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

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.

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 the 'progressive' ones.The aim of this research is the description of the earliest psammosteid species with a 'progressive' type of tesserae, Psammosteus ramosus sp.nov.Glinskiy, and comparative description of morphohistological types of discrete micromeric elements.

MATERIALS AND METHODS
The ornamentation of the specimens was cleaned mechanically by an instrument consisting of an air pump and a hypodermic needle (Selden 2003).The material was studied mainly under the stereo zoom microscope Leica М205 C, and also under the scanning electron microscope (SEM) Hitachi S-3400N in SE and BSE mode.The study of the inner structure of some micromeric specimens was undertaken with Bruker-microCT SkyScan 1172.Projection images were reconstructed with the software NRecon into cross section images rotated upright in the software DataViewer.The software CTAnalyzer was used to correct the visualization and interpretation of the reconstructed images.CTVox software was used for 3D volume rendering.The specimens were scanned by applying different parameters according to the sample type at a voltage of 40-100 kV and a current of 100-250 μA, with a copper and aluminium filter for a half rotation of 180° at the highest camera resolution.Thin sections were made with the use of Technovit® EPOX Resin and Technovit® EPOX Hardener fast.The specimens enclosed in the epoxy resin were wet-ground on a glass slide to a needed level with the use of Mirka WPF finishing sandpaper P 120-1000.The sandpaper with a grade P 1200-2000 was used subsequently to polish a thin section.The photographs of thin section PM SPU 80-7 were taken with a polarization microscope Leica DМ4500 P with the use of aniseed oil.
All specimens studied are stored in the collections of the Palaeontological Museum of St. Petersburg State University (the new taxon: PM SPU 80; material for comparison: PM SPU 71, 72, 75), St. Petersburg, Russia; Natural History Museum of Latvia (G 43), Riga, Latvia and Department of Geology, Tallinn University of Technology (GIT 680), Tallinn, Estonia.

GEOLOGICAL BACKGROUND
The material comes from two localities of the Amata Regional Stage (RS) in Northwest Russia and one locality in eastern Latvia (Fig. 1А).Most specimens (including the holotype) were found at the Andoma Hill locality on the southeastern bank of Lake Onega in the Vytegra District, Vologda Region, NW Russia (Fig. 1D).
The Andoma Hill locality is represented by a series of outcrops, stretching along the lake for a couple of kilometres.Remains of Psammosteus ramosus sp.nov.Glinskiy (Fig. 2A-F 6A-E, G, H) occur in layer AG 1-2, in the upper part of outcrop N-2 at Gnevashevskaya village (Ivanov et al. 2006).This layer is composed of red, fine and very fine, poorly cemented sandstones intercalated with thin layers of silty clay and siltstone.These deposits are assigned to the lower part of the Andoma Formation, which is correlated with the Amata-Pļaviņas interval (Lukševičs et al. 2012).Remains of the new species were found by A. Ivanov in 1984 in association with numerous isolated branchial plates, lateral scales and various micromeric elements of Psammolepis undulata (Agassiz, 1844), Psammosteus livonicus Obruchev, 1965(see hereinafter in Obruchev & Mark-Kurik 1965), P. praecursor Obruchev, 1947aand P. cf. cuneatus Obruchev, 1965(Glinskiy & Ivanov 2015).
The Borschovo locality is situated in the Luga District of the Leningrad Region, NW Russia.It is represented by a series of outcrops on the south bank of Lake Antonovskoe, in the Oredezh River basin (Fig. 1C).Remains of P. ramosus sp.nov.were found in the terminal eastern outcrop 3, situated to the east of the road from Borschovo village to the lake (Fig. 2N-P; Fig. 3K, L; Fig. 4H-O).The deposits in this outcrop are represented by variegated (pink, yellowish-grey, reddishbrown to light bluish-grey), cross-bedded, medium-and fine-grained sandstones, with intercalations of purple and greenish clay.Vertebrate remains, including those of the new species, were collected by a team of Russian-French-Latvian expedition in 2010 at the level (No. 12) of light bluish-grey sandstones of the Amata RS in the middle part of the outcrop.The only branchial plate of P. ramosus sp.nov.was found in association with numerous branchial plates, dorsal plates, ridge scales and rare micromeric elements of different psammosteids (Glinskiy 2013): Psammolepis venyukovi Obruchev, 1965, Psammolepis undulata, Psammosteus livonicus, P. praecursor.The presence of P. cf. cuneatus Obruchev at the same level needs further confirmation.One of the branchial plates of P. praecursor from this assemblage was previously misdiagnosed by V. Glinskiy as that of P. levis (Ivanov et al. 2011).
