Niurolepis susanae, Hairapetian & Blom & Miller, 2008

Hairapetian, Vachik, Blom, Henning & Miller, C. Giles, 2008, Silurian thelodonts from the Niur Formation, central Iran, Acta Palaeontologica Polonica 53 (1), pp. 85-95 : 87-92

Niurolepis susanae gen. et sp. nov.

Figs. 2, 3 .

Derivation of the name: After Dr. Susan Turner (Brisbane), in recognition of her contributions to Lower–Middle Palaeozoic agnathan fish studies.

Holotype: AEU 4105, trunk scale ( Fig. 2F).

Type locality: Eastern side of the Dahaneh Kolut Gorge, Derenjal Mountains, east central Iran.

Type horizon: S23, grey sandy bioclastic limestone, ca. 55 m above the base of Hill C, reference section of the Niur Formation.

Material.—165 scales from sample S23.

Diagnosis.—Katoporodid with small to medium sized scales (up to 1 mm length); trident−like trunk scales with flat or slightly convex median crown area and one pair of lateral wings (areas); median crown area broad and elevated; lateral wings with posteriorly pointing apices; lower crown surface with fine longitudinal ridges beneath each narrow lateral wing and a median crest. The histological structure is characterised by branched wide dentine canals.

Description lateral wing. An asymmetrical scale shows a relatively narrow and smooth median area with flattened lateral wings and Morphology.—The morphological varieties recovered in−

a distinct rim around the basal opening, perhaps from the clude head, transitional and trunk scales. Head scales (0.3–

pectoral or caudal fin regions ( Fig. 2I). It has an irregular 0.8 mm long, 0.25–0.4 mm wide) are rounded or oval, some−

quadrangular base and an almost triangular pulp depression. times slightly elongated ( Fig. 2A–D). The crown margins are

The scale ( Fig. 2K; AEU 4110) has an elliptical and concave deeply incised and have six to twelve, often bifurcating ribs.

median area lacking lateral wings. Instead, four distinct short The crown surface is smooth and flat in small scales but more

ridges on the lateral sides of the crown and neck curve upconvex in elongated forms with a posteriorly elevated area.

wards posteriorly; each ridge raised centrally. The neck is low and starts abruptly below the crown with a distinct step. The base varies from low ( Fig. 2A, B) to rela− Histology.—Scales have branched wide dentine canals, which tively high ( Fig. 2C, D). It has a large pulp depression, which spring from the pulp depression and the pulp canal ( Fig. 3A ; is rounded or elliptical in outline. AEU 4111). The dentine canals branch to numerous fine

Transitional scales (0.6–0.9 mm long, 0.4–0.5 mm wide) dentine tubules in the upper part of crown. The growth lines are fairly rare in residue samples, but one scale ( Fig. 2E) are distinct ( Fig. 3B, C ; AEU 4112–4113). The pulp canal is of shows rounded ribs, bifurcated at the crown anterior margin. medium length (1/3–1/4 of the scale length). The uppermost The neck is low and the base is shallow with an elliptical pulp part of the crown is an enamel−like tissue and the boundary depression. An anterior spur is not developed in transitional with the dentine portion seems to be transitional. A few wide scales but the base protrudes slightly anteriorly. dentine canals are present in the neck region. Tubules of

The trident−like trunk scales (0.5–1 mm long and 0.3–0.5 Sharpey’s fibres are placed on the base. mm wide) are diagnostic and are characterised by three dis− Discussion.—The histology of Niurolepis is most diagnostic tinct crown areas; a raised broad median area and two lateral and similar to that of the katoporodid−type structure. It differs areas (wings) at a lower level. The flat or slightly convex me− from other groups of thelodonts, including typical taxa such dian area covers three quarters of the crown surface and has a as Shielia Märss in Märss and Ritchie, 1998, Paralogania posteriorly pointing apex. The anteriormost part of the me− Karatajātė− Talimaa, 1997, Chattertonodus Märss, Wilson , dian area has two to four very short parallel ridges (e.g., Fig. and Thorsteinsson, 2002, Nikolivia Karatajātė− Talimaa, 2F 3, F 4; AEU 4105); sometimes a shallow notch (e.g., Fig. 1978, Canonia Vieth, 1980 , and Glacialepis Märss, Wilson , 2H; AEU 4107) occurs. Each lateral wing has a posteriorly and Thorsteinsson, 2002, by having branched wide dentine pointed spine and is separated from the median area and the canals with openings distributed evenly in the pulp depresneck by shallow grooves. sion. Niurolepis susanae gen. et sp. nov. differs from the

The lateral wing with smooth surface extends backwards other katoporodids, Katoporodus Turner and Peel, 1986 , from the middle of the crown and is rather narrow for most of Goniporus Gross, 1967 , Overia Soehn, Märss, Caldwell , and its length. Behind the spiny end, the lateral wing is reduced to Wilson, 2001, and Zuegelepis Turner in Turner et al., 1999, a narrow rim that extends to the crown posterior, just below by having a broader median area, and by lacking well develthe main surface of the median crown area ( Fig. 2F 3 , F 4). oped and deeply ridged lateral areas. The phlebolepidid gen−

