Article: Comparative taxonomy of the bivalve families Isognomonidae, Inoceramidae, and Retroceramidae

Fossil Isognomonidae (Pteriacea) can be difficult to distinguish externally from the biostratigraphically important Mesozoic family Inoceramidae (Ambonychiacea?). Internal details of ligament area morphology provide valuable taxonomic data at the family and species levels, as documented for many New Zealand species. Definitive distinction between these two families is furnished by the shell structure underlying the ligament area: in Isognomonidae the ligament attaches to the inner (presumed nacreous) shell layer, whereas in Inoceramidae it attaches to the outer prismatic shell layer. Retroceramus, formerly included in Inoceramidae, has the ligament attached to the inner shell layer, and should be placed in the Pteriacean family Retroceramidae. These findings are consistent with a polyphyletic origin for the multivincular ligament in Isognomonidae and Inoceramidae.Two new species of Isognomon are described from New Zealand, I. wellmani (Palaeocene) and I. rekohuensis (Late Cretaceous). They probably lived on soft or shelly substrates with otherwise similar life habits to Recent forms.