Estonian Journal of Earth Sciences, 2017, 66, 2, 59–76 https://doi.org/10.3176/earth.2017.05 59 A new psammosteid (Agnatha, Heterostraci) from the Amata Regional Stage of the Main Devonian Field and morpho-histological types of discrete micromeric elements in the family Psammosteidae In memory of an outstanding palaeoichthyologist Elga Mark-Kurik Vadim N. Glinskiy and Sergey P. Nilov Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034 Russia; vadim.glinskiy@gmail.com Received 28 October 2016, accepted 8 February 2017 Abstract. The range of diversity of psammosteids from the family Psammosteidae is still poorly known. Here a new species, Psammosteus ramosus sp. nov. Glinskiy, from the Amata Regional Stage of the Main Devonian Field is described. Its morphology, ornamentation, histology of exoskeletal plates, and micromeric elements are compared with those of other representatives of the family Psammosteidae. The comparison shows a close relationship of the new species with Psammosteus falcatus Obruchev, P. kiaeri Halstead Tarlo and P. pectinatus Obruchev, a group of species that is significantly different from other representatives of the genus Psammosteus and constitutes a separate evolutionary lineage. On the basis of morphological and histological features we here differentiate in the fields of tesserae of Psammosteidae the discrete micromeric elements of the ‘basic type’, known in Psammosteus bergi (Obruchev), P. levis Obruchev, P. livonicus Obruchev, P. maeandrinus Agassiz, P. megalopteryx (Trautschold), P. praecursor Obruchev and Karelosteus weberi Obruchev, and micromeric elements of the ‘progressive type’, known in Psammosteus falcatus, P. cf. kiaeri and P. ramosus sp. nov. Glinskiy. Key words: Agnatha, Heterostraci, Psammosteida, Late Devonian, Main Devonian Field, new species. INTRODUCTION Psammosteids are a group of jawless vertebrates from the order Pteraspidiformes (suborder Psammosteida sensu Tarlo 1962). They are known from the Early–Late Devonian (from Pragian to Frasnian stages), primarily of Laurussia (Halstead Tarlo 1967a; Halstead 1987) and, to a lesser extent, of the peri-Gondwanan shelf, Armorica, Barentsian, Kara-Tajmyr and Siberian palaeo- continents (Bystrow 1959; Blieck et al. 2002; Delsate et al. 2004; Vaškaninová & Kraft 2016). The exoskeleton of psammosteids is composed of macromeric and mesomeric elements (plates of the cephalothorax, scales in the tail region) and micromeric elements (mostly tesserae). Discrete micromeric elements, situated between the main plates of the cephalothorax, form so-called ‘fields of tesserae’. Praepineal and paired lateral fields on the dorsal surface, and postoral and ventrolateral fields on the ventral surface of the cephalothorax can be distinguished (Obruchev & Mark-Kurik 1965, p. 40). In numerous articulated specimens of Drepanaspis gemuendenensis Schlüter, 1887 and in a single articulated cephalothorax of Psammosteus megalopteryx (Trautschold, 1880), fields of tesserae may be found in their original articulation (Gross 1963; Halstead Tarlo 1965, pl. XVI, figs 1, 2), but in most cases their micromeric elements are found isolated (e.g. Mark- Kurik 1999; Blom et al. 2006). As a rule, these elements can be determined to the species level (Halstead Tarlo 1965; Obruchev & Mark-Kurik 1965; Glinskiy & Mark- Kurik 2016) on the basis of species-specific tubercles. Discrete micromeric elements in the family Psammosteidae sensu Novitskaya (2004) are the most diverse morphologically. They possess a smaller base than other psammosteids. Rooted tesserae are also weakly connected with the cancellous aspidin of the plates (Gross 1933, p. 15, fig. 4; Obruchev 1947a, p. 197; Mark-Kurik 1999, p. 7). The small base size of discrete micromeric elements in representatives of Psammosteidae results in their higher concentration in the fields of tesserae and on the plates of the cephalothorax. Our study of micromeric elements of psammosteids from Frasnian deposits of the Main Devonian Field has shown that discrete elements of the representatives of the family Psammosteidae can generally be classified into two morpho-histological types: the ‘basal’ and © 2017 Authors. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0).