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 Pärnu Formation, Estonia, are described. New data on the topography of ornamentation and histology of psammosteids from the Pärnu 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, Pärnu County, southwestern Estonia) is described for the first time. Along with the taxa characteristic of the Pärnu Regional Stage, the acanthodian Cheiracanthus sp. cf. C. splendens is first recorded in this interval.


INTRODUCTION
Remains of psammosteid heterostracans and acanthodians are very diverse and abundant in the Middle and Upper Devonian (Eifelian, Givetian and Frasnian stages) of the Baltic area, i.e. the Main Devonian Field (MDF). These remains characterize regional stratigraphic units, with several psammosteid and acanthodian species being used as index fossils (e.g. Gross 1942;Mark-Kurik 2000;Mark-Kurik & Põldvere 2012;Glinskiy 2013). Early Eifelian vertebrates are known from the Pärnu Regional Stage (RS) in the western part of the MDF (territory of Estonia and Latvia). They are still rather poorly studied due to the state of preservation of vertebrate remains, their relatively rare occurrence and small range of outcrops of the Pärnu Formation, confined only to Estonia (Sammet 1973;Kleesment & Mark-Kurik 1997).
Vertebrate assemblages of the Pärnu RS were described in detail by W. Gross in his works, dedicated to the biostratigraphy of the MDF (Gross 1933(Gross , p. 65, 1942. For the Pärnu RS, a psammosteid zone 'Schizosteus heterolepis (Preobrazhensky, 1911)' was proposed by this author (Gross 1942). Later E. Mark-Kurik (Obruchev & Mark-Kurik 1965;Mark-Kurik 1968) described another psammosteid taxon -Psammolepis toriensis (Mark-Kurik, 1965) from this interval. The only author, who provided detailed descriptions of acanthodian assemblages of the Pärnu RS in the Baltic area, is J. Valiukevičius. According to him (Valiukevičius 1998), the Laliacanthus singularis acanthodian Zone corresponds to both the Rēzekne and Pärnu RSs. The zone has ten diagnostic species, but only four of them had been known from the Pärnu RS. Besides the index species, these are Rhadinacanthus primaris Valiukevičius, Diplacanthus kleesmentae Valiukevičius and Watsonacanthus cf. oervigi Valiukevičius. Valiukevičius mentioned that acanthodians are very poorly represented in the terrigenous deposits of the Pärnu RS. Thus, any new data on the acanthodian diversity of this interval is of particular interest.
The aim of this study is the description and analysis of material on psammosteids and acanthodians from the typical (Tori) and new (Taali) localities of the Pärnu RS in Estonia.
(PMO), Norway. Most of the acanthodian and some psammosteid remains were collected by the authors of the article in the Taali locality (Tori Parish, Pärnu County, southwestern Estonia) during the expedition in August 2016.
Psammosteid and acanthodian material was studied under the stereo zoom microscope Leica М205 C and under the scanning electron microscope (SEM) Hitachi S-3400N in secondary electrons (SE) and back-scattered electrons (BSE) detection modes. Thin sections were made using 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 800-1000. The sandpaper with a grade P 1200-2000 was used subsequently to polish a thin section. Thin sections of acanthodian scales were made using a polymethyl methacrylate thermoplastic Glass Fil (Print Product ® ) as a mounting medium and Mirka ® Waterproof P1500 and P2500 finishing sandpapers. The photographs of thin sections were taken mainly with a polarization microscope Leica DМ4500 P with the use of aniseed oil. Some of the psammosteid sections were photographed with the SEM in the BSE detection mode.

