Early Ordovician (Tremadocian) brachiopods from the Eastern Alborz Mountains, Iran

L E Popov,M G Pour,M R Kebriaee-Zadeh

doi:10.3176/earth.2011.2.01

Abstract

Early Ordovician (Tremadocian) brachiopods from the Eastern Alborz Mountains, Iran

Highlights

GEOLOGICAL SETTINGThe Ordovician brachiopod faunas of Iran are still very poorly documented
In spite of incomplete preservation, the specimens of Acrotreta from the Deh-Molla section show close similarity to the topotypes of Acrotreta dissimilis (Biernat, 1973) in characteristic features of shell morphology and they are considered here as conspecific. They are characterized by (1) a procline to catacline ventral valve with height not exceeding commissural shell length and an evenly convex sagittal profile of the ventral pseudointerarea, (2) a narrow dorsal pseudointerarea occupying less than half of maximum shell width, (3) a poorly defined median buttress and (4) a low dorsal median septum
Acrotreta dissimilis is considered as the earliest known species of the genus (Holmer & Biernat 2002). It differs from Acrotreta curvata Klouček, 1919 from the Lower Ordovician, Třenice Formation of Bohemia revised by Mergl (2002) in having a ventral valve as a low cone with not exceeding commissural shell length in height, an almost flat sagittal profile of the dorsal valve, a significantly lower and shorter apical process, weakly impressed ventral mantle canals and dorsal cardinal and central muscle scars

Summary

INTRODUCTION

The Ordovician brachiopod faunas of Iran are still very poorly documented. In particular, Early Ordovician brachiopods are presently known only from a single section in the Eastern Alborz Mountains, where they were reported from the uppermost Tremadocian Paroistodus proteus Biozone and overlying Floian deposits (Popov et al 2008, 2009a). Sample DM-A/9B was taken from the base of the uppermost limestone bed at 41.75 m above the base of the unit and sample DM-A/9A was collected from the same bed just 5 cm above the previous sample (geographical coordinates 36°21′22.5′′N; 54°44′44.88′′E, altitude 1645 m) They contain rhynchonelliform brachiopods including abundant Nanorthis bastamensis and a few Tritoechia sp., linguliform brachiopods Acrotreta dissimilis, Akmolina minor, Diencobolus sp., Eurytreta ahmadii and Wahwahlingula kharbashi. Emanuelensis Brock & Holmer, 2004, from the Floian, Emanuel Formation in Western Australia, is characterized by a smooth larval shell and finely pitted microornament of the brephic and adult shell, but it differs from the newly described species in having an elongate suboval, rather than a teardropshaped outline, a wider ventral pseudointerarea and a long median ridge bisecting the dorsal visceral area.

Discussion

Findings

DISCUSSION AND CONCLUSION