Mid-Paleozoic dimerocrinitid crinoids from North Gondwana: evolution, biostratigraphy and paleobiogeography

Abstract

The data set accumulated during the last decade on dimerocrinitid crinoids from the northern margin of Gondwana ranging from Wenlockian lo Pragian has been worked out. Two new species, Dimerocrinites aragonensis n.sp. and D. sartensis n.sp., are described. The following evolutionary trends of the theta are evidenced: significant increase of dorsal cup size and piale thickness, smoothening and loss of plate ornamentation, columnal articular development, and related epifacet reduction. The vertical distribution of the Devonian species established in the Armorican sections shows that D. lanveocensis, which originates close to the Silurian/Devonian boundary, may be used for the characterization of the Lower Lochkovian in the Ibero-Armorican domain. The wide geographical distribution of the dimerocrinitids from South Algeria to the Armorican massif in addition to the robustness of their morphological features, clearly distinct from those of the South Laurussian species, make them paleobiogeographical indicators for the North Gondwanan province. The abundance of dissociated plates and columnals in Lower Lochkovian fine to coarse terrigenous sediments are indicative of proximal paleoenvironments in contrast with the distal open shelf scyphocrinitid biofacies.