Late Ordovician trilobites from the Karagach Formation of the western Tarbagatai Range, Kazakhstan

Abstract

Two trilobite faunas of late Ordovician (Sandbian and Katian) age are described from the siliciclastic Karagach Formation, western Tarbagatai Range, eastern Kazakhstan. They comprise 15 families and 24 genera and include the new taxa Agerina acutilimbata sp. nov., Birmanites akchiensis sp. nov., Dulanaspis karagachensis sp. nov. and Kimakaspis kovalevskyi gen. et sp. nov. Most of the Karagach Formation yields graptolites characteristic of the Diplograptus foliaceus [multidens] Biozone, which are associated with the older trilobite fauna; the uppermost part, which is the source of the younger trilobite fauna, contains Orthograptus quadrimucronatus and Dicranograptus hians which suggest a younger age, equating with the lowermost Ensigraptus caudatus Biozone, and the base of the Katian Stage. Most of the trilobite genera in both faunas have a wide geographical distribution in the late Ordovician, although Dulanaspis and Sinocybele are characteristic of low latitude eastern peri-Gondwanan faunas.