An extinct group called graptolites are the major fossil representatives of this phylum (see Fig. 4.31). They are considered hernichordates because they secreted tubes of protein similar to the modern hemichordate, Rhabdopleura. Graptolites are often found preserved in black shale as compressed carbonized impressions, sometimes replaced by pyrite.
Graptolites are thought to have been colonial marine organisms. The colonies were composed of short tubes connected by a common canal. There are two main orders of graptolites: (1) Dendroidea, bushlike graptolites that attached to the ocean floor or to floating seaweed, and (2) Graptoloidea, colonies consisting of branching stipes from an initial chamber, which floated or swam weakly in the open ocean.
Graptolites are good index fossils for the lower Paleozoic. Graptoloids declined and became extinct in the early Devonian. Dendroids became extinct in Permian times.