Tuesday 7 July 2020

Placoderms from the Early Devonian of Belarus.

Little work has been done on the Early Devonian (late Emsian) Placoderm fishes of Belarus despite these being relatively common in Belarus and their use in regional biostratigraphy elsewhere in the Old Red Sandston continent. Specimens from the orders Acanthothoraci, Ptyctodontida, Arthrodira, and Antiarcha are all recorded from the Lower Devonian deposits of Belarus. Arthrodires are the most common Placoderm group found in the upper Emsian deposits of Belarus. The first recorded find of an Arthrodire in the Emsian of Belarus was in 1963 from the Vilchitsy 1 borehole in the Obol and Lepel beds of the Vitebsk Regional Stage. These remains consisted of a skull roof, trunk shield, and a number of isolated plates from the trunk shield, as well as endocranium remains.

In a paper published in the Journal of Paleontology on 31 March 2020, Dmitry Plax of the Belarusian National Technical University and Michael Newman of Haverfordwest in Pembrokeshire, Wales, describe four new Placoderms from from the Lepel Beds of the Vitebsk Regional Stage of the Lower Devonian (upper Emsian) of Belarus. They erect two new species of arthrodire and describe two other forms in open nomenclature.

The Vilchitsy 1 borehole remains were considered by Elga Mark-Kurik to be a single species and referred to it as 'Kartalaspis belarusica'. She considered it a Phlyctaeniid Placoderm. However, the species was not formerly described despite it being mentioned in many publications and so it must be considered a nomen nudum. Even the species name of this fish in the literature has been spelled in several ways, e.g., belorussica, byelorussica, and belarussica, adding to the confusion. It is in need of being properly described.

Later,  Lyubov Lyarskaya  recorded Ptyctodontida indet., Phyctaeniina indet., and Antiarcha indet. in the Vitebsk Formation of the Orsha Depression (in the Vilchitsy 1, Pochtari 1 and Liozno 1 boreholes). 

Subsequently Millerosteus orvikui was recorded from Mstislavl 1 borehole in the Lepel Beds of the Vitebsk Regional Stage. This species was originally described from strata of Eifelian age (Kernavė Regional Substage) of the Baltic region, and this species is now considered a subjective junior synonym of Coccosteus cuspidatus. The genus Millerosteus is known from the Givetian of both Estonia and Scotland; it thus seems unlikely that it is present in the Emsian of Belarus and the remains recorded there must be considered for now as indeterminate.

Dmitry Plax started a study of the upper Emsian deposits of Belarus in 2002, and has reviewed and studied about 25 boreholes containing deposits of this age. Remains of late Emsian placoderms in various states of preservation were recovered from 13 boreholes, all within the Lepel Beds of the Vitebsk Regional Stage. Acanthothoracid remains were recovered from the Berdyzh 1 borehole. Ptyctodontida remains were recovered fromthe Chashniki 53, Lepel 1, and Latvishi 12 boreholes. One undefinable plate fragment of a Ptyctodontid was also recovered from the rocks of the Lepel Beds of the Vitebsk Regional Stage in the Rudnya 14 borehole. Arthrodiran remains are the most commonly recovered remains found in boreholes. They have been recovered from the Osipovichi 6, Bobruysk 691, Bobruysk 691/2, Lepel 1, Bykhov 151, Rogachev 736, Buda Dal’nyaya 35, Bykhov 1, and Korma 1 boreholes. Finally, a few Antiarch remains have been recovered from the Osipovichi 6, Buda Dal’nyaya 35, Berdyzh 1, Bykhov 1, and Korma 1 boreholes.

