New genus of homostiid arthrodire contributes to the revision of placoderm diversity in the Early Devonian of the Prague Basin
Abstrakt
This study adds new data to complete the taxonomic revision of arthrodire placoderms from the Early Devonian of the Prague Basin (Czech Republic). It focuses on re-describing arthrodire specimens from the Emsian strata published more than a hundred years ago and associating them with unpublished material.
The observations contradict previous taxonomic determinations of the Emsian arthrodires, which distinguished four species of three genera. Only one new genus and species is recovered-Johannaspis bohemicus (Barrande, 1872).
The previous taxonomic splitting of the material into various genera is explained as a factor of specific taphonomical preservation in different localities of the Prague Basin. Another factor enabling the artificial splitting was the present location of the material in different institutions around the world.
Together with Antineosteus rufus Vaskaninova & Kraft, 2014 based on previously unpublished specimens, they are the only two placoderm species known from the Emsian of the Prague Basin. Both are assigned to the homostiid arthrodires-large suspension feeders.
The abundance of the Emsian placoderm fauna is very low in the Prague Basin; only a few specimens are known from each locality. However, they occur in a number of localities of Emsian age in the Prague Basin and in different environments, from very shallow (A. rufus) to relatively deeper marine environments, with all the occurrences being in limestones.
The palaeogeographical distribution of homostiid arthrodires in the late Early Devonian was limited to tropical and subtropical shallow marine environments mainly in the southern hemisphere. In the Middle Devonian, the homostiids are no longer recorded from the northern continental margin of Gondwana including the Prague Basin area.
However, they appear in Laurussia, Siberia and eastern Gondwana. The disappearance of environments with abundant food resources favourable for these large suspension feeders in the Prague Basin area could be related to the narrowing of the Rheic Ocean and gradual emerging of dry land as an early manifestation of the proceeding Variscan orogeny.
This specific local palaeogeographical development of the Prague Basin area is considered the main reason for the significant decrease in diversity and abundance of placoderm fauna, which is in contradiction to the general trend. Valeria Vaskaninova [va.vaska@gmail.com], Institute of Geology and Palaeontology, Faculty of Science, Charles University, Albertov 6, Prague, 128 43, Czech Republic.