The Ķūķu locality is situated on the right bank of the Gauja River, Raiskums parish, Pārgauja municipality, eastern Latvia (Fig. 1B).In 1965 fragments of branchial plates of the new species were gathered there by L. A. Lyarskaya (Fig. 2G-M; Fig. 3E-H; Fig. 5B-F).They were previously identified as the scales of Psammosteidae indet.These remains can be found in the upper part of the outcrop in white fine-grained sandstones (at the base of the Amata RS) together with   numerous branchial and median plates of Psammolepis undulata (Lyarskaya 1981).
Three positions of the boundary between the Middle and Upper Devonian in the Main Devonian Field have been proposed: at the base of the Amata RS, in the Amata RS at the base of the Podsnetogorskie Regional Beds (RB) and at the base of the Snetnaya Gora RB of the Pļaviņas RS (Ivanov & Lebedev 2011).The first position is accepted in the current research (Fig. 1E).

MORPHOLOGY AND TERMINOLOGY OF MICROMERIC ELEMENTS
The following morphological structures can be distinguished in the discrete and rooted micromeric elements of the family Psammosteidae sensu Novitskaya (2004): a crown, formed by one or several dentine tubercles and a base composed of cancellous aspidin (Figs 5A, G; 6A).Discrete micromeric elements are characterized also by the presence of a neck (a transitional part between a crown and a base) composed of cancellous aspidin and by a comparatively short base.Rooted tesserae are usually recognized by a missing neck, and a large base area.Crown tubercles of all micromeric elements in the representatives of Psammosteidae vary substantially in shape (dome-shaped, cone-or peak-shaped) and orientation, which can be connected with their position on the body.
Various exoskeletal elements, including micromeric elements of psammosteids, can be formed by cyclomorial or synchronomorial (also syncronomorial) growth (Ørvig 1951;Halstead Tarlo 1964, p. 44).Cyclomorial elements possess a centre of growth or primordium.There is usually a primordial dentine tubercle in the superficial layer, which was formed prior to the other tubercles and around which the subsequent growth of an element took place.Main plates, scales and cyclomorial tesserae of psammosteids are characterized by cyclomorial growth.This type of growth usually manifests itself in areal (concentric) zones of growth of tubercles in the superficial layer.Cancellous and lamellar aspidin layers may also have areal zones of growth, visible on the visceral side of macromeric elements.Sometimes these zones are weakly developed, but the primordial tubercle, which gives evidence of cyclomorial growth type, can be distinguished.Synchronomorial elements are characterized by the absence of a centre of growth.Growth of the whole element is contemporaneous.The synchronomorial type of growth is characteristic of mesomeric unnamed plates, positioned on the periphery of dorsal and ventral plates and often rooted (fused) with them (Halstead Tarlo 1964, p. 44), and of some micromeric elements, such as synchronomorial tesserae and simpler complex elements (Mark-Kurik 1999, p. 16).
