Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca

Abstract

Pojetaia and Fordilla are the oldest bivalve molluscs, occurring in roughly co-eval rocks from the Tommotian, and are the only undisputed, well-known bivalves from the Cambrian. New specimens reveal that Pojetaia had a laminar inner shell microstructure reminiscent of the foliated aragonite of modern monoplacophorans, and the same is true for Fordilla. A similar shell microstructure is seen in Anabarella and Watsonella, providing support for the hypothesis that they are the ancestors of bivalves. Foliated aragonite shares many similarities with nacre, and it may have been the precursor to nacre in bivalves. No cases of undisputed nacre occur in the Cambrian, in spite of much shell microstructure data from molluscs of this time period. Thus, although considered by many to be homologous among molluscs, we conclude that nacre convergently evolved in monoplacophorans, gastropods, bivalves, and cephalopods. This independent origin of nacre appears to have taken place during, or just prior to, the Great Ordovician Biodiversification Event and represents a significant step in the arms race between predators and molluscan prey.

A bradoriid and brachiopod dominated shelly fauna from the Furongian (Cambrian) of Västergötland, Sweden

“…The occurrence of this pattern of microstructure in Fordilla cf. F. sibirica supports the arguments that it is a diagnostic character of the Family Fordillidae and that this was the primitive type of shell microstructure in bivalves, which may have been a precursor to nacre (Vendrasco et al 2011a). …”

“…21A 4 ). Elsewhere the shell microstructure is less well organised (Runnegar and Pojeta 1992;Vendrasco et al 2011a). This pattern has been described in Pojetaia runnegari, Fordilla troyensis, and now Fordilla cf.…”

Nacre in Molluscs from the Ordovician of the Midwestern United States

“…Thus it may be that the transition from foliated (or semi-foliated) aragonite to nacre began in Pojetaia and its kin, and was finalized in the close ancestors of the Cyclora-bed bivalves. This transition from foliated aragonite to nacre was hypothesized by Vendrasco et al [9] and is illustrated in Figure 10.…”

“…Nacre was once thought to be primitive in the Mollusca, but a closer analysis of purported Cambrian molluscs with nacre revealed the evidence is equivocal at best [9].…”

“…This microstructure of elongate blades of aragonite arranged in layers was termed "foliated aragonite" by Checa et al [34,35], although Carter et al [36] suggested the proper term for this shell microstructure should be "semi-foliated aragonite". In addition to the blade-like laths, in some regions of fossils of Pojetaia, large, isolated, rounded tablets also occur [9], a type of microstructure that Carter et al [36] classified as "large tablet, imbricated nacre" (p. 88). Carter [33] previously suggested that this type of microstructure was transitional to nacre.…”