All of the specimens described in this work were collected from core samples extracted from the Lepel Beds. The Lepel Beds form the upper part of the Vitebsk Regional Stage, with the lower part being the Obol Beds. The Vitebsk Regional Stage constitutes the upper Emsian (Lower Devonian) in Belarus. These deposits are widespread throughout Belarus. The Eifelian Adrov Regional Stage lies above the Vitebsk Regional Stage. The Lepel Beds correspond to the upper part of the Rhabdosporites mirus-Gneudnaspora divellomedium Miospore Biozone and approximately to the upper part of the Polygnathus costatus patulus Conodont Biozone. This is approximately equivalent to the upper part of the Rezēkne Regional Stage in the Baltic region and the upper part of the Novobasovo Beds of the Ryazhsk Regional Stage in the central part of Russia.

Stratification of the Upper Emsian deposits of Belarus and their correlation with the synchronous deposits from the adjacent territories. Plax & Newman (2020).

The first new species described is named Stipatosteus svidunovitchi, where 'Stipatosteus' comes from the Latin, ;'stipa', packed and 'osteus', bone, and honours Igor Svidunovitch who participated in the sampling of paleoichthyological material from the cores of boreholes. All specimens came from the Korma 1 borehole at depths of 340.2 m, 340.8 m, 337.7 m, 337.8 m, and 337.9 m.

Map of Belarus showing the location of the borehole sections where the skeletal elements of the Placoderm Fish were found. Boreholes: (1) Osipovichi 1; (2) Bobruysk 691; (3) Rogachev 736; (4) Korma 1; (5) Bykhov 1; (6) Buda Dal’nyaya 35. Plax & Newman (2020).

Only four near complete identifiable plates are known, all from the trunk armor. These include a right anterior lateral plate, a right anterior ventrolateral plate with attached right spinal plate, and a right posterior ventrolateral plate. There are also fragments of spinal plates and other unidentifiable plate fragments. All plates are characterized by ornamentation of densely packed tubercles forming distinct rows. The tubercles are star-shaped at their bases. Some of the individual tubercles have an unusual crown ornament of sinuous, bifurcating ridges emanating from the center. This is due to tubercles overgrowing older tubercles, indicating that these plates do not belong to juvenile individuals.

Stipatosteus svidunovitchi, dermal plate remains: (1)–(3) indefinable dermal plate fragments: (1) BNTU 121/44-23; (2) BNTU 121/44-18; (3) BNTU 121/44-16; (4) right anterior lateral plate (BNTU 121/38-2); (5)–(7) holotype, a right anterior ventrolateral plate with attached right spinal plate (BNTU 121/38-1): (5) ventral view with impression of visceral armor; (6) view of the visceral armor; (7) detail of right spinal plate; incomplete right spinal plate (BNTU 121/46-1): (8) probable dorsal view; (9) probable ventral view; (10), (11) both sides of incomplete spinal plate, orientation unclear (BNTU 121/46-3); (12) left posterior ventrolateral plate (BNTU 121/40-1). gl,  growth lines. Scale bars, 5 mm (4)–(6), (8)–(12), 1 mm (1), (7), 0.5 mm (2), (3). Plax & Newman (2020).

BNTU 121/38-2 is a right anterior lateral plate with a broken posterior edge. The edge contacting the spinal plate is also slightly broken and irregular. This makes it difficult to estimate a breadth/length index. The ornamentation consists of tightly packed tubercles that extend in rows subparallel to the outline of the plate. The anterior ventral corner has a significant process pointing anteriorly from which extrudes a ridge. The anterior dorsal corner is rounded with the dorsal edge extending diagonally ventrally.

BNTU 121/38-1 is a right anterior ventrolateral plate attached to a right spinal plate preserved in part and counterpart. The plate is worn at the edges, but its overall shape is discernible. The plate is quite long with a breadth/length index of about.8. The plate is quite flat-bottomed. The plate is very rounded mesially and anteriorly with no evidence for a contact face for an anterior ventral plate. Little or no ornamentation is preserved on the part althoughit ispresent on the attached spinal plate The suture between the anterior ventrolateral plate and spinal plate is difficult to discern. The rest of the specimen has an impression of the visceral surface of the bone. The counterpart shows the visceral side of the plate. The bone is at full thickness at the center of the plate but is worn and delaminated toward the edges. Growth lines can be seen which indicate that the bone grew from the center of the plate. The visceral surface has a ridge that extends from the posterior contact of the spinal plate a short distance into the plate.

