The Lower-Middle Cambrian animal ''Rhabdotubus'' is the earliest known

pterobranch Pterobranchia, members of which are often called pterobranchs, is a class of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water wi ...

. It bears strong similarity to the

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

s - indeed for some time it was misclassified as a dendroid graptolite.

References

Cambrian invertebrates Prehistoric hemichordate genera Fossils of Sweden {{Hemichordate-stub