Dactylioceras (Orthodactylites) cf. tenuicostatum ( Young and Bird, 1822 )

Seyed-Emami, Kazem, Fürsich, Franz T., Wilmsen, Markus, Majidifard, Mahmoud R. & Shekarifard, Ali, 2008, Lower and Middle Jurassic ammonoids of the Shemshak Group in Alborz, Iran and their palaeobiogeographical and biostratigraphical importance, Acta Palaeontologica Polonica 53 (2), pp. 237-260 : 247

Dactylioceras (Orthodactylites) cf. tenuicostatum ( Young and Bird, 1822)

Fig. 4N, R.

Material.—Four fragmentary and crushed specimens from Kuhe Bashm (050510−13/1, 050513/2, and from earlier collections by K.S.−E. (80−SE−5, 80−SE−19).

Discussion.—The wide umbilicus with extremely fine and dense ribs allows the fragmentary specimens to be compared with D. (O.) tenuicostatum and allied forms, discussed and figured by Howarth (1973: 258, pl. 5: 1, 2; pl. 6: 2, 3) and Howarth (1980: 650, pl. 82: 1–10, 13, 14).

Stratigraphic and geographic range.— Kuhe Bashm, level III. D. (O.) tenuicostatum has been recorded from the Lower Toarcian Dactylioceras tenuicostatum Zone.

Dactylioceratidae indet.

Fig. 4O.

Material.—One fragmentary phragmocone collected by Hosseinzadeh (2003) from Kuhe Bashm (H−204).

Description.—At a diameter of 20 mm the specimen is relatively evolute with an ovate whorl cross−section and flanks converging slightly towards the arched venter. The last preserved whorl becomes distinctly involute with a deep umbilicus and a vertical umbilical wall. Moreover, the whorls become considerably wider (W/H ratio = 92%), with flanks converging strongly towards the venter, resulting in a triangular−ovate whorl cross−section. The ribbing is extremely fine, dense, and sharp. The slightly prorsiradiate primaries (22 per half−whorl at D = 20 mm) bifurcate almost regularly about the mid−flank. The slightly finer secondaries cross the arched venter in a straight line. No tubercles are developed at the bifurcation point. Discussion.—To some extent, the specimen has the appearance of a Macrocephalites . The narrow umbilicus and the fine ribbing of the last preserved whorl resembles Dactylioceras (Iranodactylites) ketevanae Repin (2000: 40 , pl. 3: 1, 2), from which our specimen differs by its distinct broadness and a nearly triangular whorl cross−section. A very similar, perhaps conspecific specimen is Dactylioceras sp. nov. from the Shemshak Group of the Lut Block, East Central Iran ( Seyed−Emami et al. 2004: 81, pl. 1: 3a, b). However, the latter has a finer and denser ribbing.

Stratigraphic and geographic range.— Repin (2000: 41) recorded D. (I.) ketevanae from the Hildoceras bifrons Zone. The specimen from the Lut Block comes from the Lower Toarcian ( Seyed−Emami et al. 2004). Hosseinzadeh (2003: 67) found the described specimen, together with D. (O.) semicelatum , in the Dactylioceras tenuicostatum Zone of the Early Toarcian.