Arenaceacanthus arcuatacanalis, Valiukevičius, 2004

Valiukevičius, Juozas, 2004, New Wenlock-Pridoli (Silurian) acanthodian fishes from Lithuania, Acta Palaeontologica Polonica 49 (1), pp. 147-160 : 153

Arenaceacanthus arcuatacanalis gen. et sp. nov.

Fig. 6 .

Cheiracanthoides sp. nov. 1 and Cheiracanthoides sp. nov. 3; Valiukevičius in Karatajūtė− Talimaa et al.1987:67–68.

Holotype: LIGG 25 −A−2443, trunk scale ( Fig. 6A ).

Type locality: Ledai−179 borehole, depth 547.6 m.

Type horizon: Upper Silurian, Pridoli, Vievis Formation.

Range:FromLowerSilurian,WenlockGėluvaRegionalStage,through Upper Silurian, Ludlow, Dubysa and Pagėgiai regional stages, to Pridoli, Minija Regional Stage.

Derivation of name: From Latin arcuatus, bent, curved, referring to characteristic form of branching radial vascular canals.

Material.—96 scales.

Diagnosis.— Arenaceacanthus with tiny scales and a crown sculpture of 6 to 8 short, low, parallel or fan−like anterior ridges. Scale crown composed of simple acellular mesodentine and durodentine, and base of acellular bone. Vascular system distinguished by multibranched net−like ascending and enlarged arcuate radial canals at the base/crown junction.

Description.—Tiny scales with well developed medium to high neck. Crown plate is horizontal, with a posteriorly elongaterhomboid toelipsoidal orroundoutline, 0.3to0.54mm in length and 0.3 to 0.5 mm wide, with a sculpture of 6 to 8 short parallel anterior ridgelets that fade out at one−third of the crown length. Ridgelets are low, of angular profile and separated by shallow grooves. Scales with parallel or sub−parallel ridgelets ( Fig. 6A, B ) are more common as compared to those with radiating ones ( Fig. 6D ). Up to half the length of the crown overhangs base posteriorly. Scale bases are rhomboid in outline, with a sharp junction with the neck. Bases are moderately to deeply convex and project, at the deepest point, anterior to the crown.

Up to six crown growth lamellae consisting of net−like mesodentine ( Fig. 6E 1 ) show superpositional growth, are very thin towards crown edges and considerably thicker in the neck. Thin superficial durodentine strips ( Fig. 6F 2 ) are seen in two to three of the youngest growth lamellae. Adensenetwork is formed of ascending vascular canals, long principal branches, many smaller branchings and interspersed dentine tubules ( Fig. 6E 2 ). No osteocyte spaces have been observed in crown tissue. The radial vascular canals at the base/neck junction are multibranched, winding and arc towards the base ( Fig. 6E 1 , E 2 ). These arcuate radial canals make interconnections with small canaliculae in the base. The oldest growth lamella does not differ in this respect from the younger ones, but sometimes contains an enlarged vascular knot ( Fig. 6F 2 ) with thin dentine tubules emanating from all sides. Scale bases are composed of moderately dense acellular bone pierced by short winding canaliculae, which are mostly orientated along the growth lines. Sharpey’s fibre traces are preservedonly in short segments which may be due to preservation conditions.

Discussion.—Most ischnacanthid scale taxa were poorly studied with the exception of Gomphonchus (Pander) and Poracanthodes Brotzen. Previously scales with short, parallel, fading crown ridges and a lack of details of microstructure, were attributed to the genus Cheiracanthoides Wells. This practice was perpetuated in a biostratigraphic review of the Silurian acanthodians of Lithuania ( Karatajūtė−Talimaa et al. 1987), where scales like those of Arenaceacanthus arcuatacanalis were identified as Cheiracanthoides sp. nov. 1 and C. sp. nov. 3. Thin−sectioned scales show clearly that the putative Cheiracanthoides scales do not belong to a climatiiform acanthodian. Here, such scales are considered to be from an ischnacanthiform, as suggested by their histology and the similarity of vascular canals differing from climatiiforms and acanthodiforms.

Summing all the distinctive characters of Arenaceacanthus arcuatacanalis , it has no closely related taxa within the family Ischnacanthidae . The crown ornamentation of A. arcuatacanalis scales is unlike that of known ischnacanthids and, to the contrary, more characteristic of the climatiiform Cheiracanthoides . Otherwise, the histological characteristics of the vascular system in scales (arcs of radial canals at the base/neck junction, network of ascending vascular canals interspersed with dentine tubules, the style of mesodentine in sum) distinguishes A. arcuatacanalis from other ischnacanthids. Only the details of the branching of the vascular canals makes A. arcuatacanalis scales comparable to representatives of the genera Poracanthodes Brotzen , Gomphonchus Gross or Lietuvacanthus Valiukevičius. The arcuate radial canals over the base are unique to A. arcuatacanalis .

Occurrence.—Ledai−179 borehole, depth 544.1–567.6 m; Gėluva−99: 735.2–806.0 m; Sutkai−87: 666.6 m; Kurtuvėnai−162: 1132.0 m; Butkūnai−241: 416.4–457.0 m; Viduklė−61: 1124.0– 1161.6 m.

Figures

Fig. 6. Arenaceacanthus arcuatacanalis gen. et sp. nov. A–D. SEM micrographs of trunk scales, crown views, anterior upwards. A. Holotype, LIGG 25−A−2443. B. LIGG 25−A−2444. C. LIGG 25−A−2446. D. LIGG 25−A−2447. E, F. Microstructure of scales in vertical longitudinal (E) and vertical transverse (F) sections. E1. General view, thin section LIGG 3731. E2. Detail of the left side, the lower part of crown and bordering base strip of the same scale. F1. Thin sectionLIGG 3733, general view. F2. Enlarged mainpartof the samescale. Scale bars 0.1 mm. Ledai−179 borehole, depth 547.6 m. Upper Silurian, Pridoli, Vievis Formation.

Fig. 1. Sketch map showing location of the boreholes yielding the majority of the material studied. Borehole and its original number: 1−Stoniškiai, 7−Krekenava, 8−Stačiūnai, 11−Šešuvis, 12−Kunkojai, 44−Nida, 61−Viduklė, 87−Sutkai, 99−Gėluva, 128−Vilkaviškis, 137−Liepkalnis, 162−Kurtuvėnai, 179−Ledai, 241−Butkūnai, 252−Svėdasai.

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.