GEOLOGICAL BACKGROUND
In Estonia the Pärnu RS (Lower Eifelian, Middle Devonian) is represented by the Pärnu Formation, which is divided into the Tori (below) and Tamme (above) members (Mb.) (Mark-Kurik & Põldvere 2012). Most of the vertebrate material comes from the Tori Mb. It crops out mainly on the left bank of the Pärnu River (Tori Parish, Pärnu County) in southwestern Estonia (Fig. 1А, B). The Tori Mb. consists of light-yellow finegrained cross-bedded sandstone with interlayers of grey clay (Kleesment & Mark-Kurik 1997) (Fig. 1C). It is overlain by yellow, greenish and purplish-grey horizontally-bedded sandstones and dolostones of the Tamme Mb. It is almost devoid of vertebrate remains (Kleesment & Mark-Kurik 1997). B, Tori and Taali localities on the left bank of the Pärnu River. C, the stratigraphical position of the studied localities. 1 , Regional stratigraphical units of Estonia. Black fish figures mark the approximate stratigraphic levels in which the vertebrates are found. Abbreviations for the generalized lithologic column: 1, cross-bedded sandstone; 2, dense parallel-bedded sandstone; 3, lenses of clay (up to 1 m in width).
The Tori (Tori Põrgu) locality is the stratotype of the Pärnu RS. It is represented by a cliff which stretches along the river for 1 km upstream of the bridge in the Tori settlement. The section of this locality is formed by the rocks of the Tori Mb. (7.5 m in thickness) and Tamme Mb. (0.75 m in thickness). Vertebrate remains occur there in the cross-bedded sandstone in the lower and middle parts of the Tori Mb. (Kleesment 1991). The following taxa are reliably identified here (Kleesment & Mark-Kurik 1997;Mark-Kurik 2000): psammosteids Guerichosteus heterolepis (Preobrazhensky) ( Fig. 2A- The Taali locality is situated downstream of the Tori locality. It is represented by a quarry, situated at the edge of the Tori settlement. The walls of the quarry (4.5 m in height) are formed by the rocks of the Tori (4.3 m thick) and Tamme Mbs (0.5 m thick). Vertebrate remains occur in the cross-bedded sandstone in the lower and middle parts of the Tori Mb. (about 0.5 and 1.8-2 m from the base of the quarry). The following taxa have been recorded here: Guerichosteus heterolepis, Psammolepis toriensis, Actinolepis tuberculata, Byssacanthus dilatatus, Glyptolepis sp., Osteolepididae indet., Rhadinacanthus primaris Valiukevičius (Fig. 3A, B, G, H), Cheiracanthus gibbosus Valiukevičius (Fig. 3C, I-L), C. sp. cf. C. splendens Gross (Fig. 3D,E), Acanthodes? sp. C Valiukevičius (Fig. 3F). To Acanthodes? sp. C are assigned acanthodiform scales with a smooth flat crown, high, well-defined neck and convex base similar to those described by Valiukevičius (1985). The fossils are represented by fragments of plates and complete micromeric elements (scales and tesserae of psammosteids) up to 2-3 cm in length, and scales of acanthodians. Large fragments (more than 10 cm) of psammosteid plates occur at the level 1.8-2 m.
Remarks. Guerichosteus heterolepis is attributed to the genus on the basis of similarities in the morphology of the branchial plates, postorbital plates and in the ornamentation characters (size and morphology of tubercles) and their topography. The ornamentation of Guerichosteus is very similar to that of 'Hariosteus'.
Apparently they belong to one genus, because these taxa differ only in the density of tubercules. The topography of the ornamentation of Guerichosteus heterolepis shows that the sizes, shape of bases and density of tubercles may change on different plates of one species. Thus G. kotanskii Halstead Tarlo 1964 andG. kulczyckii Halstead Tarlo 1964 are apparently the junior synonyms of G. kozlowskii, because these taxa have identical tubercles and details of the ornamentation (including short ridgelets, accessory small tubercles), whereas the difference in sizes are primarily the result of the topographical mutability of their plate fragments. Areal zones of tubercles are visible on the parts of plates with loosely packed tubercles of 'Hariosteus'. This type of ornamentation is characteristic of e.g. Drepanaspis, Guerichosteus, however, 'Hariosteus', one species of Ganosteus, scales of 'Psammolepis' undulata Agassiz, 1844, tesserae of Drepanaspis and Psammosteidae sensu stricto also have the areal zones with graduated sizes of tubercles (Gross 1963, p. 146;Halstead Tarlo 1964, pp. 38-39, 1965Glinskiy & Nilov 2017). Areal zones demonstrate the stage of ontogenetic development -widening of the aspidin layers of the plate. If the psammosteids have the similar morphology of plates and tubercles, the areal zones or growth zones (bands) with graduated sizes of tubercles should be considered as the population variability or as the diagnostic character for the species. Guerichosteus can be distinguished from Schizosteus by the presence of the dorsal lamella on the branchial plates and the absence of tubercles' marginal crenulations.
