New data on psammosteid heterostracans (Pteraspidomorpha) and acanthodians (Acanthodii) from the Pärnu Regional Stage (Lower Eifelian, Middle Devonian) of Estonia

Vertebrate remains from the Lower Eifelian Parnu Formation, Estonia, are described. New data on the topography of ornamentation and histology of psammosteids from the Parnu Regional Stage are presented. The occurrence of the genus Guerichosteus Halstead Tarlo in the Lower Eifelian of the Main Devonian Field is established. Representatives of this taxon had previously been known from the Emsian (Lower Devonian) of the Holy Cross Mountains, Poland. The histological study of the exoskeleton of Guerichosteus heterolepis (Preobrazhensky) showed the presence of a thin layer of hypermineralized tissue (enameloid) on the surface of dentine tubercles. The taxonomic composition of the vertebrate assemblage from the Taali locality (Tori Parish, Parnu County, southwestern Estonia) is described for the first time. Along with the taxa characteristic of the Parnu Regional Stage, the acanthodian Cheiracanthus sp. cf. C. splendens is first recorded in this interval.