Evolution & Systematics

A Hypothetical Ancestral Mollusc (HAM) has been developed from which all other molluscs radiated. The most primitive known bivalve is Pojetaia, found in the lower Cambrian of Australia, and is similar to the presumed ancestral mollusc in which the radula had been lost and the univalve shell divided into 2 valves. The ancestral mollusc was presumed to be a selective deposit feeder. The radiation of bivalves has been closely tied to the evolution of the use of their gills for feeding with the development of Protobranch, lamellibranch and septibranch gill structures. Specialization has been identified as a significant factor in the speciation of bivalves (Mikkelsen 2011). 

Recent research has indicated that eumellibranch gills and heterodont (specialized) hinge are ancestral characters for all Auto branchia which would require the revision of bivalve symplesiomorphies (a trait found that two or more taxa have in common) (Plazzi et al. 2011). 

There is little known on the evolution of Scintilla or on the family to which it belongs, Galeommatidae. The earliest probable galeommatid fossils are from the Cretaceous however known definite fossils occurred in the Palaeocene. A new study by Mikkelsen has concluded that Galeommaeoidea (the superfamily containing Scintilla) is sister tax to Lucinoidea and Pholadoidea as a result of speciation driven by sulfide-oxidizing bacteria and mucus-tube feeding during the Ordovician. 

No statement can be made regarding local fossil history of Galeommatidae due to the poor level of knowledge regarding this group in Australia.