Loganellia sp.

Hairapetian, Vachik, Blom, Henning & Miller, C. Giles, 2008, Silurian thelodonts from the Niur Formation, central Iran, Acta Palaeontologica Polonica 53 (1), pp. 85-95 : 92-93

Loganellia sp. cf. L. grossi Fredholm, 1990

Fig. 4.

Material.— Seven scales from sample S28, 314 scales from sample S32, late Ludlow , Niur Formation, Derenjal Mountains, east central Iran .

Description

Morphology.—The collection from the Niur Formation contains all the main scale varieties. The head and transitional scales that dominate the collection, range from 0.15 to 0.9 mm in length, are rounded or elongate in outline, with a flat or convex crown surface and deeply crenulated margins ( Fig. 4A–E; AEU 4115–4118). Diagnostic trunk scales (0.5–0.8 mm long and 0.3–0.4 mm wide) are characterised by a smooth rhomboidal outline and a flat or slightly convex crown ( Fig. 4F–I, L–N; AEU 4119–4122, 4125–4127). A deep downward notch occurs at the anterior of the crown. Sometimes two short ribs appear on each side of the deep notch at the beginning of a wider crown anterior ( Fig. 4K 1 ). A postero−laterally down−stepped lateral rim runs each side of the crown, extending at least two thirds the length of the crown. The posterior end of the crown is pointed. Possible fin scales ( Fig. 4J, N) are smaller, and have a narrower crown. Scales have a deep base with one ( Fig. 4A 2), two ( Fig. 4B 2) or three pulp openings ( Fig. 4F 2). Some irregularly placed hollows ( Fig. 4I 2, I 3 ) are situated on the lateral side of the base which seem to be taphonomic artefacts.

Histology.—Due to poorly preserved material, histological studies have not been undertaken, but a pulp canal can be recognised in some examined thin sections.

Discussion.—The set of scales from the Niur Formation described here is similar to the type material of Loganellia grossi , from the Wenlock of the Slite Beds of Gotland, Sweden ( Fredholm 1990; redescribed by Märss 1996) in most details of the crown structure of trunk scales, including the anterior notch. The specimens from the Baltic region, including those from Estonia ( Märss 1996), differ from the Iranian specimens in having a shallower base.

Scales from the Samoilovich Formation of October Revolution Island, Severnaya Zemlya (Märss and Karatajātė− Talimaa 2002), the Cape Philips Formation of Canadian Arctic Archipelago ( Märss et al. 2006), and the Kap Moton Formation of Washington Land, north Greenland ( Blom 1999), all of Wenlock age, have also been assigned to L. grossi . However, the Russian and the Canadian specimens differ from the Iranian scales in having a shallower anterior notch, a wider crown, and better developed lateral rims (Märss and Karatajātė− Talimaa 2002: figs. 2N–S; Märss et al. 2006: pl. 3: 14, 15). A median posterior crest on the lower crown surface is also recognised in the north Greenland material ( Blom 1999: fig. 3.3) and is not seen in the material described here.

Indeterminable acanthodians

Fig. 5 .

Material.—Two scales and a jaw bone fragment from sample S30, nine scales from sample S32, late Ludlow, Niur Formation, Derenjal Mountains, east central Iran.

Discussion.—These findings are the first illustrated acanthodians from the Silurian of Iran and the Middle East. The scales are of climatiid−type and have a diamond−shaped crown with a distinct neck ( Fig. 5A ; AEU 4128). The anterior part of the crown of the best preserved scale is badly damaged, so the positions of the anterior ridges are unknown. The crown slopes anteriorly toward the protruding base, which is deep anteriorly and shallow posteriorly, and separated from the neck by a pronounced rim.

One specimen ( Fig. 5B ; AEU 4129) is possibly a fragment of dentigerous jaw bone, in which a triangular tooth is ankylosed to the bone. The tooth is damaged or worn, but shows a semicircular horizontal section and irregular radial ridges.

Histological studies have not been attempted on these poorly preserved micro−remains.

Figures

Fig. 1. A, B. The position of the Derenjal Mountains, north of Tabas, east central Iran, and the sampled section of the Niur Formation. C. Composite stratigraphic column of the sampled section showing fish−bearing beds.

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.

Fig. 5. Indeterminable acanthodians, SEM micrographs of micro−remains from sample S30, late Ludlow of the Niur Formation, east central Iran. A. Scale AEU 4128 in crown (A1) and lateral (A2) views. B.?dentigerous jaw bone fragment AEU 4129 in occlusal (B1) and lateral (B2) views.