evolution, their abundance — the most productive samples bearing almost a thousand tests per gram — and the easy identification (due largely to the large variation in shapes) and short lifetimes (<10 million years) of most species. They are also widely distributed and appear in a variety of marine depositional settings, making correlation easier; better still, they can often be recognised in even quite strongly metamorphosed rocks. However, convergence of morphological form to similar environments sometimes leads to the mistaken identification of a species in several areas separated by vast differences in space and time, but sharing a similar depositional environment; clearly, this can cause major problems if the organisms are interpreted as being the same species. Aside from the acritarchs, chitinozoans were the only reliable means of correlating palæozoic units until the late 1960s, when the detailed study of conodonts and graptolites fully unleashed their stratigraphic potential. The oldest know chitinozoans appear to be phosphatized remains tentatively referred to the genus Eisenackitina. They were recovered from the Middle Cambrian (Stage 5)-age Gaotai Formation, more than 20 million years before the group is found elsewhere in the Ordovician. Chitinozoans appear to have become extinct at the end of the Devonian; rare Carboniferous and Permian remains may represent reworked fossils or fungal spores. References External links Commission Internationale de Microflore du Paléozoique (CIMP), international commission for Palaeozoic palynology. Prehistoric marine animals Enigmatic animal taxa Palynology Early Ordovician first appearances Pennsylvanian extinctions