BNTU 121/46-1 is the most complete of the known isolated spinal plates, and the species description is mostly based on this specimen. It is preserved in three dimensions, and oval in transverse section. It is probably a right spinal plate because one surface is smoother than the other. The anterior section of the ‘free’ part of the plate is preserved with little or no contact face to the anterior ventrolateral plate visible. The tip of the distal end is also missing. BNTU 121/46-1 is long and slender with a gentle mesial curve. Rows of rounded tubercles extend the length of the spinal plate. There are no spinelets on the inner edge of the plate. Similar ornamentation is seen in BNTU/46-3, which shows BNTU 121/46-1 is neither worn nor aberrant. BNTU 121/38-1 is a third right spinal plate attached to an anterior ventrolateral plate and shows similar ornamentation. However, it has been crushed flat and shows no curvature. Also, it is not clear how complete it is. Certainly, some of the free end is missing. It has a long contact with the anterior ventrolateral plate at about half of its preserved length.

BNTU 121/40-1 is a left posterior ventrolateral plate with preserved lateral and ventral laminae. The plate is fairly complete with a breadth/length index of at least 0.5, although the posterior end of the plate is missing. The ventral lamina is approximately twice as wide as the lateral lamina and is as long as two thirds of the ventral lamina. The ventral lamina is flat-bottomed. The ventrolateral ridge is quite pronounced posteriorly. Where preserved, the plate is densely packed with tubercles. The visceral side is still buried within the matrix.

Because a number of the ventral trunk plates areknown, Plax and Newman were able to reconstruct the ventral trunk armor. It is clear the trunk armor is quite short with a breadth/ length index of about 1.0 at the anterior ventral plate’s widest points. The anterior ventrolateral plates are particularly short, and shorter than the posterior ventrolateral plates. Another distinctive character is the long spinal plates, which are only gently curved without mesial denticles. The free end of the spinal plate is approximately the same length as the spinal plate attached to the anterior ventrolateral plate. These characters are incompatible with Stipatosteus svidunovitchi being a Petalichthyid. The ventral armor is most similar to that of the Arthrodiran infraorder Actinolepina, but the lack of an anterior ventral plate clearly places it in the suborder Phlyctaenioidei. The long, thin spinal plates with long, thin, free distal ends are characteristic of the Phlyctaeniidae.

Stipatosteus svidunovitchi n. gen. n. sp., reconstruction of ventral trunk armor. Abbreviations: AMV, anterior median ventral plate; AVL, anterior ventrolateral plate; IL, interolateral plate; PMV, posterior median ventral plate; PVL, posterior ventrolateral plate; Sp, spinal plate. Dashed lines indicate the boundaries of unknown plates. Plax & Newman (2020).

Approximately 18 genera are known in the Phlyctaeniidae with the highest diversity being known from the Early Devonian of Spitsbergen and Campbellton in New Brunswick, Canada. Stipatosteus svidunovitchi displays some similarity to species of Phlyctaenius. There are three species known: Phlyctaenius acadicus, Phlyctaenius atholi, and Phlyctaenius stenosus. The closely related form Gaspeaspis cassivii is probably a junior subjective synonym of Phlyctaenius atholi. Stipatosteus svidunovitchi differs from Phlyctaenius in several ways. The anterior ventrolateral plates are shorter, but also wider in Stipatosteus svidunovitchi, making the whole trunk shorter than in Phlyctaenius. The posterior ventrolateral plates form an embayment at their posterior ends, absent in Phlyctaenius stenosus. The spinal plates are also slenderer than in Phlyctaenius. The arrangement of the tubercles is uniform and close together in both Phlyctaenius acadicus and Phlyctaenius atholi. However, in Phlyctaenius stenosus, they are arranged parallel to the plate margins, similar to those in Stipatosteus svidunovitchi. Some of the tubercles of Phlyctaenius show a similar crown ornament of sinuous ridges due to tubercle overgrowth as seen in Stipatosteus svidunovitchi. Stipatosteus svidunovitchi slightly differs from Phlyctaenius in having a shorter body trunk with, therefore, shorter trunk plates. The spinal plates are also narrower.

