Figs. 4, 5.

Gomphonchus sp. A; Märss 1997: pl. 2: 13–15.

Holotype: LIGG 25 −A−2405, trunk scale (Fig. 4A).

Type locality: Kurtuvėnai−162 borehole, depth 1052.2–1072.4 m.

Type horizon:TheupperpartofMinijaRegionalStage(VarniaiBedsor theircorrelatives)tothelowerpartofJūraRegionalStage(lowerpartof the Jūra or Lapės formations).

Range:MinijaRegionalStagetothelowerpartofJūraRegionalStage.

Derivation of name: From Latin basis, base and globosus, round, globose, referring to the scale base shape.

Material.—More than 50 000 scales.

Diagnosis.— Fecundosquama scales having low horizontal crowns with crenulated sculpture anteriorly or around the entire crown margin, lacking a neck; deep convex base present. Scale crownscomposed of simple bone−like mesodentine with abundant osteocyte cavities and enlarged vascular canals present in the oldest growth lamellae only. Crown mesodentine merges gradually into highly cellular thin−lamellar bone in the base that is penetrated by long traces of Sharpey’s fibres.

Description.—Scales have a very low crown and a large, convex base protruding beyond the crown on all sides. Crown length varies from 0.35 to 0.7 mm, and can be 0.3 to 0.77 mm wide. The isometric rhomboid crown plate of trunk scales (Fig. 4A, B, E) is almost flat, very slightly curved downwards at the anterior margins, and posteriorly the crowns are elongated and rhomboidal to elipsoidal. The anterior margin of the crown has short but comparatively wide notches which extend to the surface of the scale base. The number of notches varies from 5 to 6 (Fig. 4E) to 10 (Fig. 4B). The putative head scales (Fig. 4C, D) differ from the trunk scales in having isometric rhomboid to round crowns with notches on all margins. The deep,convexscale baseis rhomboid to round in outline, with a prominent lateral rim at the deepest point projecting anterior to the crown. In rare cases the deepest point of the base is situated posteriorly, so the base protrudes beyond the posterior margin of the crown (Fig. 4E). Scales lack necks.

Simple mesodentine crown tissue (neither stranggewebe nor durodentine present) and cellular bone in scale bases can be seen in the sectioned scales. Up to six thin lamellae of superpositional growth in crowns are composed of bone−like mesodentine containing numerous multi−angular osteocyte spaces even in the latest, outer lamella (Fig. 5A 2) and short radiating and winding dentine tubules form a fine network. The scale primordium does not differ from others in respect to tissue structure. Most scales sectioned have no enlarged vascular canal system in crowns (Fig. 5A 1, B 1), and only one (Fig. 5C 1, C 2) demonstrates wide radial vascular canals in the two oldest growth lamellae. There is no sharp boundary between the crown mesodentine and bone of the base: the first tissue grades gradually into the second. The cellular bone of the base is composed of numerous very fine growth zones (Fig. 5A 1, A 2) penetrated by long Sharpey’s fibre traces (Fig. 5B 2, B 3) and enclosed abundant osteocyte cavities with short cell processes. Osteocytes are oriented along growth lines.

Discussion.—This new genus and species is based on scales with clear morphologic (center of flat crown without ridge ornamentation, fine ridges confined to crown margins, no neck, deep convex base protruding beyond crown on all sides) and histologic characters (neither stranggewebe nor durodentine present in crowns, vascular canal system weakly developed, well−developed net of bone−like mesodentine with many osteocyte spaces, forming a gradual transition to cellular bone in scale bases). The comparatively short stratigraphic range of Fecundosquama basiglobosa gen. et sp. nov. together with diagnostic features makes it a potentially good biostratigraphic taxon for more detailed vertebrate zonations of the Late Silurian.

F. basiglobosa gen. et sp. nov. has outstanding diagnostic features, and no related taxa are known to date. In terms of histology, there are some known climatiids similar F. basiglobosa in some ways, for example, bone−like mesodentine, absence of stranggewebe or reduced principal vascular canal system. The presence of such scale characters was demonstrated in a study on phylogenetical development of nostolepids from the Timan−Pechora region spanning the Late Silurian–Early Devonian (Valiukevičius 2000). All the characters together in F. basiglobosa, especially the distinctive features of bone tissue in scale bases (density and fine−lamellar cellular bone and gradual transition to the crown mesodentine), are observed in acanthodians for the first time.

Märss (1997) presented morphologically similar specimens (histology not examined) from the same region of the Baltic, in the Dubovskoye borehole of the Kaliningrad District (former East Prussia). The rocks are from the Kaugatuma Stage, Pridoli, from depth 1064.7–1085.5 m and originally were identified as Gomphonchus sp. A (Märss 1997: fig. 4). These scales are referred to F. basiglobosa gen. et sp. nov. Some characters (low horizontal crown, no neck, large convex base and proportions of crown/base development) demonstrate similarity between these scales, however, the scales are not identical. The Dubovskoye specimens (Märss 1997: pl. 2: 13–15)not only havenotches onthe anterior part ofthe crown, but also have short surface ridgelets. The main difference between F. basiglobosa and the Dubovskoye scales is the presenceofsmallbulbsorsometimes cone−likespineletsdisplaced in lines on the postero−lateral crown walls in the latter. These specimens, in my opinion, are similar to and might belong to F. basiglobosa, what suggests a climatiid rather than an ischnacanthid affinity for the Dubovskoye scales.

Occurrence.—Ledai−179 borehole, depth 537.4–547.6 m; Gėluva−99: 681.6–685.3m;Sutkai−87: 609.3–615.3m;Kurtuvėnai−162: 1052.2–1072.4 m; Stoniškiai−1: 1311.0– 1359.0 m; Šešuvis−11: 1072.9–1119.5 m; Liepkalnis−137: 892.1–893.9 m; Vilkaviškis−128: 706.2–714.0 m.