Discrete cyclomorial tesserae are dominant in the fields of tesserae in Drepanaspis and can also be rooted with larger median plates (Halstead Tarlo 1964, p. 44).In Schizosteus striatus (Gross, 1933), Schizosteus splendens (Eichwald, 1844) (family Guerichosteidae) and Psammolepis venyukovi Obruchev, 1965 (family Psammolepididae), the tesserae, rooted with rostral and postorbital plates are also cyclomorial and belong to praepineal and lateral fields (Obruchev & Mark-Kurik 1965).Psammosteids from the family Drepanaspididae (e.g.Drepanaspis) and Guerichosteidae (e.g.Schizosteus) possessed rooted unnamed plates and tesserae showing a synchronomorial type of growth in the vicinity and on the margins of dorsal and ventral plates (Halstead Tarlo 1964, p. 44).In the representatives of the families Psammolepididae and Psammosteidae, rooted tesserae (Obruchev & Mark-Kurik 1965, p. 49) of synchronomorial and/or cyclomorial types of growth partially or completely cover the dorsal and ventral plates.This evolutionary tendency in the psammosteid lineage is called 'advanced tesserization' (Obruchev & Mark-Kurik 1965, p. 64).Thus, in the representatives of Psammosteus it is expressed in full substitution of median plates by cyclomorial tesserae and in coverage of the most part of the ventral side of branchial plates by them.
The following morphological terms were proposed to distinguish various isolated micromeric elements of Psammosteus sp. from Givetian deposits (Burtnieki RS) of the Essi locality, Estonia: single elements, complex elements and 'tesserae' (Mark-Kurik 1999).A single element is a solitary primordial odontode (Ørvig 1977), mounted on an aspidin base (Mark-Kurik 1999, p. 9).A complex element has a crown that consists of two or more primordial odontodes (Mark-Kurik 1999).In the case of simpler (basic) complex elements, the bases are fused to form one common base and developed synchronomorially (Ibid., p. 18).Mark-Kurik (1999) distinguishes one more type of complex elements.Such elements are developed cyclomorially; they have a narrow overlap area and the primordial tubercle, which is situated at their posterior margin (Ibid.,pp. 16,17).A cyclomorial platelet or 'tessera' is a polygonal element with a primordial tubercle (it can be situated concentrically or eccentrically in relation to a geometric centre of a tessera), concentrically surrounded by much smaller and simpler tubercles (Ibid.,pp. 15,16).
The terms proposed by Mark-Kurik (1999) for describing micromeric elements are used in this article.It should be noted that the term 'сomplex element' is used by that author to refer to several types of micromeric elements in Psammosteus, namely: (1) basic complex elements with the synchronomorial type of growth and (2) scales and scale-like elements with the cyclomorial type of growth, which probably come from the tail and the caudal part of the cephalothorax.The description of these elements is the same as that of the lateral scales of Psammosteus sp. from Givetian deposits (Burtnieki RS) of the Joosu quarry locality, Estonia (Mark-Kurik 1993).
In the text below, we use the term 'сomplex element' to refer to the elements of the first type mentioned above.
On the basis of morphological and histological differences it is proposed to differentiate two types of discrete micromeric elements in the fields of tesserae in representatives of Psammosteidae sensu Novitskaya (2004): the 'basic type' and the 'progressive type'.In micromeric elements belonging to the 'basic type', tubercles are similar both in general shape and in inner structure.In the discrete tesserae of the 'basic type', the crown is formed by a primordial tubercle and satellite tubercles (Fig. 5A-C, E-G, I, J).The primordial tubercle is as a rule larger than satellite tubercles or has a comparable size.Pulp cavities of the tubercles are identical, usually hollow; rarely trabeculae of cancellous aspidin can penetrate into them (Fig. 5J, Q).Micromeric elements belonging to the 'basic type' are known in early representatives of the genus, such as Psammosteus sp. from the Early Devonian (Givetian) of Estonia (Mark-Kurik 1999, fig.8).We assign these micromeric elements, defined by Mark-Kurik (1999) as those of Psammosteus sp., to Psammosteus bergi (Obruchev, 1943) (Fig. 5R, S).The same micromeric elements are also present in Psammosteus levis Obruchev 1965, P. livonicus Obruchev, 1965, P. maeandrinus Agassiz, 1844, P. megalopteryx (Trautschold, 1880), P. praecursor Obruchev, 1947b (Fig. 5G-Q) and also Karelosteus weberi Obruchev, 1933(q.v. Glinskiy 2014, p. 23, fig. 1:3).
