Because of the absence of vitrinite in lower Paleozoic sediments, the reflectance of pyrobitumen is used as an indicator of thermal maturation. A new technique of preparing organic matter permits discrimination among types of zooclasts and solid bitumen, including pyrobitumen, and leads to more precise reflectance-histograms. Studies of organic matter from Ordovician-Silurian strata of Anticosti Island show that reflectance of zooclasts is different from that of solid bitumen, and that each group of the following zooclasts, chitinozoans, graptolites, and scolecodonts, has a specific reflectance evolution path with increasing depth. On a logarithmic scale, the paths of the three groups of zooclasts are linear, highly correlated, and converging. However, reflectance of solid bitumen is not linear throughout the range studied; solid bitumen becomes linear only where the reflectance is greater than 0.75%. Comparing the relations between zooclasts and solid bitumen with the vitrinite-solid bitumen correlation in the literature suggests that a hypothetical vitrinite evolution line in the Anticosti Island basin should be subparallel with that of chitinozoans and graptolites, but 0.4-0.8% lower, in the 1-2% range. However, scolecodonts and vitrinite should have the same reflectance at 1%, but scolecodont reflectance should be slightly (0.2%) higher for a vitrinite reflectance of 2%.