Iranolepis ginteri, Hairapetian & Valiukevičius & Burrow, 2006

Hairapetian, Vachik, Valiukevičius, Juozas & Burrow, Carole J., 2006, Early Frasnian acanthodians from central Iran, Acta Palaeontologica Polonica 51 (3), pp. 499-520 : 501-508

Iranolepis ginteri gen. et sp. nov.

Figs. 3 , 4.

Holotype: AEU 301 , flank scale ( Fig. 3A ).

Type locality: Chahriseh section, Kaftari Mt., central Iran.

Type horizon: GI4, the middle Mesotaxis falsiovalis to Palmatolepis hassi conodont zones.

Derivation of the name: For Michał Ginter, in recognition of his work on teeth of Palaeozoic vertebrates.

Material.—Three scales from sample H1, 33 scales from sample GI4.

Diagnosis.— As for the genus Iranolepis gen. nov., by monotypy.

Description

Morphology.—All specimens examined are medium to largesized, and identified as flank scales. The crowns are slightly elliptical, 0.6–1.7 mm long and 0.5–1.5 mm wide. A few scales have an elongated rhomboidal crown. Inclination of the crown plane varies from flat, to down sloped anteriorly toward the protruding base. An elliptical, highly raised medial area forms most of the crown plate; it is separated by steep slopes from the narrower lateral areas ( Fig. 3A 1 ; AEU 301). Medial and lateral areas reach the posterior crown tip. The medial area is often shallowly depressed centrally and may carry one short anteriormost ridgelet. Rarely, the central depression is deep and furrow−like ( Fig. 3C ; AEU 303). Lateral crown areas are generally smooth, without ridges even along the anterior edge. A pair of additional weakly expressed oblique neck ridges may be present symmetrically on both sides of scales ( Fig. 3A 3 ). Scale neck is slightly concave, decreasing from a medium height posteriorly to almost negligible where the anterior edge of the crown reaches the base. The lowermost neck has a row of large pores, with up to four opening on each lateral face at the base−neck junction ( Fig. 3A 3 , B 2 ). Scale bases are rounded rhomboidal, or rarely asymmetric rhomboidal, and separated from the anterior crown edge by a pronounced rim. Bases are shallow posteriorly (sometimes transversely concave), deepening to convex anteriorly and protruding beyond the crown antero−laterally, with the deepest point in front of or just below the anterior edge of the crown.

Histology. —The crown has up to six superpositional growth lamellae ( Fig. 4D; EUIV 411), composed of odontocytic mesodentine (this mesodentine type with lacunae we distinguish to separate it from the syncitial mesodentine lacking lacunae: Karatajūtė− Talimaa and Smith 2003; Valiukevičius and Burrow 2005) and stranggewebe. Mesodentine is characterised by a dense network of fine, narrow and winding dentinal canaliculi ( Fig. 4B 1 , B 3 ; LIGG 3804) which extend from all sides of the abundant and small polygonal odontocytes. Odontocytes are large in the lower crown (neck) only, and are abundant in all growth zones (though fewer in the youngest lamellae). In the outer parts of the lamellae, canaliculi are mostly directed superficially ( Fig. 4C 2 ; LIGG 3805), durodentine (sensu Gross 1971) is lacking. Stranggewebe is only present low in the posterior part of the crown/neck, and not in the primordial growth zone. The stranglakunae are short and narrow, and comparatively densely packed ( Fig. 4C 1 , D). Scales with reasonable preservation show wide, long radial vascular canals at the base−crown/neck junction and single branches of ascending and circular canals positioned in the inner/basal parts of growth zones ( Fig. 4A). Scale bases are composed of highly cellular bone, and osteocyte cavities are distributed evenly throughout; the base forms a very low−angled pyramid ( Fig. 4A; LIGG 3802). Osteocytes are of polygonal or spindle−like form ( Fig. 4B 2 ) and oriented along the dense, thin growth lamellae. Osteocyte cells are interconnected by winding, narrow processes. Sharpey’s fibre traces are bundled into long and narrow strips.