In micromeric elements belonging to the 'progressive type', the crown is formed by a large primordial tubercle (Fig. 6A-C, G, H, I-K, N, O).It is situated centrally on the top of a conical aspidin mound (neck) and covers the upper half of the neck by its long and complicated marginal crenulations (Fig. 6E, M, Q).A primordial tubercle can have one or several pulp cavities, usually filled in by aspidin trabeculae.Satellite tubercles in the tesserae belonging to the 'progressive type' can retain simple inner structure, characteristic of the 'basic type'.Discrete tesserae of 'progressive type' differ from discrete single elements by the presence of satellite tubercles, arranged in concentric zones on the neck (Fig. 6A, B, G).Often in the fossil record, elements of 'progressive type' that are devoid of dentine are found, with the remaining aspidin part represented by a conical aspidin mound (neck) and a base (Fig. 6F).

SYSTEMATIC PALAEONTOLOGY
Class PTERASPIDOMORPHI Goodrich, 1909 Subclass HETEROSTRACI Lankester, 1868 Order PTERASPIDIFORMES Berg, 1937 Suborder PSAMMOSTEIDA Kiaer, 1932Family PSAMMOSTEIDAE Traquair, 1896 Genus Psammosteus Agassiz, 1844 Psammosteus ramosus sp.nov.2D, E; Fig. 3C, D).The posterior margin is concave.The distal tip of the plates is curved backwards or medially (Fig. 2A, B, N, O).Branchial plates without deformation are insignificantly dorsally convex in width (0.5 mm) (Fig. 2С, F).The distal part of the branchial plates (PM SPU 80-2) is slightly dorsally convex in length (1.4 mm from the proximal side, less than 1 mm from the distal side).The ornamented surface is developed on the distal part of the branchial plates.The area of the ornamented surface on the dorsal side is more or less equal to that on the ventral side of the plates (Fig. 3A, B, E-H, K, L) or slightly smaller (Fig. 3С, D).
________________________________________________________________________________________________________ Fig. 6.Discrete micromeric elements of the 'progressive type' in some species of Psammosteus and their microstructure.
A-E, PM SPU 80-5, Psammosteus ramosus sp.nov.The boundary between the ornamented and unornamented surfaces of the branchial plates is usually distally convex.Branchial plates are of moderate sizes.The measurements of the fragments are given in Table 1.Lateral scales have rounded margins; they are short, wide and flattened.In specimen PM SPU 80-4 the right margin is concave (Fig. 2Q, R).Ornamentation is developed in the posterior part on both sides of scales (Fig. 3I, J).On the external side the area of the ornamented surface slightly exceeds half of the total area of this side of the plate (shortened ornamented zone).The area of the anterior unornamented surface is large.On the visceral side the ornamented surface covers approximately a quarter of the total area.The length of scale PM SPU 80-4 is 6.8 mm; the width is 8.9 mm.The thickness of the plate in the unornamented part is 0.8 mm; it is 2.5 mm in the zone with ornamentation.
The micromeric elements belonging to the species are discrete tesserae (Fig. 6A-H) and rooted tessera (Fig. 5A), preserved on the ventral side of branchial plate PM SPU 80-2.Discrete tesserae of 'progressive type' have a high crown, formed by a large single primordial tubercle, a high neck and a rounded small base (Fig. 6A-D, G, H).The crown is subvertically oriented (Fig. 6A, B, G) or slanted (PM SPU 80-6).Satellite tubercles sit on the neck, forming up to two areal zones (Fig. 6A, B, G).The area of the base is equal to or slightly exceeds that of the crown.Its visceral side has a medial depression (Fig. 6D, E).The most complete specimen PM SPU 80-5 is 4.5 mm in height and 4 mm in length of the base.The rooted tesserae on the ventral side of the branchial plates are low and small (3 mm in length).Tubercles are situated concentrically; the primordial tubercle does not differ from them either in shape or in size.

Ornamentation.