Ornamentation and topography. The tubercles are fairly large (from 0.4 to 1.7-2 mm, usually approximately 1 mm). The crowns of the tubercles are dome-or blistershaped. They are oriented vertically or can be slightly slanted towards the surface of the plate ( Fig. 2H-J, N, O). Radial ribs (12-17) are robust and can ramify (Fig. 2I, J). Ribs bear rare microtubercles (up to 5) (Fig. 2O, P) and can reach the base of the tubercle, forming marginal serrations. However, in most cases (branchial plates, body scales) radial ribs are short and the base of the tubercle is smooth (Fig. 2N, O). Tubercles usually have rounded and oval (more rarely irregular and polygonal) bases. Rarely two tubercles fuse, forming short ridgelets (Fig. 2H). Small, mostly angular, tubercles, filling the spaces between the larger tubercles are occasionally located concentrically around large ones (Fig. 2K, L, M). They are usually closely-packed on the orbital, branchial, dorsal plates, scales and loosely packed on the postorbital plate. On the postorbital plate they can form very tight longitudinal rows (Fig. 2M). On the dorsal side of the branchial plates (TUG 1552-2, GIT 116-13) there are growth zones with different sizes of tubercles. Tubercle surfaces do not bear cell imprints.
Histology. The superficial layer of the plates consists of orthodentine tubercles covered by a thin layer (from 8 to approximately 20 µm) of enameloid ( Fig. 2Q-V). The latter covers the whole tubercle down to its base (Fig. 2T). It reaches the maximum thickness in the upper part of the tubercle. Upper and lower layers can be distinguished in the enameloid (Fig. 2U, V). The upper one (up to 1.65 µm in thickness) is the most hypermineralized. The thicker (approximately 10 µm) lower layer is less mineralized. It is penetrated by dentine tubules (Fig. 2R, S) as in durodentine tissue. Orthodentine comprises dentine tubules, which start to branch intensively near the enameloid-dentine boundary.
Their ramifications are short in other regions of dentine (Fig. 2S). Each tubercle has a large and simple pulp cavity. The thickness of the L1 layer is 0.2 mm. The aspidin trabeculae of the L2 layer form large cavities (up to 0.4 mm).
Comparison. The branchial plates of Guerichosteus heterolepis have a free anterolateral ledge with the distal corner and dorsal lamella, as can be seen on the adult specimens of G. kozlowskii (Halstead Tarlo 1965, pl. 9, figs 2, 3, 6). Halstead Tarlo (1964, p. 99) mentioned that the ornamentation of G. heterolepis is similar to that of G. kozlowskii. Guerichosteus heterolepis is also similar to G. kozlowskii (Halstead Tarlo 1965, text- fig. 8B, pl. 10, fig. 3, pl. 12, fig. 7) in the presence of small tubercles between larger ones (it is not the specific character) and to other specimens (Ibid.,pl. 7,figs 3,4) in the presence of small (about 0.5 mm) tubercles. The species differs from G. lefeldi Halstead Tarlo, 1964 in the size of ornamentation, which is larger. Guerichosteus heterolepis probably differs from the other species of Guerichosteus in higher topographic mutability (shape of tubercle bases, density of ornamentation).