Of the Spitsbergen species, some of the genus Heterogaspis acuticornis, Heterogaspis minutus, Heterogaspis hornsundi, are somewhat similar to Stipatosteus svidunovitchi. Stipatosteus svidunovitchi differs from Heterogaspis acuticornis in having a longer posterior ventrolateral plate. However, as in Stipatosteus svidunovitchi, there is an embayment at the posterior median end of the posterior ventrolateral plates. However, the spinal plate of Heterogaspis acuticornis is strongly curved, making it quite different from Stipatosteus svidunovitchi. In Heterogaspis minutus, the ventral trunk armor is unknown but the spinal plate is strongly curved, unlike that of Stipatosteus svidunovitchi. Heterogaspis hornsundi is poorly known. with many missing characters. Like other Heterogaspis species, it has strongly curved spinal plates. Thus, whereas some Heterogaspis species are similar to Stipatosteus svidunovitchi, they differ in having strongly curved spinal plates.

The second new species described is placed in the genus Actinolepis, and given the specific name zaikai, in honour of the Belarusian palaeontologist Yury Zaika. The species is based upon 22 specimens from the boreholes Osipovichi 6 at depths of 118.8 m, 113.8 m, and 113.5 m, Bobruysk 691 at a depth of 234.5 m, and Bykhov 1 at a depth of 307.3 m, all from the Mogilev region. Also, the boreholes Rogachev 736 at a depth of 293.0 m and Korma 1 at a depth of 322.3 m, both from the Gomel region. All occurrences are within the Lepel Beds of the Vitebsk Regional Stage, which is in the upper Emsian of the Lower Devonian.

Seventeen of the 22 specimens are fragmentary and not identifiable as specific plates. However, they each have a similar external ornamentation allowing a fair degree of confidence that they all belong to the same species. The ornamentation consists of narrow ridges topped by a single row of tubercles. Five specimens are identifiable plates.

Actinolepis zaikai, dermal plate remains: (1)–(4) plate fragments: (1) BNTU 44/1-7f.2; (2) BNTU 44/1-6a; (3) BNTU 44/1-16; (4) BNTU 44/1-16a; (5), (6) fused left and right preorbital plate (BNTU 44/1-12): (5) dorsal view; (6) visceral view; (7), (8) right paranuchal plate (BNTU 44/1-21): (7) dorsal view; (8) visceral view; (9) left anterior dorsolateral plate (BNTU 44/1-13); (10) anterior lateral plate (BNTU 6/1-1); (11) indefinable plate fragment (BNTU 44/1-6). Abbreviations: af, articular flange; lc, main lateral line groove; oa, overlap area; oa.C, area overlapped by central plate; oa.MD, area overlapped by median dorsal plate; occ, occipital cross commissure canal; pp, posterior pit line; r, ridges under supraorbital sensory line on visceral side; sl, sensory line; soc, supraorbital sensory line. Scale bars are 5 mm. Plax & Newman (2020).

BNTU 44/1-12 consists of a fused left and right preorbital plate and is the only specimen of this plate available to study. The plate is V-shaped and fairly well preserved. It has a maximum length of 24 mm with a maximum width of 16 mm. The width/length ratio is 0.67. The dorsal surface has an ornamentation of narrow ridges each topped by a single row of small tubercles. These tubercles are somewhat worn. The left and right branches of the supraorbital sensory line meet in the middle of the plate toward the posterior. On the visceral surface are two low longitudinal ridges under the soc forming the thickest part of the plate. The visceral surface is relatively smooth on both sides of these ridges.

