Evolution of molluscs

The origin and diversification of molluscs through geologic time

The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. The fossil record of mollusks is relatively complete, and they are well represented in most fossil bearing marine strata. Very early organisms which have dubiously been compared to molluscs include Kimberella and Odontogriphus.

Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well known living and fossil forms, is still vigorously debated. Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about , has been described by some paleontologists as "mollusc like", but others are unwilling to go further than "probable bilaterian". Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from are fragments of a genuinely mollusc like radula. This appears to contradict the concept that the ancestral molluscan radula was mineralized. However, the Helcionellids, which first appear over in Early Cambrian rocks from Siberia and China, are thought to be early molluscs with rather snail like shells. Shelled molluscs therefore predate the earliest trilobites. Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults. Some analyses of helcionellids concluded these were the earliest gastropods. However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion characteristic of modern gastropods, that twists the internal organs so the anus lies above the head. Volborthella, some fossils of which predate , was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. Volborthella'''s classification is uncertain. The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite shells. However, Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface. All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period However, the shell less Coleoidea (squid, octopus, cuttlefish) are abundant today. The Early Cambrian fossils Fordilla and Pojetaia'' are regarded as bivalves. "Modern looking" bivalves appeared in the Ordovician period, One bivalve group, the rudists, became major reef builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event. Even so, bivalves remain abundant and diverse. The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life