Extra-lobal and complex dimorphic features in Middle Devonian palaeocopine ostracods

Abstract

With respect to Middle Devonian ostracods from western Canada, special cases of palaeocopine extradomiciliar dimorphism are discussed, as in the opinion of many workers are unusual for younger hollinoideans and, in particular, primitiopsoideans. Although the general view that velar dimorphism is the main characteristic of Mid Palaeozoic Palaeocopina is basically not wrong, dimorphic histial structures, even though less conspicuous in aspect, have been recognised in some Mid-Devonian hollinoideans, specifically hollinids and ctenoloculinids. This is now also proven to be true of distinct hollinellids and, by the way, discrete primitiopsids, while distinguishing between narrow-based projections (ridges, flanges, spines) and broad-based protrusions (e.g. lobes). Whereas the former may occur in velar as well as histial situations, lobal features are especially found in the histial position. Special attention is paid to false primitiopsids, being (with some caution) grouped in the Hollinoidea (Family unknown). Essentially, three genera are new or redefined (former left under open nomenclature): Gen. aff. Abditoloculina: multilobate in aspect, lobal (histial) dimorphic; Gen. aff. Gravia: coalesced velar dimorphic; Bisphenella: histial dimorphic. With respect to the hold discussion about ostracod phylogeny, the updated standard classification scheme is given, as applied to Palaeozoic, non-myodocopine ostracods. Reduction of dimorphism in palaeocopines is considered to be an apomorphic character, lack of dimorphism assumed to indicate asexual reproduction. At the same time, a plea is made in favour of reviving Treatise revision.