BNTU 44/1-21 is a right paranuchal plate, irregularly oval in shape with a rounded outer margin. The ornamentation of the dorsal surface is formed by concentrically located, narrow ridges, each topped by a single row of tubercles. The tubercles are somewhat smoothed. Three grooves of the lateral line system pass on the external surface of the plate: the posterior pit line extending approximately from the center of ossification of the plate upward to the central plate, the main  lateral line canal extending to the marginal plate and anterior dorsolateral plate, and the occipital cross commissure canal. The grooves connect with each other over the center of ossification at different angles: the angle between occipital cross commissure canal and lateral line canal is about 60°, the angle between occipital cross commissure canal. and pp is about 70°, and the angle between posterior pit line and lateral line canal is about 100°. The area overlapped by the central plate is a depression on the anterior mesial margin. The rest of the mesial margin is broken. The visceral surface is worn and delaminated in places.

BNTU 44/1-13 is the most dorsal part of a left anterior dorsolateral plate. The preserved area is triangular, with only a small ornamented area. Most of the posterior portion of the preserved plate consists of the overlap area for the median dorsal plate, and the anterior portion for the articular flange. These two areas are separated by a low ridge. The ornamentation consists of narrow sinuous ridges with a single row of small, numerous tubercles on their tops, which for the most part are worn.Within these ridges in the center of the ornamented area is the anterior part of the main lateral line groove. The visceral surface is uninformative. In some places, this plate is slightly rough on the margins.

BNTU 6/1-1 is a very incomplete anterior lateral plate. It is flattened, rather large, with an ornamentation of concentrically orientated ridges topped by a single row of tubercles. The tubercles are quite worn. There is a pair of ridges that join to form a ‘V’ shape. This character is only seen on anterior lateral plates. However, because the plate is incomplete, we are unable to determine whether it is a right or left plate or even the original orientation. The visceral surface is obscured by matrix.

BNTU 44/1-6 is an incomplete unidentified plate. The ornamentation is well preserved consisting of narrow, concentrically orientated ridges topped by a single row of tubercles. Part of an overlap area and a sensory line are visible. However, due to the incomplete nature of the plate, we cannot determine what the sensory line was or what plate overlapped this plate. The visceral surface is uninformative.

The first unnamed form is placed in the family Coccosteidae, but not assigned to a genus or species due to the limited and fragmentary nature of the material, which comprises seven identifiable plates were found, one from the cranial roof and six from the trunk armor, as well as two unidentifiable plate fragments, all from the Buda Dal’nyaya 35 borehole at a depth of 233.2 m; also from the Vitebsk region in the Lepel Beds of the Vitebsk Regional Stage, upper Emsian, Lower Devonian.

BNTU 51/2-2c is an incomplete right central plate that is slightly convex. It is about 10 mm long and 8 mm wide. The supraorbital sensory line, the central sensory canal, and the middle pit-line groove are well developed on the dorsal side of the plate. The central sensory canal and mp branch off from one another on the left side of the plate and then extend parallel to one another to the right edge of the preserved plate. The supraorbital sensory line curves anteriorly from the csc toward the right edge of the plate. The angle between the central sensory canal and supraorbital sensory line at the right lateral edge is 30°. The tubercles are rounded and of approximately the same size. They are evenly distributed across the plate surface. These form weakly expressed rows near the sensory grooves.

