Heteromorph Ammonites of Albian-Cenomanian in the Aitamir Formation (Koppeh Dagh)

Cretaceous Heteromorph Ammonites are known as index fossils with widely distribution and narrow range in times which play an important role in Cretaceous biostratigraphy (Seyed-Emami & Aryai, 1981; Seyed-Emami et al., 1984; Immel et al., 1997; Mosavinia et al., 2014). Most of these specimens have short age with order of a sub-stage or a zone. More than ten stratigraphic sections from Kopeh-Dagh sedimentary basin were investigated in order to find the Heteromorph forms in the area, but Heteromorph Ammonites were observed in only three of them. Three stratigraphic sections have been measured and logged in the Koppeh Dagh Mountains north, northeast and east of Mashhad at the following localities: 1- Ghorghoreh, locatedabout 70 km E of Mashhad (coordinates: N 36° 13′ 34″, E 60° 24′ 18″)
2- Taherabad, located about 50 km NE of Mashhad (coordinates: N 36° 37′ 05″, E 60° 04′ 03″)
3- Sheikh, located about 20 km NE of Bojnourd (co-ordinates: N 37° 33′ 51″, E 67° 32′ 46″).
62 Specimen of Heteromorh Ammonites are collected from these sections and described which are systematically arranged in genus, species and subspecies as below:4 specimen of Pseudhelicoceras robertianum (Orbignyi, 1842) from Sheikh Section with the age of Early Late Albian, 3 specimen of Mariella (Mariella) cf. bergeri (Brongniart, 1822) from Taherabad and Ghorghoreh sections occur in Late Albian, 2 specimen of Mariella (Mariella) cf. bicarinata (Kner, 1852) from Ghorghoreh section which occur in Early Cenomanian, 8 specimens of Marialla (Mariella) lewesiensis (Spath, 1926) from Ghorghoreh section show the age of Early Cenomanian, 19 specimens of Mariella (Mariella) cenomanensis (Schluter, 1876) from Ghorghoreh section occur in Early Cenomanian, 11 specimens of Turrilitess cheuchzerianus (Bosc, 1801) from Ghorghoreh and Taherabad sections identifying the age of Late Early to Early Middle Cenomanian, 2 specimens of Turrilites costatus Lamark, (Bosc, 1801) from Taherabad section with the age of Middle Cenomanian, 8 specimens of Hypoturrilites cf. tuberculatus (Bosc, 1801) from Ghorghoreh section occur in Early Cenomanian and 5 specimens of Hypoturrilites wiedmanni (Collignon, 1964) from Ghorghoreh section occur in Early Cenomanian. Well preserved Heteromorph Ammonites have been recorded in this studyarea, consequently 9 taxa are described. The oldest one is Mariella (Mariella) bergeri of Stoliczkaia dispar zone, indicates the Late Albian age of the strata related to 100 ma. The youngest species which has been obtained is Turrilites costatus of Acanthoceras rhotomagense zone and Turrilites costatus subzone of the Middle Cenomanian estimates an age of 95.5 Ma.
The specimens belong to Mantelliceras mantelli zone, are relatively more abundant which are comprised of Mariella (Mariella) cenomanensis, Hypoturrilites cf. tuberculatus, Hypoturrilites wiedmanni, Mariella (Mariella) cf. bicarinata and Marialla (Mariella) lewesiensis. This Zone indicates the base of Early Cenomanian.