Morphology, morphoclines and a new classification of the Pteriomorphia (Mollusca: Bivalvia)

Abstract

Two morphological paradigms have long been used in comparative anatomical studies of bivalves: (1) the primary ligament is three-layered, with the layers corresponding to three shell layers; and (2) the primary mantle edge is composed of three folds with clearly defined functions. The results of studies of larval development indicate, on the contrary, that the primary ligament is completely organic. Calcified, fibrous ligamental material develops from lamellar material near areas of contact between ligament and shell, and development of the fibrous portions then proceeds toward the midline, finally achieving in many lineages a continuous fibrous bridge between valves. Furthermore, these results suggest that the mantle edge in the Bivalvia is primarily twofold and that the only clearly homologous structure between major groups is the periostracal groove itself. These morphological concepts, with other new and previously published data on shell ultrastructure, ligaments, mantle edges, ctenidia, palps, lips, stomachs, muscles, and photoreceptors, lead to a new picture of the evolution of primitive and derived character states in groups previously included in a subclass Pteriomorphia. Furthermore, a cladistic analysis of these data allows predictions of the morphology of ancestors which can be tested by reference to the preserved morphology and sequence of fossils. A new phylogenetic classification separates these groups into three superorders within the subclass Autobranchia: Isofilibranchia (mytiloids), Prionodonta (arcoids), and Pteriomorphia. The most complex radiation has been in the Pteriomorphia. Three orders originated in the early Palaeozoic: Pterioida, Limoida, and Ostreoida. The Pterioida and Ostreoida developed monomyarian, pleurothetic states independently, and each order developed its own mode of shell secretion. Further differentiation in the order Ostreoida occurred in the mid-Palaeozoic, producing two suborders, the Ostreina and Pectinina, both of which had already developed foliated calcitic ultrastructure from simple prismatic structure. By the early Mesozoic, the Ostreina had given rise to three extant superfamilies - the Ostreacea (true oysters), Dimyacea, and Plicatulacea - through atrophy of the foot, the assumption of a pleurothetic state on either the left or right side, and early obligate cementation. The Pectinina, through retention of the foot and the assumption of a pleurothetic mode of life, had evolved before the late Palaeozoic to the Anomiacea and Pectinacea. Within the superfamily Pectinacea, four extant families have origins ranging from early Carboniferous to Cretaceous in age: Propeamussiidae, Pectinidae, Syncyclonemidae, and Spondylidae. The new family Syncyclonemidae, which contains a genus long assumed to have become extinct at the end of the Cretaceous, is here recognized in the Recent and late Pleistocene on opposite sides of the Earth. With regard to extinct groups, many genera previously assigned to the Pteriacean family Malleidae belong in the Ostreacea on the basis of shared derived character states. Incorporation of these taxa as well as the Dimyacea in the Ostreina suggests that oysters have a dimyarian, possibly non-pleurothetic, origin and cannot have evolved from forms like the Pseudomonotids, which retained their foot and became pleurothetic. The new name Buchiacea is introduced for a set of extinct taxa within the suborder Pectinina including the Buchiidae, Monotidae, Oxytomidae, and Pseudomonotidae of previous authors. Derivation of this group is from the common ancestry of the Anomiacea and Pectinacea. The extinct Palaeozoic Aviculopectinidae, Pterinopectinidae, Deltopectinidae, and Leiopectinidae are grouped in a superfamily Aviculopectinacea, which also appears to have branched from the early ancestry of the Pectinina.

Taxonomy and phylogeny of cementing Triassic bivalves (families Prospondylidae, Plicatulidae, Dimyidae and Ostreidae)

“…Consequently, most authors have based the taxonomic placement of these families chie¯y on observations of Recent forms. It has been proposed that the Ostreidae and Dimyidae were sister taxa, linked by their continuously ®brous resilia, and that the Plicatulidae was the sister taxon of this clade, sharing with it the absence of a foot (Waller 1978). However, these characters do not conclusively indicate a phylogenetic relationship, as continuously ®brous resilia evolved more than once (e.g.…”

“…Pallial line missing except in Recent Saccostrea in which a disjunct line of pallial muscle insertions is present (Stenzel 1971, p. N968). Ligament submarginal, alivincular, resilium completely ®brous and continuous between valves, without a lamellar junction (Waller 1978). Ligament area with a broad resilifer, that of right valve elevated.…”

“…When uniting the Plicatuloidea, Dimyoidea and Ostreoidea in the suborder Ostreina, Waller (1978Waller ( , 1985 claimed that aragonitic layers are, where present, generally crossed-lamellar in this suborder. However, nacreous structures have since been documented in a Triassic Gryphaea (McRoberts and Carter 1994) as well as in the oyster-like specimen UNC 13497b (Carter 1990, p. 217).…”

Treatise Online no. 48: Part N, Revised, Volume 1, Chapter 31: Illustrated Glossary of the Bivalvia

“…hautMaNN (2004, fig. 1D) indicated that the hinges are united along their dorsal margin by secondary (i.e., periostracal) ligament, rather than by lamellar ligament, but Waller (1978) maintained that in Dimyidae, Plicatulidae, and Spondylidae, they are united dorsally by lamellar ligament. Note that lamellar ligament is present as a medial core in the resilium in Spondylidae (as in Pectinidae), but not in Dimyidae and Plicatulidae.…”

“…simple crossed lamellar microstructure (abbr., simple CL, or just CL). An aragontic crossed lamellar shell microstructure in which the elongate, basic structural units (third-order lamellae) are arranged into sheetlike second-order lamellae, the secondorder lamellae are mutually parallel within each first-order lamella, and the third-order lamellae in adjacent first-order lamellae show two and only two predominant dip directions relative to the shell margin (Wise, 1968, p. 325;Waller, 1978;Carter & Clark, 1985;Carter & others, 1990, p. 612, 658), e.g., in the trapeziid Trapezium (Neotrapezium) sublaevigatum (laMarCk, 1819 in 1818-1822) (Fig. 283) (Carter, 1990a;Waller, 1990); e.g., the submarginal simple ligament in Lucina pensylvanica (liNNaeus, 1758) (Fig.…”