New Wenlock-Pridoli (Silurian) acanthodian fishes from Lithuania Author Valiukevičius, Juozas text Acta Palaeontologica Polonica 2004 2004-12-31 49 1 147 160 journal article 10.5281/zenodo.13406145 1732-2421 13406145 Vesperalia perplexa gen. et sp. nov. Figs. 2 , 3 . Holotype : LIGG 25 −A−2413, trunk scale ( Fig. 2A ). Type locality : Stoniškiai−1 borehole, depth 1211.0–1217.0 m. Type horizon :RietavasBedsoftheJūraFormation,Pridoli,UpperSilurian. Range : Type horizon only. Derivation of name : From Latin perplexus , tangled, intricate. Material .—Total about 600 scales. Diagnosis .— Vesperalia with high−crowned scales, porous neck and thick crown with a sculpture of four to six linear or wavy longitudinal ridges, of which two converging ones may form a raised medial area. Ridges usually cross the entire length of the crown or fade out mid−length, continuing after a smooth area on the posterior edge of the crown. Scales grow superpositionally or both superpositionally and areally. Stranggewebe with unusually large oriented lacunae and simple mesodentine form both posterior and anterior parts of the crown. The crown contains a system of complicated and enlarged vascular canals. Description .— Most trunk scales have rounded rhomboidal crowns varying from isometric to elongate, with length of 0.5–0.87mm andwidth of 0.61–0.9mm . Thelength andwidth of the larger scale specimens is 0.93–1.47 mm and 0.95– 1.4 mm , respectively. The crown surface is broad and flat, with slightly downsloped lateral and anterior margins. Four to six sharp sub−parallel straight or wavy ridges extend the whole crown length. The ridges become more robust towards the scale base.They are usually of the same height across the crown but may be more pronounced and higher posteriorly ( Fig. 2C, D, G ). The two most conspicuous ridges converge at the posterior tip of the crown and may form a slightly raised triangular medial plateau ( Fig. 2B, D ), which has a sculpture of weak anterior ridgelets ( Fig. 2B ) or has an anteriorly widened shallow groove ( Fig. 2D ). The holotype and other specimens have ridges that fade out at the mid−length of the crown andafteracentralsmooth area,continueandincreaseinheight posteriorly ( Fig. 2A, E ). Of particular interest are the specimens with crown ridges and grooves on the anterior and posterior areas that do not join, but are shifted laterally relative to the long axis of the scale ( Fig. 2E ). The grooves between ridges are of irregular depth and width, and sometimes form deeper troughs at their anterior end ( Fig. 2E, G, H ). Fig. 2. Vesperalia perplexa gen. et sp. nov. SEM micrographs, trunk scales except for F, head? scale, crown views, anterior upwards: holotype, LIGG 25−A−2413 ( A ), LIGG 25−A−2416 ( B ), LIGG 25−A−2417 ( C ), LIGG 25−A−2418 ( D ), LIGG 25−A−2420 ( E ), LIGG 25−A−2422 ( F ), LIGG 25−A−2421 ( G ), LIGG 25−A−2414( H ). Scalebars0.1 mm.Nida−44borehole, depth1213.0 m(A, E–H) andStoniškiai−1borehole, depth1211.0–1217.0 m(B–D). Upper Silurian, Pridoli, Rietavas Beds of the Jūra Formation. Fig. 3. Vesperalia perplexa gen. et sp. nov. Microstructure of scales in vertical longitudinal ( A ) and transverse ( B ) sections. A 1 . Holotype−like scale with uniformbent, sub−parallelridgesprominentattheanteriorand posterioredges ofthecrown, and almostfadingoutnear thecrown center. ThinsectionLIGG 3678. A 2 . Detail of posterior edge of crown and a small portion of base of the same scale. B 1 . Widened scale with flattened ridges like those in Fig. 2E. Thin sectionLIGG 3679. B 2 . Detail of the leftpart of the same scale. B 3 . Detail of the rightpart of the same scale. Scale bars 0.1 mm. Stoniškiai−1borehole, depth 1211.0–1217.0 m. Upper Silurian, Pridoli, Rietavas Beds of the Jūra Formation. Scale necks are tall and contain large linearly opening basal pores on all the walls, with up to four on each side ( Fig. 2A, E ). The scale base is rhombic in outline, moderately to greatly convex, usually protruding slightly beyond the crown antero−laterally. The deepest point of the base is centrally positioned. Putative headscales(veryrareinthestudiedsamples)have isometric rhomboid, thickened crowns (up to 0.65–0.82 mm long and wide) and low porous necks as compared to trunk scales. Head scale crowns have three to four prominent, robust and rounded ridges of even height extending over the entire crown length. These robust ridges bend medially towards the posterior edge ( Fig. 2F ), but do not join each other. Additional lateral oblique riblets occur on the side of each ridge. The postero−lateral crown edge is multicuspid with several (up to four on each side) rough spinelets. The histology of all V. perplexa scales ( Fig. 3 ) is uniform. Crowns are composed of stranggewebe (mesodentinal tissue containing elongate, linearly oriented lacunae and incorporating osteocytes; for histologic terminology see Gross 1971 and Denison 1979 ) and simple mesodentine arranged in four thick growth lamellae. The growth type differs slightly among scales. Scales with interrupted ridges ( Fig. 3A 1 , B 1 ) demonstrate