Dactylioceras (Dactylioceras)

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 : 246

Dactylioceras (Dactylioceras) sp.

Fig. 4Q.

Material.— One specimen from earlier collections by K.S.−E. from Kuhe Bashm: (80−SE−16).

Dimensions (in mm)

Specimen D U H W

80−SE−16 41 48 29 29

Description.—Evolute Dactylioceras with nearly flat flanks, a quadratic whorl cross−section and a slightly rounded venter. On the last preserved whorl the ribbing is relatively coarse and widely spaced. The slightly prorsiradiate primaries bifurcate ventrolaterally.

Discussion.—The coarse ribbing and the quadratic whorl cross−section are similar to the lectotype of D. commune (J. Sowerby, 1815) refigured by Dean et al. (1961: pl. 72: 5a, b).

Stratigraphic and geographic range.—The precise stratigraphic position of the loosely collected specimen is not known.

Elsewhere, D. commune occurs in the Middle Toarcian ( D. commune Subzone ).

Subgenus Eodactylites Schmidt−Effing, 1972 Dactylioceras ( Eodactylites ?) pseudocommune Fucini, 1935

Fig. 4L.

1935 Dactylioceras pseudocommune sp. nov.; Fucini 1935: 86, pl. 9: 1–3.

1966 Dactylioceras pseudocommune Fucini 1935 ; Fischer 1966: 26, pl. 1: 5; pl. 4: 3, 6.

1972 Dactylioceras (Eodactylites) pseudocommune Fucini, 1935 ; Schmidt−Effing 1972: 91, pl. 3: 1a–c; pl. 18: 7; text−fig. 15.

1973 Dactylioceras (Dactylioceras) pseudocommune Fucini ; Howarth 1973: 253, pl. 1: 1 (with synonymy).

1994 Eodactylites pseudocommunis (Fucini) ; Faraoni et al. 1994: pl. 3: 1; pl. 4: 1–3, 5.

Material.— Two specimens from Sharif−Abad (050512−6/1, 050506/2).

Dimensions (in mm)

Specimen D U H W

050512−6/1 79 54 26 23

Description.—The figured specimen is a very evolute Dactylioceras , with nearly parallel, flat flanks and an arched venter. The whorl cross−section is rectangular to ovate. On the last preserved whorl the ribs are coarse, rectiradiate, rather sharp, and widely spaced. Most of the ribs bifurcate at the ventrolateral edge, but a few ribs are simple. There are no distinct tubercles at the point of bifurcation. On the crushed inner whorl the ribbing is relatively fine and dense.

Discussion.—Our specimen closely matches the one figured by Fucini (1935: pl. 9: 1), which has been designated as lectotype by Schmidt−Effing (1972: 91). The characteristic gable−shaped venter, being discussed by Fischer (1966) and Schmidt−Effing (1972), cannot be observed in our specimen, partly because of erosion.

Stratigraphic and geographic range.— Dactylioceras (E.) pseudocommune is largely a Mediterranean form, known to occur in the Late Pliensbachian (Domerian) or Early Toarcian (Fucini 1935; Schmidt−Effing 1972; Faraoni et al. 1994). Howarth (1973) recorded it from the base of the Dactylioceras tenuicostatum Zone. At Sharif−Abad it was found at level B, above the beds with Amaltheus and below the beds with Dactylioceras semicelatum .