Polychaete annelids from the earliest Cambrian Period.

Abstract

Unambiguous body fossils of annelids (ringed worms) first appear in the Cambrian Period but are so far known exclusively as flattened specimens preserved in Burgess Shale-type Lagerstätten. Here, we report phosphatized microfossils, interpreted as polychaete annelids, from a distinct taphonomic window (i.e., three-dimensional Orsten-type preservation) in the earliest Cambrian (early Fortunian Age, ca. 535 Ma) Zhangjiagou Lagerstätte of South China. The fossils are millimetric in length and preserved as three-dimensional endocasts of the body. They exhibit clear trunk segmentation, with each segment bearing a pair of lateral or ventrolateral outgrowths. Two species, <i>Kuanchuanpivermis brevicruris</i> gen. et sp. nov. and <i>Zhangjiagoivermis longicruris</i> gen. et sp. nov., are distinguished based on the relative length of these outgrowths. The lateral outgrowths terminate in a bifurcation into two lobes of equal, subequal, or unequal size, closely resembling the notopodium and neuropodium of annelid parapodia. Both species are interpreted as members of the total-group Annelida. <i>Kuanchuanpivermis brevicruris</i> has relatively short appendages and may have been a benthic annelid similar to modern nereids. <i>Zhangjiagoivermis longicruris</i> closely resembles fossil and extant polychaetes (e.g., <i>Eotomopteris</i> and <i>Tomopteris</i>) in their relatively long appendages, suggesting a swimming lifestyle and representing the earliest known semi-pelagic annelid. These findings indicate that annelids had already acquired a polychaete-like body plan in the Fortunian and that early members of the clade had diverged from their living sister group and differentiated into forms with both short and elongate parapodia by the Fortunian Age.