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 Rohonilepis breviornatus gen. et sp. nov. Fig. 8 . Holotype : LIGG 25 −A−2459, trunk scale ( Fig. 8C ). Type locality : Kurtuvėnai−162 borehole, depth 1091.3–1117.8 m . Type horizon : Ventspils and Minija formations, Ludlow and Pridoli, Upper Silurian. Range :UpperSilurian,Ludlow,PagėgiaiRegionalStage,toPridoli,the lower part of Jūra Regional Stage. Derivation of name : From Latin brevis , short, and ornatus decorated, ornamented. Material . Total more than 2100 scales. Diagnosis .— Rohonilepis with tiny scales sculptured by 6 to 10 short, sharp, symmetrical sub−parallel anterior ridges turning anteriorly toward the neck−base junction. The two longest posteriorly pointing ridges can form an elevated medial area separating narrow, lowered lateral folds. Crown dentine contains medium−branched linear ascending and simple widened radial vascular canals over the flat−pyramidal base, that is composed of dense acellular bone. Fig. 7. Bracteatacanthus assiduus gen. et sp. nov. A–E . SEM micrographs of trunk scales; crown views, anterior upwards, except for B , lateral view, anterior to the left. A . LIGG 25−A−2450. B . LIGG 25−A−2451. C . LIGG 25−A−2452. D . Holotype, LIGG 25−A−2453. E . LIGG 25−A−2454. F , G . Microstructure of scales in vertical longitudinal sections. F 1 . Thin section LIGG 3705, general view. F 2 . Detail of the left part of crown and base apex of the same scale. G 1 . Thin section LIGG 3708. G 2 . Detail of the left part of crown and base apex of the same scale. G 3 . The same scale, detail of the right part. Scale bars 0.1 mm. Nida−44 borehole, depth 1213.0 m. Upper Silurian, Pridoli, Rietavas Beds of the Jūra Formation. Description .—Tiny scales with crowns that only rarely reach 0.5 mm in length and scale width varies from 0.22 to 0.38 mm . The crown plate is horizontal, with up to one−third of the posterior unsculptured part overhanging the base. A quarter to one−third of the crown length has 6 to 10 short, sharp anterior ridges with a symmetrical profile, which fade posteriorly. Crown ridges may be straight ( Fig. 8D ) or slightly curved towards mid−scale ( Fig. 8B, C ). The holotype−like specimens, with sub−parallel ridges are more abundant in samples studied, however, scales with almost parallel ridges also are known. Ridges are characteristically downwardly sloped anteriorly, sometimes reaching the base of the neck. Grooves between ridges are moderately deep and wide. Some scales have narrow lowered lateral ledges ( Fig. 8A, C ), separated from the remainder of the crown by the two longest, posteriorly pointing ridges. Moderately high to low neck usually has linear rows of pores. The scale base slightly protrudes beyond the crown antero−laterally, but may be also displaced far in advance of the anterior margin of the crown. Fig. 8. Rohonilepis breviornatus gen. et sp. nov. A–D . SEM micrographs of scales, crown views, anterior upwards. A . LIGG 25−A−2457. B . LIGG 25−A−2458. C . Holotype, LIGG 25−A−2459. D . LIGG 25−A−2461. E , F . Microstructure of scales in vertical longitudinal sections. E 1 . Thin section LIGG 3726, general view. E 2 . Detail of the upper right crown part of the same scale. F . Thin section LIGG 3727. Scale bars 0.1 mm. Kurtuvėnai−162 borehole, depth 1103.0 m. Upper Silurian, Ludlow, Ventspils Formation. Histologic microstructure has not been sufficiently studied yet because of the preservation of the scales, but they are certainly of the Acanthodes − type . At least, 6 growth lamellae in crowns composed of dentine can be counted ( Fig. 8F ), with the oldest lamella unusually large. The vascular system is poorly defined due to re−mineralization of tissues and only short segments ( Fig. 8E 2 , F) of ascending and other vascular canals are visible. Ascending canals are moderately branched and mostly linear along growth lamellae. Radial vascular canals are simple and located just over the basal surface ( Fig. 8F , right side). Durodentine has not been definitely recognized, but may be present. Acellular basal bone is dense and forms a flat−pyramid below the crown. Discussion .— Rohonilepis breviornatus is comparable to the above described Bracteatacanthus assiduus , as histologically they both have many characters in common. The latter is distinguished by the peculiar arrangement and position of radial vascular canals in the dentine of the crown, that is definitely not present in R. breviornatus . Morphological differences are more substantial: asymmetrically sloped (in profile) rounded ridges do not extend baseward in Bracteatacanthus scales and sharper, symmetric, frequently centrally curved ridges sometimes reach the neck base in Rohonilepis . The lateral lower step−like margins of R. breviornatus scales have not been observed on those of Bracteatacanthus . The sculpture of some scales of R. breviornatus resembles several specimens of Gomphonchus from the Hviždalka locality, Czech Republic ( Märss 1997 : pl. 7: 3, 10). Vergoossen (2002) illustrated specimens of Acanthodii gen. et sp. indet. that are similar but not as well preserved as the material figured here, but suggest, that some comparable varieties of scales might also be present in southern Sweden . Occurrence .—Kurtuvėnai−162borehole,depth 1091.3–1137.5 m ; Sutkai−87: 612.9–623.6 m ; Gėluva−99: 654.1– 685.3 m ; Stoniškiai−1: 1224.0– 1308.9 m ; Šešuvis−11: 1047.0– 1049.5 m ; Ledai−179: 525.7–544.1 m ; Liepkalnis−137: 900.1–909.2 m .