Remarks. Previous reconstructions of the dorsal side of the branchial plate of Guerichosteus heterolepis (Obruchev & Mark-Kurik 1965;Novitskaya 2004, textfig. 92a) were made on the basis on two different specimens (TUG 1552-2 and GIT 116-1) without taking into account the tubercles that have dropped off from the medial region. The dorsal side of the branchial plates of Guerichosteus is oriented at a larger angle towards the ventral side, not like in Schizosteus (Halstead Tarlo 1964, p. 28;Glinskiy 2014). It should also be noted that the orbitale of G. heterolepis bears several radial lateral line canals (Obruchev & Mark-Kurik 1965), and not one, as suggested by Halstead Tarlo (1965). The pattern of the growth lines on the postorbital plate of this species is identical to that of Guerichosteus (Halstead Tarlo 1965, pp. 58-59). The ornamention of the postorbital plate of G. heterolepis is close to that of 'Hariosteus' kielanae Halstead Tarlo, 1964 and'H'. lobanowskii Halstead Tarlo, 1964. Guerichosteus heterolepis differs from them in the dominance of angular-shaped small tubercles and a generally relatively denser arrangement of tubercles. The species 'H'. kielanae and 'H'. lobanowskii are described based on fragmentary material. They are distinguished by the characters of ornamentation topography. However, the material of G. heterolepis shows that the ornamentation, similar to either G. kozlowskii, 'H'. kielanae (Halstead Tarlo 1965, text- fig. 9) or 'H'. lobanowskii (straight rows of closely-packed tubercles) can be present on certain regions of different plates of the same species (Fig. 2H, K-M). The topographical study of the ornamentation of the more complete material of Guerichosteus and 'Hariosteus' is needed.

Description
Morphology of the scales. Small scales with the length of the crown about 0.3-0.4 mm. The crown is flat and subhorizontally oriented. The sculpture on its surface is represented by very thin ridges, which run parallel to each other in the posterior part of the crown and diverge in a fan-like manner in its anterior part (Fig. 3A). Most of the ridges in the medial part of the crown dichotomize towards its anterior margin, giving off numerous finer ridges. The ridges gradually fade posteriorly. An unornamented rim can be developed along the anterior margin of the crown. The neck is well defined and high, with vertical grooves running in its lower part (Fig. 3B). Small rounded prominences can be present on the posterolateral walls of the neck. The base is convex, its tip is well defined and subcentrally situated.
Histology of the scales. Due to scarcity of the material only one transverse vertical section was studied. It shows at least two relatively wide growth zones in the crown. The crown is composed of mesodentine. Horizontal canals of the crown are very narrow, almost as narrow as dentine tubules. They give off numerous intensively branching winding dentine tubules. Neighbouring horizontal canals are connected with very thin circular canals (they are almost as thin as dentine tubules). The tubules do not form distinct 'Scheitelung' (sensu Gross 1973) under the ridges (Fig. 3G, H). The most outstanding feature of the vascular canal system of the neck is a wide circular canal, situated in its lower part above the base (Fig. 3G). Wide ascending canals branch upwards from it. They ramify, giving off branches in different directions. These canals are not oriented strictly in relation to growth lamellae: some of them cross these zones diagonally (though the growth lines are not distinct in the neck). A very dense network of relatively wide winding canals is developed in the base.
Comparison. Rhadinacanthus primaris most closely resembles R. longispinus (Agassiz, 1844). These scales are very similar in morphology, though R. primaris scales have a more delicate ornamentation, with distinct fan-like divergence of ridges near the anterior margin of the crown. As suggested by Burrow et al. (2016), there is no distinct 'Scheitelung' (sensu Gross 1973) under the ridges of R. primaris scales, which is well developed in the scales of R. longispinus (Burrow et al. 2016). Each growth zone in R. longispinus scales has its own vascular and dentinal canal system, whereas in the studied scales of R. primaris many of the vascular canals seem to cross growth zone margins.

Description
Morphology of the scales. The scales are large and robust; the length of the crown is about 0.6 mm. It is thick, rhomboidal in outline, with a rounded anterior margin. The crown sculpture is represented by two broad low medial ridges, which are separated by a short narrow U-shaped or slit-like groove near the posterior angle of the crown and fuse anteriorly to form a wide flat prominence, which covers almost the entire crown area (Fig. 3C). These ridges gradually increase in height posteriorly. Short fine ridges are developed near the anterior margin of the crown. They are oriented subperpendicular to it and cover both the lateral regions of the crown and the anterior part of the fused medial ridges. The neck is well defined and considerably high; the base is convex; its tip is slightly shifted anteriorly.
