New genus of homostiid arthrodire contributes to the revision of placoderm diversity in the Early Devonian of the Prague Basin

Abstract

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arthrodire specimens from the Emsian strata published more than a hundred years ago and associating them with unpublished material.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 Together with Antineosteus rufus Vaškaninová & Kraft, 2014 based on previously unpublished specimens, they are the only two placoderm species known from the Emsian of the Prague Basin.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.

Valéria Vaškaninová [[email protected]], Institute of Geology and Palaeontology, Faculty of Science, Charles University, Albertov 6, Prague, 128 43, Czech Republic.

Keywords:

• Vertebrata
• Arthrodira
• Emsian
• Czech Republic
• paleobiogeography
• peri-Gondwana
• Variscan orogeny

Acknowledgments

I am indebted to Boris Ekrt, Vertebrate Palaeontology Curator at Národní Muzeum (Prague) for his extensive help with the specimens and the SEM photographs (Fig. 6). Jessica Cundiff (Museum of Comparative Zoology, Harvard University), Ursula Göhlich (Naturhistorische Museum Wien) and Martin Mazuch (Chlupáčovo Muzeum Historie Země, Prague) kindly provided specimens for study. Petr Kraft (Charles University, Prague) and Daniel Goujet (Paris) are thanked for valuable discussions and advice. Jaroslav Marek (Charles University, Prague), Vojtěch Turek (Národní Muzeum, Prague) and Pavel Lukeš (Prague) provided rare insights on the historical localities in the Prague Basin. Photographs were taken by Lenka Váchová (Národní Muzeum, Prague; Figs 3, 4A, D, E, 5C, D, and 8) and Katherine Turk (Museum of Comparative Zoology, Harvard University; Figs 4C, 5A, B). I am grateful to Kate Trinajstic (Curtin University, Perth), Vincent Dupret (Uppsala University) and two anonymous reviewers for valuable comments and suggestions considerably improving the manuscript, and to Hannah Byrne (Uppsala University) for language proofreading.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was supported by the Center for Geosphere Dynamics [UNCE/SCI/006] and PROGRES Q45.