PHYLUM HEMICHORDATA
The photograph below shows an example of a type of important guide fossil for Paleozoic rocks -- a graptolite (Class Graptolithina). They are usually preserved, as here, as carbon films in black shales, and may be the only fossils in those shales. They most often resemble small segments of small saw blades, though other shapes do occur. The scale bar is 1cm. (Photo by B. Carter.)