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