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

pterobranch Pterobranchia 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 with the help of cilia attached to tentacles. There ...

. 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 the L ...

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

References

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