Sibling echinoderm taxa on isolated Ordovician continents: Problem of center of origin

Morphologically similar echinoderm genera are mostly known from the Ordovician of two isolated continents, Baltica and Laurentia, although these landmasses were strictly isolated biogeographically by the Iapetus Ocean and had different climates during the Early and Middle Ordovician. The morphological characters of most of these echinoderm genera exclude the possibility that Baltic genera evolved from Laurentian forms or vice versa and suggest that their common ancestral group was from a different region. I proposed to name such genera ‘sibling genera’, or twin genera by analogy
with sibling species. Eocrinoid sibling genera are representatives of the cryptocrinitid-rhipidocystid clade. Baltic Paracryptocrinites and Cryptocrinites are sibling genera of Laurentian Columbocystis, Springerocystis and Foerstecystis, Baltic Rhipidocystis is a sibling genus of Laurentian Mandalacystis and Baltic Neorhipidocystis is sibling genus of Laurentian Batherocystis. It is assumed that North American platicystid paracrinoids, cryptocrinid and rhipidocystid eocrinoids evolved from a common ancestral eocrinoid, similar to Paracryptocrinites. Sibling genera are also known among other Ordovician echinoderms, for example, crinoids (hybocrinids), edrioasteroids (edrioblastoids) and rhombiferans. Some genera migrated from Baltica to Laurentia and vice versa. The temperate warm water seas of eastern Gondwana in the northern China-Australian region were a possible biogeographical center of origin, diversification or distribution of Laurentian and Baltic sibling genera. Biogeographical analysis of Baltic echinoderms shows that the Baltic Region could be viewed as a ‘museum’ or ‘storehouse’ for many of them, they immigrated and survived there
for some considerable time, rather then a ‘cradle’, where they arose and from where they migrated to other continents. Laurentia, Gondwana.