Article: Systematics, affinities, and life habits of Babinka, a transitional Ordovician lucinoid bivalve

The rare bivalve genus Babinka from lowest Middle Ordovician rocks of the Bohemian Basin shows multiple muscle scars which have led several palaeontologists to suggest a relationship to some metameric mol-luscan ancestor. A systematic and morphologic revision reveals that Babinka is a typical bivalve in all features except the pedal and gill muscle-scar patterns. These scars are not like those of other bivalves, but are almost identical to the pattern found in the recent monoplacophoran Neopilina, and in some early Palaeozoic Mono-placophora. This close similarity confirms the suggestion that the muscle pattern in Babinka is an inheritance from a monoplacophora-like ancestral mollusc.Babinka is among the first bivalves to appear in the fossil record. The genus is both chronologically and morphologically an ideal ancestor for the earliest lucinoid bivalves which appear abruptly in Middle Silurian deposits. Morphological features of Babinka which are strongly suggestive of lucinoid affinities are: the anteriorly-expanded shell shape; the elongate anterior adductor muscle with associated 'elongate impression'; the non-sinuate pallial line; and the typical lucinoid hinge, dentition, and ligament. Babinka provides the first direct evidence of an evolutionary transition between the Bivalvia and more primitive molluscs.Functional comparison with recent lucinoid bivalves suggests that Babinka was a mobile, infaunal suspension feeder that received nutrient-laden water into the mantle cavity through an anterior inhalent opening maintained by extrusion of the foot.