Ornamentation is represented by large tubercles with the area of the base up to 1-1.5 mm in diameter.The mean length of marginal crenulations is 0.8-1 mm, ranging in number (without their branch tips) from 6 to 19, 13-16 on average.The general size of the tubercles including the length of marginal crenulations can attain 2.6 mm in total (Fig. 4B, I).The crowns of the tubercles have a conical and dome-like, rarely ___________________________________________ peaked shape.They are oriented vertically or can be slightly slanted towards the surface of the plate (Fig. 4H).The mean height of the tubercles is 1 mm.Tubercles with round or oval bases are dominant (Fig. 4E, F, J, O) and those with angular, fan-or poleaxe-shaped bases are more rare (Fig. 4I-K).As a rule, thin radial ribs diverge from the top of the crown, which can be observed on low-crowned unabraded tubercles (Fig. 4L-O).Ribs branch on the marginal crenulations and ramifications (second-order marginal crenulations) forming fine plumose wrinkles (Fig. 4H, M-O; Fig. 5B, C, E).Marginal crenulations are massive and complicated, but not tapered.Finer and simpler crenulations may be present between them (Fig. 4B, C, I).The crenulations on the proximal side of branchial plates tubercles attain substantial length (up to 2.2 mm) and are more complicated (Fig. 4A, B, F, I), in comparison with those on the distal side of these tubercles.The tips of marginal crenulations often ramify into two or three long branches (Fig. 4A, B, F, I, L, O).Massive ramifications occur on the flanks of marginal crenulations.They can attain substantial length and can bear indentations on their margins in large tubercles (Fig. 4A, B).Ramifications are usually densely distributed along the whole length of marginal crenulations (Fig. 4C-G, I-O; Fig. 5B, C, D, F; Fig. 6A, B, F, G).Large microtubercles are situated along the whole length of marginal crenulations (up to 12 in discrete tesserae); small, scarcely discernible microtubercles are also observed on the ramifications (Fig. 6A, B).
Tubercles on the branchial plates often fuse into long ridges.The length of the ridges is up to 1 cm (Fig. 2A; Fig. 4B, C, G, I, N).The number of crenulations of fused tubercles exceeds 17.The tubercles of the lateral scale have predominantly round or oval bases; the crowns are oriented vertically; short ridges are present.Discrete tesserae bear a large primordial tubercle with very long marginal crenulations, covering the conical aspidin mound almost totally.Small tubercles of later generations on the branchial plates and tesserae are usually rounded, more rarely elongated; they bear 6-11 marginal crenulations, which are usually short, simple and unbranched (Fig. 4B, J, K; Fig. 6A, B, F).Some small tubercles on the ventral side of branchial plate PM SPU 80-2 (holotype) have a low (Fig. 4E-H) count of marginal crenulations, which intensely ramify towards their tips (Fig. 4E, F).The diameter of the bases of these small tubercles of later generations is 0.3-0.5 mm.There are also blisters with simple marginal crenulations, covering injuries on the plates (Fig. 2E, G, H; Fig. 3D, G, H).
3. Ornamentation topography.On the plates and micromeric elements of the new species known to date tubercles are arranged predominantly in regular concentric rows.The tubercles are not situated densely relative to each other (Fig. 4F, K, O).Rows of tubercles are oriented generally subperpendicular to the long axis (=to anterolateral and posterior margins) of branchial plates (Fig. 2A, G).The tubercles in rows are larger and have more massive and pronounced ridges on the dorsal side of the plates.The rows of the branchial plates can also have a slight proximal curvature (Fig. 3A, B, G, H).Small tubercles of later generations are situated between the large ones on the branchial plates.The ornamented surface on the branchial plates can generally be divided into two zones, the distal and the proximal one.In the distal zone the tubercles have a predominantly conical and peak-like shape; the crowns are usually slanted distally, the ridges in the rows are large and long, and the rows stand far from each other due to proximally developed marginal crenulations.In the proximal zone the tubercles have predominantly a simpler structure.They are domeshaped and low cone-shaped with a vertically oriented crown.They possess short marginal crenulations, weakly growing longer in the proximal direction.The shapes of the bases are various in both zones and their topography is unique for every specimen.Small tubercles of late generations are situated in the proximal zone.The shape and orientation of the tubercles' crowns on the lateral scales is the same as in the proximal zones of the ornamented parts of the branchial plates.On the external surface of the lateral scale, the tubercles with conical and dome-shaped crowns are arranged in concentric rows around the large primordial tubercle (Figs 2Q, 3I). 4. Internal structure.The superficial layer of the plates consists of orthodentine tubercles, covered by a thin layer (up to 1.5 μm) of hypermineralized tissue (PM SPU 80-8).The pulp cavities of dentine tubercles can be either hollow (Fig. 7D) or filled with aspidin trabeculae (Fig. 6E; Fig. 7B, C).Pulp cavities are connected with radial pulp canals, which run under marginal crenulations.Large dentine tubercles on the branchial plates, lateral scales and the primordial tubercle of micromeric elements are situated on cancellous aspidin mounds (Fig. 6E; Fig. 7A, D).The mounds on the branchial plates can also fuse into aspidin ridges (Fig. 2A, B).Satellite tubercles of discrete tesserae and small tubercles on the branchial plates generally have hollow pulp cavities.Cancellous aspidin has large spaces and vascular canals of aspidones; the reticular layer of the branchial plates is weakly developed (Fig. 7D).
