Is Neckajatia an Ancestor of the Platycope Ostracodes?

Abstract

Neckajatia Schallreuter, 1974 and a closely related genus are known from a complex of at least four species from the Silurian of Gotland, Great Britain (where the related genus has been recorded as species of Primitid ), Podolia and the eastern Baltic states of the U. S. S. R. The oldest of these species, N. modesta Neckaja, 1958, occurs in rocks older than the rocks which yield the oldest true platycopes which are from the upper Llandoverian of Gotland and Estonia. Neckajatia and the related genus possess characters which are basic to the definition of the platycope ostracodes, but lack others which are equally fundamental to that definition. The combination of characters, which is a) right-over-left overlap, b) a contact groove along all or most of the free margin of the right valve, c) a relatively long straight hinge with a groove in the right valve hinge, d) poorly-developed straguloid processes, but e) the absence of any well-defined dimorphic characters, suggests Neckajatia and related forms are a primitive stock from which the platycopes evolved. It is suggested that this occurred through reduction of the straight hinge, improvement of the contact groove (holoselenic groove) in the right valve and introduction of a distinctive domiciliar dimorphism. Present knowledge indicates that this occurred during the Llandoverian.