Graptolithina, gen. indet.

Graptolites are colonial hemichordates — not molluscs, corals or plants, but distant relatives of the living pterobranchs. Each small cup, or theca, along the branch housed one zooid, and the colony grew by budding.

The skeleton is built of collagen, laid down in overlapping half-rings. Because it is organic rather than mineral, graptolites are usually preserved as a flattened carbon film on the bedding plane, looking, as the name says, like pencil marks on the rock.

The graptoloids — the branched, floating forms — drifted in the open ocean, spread worldwide quickly and evolved fast, which makes them among the most useful of all fossils for dating and correlating Ordovician and Silurian rocks. The Yass succession is dated partly on its graptolites.