Indeterminate Coccosteidae form 1, dermal plate remains: (1) right central plate (BNTU 51/2-2c); (2), (3) posterior part of a median dorsal plate (BNTU 51/2-2b); (2) dorsal view; (3) visceral view; (4), (5) left anterior dorsolateral plate (BNTU 51/2-2): (4) lateral view; (5) visceral view; (6, 7) right anterior dorsolateral plate (BNTU 51/2-2a): (6) lateral view; (7) visceral view; (8), (9) left posterior ventrolateral plate (BNTU 51/2-2e): (8) ventral view; (9) visceral view; (10), (11) probable left posterior lateral plate (BNTU 51/2-2g): (10) lateral view; (11) visceral view; (12), (13) spinal plate (BNTU 51/2-2f): (12) outer view; (13) visceral view; (14), (15) indefinable plate fragment (BNTU 51/2-2d): (14) external view; (15) visceral view. Abbreviayions: af.smd.pl, articular face for submedian dorsal plate; cf.PDL, area overlapping posterior dorsolateral plate; cf.PVL, area overlapping posterior ventrolateral plate; cr.pr, carinal process of median dorsal plate; csc, central sensory canal; k, keel; kd, glenoid condyle; lc, main lateral line canal; ld, dorsal branch of main lateral sensory line groove; mp, middle pit line groove; oa.AL, area overlapped by anterior lateral plate; oa.AVL, overlap area for anterior ventrolateral plate; oa.MD, area overlapped by median dorsal plate; os, osteolepid scale; pr.p, cartilaginous prepectoral process of endoskeletal scapular belt; pr.sbgl, subglenoid condyle; soc, supraorbital sensory line. Scale bars: 5 mm (1)–(11), (14), (15), 1 mm (12), (13). Dashed lines indicate the estimated boundary of the missing parts of the plate. Plax & Newman (2020).

BNTU 51/2-2b is a fragment of the posterior part of amedian dorsal plate. The specimen is 21 mm long and 7 mm wide at its widest point. With extrapolation, it can be argued that the plate is relatively long and slightly concave in profile. The posterior part of the plate tapers to a point, although the tip is missing. The dorsal surface is covered with small round tubercles, which form rows parallel to the lateral edges. Toward the center of the plate, the tubercles are more random and much scarcer. The right side of the dorsal branch of the main lateral sensory line groove is present as a gentle curve. Although the left side of the plate is missing, the lateral sensory line groove do not connect to the midline. The lateral sensory line groove would have intersected the right and left posterior dorsolateral plates. The visceral surface is smooth, except for a slight depression for the area overlapping the posterior ventrolateral plate. The median ventral keel is sharp and well developed. It begins near the anterior margin of the missing part of the plate, and continues to the posterior, gradually deepening. It ends posteriorly with a carinal process. The posterior face of the carinal process flares out and is hollowed to make an articular surface for the submedian dorsal plate.

Two anterior dorsolateral plates have been identified. BNTU 51/2-2 is a left anterior dorsolateral plate and BNTU 51/2-2a a right anterior dorsolateral plate. BNTU 51/2-2 is the better-preserved specimen. It is convex on its lateral surface and is quadrangular overall. The plate is 10 mm long and 14 mm wide. The glenoid condyle and the subglenoid process are not preserved. Also, the most anterior ornamented part of the plate is not preserved. The lateral surface is covered by numerous, well developed, small, round tubercles arranged in concentric rows around a center of ossification. The areas overlapped by the median dorsal plate and the left anterior lateral plate are partially preserved. The dorsal branch of the main lateral line canal and main lateral line canal are clearly seen on the lateral surface. The main lateral line canal originates where the glenoid condyle would be and extends posteriorly in a downward diagonal direction to where the posterior lateral plate would have been. The dorsal branch of the main lateral line canal extends posteriorly from near the anterior end of the main lateral line canal diagonally upward toward where the posterior dorsolateral plate would have been. The angle between the main lateral line canal diagonally and dorsal branch of the main lateral line canal is about 60°. The visceral surface is smooth. The area overlapping the posterior dorsolateral plate is visible on the visceral surface on the posterior margin.