a superpositional growth in only the two oldest lamellae, whereas the two youngest are added areally. Scales with continuous ridges grow superpositionally. Thestranggewebeischaracterizedbyenlargedandelongate stranglacunae with interconnecting short processes. An unevenly thick mantle of simple mesodentine is superficial to the stranggewebe in each growth lamella ( Fig. 3B 2 ). In scales of this species, the stranggewebe develops with a similar intensity not only in the posterior part of the crown, as it is characteristic of most climatiids, but also in its anterior parts ( Fig. 3A 1 ). The basal part of the neck has rare, typical mesodentine osteocyte cavities. The principal vascular system is formed of enlarged radial vascular canals positioned over the base, and wide ascending and slightly branched main canals supplying each growth lamella ( Fig. 3A 2 , B 3 ). This system is complicated intheearliest/oldestlamellaebecauseofadditionalbranchings. The simple mesodentine enclosing stranggewebe in the outer parts of lamellae contains a characteristic net of winding dentine tubules with many osteocyte cavities even in the superficial layers ( Fig. 3A 2 , B 2 ). Durodentine is absent. Scale bases are composed of cellular bone arranged in very thin and dense growth lamellae with plenty of osteocyte cavities and medium to long Sharpey’s fibre traces. The osteocyte density increases towards the tip of pyramidal base, where they become oriented along the growth lines, and approximate thestructureofbothbonetissueandstranggewebe( Fig.3B 2 ). Discussion .—The trunk scales of Vesperalia differ from those of all known climatiids in having a high and linearly porous neckdistinct fromtheconvexbasebyasharprim andirregular ridges ornamenting the horizontal plate of the crown. They also have wide, pronounced, often wavy subparallel ridges; the two central ridges may form an elevated medial area and point towards the posterior end of the crown. Ridges of some scale specimens aredeveloped along the anterior and posterior edges of the crown, leaving the median part of the crown unsculptured a character not previously observed in acanthodians. Histological microstructure corresponds to the Nostolepis− type , distinguished by very large ascending and radial vascular canals, unusually elongate lacunae of stranggewebe covered by a mantle of osteocyte−rich mesodentine in scale crowns (the tissue has bone−like characteristics). The Nostolepis− type histology suggests that Vesperalia perplexa gen. et sp. nov. can be asigned to the climatiids as presently classified. Based on morphologic characters, particularly the distribution of crown ridges, the scales of V. perplexa differfromknownnostolepids,andshouldbeconsideredgenerically distinct. Other morphological features, that are worth mentioning are the high and porous necks (not characteristic of most climatiids), and the crown sculpture of interrupted ridges. In addition, both superpositional and areal growth types are developed in scales of V. perplexa . It is obvious, that the replacement of the superpositional growth by the areal one takes always place after the formation of two the oldest lamellae (of four), and can not be attached to the ontogenetic development stage(youngerorolder)ofscales.Histologically,scalesaredistinguished from those of other Climatiiformes not only by an exhuberant development of stranggewebe, a unique character among climatiids, but also by the particularly large oriented lacunae, clearly separated strips of stranggewebe and thick mantle of simple mesodentine in growth lamellae. There are no closely related climatiids to be compared. According to several characters, Vesperalia perplexa gen. et sp. nov. resembles representatives of Endemolepis and Tareyacanthus ( Valiukevičius 1994 , 1998 ). The former, represented by a single species Endemolepis inconstans Valiukevičius, 1998 , ranges across the Silurian–Devonian boundary in the Baltic, is distinguished by crown ridges of changeable form and length (sharp narrow, broad rounded, single or meeting in pairs, frequently with additional oblique branchings) extending the entire crown length, except for a diagnostic, narrow, unornamented anterior strip ( Valiukevičius 1998 : pl. 6: 3–7). Two known Devonian representatives of Tareyacanthus , T. magnificus Valiukevičius, 1994 (pl. 23: 9–14) from Taimyr Peninsula ( Russia ) and T. dissectus Valiukevičius, 1998 (pl. 3: 22–24 and pl. 4: 7–9) from the Stoniškiai Formation of the Baltic coast, have scales ornamented with high, sharp parallel or slightly fan−shaped ridges (up to eight per scale), that may bifurcate anteriorly and are separated by deep grooves. The histologic microstructure of both taxa is only comparable with Vesperalia perplexa gen. et sp. nov. as they all have a complicated system of principal vascular canals in crowns, but the details mentioned previously distinguish Vesperalia and its single species from all known climatiids. Occurrence .—Stoniškiai−1 borehole, depth 1211.0–1217.0 m; Nida−44: 1213.0 m and Šešuvis−11: 1005.0– 1015.2 m .