Comparison.The branchial plates of the new species are more stenobasal than those of typical representatives of the genus Psammosteus.Psammosteus ramosus sp.nov.Glinskiy resembles P. falcatus Obruchev in Gross, 1942, P. kiaeri Halstead Tarlo, 1964, P. pectinatus Obruchev, 1965 and, to a lesser extent, P. tenuis Obruchev, 1965 in the extremely stenobasal shape of the branchial plates, details of ornamentation and its topography (Kiaer 1915, pl. 5, fig. 5;Halstead Tarlo 1964, pl. 2, fig. 2;Obruchev & Mark-Kurik 1965, pls 87-89, 91, 93).The rows of tubercles on branchial plates of P. ramosus sp.nov.can have a slight proximal curvature, as opposed to what can be seen in P. kiaeri and P. pectinatus.In the new species the tubercles in the rows on the branchial plates fuse into rather long ridges, just like in P. falcatus, P. kiaeri and P. pectinatus.However, in the new species these ridges are shorter than in P. kiaeri and they are very weakly developed on the proximal side of the plates, which differentiates it from P. pectinatus and places it somewhat closer to P. falcatus.The new species also differs from P. pectinatus in the size of the branchial plates, in the latter species they are small (Obruchev & Mark-Kurik 1965, pl. 93, fig. 2;Glinskiy & Ivanov 2015, fig. 1: 6).Lateral scales, both in size and in shape, most closely resemble those of Psammosteus bergi (Obruchev, 1943) and P. megalopteryx (Trautschold, 1880).The concentric arrangement of tubercles around the primordial tubercle (like on the tesserae) and their position at the posterior end of scales is seen in some scales of Psammosteus sp.(Mark-Kurik 1993, fig.3B) and also in isolated scales of P. bergi (Mark-Kurik 1999, fig.7D, E, G-I).Lateral scales of P. ramosus sp.nov.are also similar to those of P. bergi (Mark-Kurik 1999, fig.7D) in a greater reduction of the external ornamented surface.The new species comes close to P. cf. kiaeri and P. falcatus (Fig. 6 A-Q) in terms of morphology and inner structure of discrete elements, and is substantially different from other species of Psammosteus.High and complicated tubercles of exoskeletal elements in P. ramosus sp.nov.are situated on cancellous aspidin mounds or ridges, like in P. falcatus (Novitskaya 1965, fig. 227), P. cf. kiaeri and P. pectinatus.
