Fossil record Mollusca

the tiny helcionellid fossil yochelcionella thought mollusc

however, helcionellids, first appear on 540 million years ago in cambrian rocks siberia , china, thought molluscs rather snail-like shells. shelled molluscs therefore predate earliest trilobites. although helcionellid fossils few millimeters long, specimens few centimeters long have been found, more limpet-like shapes. tiny specimens have been suggested juveniles , larger ones adults.

some analyses of helcionellids concluded these earliest gastropods. however, other scientists not convinced these cambrian fossils show clear signs of torsion identifies modern gastropods twists internal organs anus lies above head.

septa , siphuncle in nautiloid shell

volborthella, fossils of predate 530 million years ago, long thought cephalopod, discoveries of more detailed fossils showed shell not secreted, built grains of mineral silicon dioxide (silica), , not divided series of compartments septa of fossil shelled cephalopods , living nautilus are. volborthella s classification uncertain. late cambrian fossil plectronoceras thought earliest cephalopod fossil, shell had septa , siphuncle, strand of tissue nautilus uses remove water compartments has vacated grows, , visible in fossil ammonite shells. however, plectronoceras , other cephalopods crept along seafloor instead of swimming, shells contained ballast of stony deposits on thought underside, , had stripes , blotches on thought upper surface. cephalopods external shells except nautiloids became extinct end of cretaceous period 65 million years ago. however, shell-less coleoidea (squid, octopus, cuttlefish) abundant today.

the cambrian fossils fordilla , pojetaia regarded bivalves. modern-looking bivalves appeared in ordovician period, 488 to 443 million years ago. 1 bivalve group, rudists, became major reef-builders in cretaceous, became extinct in cretaceous–paleogene extinction event. so, bivalves remain abundant , diverse.

the hyolitha class of extinct animals shell , operculum may molluscs. authors suggest deserve own phylum not comment on position of phylum in tree of life