A formal Ordovician–Silurian chitinozoan biozonation for western Gondwana is proposed. This palaeogeographic province includes South America, and was located in medium to high latitudes during Ordovician and Silurian times. Ordovician chitinozoans are known from northern Argentina, southern Bolivia, and Brazil. Silurian chitinozoans occur in Brazil, northern Argentina, southern Bolivia and southern Peru. No published information is available about Ordovician–Silurian chitinozoans from Ecuador, Colombia or Venezuela. Altogether more than 150 localities (including wells and outcrops) are included in this study, and 154 species have been encountered. A biozonation based on the first occurrence of critical chitinozoan species is introduced. Five biozones are defined in the Ordovician (zones of Desmochitina sp. gr. minor, Conochitina decipiens, Eremochitina brevis, Lagenochitina obeligis and Tanuchitina anticostiensis), and nine in the Silurian (zones of Belonechitina postrobusta, Spinachitina harringtoni, Pogonochitina djalmai, Margachitina margaritana–Salopochitina monterrosae, Angochitina echinata, Eisenackitina granulata, Fungochitina kosovensis and the subzones of Sphaerochitina solutidina and Desmochitina cf. D. densa). These biozones are compared with known graptolite, conodont, acritarch and spore zones from the same area, and chitinozoan zones on a global basis.

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