Discussion.—This taxon belongs to the climatiid acanthodians having robust but sparsely sculptured crowns, showing commonalities with representatives of some phylogenetic lineages of the Nostolepis sensu lato. Among taxa known from the southern hemisphere, Iranolepis ginteri slightly resembles scales from the Broken River Group of North Queensland, Australia which De Pomeroy (1996) assigned to Nostolepis sp. 1 and N. sp. 2. Burrow (2002: 97–99, figs. 16, 17) erected a new species Canadalepis basdenae and included these two taxa in its synonymy. Burrow suggested that the younger stratigraphic occurrence (Givetian) for the first form could have been in a limestone clast reworked from the older horizon (Emsian). Iranolepis ginteri scales resemble those of Canadalepis in having tongue−shaped crowns, but differ in having low lateral crown areas, some stranggewebe, and bun− scale morphotype, which we regard as a growth form comdled rather than separate Sharpey’s fibres. Scales tentatively mon to most climatiid and ischnacanthiform taxa (Burrow assigned to Canadalepis ? sp. were identified in calcareous ho− 2002). Scales vary greatly in shape and crown outline. rizons of the upper Lower Devonian Jawf Formation of Saudi Crowns are 0.6–1.8 mm long and 0.5–1.3 mm wide. Arabia ( Burrow et al. 2006: figs. 4.1–8, 4.14), and similar Rounded, broadly elliptical or elongated trapezoidal crowns scales are found in calcareous facies of the late Middle Devo− are common forms. The scales have no more than six longinian (Givetian) Aztec Siltstone, Antarctica (CJB personal ob− tudinal, often slightly oblique ridges, although some speciservation). The Saudi Arabian scales closely resemble those of mens have short additional ridgelets intercalated (Fig. 5G 1). Iranolepis ginteri , although the lower lateral crown areas are Many specimens have a shallow antero−median depression only variably present in the former; where these areas are lack− or sulcus at the neck−crown junction and one lateral (right or ing, multiple vertical ridgelets run from the lateral crown left) crown area unsculptured. Unlike the flank specimens edges down to the base. (Fig. 5E), asymmetrical scales have no neck ridges converg− A northern hemisphere taxon with scales morphologically ing into the postero−lateral crown edges. Scale bases are similar to those of I. ginteri is Nostolepis valentinae (Valiu− small and shallow, of irregular forms, most often narrowed kevičius 2003a: figs. 20A–G, 21A–E) from the Lower Devo− and broadly elliptical to rounded−triangular (Fig. 5F 2, G 2).

nian (Lochkovian) of the Timan−Pechora region, Russia. A re− Histology.—The three to five thick growth zones in crowns cent synopsis and revision of climatiiform acanthodians with of flank scales ( Fig. 6 ) are composed of an orthodentine−like the Nostolepis − type histological structure by Valiukevičius tissue in the outer layer and odontocytic mesodentine in the and Burrow (2005) renamed this taxon as Pechoralepis valen− deeper inner layer. The dentine is best preserved and most tinae. Like the Iranian specimens, its scales have a distinctly clearly seen in the anteriormost crown part forming almost raised, wide medial area on the crown with only one or two all of the growth lamellae; it also extends into the thinner short anterior ridgelets, but the crowns are flat without a cen− (outer) layers in the central crown part.

tral depression. Also, the lateral slopes of the Timan−Pechora

Unusually for climatiids, the dentine tubules are long, specimens are not as pronounced and rarely continue to the

straight, rarely branching and the main canaliculi emanate posterior crown tip; the scales lack large neck pores; and the odontocytic mesodentine in crowns is distinctly bone−like.

from the rounded odontocyte lacunae ( Fig. 6A 2 ; LIGG 3795).

Odontocytes and branchings of the dentine tubules are concentrated in the basal parts of growth zones and are less nu−

Genus Nostolepis Pander, 1856

merous in the youngest (last) lamella. Thus, the tissue in the Type species: Nostolepis striata Pander, 1856 ; Ohesaare Regional Stage , basal parts of growth zones resembles the odontocytic meso− Přidoli, Upper Silurian. Ohesaare Cliff, Saaremaa, Estonia .

dentine characteristic of climatiids. The stranggewebe of the Nostolepis sp. cf. N. gaujensis Valiukevičius, 1998 posterior crown part is only observed in the lower neck area Figs. 5, 6, 7. and has fine, narrow and short stranglakunae connected by 2002 Nostolepis sp. cf. N. gaujensis ; Turner et al. 2002: figs. 9A–H, fine winding dentinal processes ( Fig. 6B 2 , C; LIGG 3796, 10A, B. EUIV 412).

Material. —About 450 scales from sample H1,>800 scales The base pyramid is comparatively high and composed of from sample GI4, both middle Mesotaxis falsiovalis to Pal− cellular, fine−lamellar bone incorporating numerous osteomatolepis hassi conodont zones of Chahriseh section, Kaftari cyte spaces lengthened and oriented along the growth lines Mt., central Iran. ( Fig. 6B 2 , C).