The species in the genus Psammosteus differ from each other primarily in details of ornamentation on branchial plates (Halstead Tarlo 1964, p. 39).General details of ornamentation, its topography and histological characteristics of branchial plates of Psammosteus ramosus sp.nov.are close to those of P. falcatus Obruchev in Gross, 1942, P. kiaeri Halstead Tarlo, 1964and P. pectinatus Obruchev, 1965.In the details of tubercle morphology the new species also resembles P. asper Obruchev, 1965, P. bystrowi Obruchev, 1965, P. livoncius Obruchev, 1965 and P. tenuis, and is different from other species of the genus Psammosteus.It resembles P. asper, P. livonicus and P. tenuis in the massiveness of its crenulations and ramifications.As in P. asper, P. falcatus, P. kiaeri and P. pectinatus, marginal crenulations go deep into the crowns of the tubercles in the species described herein.The new species is similar to P. asper in its indented and rugose marginal crenulations.In the latter species such morphology of marginal crenulations is formed by microtubercles, not ramifications.The new species differs from P. bystrowi in a more complex form of marginal crenulations and the presence of ramifications.In the new species, ramifications are situated along the whole length of marginal crenulations as opposed to P. livonicus, in which they are confined to the tips of marginal crenulations (Glinskiy & Mark-Kurik 2016, fig. 6: 9, 10, fig. 7A).The new species is most closely similar to P. falcatus, P. kiaeri and P. pectinatus in the presence of ramifications along the whole length of the marginal crenulations.The main species-specific difference of the new species is that ramifications are large, massive, and densely and evenly distributed along the whole length of the flanks of marginal crenellations.Psammosteus ramosus sp.nov.differs from P. kiaeri (Kiaer 1915, pl. 5, fig. 5;Halstead Tarlo 1964, pl. 2, fig.2) in larger structural elements: each crenulation is thicker and more massive; these crenulations do not fork at the very crown base and bear larger ramifications and microtubercles.The new species is closest to P. pectinatus in details of tubercle structure.It generally differs from the latter in short branching tips of the marginal crenulations and longer ramifications (Fig. 6A, B).In their general morphology the tubercles of the new species resemble those of P. falcatus (and P. tenuis), but they differ in bearing microtubercles, in larger, short, massive ramifications without fine branching and in having more simple plumose wrinkles.
Remarks.The exoskeletal elements of P. ramosus sp.nov.include discrete micromeric elements, tesserae of 'progressive type', which are also known in P. falcatus and P. cf. kiaeri and, probably, will be discovered in P. pectinatus.This group is substantially different from the main evolutionary lineage of species in the genus Psammosteus and also Karelosteus (q.v.Glinskiy 2014) in all these characters, and especially in possessing the discrete micromeric elements of 'progressive type' instead of those of 'basic type'.Specimen PM SPU 80-9 is defined as P. cf. ramosus sp.nov., because it possesses a naked aspidin mound without dentine tubercles.
The extension of cancellous aspidin trabeculae into the pulp cavity of large dentine tubercles is well known in Ganosteus stellatus Rohon, 1901(Novitskaya 1965, figs 206, 208).The microstructure of large tubercles and micromeric elements of 'progressive type' in the described Psammosteus lineage and in the plates of Traquairosteus pustulatus (Traquair, 1897) is similar, because in both cases the undergrowth of cancellous aspidin and development of a cancellous aspidin mound are seen under growing dentine tubercles (Tarlo 1961, fig. 3C).However, dentine tubercles in T. pustulatus are simple, reduced in size (Halstead Tarlo 1965, p. 150).A similar model of skeletal development is also seen in a heterostracan Lepidaspis serrata Dineley & Loeffler, 1976: it possesses dentine ridges on the aspidin ridges, filled with aspidin trabeculae (Keating et al. 2015, fig.2A-D).

DISCUSSION
The evolution of the exoskeleton in the representatives of the family Psammosteidae sensu Novitskaya (2004) resulted in the reduction of exoskeletal structures in comparison with other psammosteids, which made it possible to lighten the skeleton and improve locomotion capabilities.A number of evolutionary transformations are observed in major plates of the exoskeleton (progressive development of stenobasal branchial plates, elongation of median plates), which are connected with contraction in width of the cephalothorax (Obruchev & Mark-Kurik 1965, pp. 54, 55).The extremely stenobasal shape of branchial plates, characteristic of the juvenile stage of development in many Psammosteus species (Obruchev & Mark-Kurik 2016), is retained in some species of Psammosteus in the adult stage (paedomorphism).This phenomenon is manifested independently in different evolutionary lineages of Psammosteus.All representatives of Psammosteus, as opposed to other groups of psammosteids, are also characterized by the reduction in size of mesomeric exoskeletal elements, the lateral scales.According to Obruchev, this is connected with an increase in the mobility of the tail (Obruchev 1945, p. 263;Obruchev & Mark-Kurik 1965, p. 217).Psammosteus is characterized by a general constriction of the base of rooted micromeric elements -tesserae.At the same time they occupy most of the surface of cephalothoracic plates (advanced tesserization).Morphotypes of micromeric exoskeletal elements in Psammosteidae are probably more varied than in other psammosteids (as they are known only in Psammosteidae to date).In discrete micromeric elements of the cephalothorax an increase in height is observed.The height of such elements corresponds to the usual thickness of a dorsal median plate in representatives of the family Psammosteidae, suggesting that discrete exoskeletal elements could be situated in the skin at approximately the same level as the plate.
