The Tremadocian of the East Alborz Region is dominated by condensed fine clasticsediments. These beds have yielded low to medium diversity trilobite associations, which belong to theolenid, nileid and raphiophorid biofacies, characteristic of an outer shelf environment. Five successivetrilobite biozones can be recognised in the Tremadocian succession of Alborz. The lower TremadocianAsaphellus inflatus–Dactylocephalus and Psilocephalina lubrica zones are characterised by mediumdiversity trilobite associations with strong links to contemporaneous faunas of South China. Three upperzones are documented in the section at Gerd-Kuh, the successive Vachikaspis insueta and Kayseraspiszones represent a low diversity interval during a time of rapid changes in the sea level changes; themedium diversity fauna of the Asaphellus fecundus–Taihungshania miqueli zone shows strong links tothe faunas of Mediterranean segment of Gondwana. Brachiopods in Gerd-Kuh are represented by themonotaxic Tarfaya Association and the low diversity Paralenorthis–Xinanorthis Association. Therecurrent oligotaxic Protambonites Association invaded the area in the late Tremadocian during shortterm regressive episodes
