An Early Cambrian Hemichordate Zooid

Hemichordates are known as fossils from at least the earliest mid-Cambrian Period (ca. 510 Ma) and are well represented in the fossil record by the graptolithinid pterobranchs ("graptolites"), which include the most abundantly preserved component of Paleozoic macroplankton. However, records of the soft tissues of fossil hemichordates are exceedingly rare and lack clear anatomical details. Galeaplumosus abilus gen. et sp. nov. from the lower Cambrian of China, an exceptionally preserved fossil with soft parts, represents by far the best-preserved, the earliest, and the largest hemichordate zooid from the fossil record; it provides new insight into the evolution of the group. The fossil is assigned to the pterobranch hemichordates on the basis of its morphological similarity to extant representatives. It has a zooidal tube (coenecium) with banding throughout comparable to that in the extant pterobranchs and a zooid with paired annulated arms bearing paired rows of annulated tentacles; it also displays a putative contractile stalk. G. abilus demonstrates stasis in pterobranch morphology, mode of coenecium construction, and probable feeding mechanism over 525 million years.