The lower surface of crown has ridgelets that run from an− era, Erepsilepis Märss, Wilson, and Thorsteinsson, 2002 , terior of the spines to the posterior part. The lower crown sur− Helenolepis Karatajātė− Talimaa, 1978, and Phlebolepis Panface also shows a very prominent and wide median crest der, 1856, which all show histology similar to katoporodids, ( Fig. 2F 2, H 2). There is no microsculpture on the crown. The differ from Niurolepis by having numerous ridges on the crown is inclined towards the anterior at an acute angle to the crown and lacking trident−like trunk scales. Niurolepis difbase. The neck is shallow and forms a distinct boundary with fers from other morphologically similar shieliid genera such the base. as, Shielia and Paralogania , by having a wider median area

In trunk scales, the base is anteriorly protruded, making a and by lacking spines at the margins of the lateral areas. single spur that varies in length from short to rather long (Fig. Scales of nikoliviids, Chattertonodus and Nikolivia can be 2F–H, J); there is no additional basal projection. The pulp de− distinguished from those of Niurolepis as they have nikopression is also elongated, has an elliptical outline, and is sit− liviid−type bases, smooth lower crown surfaces and wider uated close to the centre of the scale. lateral areas. The furcacaudiform Canonia has a flattened

There are some rare additional scale forms in the collec− crown with longitudinally ridged median and lateral areas, a tion. Only one rare scale seems to have two spines on each smooth lower crown surface, a relatively high smooth neck +

dentine tubule tubules of Sharpey’s fibres and a different base of low collar form around the pulp depression. Niurolepis differs from the talivaliid Glacialepis in having a narrower median area with spiny lateral wings, and ridged lower crown surface.

The southern hemisphere taxon, Turinia fuscina Turner, 1986 (figs. 2B–H, K–U; 3F) from the Late Silurian (most probably Ludlow–Přidoli; see also Burrow and Turner 2000 for age constraints) of the Silverband Formation, western Victoria, Australia is morphologically very similar to N. susanae . Like the Iranian scales, head and transitional scales show identical structure, and its trunk scales have a distinctly raised, wide median area on the crown with longitudinal lateral wings, but as Turner (1986: 58) stated, the lateral wings apparently do not reach the crown posterior. It seems that, the anteriormost part of the median area in eighteen paratypes of T. fuscina is smooth. Two scales illustrated by Turner (1986: figs. 2Q and 3F) seem to have more than two lateral wings. The histological structure of T. fuscina was not preserved, but Turner (1986) suggested based on broken scales that they may have turiniid−type histology. The morphological resemblances between T. fuscina and N. susanae may suggest that these two species are closely related and may be considered congeneric. However, such conclusion can only be drawn if both can be shown to have identical histological structures.

Among taxa recently described from the northern hemisphere, Niurolepis susanae shows similarities to those from the Early Devonian (Lochkovian) of the Ben Nevis Formation of Spitsbergen which Blom and Goujet (2002) attributed to a new species, Turinia barentsia . Like the Iranian specimens, its trunk scales have a distinctly raised, and anteriorly inclined median area of the crown supported by two lateral wings, but the anterior median and lower crown areas are smooth. In some figured scales ( Blom and Goujet 2002: pl. 2: 6, 7, 9), sometimes the lateral wing outwardly developed a flattened segment, midway to the crown posterior. Also, the base is oval and did not produce a spur, although to some extent, it protruded anteriorly. As with most turiniids, Turinia barentsia is also significantly larger than Niurolepis susanae .

There are other Early Devonian smooth forms of Turinia with a wide geographic distribution which can be compared to N. susanae . T. pagei ( Powrie 1870) differs from the Iranian scales in having crenulated lateral wing margins, a ridged neck and a much longer basal process (e.g., Karatajātė− Talimaa 1978). Scales of T. polita Karatajātė− Talimaa 1978 have a smooth neck and an undivided crown surface with serrated margins, a smooth lower crown with median crest, and a deeper base (Karatajātė− Talimaa 1978; Märss et al. 2006). Histologically, scale crowns of these early Turinia species (including T. barentsia ) differ from those of the Iranian Niurolepis in having turiniid−type characters, with long straight and narrow dentine tubules that are often wider near the pulp cavity and pocket−like hollows.

Figures

Fig. 3. The phlebolepidiform thelodont Niurolepis susanae gen. et sp. nov. from sample S23, late Wenlock /?early Ludlow of the Niur Formation, east central Iran. Photomicrographs of thin sections. A. Vertical longitudinal section of a trunk scale AEU 4111. B. Vertical longitudinal section of a trunk scale AEU 4112. C. Horizontal section of a head scale AEU 4113. Scale bars 0.2 mm.