Bracteatacanthus assiduus, Valiukevičius, 2004

Valiukevičius, Juozas, 2004, New Wenlock-Pridoli (Silurian) acanthodian fishes from Lithuania, Acta Palaeontologica Polonica 49 (1), pp. 147-160 : 154-155

Bracteatacanthus assiduus gen. et sp. nov.

Fig. 7 .

Holotype: LIGG 25 −A−2453, trunk scale ( Fig. 7D ).

Type locality: Nida−44 borehole, depth 1213.0 m.

Type horizon:RietavasBedsoftheJūraFormation,Pridoli,UpperSilurian.

Range: Found only at type horizon.

Derivation of name:From Latin assiduus ,regular,constant,referringto morphological uniformity of scales.

Material.—Approximately 100 scales.

Diagnosis.— Bracteatacanthus having small rhomboidal scales with a sculpture of 6 to 8 short crown ridges of asymmetric profile and an antero−median sulcus. The most convex part of the base projects in advance of crown. Crown composed of dentine with long ordinary ascending and multibranched radial vascular canals, part of which are situated high in the neck; thin outer durodentine strips present. Acellular bone in base very dense, with thin growth lamellae.

Description.— B. assiduus scales are small with well developed morphological elements: deeply convex base, tall neck and medium to thick crown. The crown plate is horizontal, almost isometrical, rhomboid, and sometimes with an elongate and narrow posterior flange. Crown length and width varies from 0.4–0.45 to 0.6–0.66 mm respectively. The posterior end of the crown overhangs the base only slightly. Anterior part of the crown has 6 to 8 short parallel to sub−radial ridges fading out at one−third of the crown length. The ridges are not sharp, and often have rounded outer crests ( Fig. 7A ) and widen anteriorly. The lateral ridge slopes are gently curved, whereas the median ones are nearly flat ( Fig. 7A, D ). The ridges are arranged symmetrically from the enlarged antero−median sulcus, which can form an anterior “beak” on the scale crown ( Fig. 7E ). The grooves in between the ridges are wide and shallow. The tall neck is porous ( Fig. 7A, B ) and has vertical grooves on the posterior face. The neck/base junction is outlined by a prominent rim ( Fig. 7C, E ). The base is rhomboidal, deeply convex, and the deepest point projects in advance of the crown.

Scales are composed of an Acanthodes − type histological structure. Up to ten lamellae of superpositional growth are formedofdentine anddurodentine. Thelong ascendingvascular canals are regularly oriented in the lamellae, but are not branched. A main central canal is always present ( Fig. 7F 2, G 2 ). There are almost no upwardly directed dentine tubules in the horizontal part of crown plate. Centripetal radial vascular canals are complex and consist of several separate branched main canals, part of which are positioned unusually high in the neck ( Fig. 7G 2 ). From the lowest of them appear some basally directed branches oriented along the growth lines of the base ( Fig. 7G 2 ). Some of the earliest growth lamellae, not only the oldest one, have a complicated network of interwoven radial canals, ascending canals and their side branches ( Fig. 7F 2 ). The acellular bone of the base is extensive, and forms a pyramid centrally, leaving only small space for crown dentine. Basal bone is very dense, with thin growth zones, penetrated by long Sharpey’s fibre traces and contains rareshort, winding vascular tubules.

Discussion.—Similar short−ridged scales from other regions have been attributed by other researchers to Gomphonchus . In my opinion, morphologic characters with definitive features of crown ridges and histological structure, distinguish B. assiduus from Gomphonchus sandelensis (Pander) and moreover from G. hoppei (Gross) , and support erecting of the new genus. Whilst having Acanthodes − type histology, there are still diagnostic features for Bracteatacanthus scales which distinguish them from those of Gomphonchus with its characteristic cellular bone in scale bases ( Denison 1979), and the composition of vascular canals in crowns.

Bracteatacanthus assiduus scales show some similarities with previously published specimens. Gomphonchus sp. 3 scales( Märss1997:pl.7:4–9)fromHviždalka,bed26(Pridoli) of the Czech Republic, resemble those of B. assiduus based on scale shape, proportions, and several of them also by the ridge characters,which,incontrasttotheLithuanianspecimens,have more relief and are sharper. Unfortunately, the Czech scales have not yet been examined histologically because of their poor preservation. Morphotype III scales of Gomphonchus ? turnerae Burrow from the Jack Formation (Ludlow, ploeckensis conodont Zone) of Queensland, Australia also are similar to thoseof B. assiduus .Thediversityofscalecrownsillustratedby Burrow and Simpson (1995: fig. 2E–K) is within the range of variation of Bracteatacanthus , but the histological microstructure is completely different: the Australian specimens are supposedly composed of mesodentine ( Burrow and Simpson 1995: fig. 3A–D). Two new Gomphonchus species, recently studied from Timan−Pechora region (Valiukevičius in press) differ from B. assiduus in that the scales have cellular basal bone and a definitive vascular canal structure in their crowns.

Occurrence.—Nida−44 borehole, depth 1213.0 m; Stoniškiai−1: 1211.0–1217.0 m; Šešuvis−11: 1005.0–1006.0 m.

Figures

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. F1. Thin section LIGG 3705, general view. F2. Detail of the left part of crown and base apex of the same scale. G1. Thin section LIGG 3708. G2. Detail of the left part of crown and base apex of the same scale. G3. 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.

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.