Histology of the scales. Due to scarcity of the material only a horizontal section of the lower part of the crown was studied. It shows a very complex dense network of ascending, circular and horizontal canals characteristic of the species (Fig. 3I-L). Many of the canals are wide and form extensive cavities (Fig. 3K, L). Horizontal canals branch intensively, giving off numerous very fine canals.
Comparison. In their morphology the scales are rather similar to those of С. latus? Egerton. The scales of the described species are much more robust, have a thicker crown and more extensive medial ridges. They differ histologically in the structure of mesodentine of the lower part of the crown, which has a characteristic dense complex network of widened canals.
Remarks. The scales of С. latus? Egerton, mentioned above, were defined as C. longicostatus Gross in Valiukevičius (1985), but in their morphology they seem to be similar to those of С. latus Egerton. More research is needed on this subject. This taxon is one of the most common cheiracanthids in the Middle Devonian of the Main Devonian Field.

Description
Morphology of the scale. The scale is very small, the length of the crown is about 0.2 mm. The crown has isometric, square, rounded outline, however, it was affected by abrasion. It is flat, horizontally oriented and devoid of sculpture. The crown surface has an ultrasculpture, represented by a network of tongueshaped depressions. Their length varies from 8 to 15 µm, the width -from 6 to 10 µm. The ʽtonguesʼ gradually deepen anteriorly. They are arranged in longitudinal rows. The neck is well defined and of medium height. The base is convex and of the same height as the neck.
Comparison. In its morphology the scale is very close to the scales of C. splendens Gross, 1973 from the erratic boulders of the Narva RS in Estonia. The only difference is that in the latter the 'tongues' are convex and partially overlap each other, whereas in the described scale they represent depressions. The 'tongues' on the scales of C. splendens are more elongated, reaching 25 µm in length (Märss 2006).

DISCUSSION
The diversification of psammosteids had taken place before the Givetian (Obruchev & Mark-Kurik 1968). Guerichosteus heterolepis (Preobrazhensky, 1911), a representative of an old evolutionary line of psammosteids (Guerichosteidae), Psammolepis toriensis Mark-Kurik, 1965, a basal representative of a progressive psammosteid group (Psammolepididae) and Tartuosteus? sp., a typical representative of the Schizosteus-lineage, are present in the sandstones of the Pärnu RS. Thus, along with last archaic psammosteids, two major lineages of more derived psammosteids are present in the Pärnu deposits. The latter became widespread from the end of the Eifelian till the Frasnian in the Baltic area and beyond it (Lukševičs et al. 2010). It may be concluded that the radiation of psammosteids with the appearance of major groups had ended by the early Eifelian (Pärnu Age). The genus Guerichosteus, which had previously been known from the Emsian of Poland and perhaps of the Baltic States (Rhinopteraspis cornubica Zone), is also found in the Eifelian of the East Baltic area (Estonia). Two of the acanthodian species diagnostic of the Laliacanthus singularis Zone are found in the Taali locality -Rhadinacanthus primaris Valiukevičius and Cheiracanthus gibbosus Valiukevičius. The first one has already been known from this interval, whereas C. gibbosus is recorded for the first time in the Pärnu RS. Another taxon, which had not been known before from this interval, is Cheiracanthus sp. сf. C. splendens Gross. It is very close to C. splendens Gross, which was described from erratic boulders of the Narva RS in Estonia (Gross 1973).

CONCLUSIONS
The genus Guerichosteus, which had previously been known from the Emsian of Poland (Holy Cross Mountains, placoderm sandstone, Rhinopteraspis cornubica Zone), is also found in the Lower Eifelian of Estonia. The presence of a hypermineralized capping layer (enameloid) in the exoskeleton of Guerichosteus heterolepis (Preobrazhensky, 1911) is shown. The acanthodians Cheiracanthus gibbosus Valiukevičius and Cheiracanthus sp. сf. C. splendens Gross are described from the Pärnu RS for the first time. The latter taxon has an ultrasculpture, formed by the network of tongue-like depressions. Such ultrasculpture has not been described in acanthodians before. It most closely resembles that of C. splendens from the Narva RS, but the ʽtonguesʼ are convex in the latter.