The asymmetrical sensory line scales ( Fig. 7 ; LIGG Description 3816–3818) are also characterised by superpositional growth Morphology.—Morphologically, specimens identified as of the crown, but growth zones are more numerous with up to flank scales (Fig. 5A–E; AEU 303–307) conform to the de− 10 noted. Unlike flank scales, the orthodentine−like tissue is scription in Turner et al. (2002), except that crowns carry 4–6 never observed in the anterior parts of crowns, which are (not 4–5) ridges which decrease in height posteriorly and may composed of dense networks of odontocytic mesodentine, fade out so that the posteriormost third of the crown is smooth. with stranggewebe in the posterior part of the crown (Fig. Large asymmetric scales (Fig. 5F–H; AEU 308–310) are 7A 2, B 2). Odontocyte cells are comparatively larger and identified as lateral sensory line scales by comparison with more densely concentrated, with more and wider processes the squamation of articulated climatiids (e.g., Nostolepis emanating from all sides. Wide branches of ascending and decora Valiukevičius 2003a) and Acritolepis (Valiukevičius radial dentine canals are also present in crowns, although 2003a) showing principal characters of the “ Pruemolepis ” only short lengths are preserved in the ground thin sections +

Fig. 5. SEM micrographs of scales of climatiid acanthodian Nostolepis sp. cf. N. gaujensis Valiukevičius, 1998 from early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. A–E. Flank scales, anterior upwards except for E, two scales in lateral view, anterior to right. A. AEU 303 in crown (A 1) and basal (A 2) views. B. AEU 304 in crown (B 1) and basal (B 2) views. C. AEU 305 in crown (C 1) and basal (C 2) views. D. AEU 306, in antero−lateral (D 1) and basal (D 2) views. E. AEU 307 in crown−lateral view. F–H. Specialised scales from the lateral sensory line, anterior upwards. F. AEU 308 in crown (F 1) and basal (F 2) views. G. AEU 309 in crown (G 1) and basal (G 2) views. H. AEU 310 in crown view.

dentine odontocyte growth odontocyte dentine tubule cell dentine? lamella cell tubule

( Fig. 7A 2 ). Bases are comparable to those of flank scales, with thinly lamellar cellular bone.

Discussion.—Undoubtedly, the scales studied herein are from the same species as those identified by Turner et al. (2002: fig. 9A–H), from the same geological horizon of the Chahriseh section. The taxon belongs to the genus Nostolepis as it has scales with all the characteristic tissues composing crown and base which are diagnostic for the genus, in both articulated specimens and isolated microremains. The new material includes more morphological varieties of flank scales than previously recognised, and also specialised asymmetrical scales of a “ Pruemolepis ”− type having similar tissue microstructure.

Nostolepis spp. flourished in the uppermost Silurian and Lower Devonian in many regions worldwide, and showed wide variations in the sculpture on scale crowns: sharp ridges extending the whole crown length; sharp or rounded ridges, only on the anterior crown part; parallel, subparallel or radially−oriented ridges. Some species have a raised median area and lower lateral areas which may or may not be ridged.

Very few Middle or Late Devonian nostolepids are known, and most have scales with a relatively robust crown sculpture. The Iranian species is distinguished by broad parallel ridges extending at least two thirds of crown length, and configuration of the two median ridges. Nostolepis gaujensis ( Valiukevičius 1998: pl. 8: 8) from the Baltic Upper Old Red Sandstone (Frasnian) facies has scales with four simple broad, rounded ridges extending almost the whole crown length with broad, shallow grooves between. It is comparable with the Iranian specimens in most details; in both, the broadest median ridges are joined by an arc−like rim along the anterior crown edge. Morphological differences between these taxa are the fewer ridges (four) and lower or almost negligible neck in scales of N. gaujensis . However, the latter was described based on only five scales, hardly encompassing the total scale diversity range in this taxon. The histological structure of N. gaujensis was not investigated. Scales from the?Frasnian Cuche Formation of Colombia in South America have also been assigned to Nostolepis sp. cf. N. gaujensis ( Burrow et al. 2003) . Unfortunately, those scales were preserved only as hollow shells, and so their internal structure is unknown, but their morphology conforms to that of the Iranian and the type N. gaujensis scales.