BNTU 51/2-2a is a right anterior dorsolateral plate  with broken lateral edges. The plate is 12 mm long and 14 mm wide. Its lateral surface is somewhat obscured by matrix although the lateral lines are visible, particularly the dorsal branch of the main lateral line canal. The plate is morphologically similar to BNTU 51/2-2 but has the glenoid condyle and the subglenoid process preserved. These two structures are morphological similar to those of the better-known ones in Coccosteus cuspidatus. The visceral surface is smooth and shows no diagnostic characters other than the area overlapping the posterior dorsolateral plate.

BNTU 51/2-2e is a left posterior ventrolateral plate lacking the anterior lateral corner. It is elongated and ventrally convex in profile. The plate is 21 mm long and 11 mm wide at the widest point. At the anterior edge, part of the overlap area for the anterior ventrolateral plate is visible. The ventral surface is covered with small, rounded tubercles that are slightly larger and more prominent along the edges of the plate. An osteolepid scale and matrix obscures part of the ventral surface of the plate, which is too delicate to allow removal. The visceral surface is mostly smooth. The area overlapping the right posterior ventrolateral plate is clearly seen on the mesial margin.

BNTU 51/2-2 g is probably a left posterior lateral plate but it could be a posterior dorsolateral plate because they are morphologically very similar. The plate is vaulted, triangular, and nearly complete. It is length is 8 mm long and 9 mm wide on the longest axis. Almost the entire lateral surface is covered with matrix, but there are small areas where several small, round tubercles can be observed. Overlap areas for other plates are visible on some of the edges but it is not certain which plates attached to these areas. The visceral surface is smooth and featureless.

BNTU 51/2-2f is a relatively short spinal plate. It is 7 mm long and about 3.5 mm wide. Its external surface has small, rounded tubercles arranged in rows parallel to the long axis. Viscerally, there is a conical cavity opening anteriorly that was filled with the cartilaginous prepectoral process of the endoskeletal scapular belt.

BNTU 51/2-2d is a small unidentifiable plate fragment. It is triangular is shape with a maximum length of 11 mm. The plate has a similar ornamentation on the external surface as the plates described above, showing that even in the smallest specimen, there is reasonable confidence in identifying it to species.

Plax and Newman are confident that all the specimens represent one Coccosteid species because all specimens have a very similar ornamentation. However, it is not possible to assign the remains to a genus and species due to the uniformity of known plates when compared with other coccosteid genera such as Coccosteus, Dickosteus and Millerosteus.

The final indeterminate form comprises a large number of small (3–20 mm), thin, unidentifiable plate fragments from the Bykhov 1 borehole at depths of 325.5 m, 324.3 m, 324.2 m, 321.3 m, 321.2 m, and 321.0 m, Mogilev region; also from the Korma 1 borehole at depths of 343.5 m and 323.3 m, Gomel region. Both borehole specimens are from the Lepel Beds of the Vitebsk Regional Stage, which is in the upper Emsian of the Lower Devonian. They have a characteristic ornamentation of variably sized, rounded tubercles, which are star-shaped at their bases. Often these tubercles form distinct rows; also larger tubercles are often present between smaller ones.

Indeterminate Placoderm, dermal plate fragments: (1), (2) fragment of a spinal plate (BNTU 116/49-8): (1) external view; (2) visceral view; (3) fragment of spinal plate (BNTU 116/54-1); (4) fragment of a plate (BNTU 116/49-7); (5), (6) fragment of a plate (BNTU 116/53-4): (5) external view; (6) visceral view. Scale bars are 1 mm. Plax & Newman (2020).

Several small fragments of spinal plates have also been collected. They are oval in cross section. The tubercles on these plates often fuse with each other to form distinct rows. Along the mesial margin of the spinal plates are rows of large, well-developed, strongly curved, evenly spaced spinelets.

Indeterminate Placoderm. Dermal plate fragments: (1) plate fragment (BNTU 116/49-1); (2)–(6) spinal plate fragments: (2) BNTU 121/48-1; (3) BNTU 116/49-2; (4) BNTU 116/48-3; (5) BNTU 116/48-5; (6) BNTU 116/ 56-1. spl.b, spinelet base. Scale bars: 1 mm (1), (3); 0.5 mm (2), (4); 0.2mm (6); 0.1 mm (5). Plax & Newman (2020).