Thus, discrete tesserae of 'progressive type', characteristic of one of the lineages in the genus Psammosteus, stand out by their height, which is attained by the enlargement of an aspidin mound (neck) and its overgrowth by the dentine of the primordial tubercle.The development of discrete tesserae of 'progressive type' in the lineage P. ramosus sp.nov.-P.falcatus was by successive accretion of satellite tubercles to the neck of single elements.Tesserae of this type, formally circular tesserae (Halstead Tarlo 1967b;Karatajūtė-Talimaa 1998), are a result of multifunctional adaptation through paedomorphism, because the reduction in the count of tubercles in the crown provides the lightening and improvement of locomotion while preserving the protective function.Single elements represent the first ontogenetic stage of development of micromeric elements in psammosteids.
According to Obruchev (Obruchev & Mark-Kurik 1965, p. 43), the presence of lateral dorsal canals of the lateral line sensory system on the dorsal plate of Psammosteus points at the general constriction of the cephalothorax.Thus, lateral fields of tesserae were much narrower in Psammosteus than in other psammosteids, in which lateral dorsal canals were situated in wide fields of tesserae.The increase in the mobility of Psammosteidae in comparison with other psammosteids is also indicated by a progressive complication of the pattern of the lateral line system on the dorsal plate.Thus, in a late representative of the group, cf.Psammosteus from the Frasnian of Arctic Canada, sensory canal grooves have a very elaborate pattern (Elliott & Mark-Kurik 2005).
Psammosteid heterostracans underwent the stages of assimilative (e.g.Drepanaspididae, Obrucheviidae) and regressive (Psammosteidae) phases of exoskeletal development during their evolution.The phase of exoskeletal regression usually coincided with the increase in the mobility of the animals.Thus the ecological specialization changed from demersal Drepanaspis (Blieck 2016) with armoured body type to the most active demersal forms like Psammosteus.

RESULTS
At least two evolutionary lineages within Psammosteidae sensu Novitskaya (2004) can be distinguished now based on the differences in the morphology and histological details of the exoskeleton.The species Psammosteus ramosus sp.nov.Glinskiy is the oldest known representative of the psammosteid lineage with discrete cyclomorial tesserae of 'progressive type' and a peculiar histological structure of the other plates.The new species comes close to P. falcatus Obruchev in Gross, 1942, P. kiaeri Halstead Tarlo, 1964and P. pectinatus Obruchev, 1965, but is strongly different from other known representatives of the genera Psammosteus and Karelosteus weberi Obruchev, 1933.Obruchev did not exclude the possibility that the genus Psammosteus may be divided into several genera in the future (Obruchev & Mark-Kurik 1965, p. 219).

Fig. 1 .
Fig. 1.A, a map showing the localities of Psammosteus ramosus sp.nov.Glinskiy from eastern Latvia and Northwest Russia.B, Ķūķu locality on the right bank of the Gauja River, Latvia.C, outcrop 3 of the Borschovo locality, Russia.D, outcrop N-2 of the Andoma Hill locality, Russia.E, the stratigraphical position of the studied localities. 1 , Regional Beds of the eastern part of the Main Devonian Field.Black psammosteid figures mark the approximate stratigraphic levels from which the specimens of the new taxon were collected.

Table 1 .
Measurements of fragments of branchial plates (in mm)