The paleontology of rostroconch mollusks and the early history of the phylum Mollusca

Abstract

Introduction _______________. Acknowledgments __________. Functional morphology _______. Orientation ________________. Larval shell __________ Metamorphosis ___________. Subsequent shell growth _____. Summary of shell growth ___ Opening of the valves ____. Musculature ________________. Pedal musculature ______. Pallial musculature ________ Alimentary canal _______. Feeding structures _____________ Cleaning the mantle cavity _____ Water currents and gills _____ Function of the hood __________ Taphonomy ________________ Phylogeny _________________. Origin of the Mollusca _______ The ancestral mollusk ________. The oldest known fossil mollusks _ Early Cambrian univalves ______ Radiation of the Monoplacophora Origin of the Gastropoda _______ Origin of the Cephalopoda ____ Origin of the Rostroconchia ___ Radiation of the Rostroconchia

Monoplacophorans and the Origin and Relationships of Mollusks

“…Whether these fossils are monoplacophorans or gastropods is debatable based on several lines of inference, and there remains some controversy as to what are and what are not monoplacophorans in the fossil record (see Signor 1985;Runnegar 1996). For example, whereas Pojeta and Runnegar (1976) and Peel (1991) considered almost all Cambrian cap-shaped taxa as well as the coiled Helcionelloida and some, if not all, of the bellerophontiform taxa to be monoplacophorans, other workers, including Knight and Yochelson (1960), Golikov and Starobogatov (1988), and Parkhaev (2002Parkhaev ( , 2008, limit the diagnosis of Monoplacophora to cap-shaped taxa and consider the remaining Helcionelloida and bellerophontiform taxa to be members of other univalved or gastropod groups. Because these positions are based on the interpretations of a small suite of muscle insertion characters and cartoonlike reconstructions of possible water-flow patterns, it is difficult to test either position.…”

A new problematic Early Ordovician univalve mollusc from France

“…Gubanov andPeel 1998, 1999) and Oelandia Westergård, 1936 (Peel and Yochelson 1987) show prominent, often pouch-like, plications on the dorso-lateral surfaces, but these structures are comarginal and quite unlike the essentially radial (spiral) lateral plications and swellings of Lamaureriella. Similar plications, however, are seen in a variety of Ordovician ribeirioid rostroconchs of the family Technophoridae Miller, 1889, well illustrated by Pojeta and Runnegar (1976), Pojeta et al (1977) and Pojeta (1987). Technophorus itself resembles many bivalves in its laterally compressed form although, as a rostroconch, it is not bivalved and lacks a true hinge with ligament and teeth.…”

“…40, figs 4, 6) which appears morphologically similar to the rostral opening of Technophorus. However, this sub-apical sinus in Lamaureriella and these other helcionelloids would be anterior following helcionelloid reconstructions proposed by Pojeta and Runnegar (1976, fig. 11) whereas these authors consider the rostrum of Technophorus to be posterior.…”

“…2B-C) where the shell is much less expanded than in Technophorus. Fine details of the protoconch, especially with regard to coiling, are not described in technophorid rostroconchs, although internal moulds often show a pronounced tubercle indicating both its presence and position (Pojeta and Runnegar 1976;Pojeta 1987, fig. 14.70).…”

“…While the shell of other ribeirioid rostroconchs may gape anteriorly and posteriorly, the shell of Technophorus is essentially closed around the anterior and ventral (abapical) margins. However, the junction between opposing plications creates small openings, or gapes, at the posterior and the rostrum itself may also carry an opening (Pojeta and Runnegar 1976;Text- fig. 2A).…”