Due to the incomplete nature of these specimens, it is not possible to assign them to a genus and species. The spinal plates are reminiscent of an Aactinolepid, Phlyctaenine, or Groenlandaspidid, but there are other possibilities, e.g., a Petalichyiid, which makes assignment even to Arthrodires impossible. They are different from the three other forms described in having spinelets on the spinal plates.

The Lepel Beds Placoderms described by Plax and Newman are found in the terrigenous and carbonate-terrigenous rocks. The lithology consists mostly of mudstones with occasional siltstones and sandstones. Associated with the Placoderms are Ostracod valves (less common), Conchostracan shells (very rare), and near-complete valves of inarticulate Brachiopods (less common). Vertebrate remains include dentine tubercles, tesserae, and small plate fragments of Psammosteid Heterostracans (quite rare); scales and fin spine fragments of Acanthodians (scales very common, fin spines quite rare); scales of Chondrichthyans (very rare); scales, teeth, jaws, and unidentifiable bone fragments of Sarcopterygians (less common); scales of Actinopterygians (very rare); and Fish otoliths (very rare). Acritarchs (less common) and Miospores (very common) are also present.

Most of the organic remains are found in massive or poorly laminated rock. The placoderm remains are usually isolated, but occasionally form small accumulations with other organic remains. The organic remains usually lie flat on the bedding planes. The placoderm remains are usually well preserved, but often fragmentary with only occasionally whole plates preserved. These remains range in size from a few to 40 mm. Although the plates are quite dense, they are often also brittle. The histological structure of the placoderm remains are well preserved. The Placoderm plates are not deformed and are yellowish orange to dark brown (sometimes black). The plates have sharp edges with little rounding, indicating that they were deposited in a quiet environment. However, because the remains are fragmentary, they must have been transported for some distance. The presence of Acritarchs indicates a marine influence on the deposit, but the lack of other true marine forms suggests that it was marginal. Conchostracans and inarticulate Brachiopods often preferred brackish-water conditions during the Devonian, and thus the deposit could represent a shallow coastal basin with an input of freshwater, perhaps even an estuary or lagoon. However, it is difficult to be certain of the precise environment because there are no surface outcrops of the Lepel Beds.

Despite disarticulated material, it is clear that at least four Placoderm species are present in the upper Emsian rocks of Belarus. There is sufficient material to erect two new taxa: Stipatosteus svidunovitchi and Actinolepis zaikai. The other two species were not well-enough preserved to allow species assignment and had to be left in open nomenclature.

The presence of a species of Actinolepis indicates at least some connection during the Devonian between Belarus and the Baltic region where the genus also occurs. Actinolepis also occurs in Scotland and Spitsbergen, indicating a possible connection during the Devonian. Whether this connection was during the upper Emsian or earlier is impossible to say at present and further research by the authors and others is presently under way to ascertain this. It has been noted that the acanthodian Diplacanthus kleesmentae was very similar, if not a subjective junior synonym of the Scottish Middle Devonian species Diplacanthus crassisimus, but that the Baltic material was not well-enough preserved to be certain. Further research is being undertaken by Burrow and her team on the Scottish Acanthodians to establish the species diversity recorded within the Old Red Sandstone. The Belarus material is particularly important to the latter investigation because some of the Devonian strata includes a fully marine fauna, thus allowing a global correlation.

Based on lithological, palaeontological, and taphonomic data, it is thought that the Placoderms described by Plax and Newman, with their associated vertebrates and invertebrates, lived in a shallow epicontinental sea basin, probably with freshwater input. These new data allow us to supplement the biostratigraphy of the subregional stratigraphic units of the most recently published stratigraphic chart of the Devonian deposits of Belarus.

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