Amaltheus margaritatus de Montfort, 1808

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 : 244

Amaltheus margaritatus de Montfort, 1808

Fig. 4C, D, G, T.

1808 Amaltheus margaritatus ; de Montfort 1808: 91, fig. 90.

1958 Amaltheus margaritatus de Montfort ; Howarth 1958: 13, pl. 3: 4–6; text−figs. 8, 9 (with synonymy).

1976 Amaltheus margaritatus de Montfort ; Pourmotamed and Motamed 1976: 106, fig. 3.

1977 Amaltheus stokesi (Sowerby) ; Nabavi and Seyed−Emami 1977: 81, fig. 9 (4, 5 only).

1986 Amaltheus margaritatus (de Montfort 1808) ; Meister 1986: 95, pl. 20: 9; pl. 22: 1; pl. 23: 6 (with synonymy).

1998 Amaltheus margaritatus de Montfort, 1808 ; Géczy and Meister 1998: 103 (with synonymy).

Material.— One specimen from Gardaneh−e−Emamzaheh− Hashem (northeast of Tehran, Central Alborz) and more than 10 poorly preserved inner whorls from Shahmirzad (050514−6) and Sharif−Abad (050512−5). Specimens H−233 and 90−SE−1 come from earlier collections made by Mohammad Hosseinzadeh from Shahmirzad and K.S. −E. from Emam−Zadeh Hashem Pass in Central Alborz .

Dimensions (in mm)

Specimen D U H W

050514−6/3 16 31 44 –

050514−6/2 24 31 42 ~25

050514−6/5 31 38 37 ~23 (evolute specimen) H− 233 38 34 41 ~21 (finely ribbed specimen) 90−SE−1 44 23 ~48 –

Description.—Specimen 90−SE−1 ( Fig. 4G) is a flat, oxycone and relatively involute Amaltheus , with a rather prominent and fairly finely crenulated keel. Ribs are mediumcoarse and slightly sigmoidal. Close to the venter the ribs curve forward and almost fade before reaching the keel.

Discussion.—The specimens from Shahmirzad are small inner whorls, with a varying degree of involution and coarseness of the ribs. Specimen H− 233 ( Fig. 4T) is very finely ribbed, looking much like A. depressus Simpson, 1843 , stadium compressum Quenstedt in Frentzen (1937: pl. 2: 4). The latter has been regarded as a synonym of A. margaritatus by Howarth (1958: 140). Nevertheless, the juvenile, poorly preserved specimens from Shahmirzad can be placed, with caution, within the range of variation of A. margaritatus of Meister (1988). On a slab with A. margaritatus (90−SE−1) there is a fragment of Amauroceras sp. , looking much like A. ferruginum ( Simpson, 1843) of Meister (1986: pl. 20: 7). The genus Amauroceras has been considered to be the microconch form to Amaltheus by Meister (1986: 125).

Stratigraphic and geographic range.—Shahmirzad, level II; Sharif−Abad, level A; Amaltheus margaritatus Zone, Late Pliensbachian (Domerian) . Amaltheids are typical Boreal and “Euroboreal” taxa ( Dagis 1976; Meister 1988; Meister and Stampfli 2000; Smith et al. 2001), which occur only sporadically in northern Iran ( Nabavi and Seyed−Emami 1977; Seyed−Emami 1988). So far, this is the southernmost occurrence of the family within the northern hemisphere (northwestern Tethys).