Morphological variability within the mitrate Mitrocystella incipiens (Echinodermata, Stylophora) from the Darriwilian (Middle Ordovician) of high-latitude peri-Gondwanan regions

Abstract

The mitrate Mitrocystella incipiens (Barrande, 1887) is a locally abundant stylophoran echinoderm, with a short stratigraphic range but a wide palaeobiogeographic distribution in the high-latitude peri-Gondwanan regions of the Mediterranean Province: Czech Republic (Prague Basin, Bohemia), France (Armorican Massif, Brittany), northern Portugal and Spain (Iberian Massif). Two regional subspecies were previously identified based on the morphology of their thecal outlines: M. incipiens incipiens (Barrande, 1887) (Prague Basin), and M. incipiens miloni Chauvel, 1937 (Armorican Massif and Iberian Peninsula). A sample of 40 complete and well-preserved individuals from the Armorican Massif, the Iberian Peninsula, and the Prague Basin was selected to assess whether these morphotypes could be distinguished through quantitative analysis. Geometric morphometric analysis of their theca confirmed the existence of morphological differences between the Armorican and Czech populations, but also identified that the Iberian specimens are morphologically intermediate between those from the Armorican Massif and Prague Basin. Furthermore, the antero-lateral margins of the Iberian specimens present a delicate pustulose ornamentation, while the lower thecal surface of the Armorican and Czech individuals is completely smooth. The Iberian specimens are thus assigned to a new subspecies: M. incipiens pustulosa ssp. nov. These results support the idea that all populations of Mitrocystella belong to the same species, M. incipiens, which is characterized by a broad phenotypic variation. Together with geographic isolation, subtle environmental differences between the three regions (e.g. depth) may have contributed to shape these ecophenotypes. Finally, the first example of abnormal thecal plating in Mitrocystella incipiens is described in a specimen from Spain and its possible phylogenetic implications are discussed.