstructure of the non-window portions of the shell of Corculum cardissa resembles other Fraginae, with predominantly fibrous prismatic outer, branching crossed lamellar middle, and complex crossed lamellar inner layers. Both the anterior and posterior windows in its shell reflect reduced pigmentation and incursion of the outer shell layer, but the posterior windows involve deeper incursion plus reduction of the outer and middle sublayers of the outer shell layer and microstructural modification of the middle shell layer to enhance light transmission. The planoconvex shape of the posterior windows has more likely evolved to direct and focus light toward the deeper, zooxanthellae-rich gills and anterior mantle, than to merely disperse light.
Observations of the hinge structure of the Middle Devonian species
Ptychodesma knappianum
Hall and Whitfield 1872 confirm the cyrtodontid affinities of this once problematic genus. The shell microstructure of
P. knappianum
supports Douvillé’s (1913) hypothesis concerning the ancestral nature of nacreous shell microstructure in the Bivalvia, and suggests further that modern arcoids and pterioids evolved from nacro-prismatic cyrtodontid ancestors. The Arcoida generally retained a rigid sub-periostracal shell margin and consequently evolved rigid crossed microstructure and a strong dentition to effect proper guidance of the shell margins upon closure. In contrast, the early Pterioida evolved a more prominent flexible outer prismatic layer to assist effective closure along the shell margins, and they retained nacreous microstructure as an adaptation for shell durability.
A correct interpretation of ligament ontogeny and structure is essential for establishing phylogenetic relationships among higher taxa in the bivalve superorder Pteriomorphia. Recent research on pteriomorphian ligaments has focused on understanding ligament morphospace (Thomas et al., 2000; Ubukata, 2003) and evolutionary pathways. In this regard, studies of the transition from larval to post-larval and adult ligaments (Malchus, 2004) have been especially fruitful.
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