Another representative from the Baltic, Nostolepis kernavensis ( Valiukevičius 1985: pl. 1: 9–13) from the topmost Eifelian upper Narva Substage, differs in having two very robust median crown ridges which sometimes are converging posteriorly, or four median ridges meeting in two pairs posteriorly. Unlike the Iranian scales, ridges in N. kernavensis are high, with sharper crests, and grooves are deeper and narrower. Histological structure of the latter shares many characters in common with the Iranian Nostolepis sp. cf. N. gaujensis , even showing the relatively straight unipolar and perpendicularlyoriented dentine tubules in the anterior crown part ( Valiukevičius 1985: fig. 2.4A, B), a feature rarely observed in climatiid acanthodians. The principal histological difference between the species is in the stranggewebe, with more elongated and dense lacunae enveloped by an odontocytic mesodentine layer in each growth zone of N. kernavensis .

In the number and robustness of crown ridges, Nostolepis sp. cf. N. gaujensis slightly resembles scales from the Lochkovian and Pragian in Yunnan, China which Wang (2003: fig. 2A–D) assigned to N. gracilis Gross, 1947 . Morphologically, the Chinese scales do not compare well with N. gracilis (shape of ridges differs, scales have a deep base protruding in front of the crown, no pores open out on the anterior crown part), and histological structure is not of the Nostolepis − type (very high apexed, thin−lamellar base composed of acellular bone, and no mesodentinal tissue in crown). The scales are comparable with the Iranian ones in having 4–6 parallel ridges extending to the posterior crown part but never reaching its end. However, they differ significantly in having sharp crown ridges, a pair of short oblique neck ridges sloping down to the lateral corners of the base, and sometimes vertical ridges on the posterior neck.

Figures

Fig. 3. Climatiid acanthodian Iranolepis ginteri gen. et sp. nov. from early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. SEM micrographs of flank scales. A. Holotype, AEU 301 in crown (A1, anterior upward), basal (A2, anterior upward), and lateral (A3, anterior to right) views. B. AEU 302 in crown (B1) and basal (B2) views. C. AEU 303 in crown view. In B1, B2, and C anterior is upward.

Fig.1. Map of Iran (A) and close−up of the Chahriseh area showing location of the Kaftari Mountain section (B).

Fig. 2. Simplified stratigraphic column of the Chahriseh area with a magnification of the basal Frasnian deposits and position of acanthodian−bearing horizons H1 and GI4.

Fig. 6. Climatiid acanthodian Nostolepis sp. cf. N. gaujensis Valiukevičius, 1998 from early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. Photomicrographs of thin sections of flank scales. Vertical longitudinal sections (A, B) and vertical transverse section (C) of the central part of scales. A. Thin section LIGG 3795. A1, structure of?orthodentine in the outer layers and odontocytic mesodentine in the inner parts of growth lamellae in crown anteriorly (left part); A2, magnified area of the anterior crown part exposing unipolar, more outwardly−oriented dentine tubules emanating from large, rounded odontocytes. B. Thin section LIGG 3796. B1, scale exposing cellular bone in base, portions of?orthodentine and stranggewebe tissues respectively in the anterior and posterior crown parts; B2, orientation of elongated osteocyte cells along the growth lines in the apex of base, and probably stranggewebe at the base/crown junction, at higher magnification; B3, magnified part of the anterior crown with the orthodentine−like tissue. C. Thin section EUIV 412 showing a dense lamellarity of cellular bone in scale base with plenty of osteocytes, stranggewebe in the lower crown, and partly bush−like odontocytic mesodentine in the outer parts of growth zones in crown. Scale bars 0.1 mm.

Fig. 7. Climatiid acanthodian Nostolepis sp. cf. N. gaujensis Valiukevičius, 1998 from early Frasnian, horizon GI4 of an unnamed formation in the Chahriseh section, Kaftari Mt., central Iran. Photomicrographs of vertical transverse sections of specialised scales from the lateral sensory line. A. Thin section LIGG 3816. A1, general view of a scale sectioned through the central base part and showing growth characters; A2, the anterior, left part of the same scale at higher magnification exposing distribution of densely cellular mesodentine and stranggewebe in growth zones of crown, and structure of adjacent base part showing large numbers of osteocytes and long Sharpey’s fibre bundles. B. Thin section LIGG 3817. B1, structure of odontocytic mesodentine in the anterior crown part; B2, finely networked mesodentine incorporating densely concentrated osteocytes in growth zones of crown, at high magnification. C. Thin section LIGG 3818 showing general view (C1) and a small portion of the short−lacunal stranggewebe developed in the crown amongst the odontocytic mesodentine (at higher magnification, C2) characteristic of most of the crown. Scale bars 0.1 mm.