A research team of scientists from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS), Virginia Tech, LMU Munich, and First Institute of Oceanography of the Ministry of Natural Resources have reported the discovery of the earliest Cambrian (ca. 535 million years ago, or Ma) microfossils interpreted as annelids (ringed worms), a group of animals that include bristle worms, earthworms, leeches, peanut worms, and many other creatures.
The discovery adds fresh insights into the origin and early evolution of the Annelida.
The study was published in Proceedings of the National Academy of Sciences of the United States of America on April 21th.
The Annelida represents one of the most speciose and ecologically widespread animal phyla. Traditionally, it is divided into Polychaeta (bristle worms), Oligochaeta (earthworms and their kin), and Hirudinea (leeches and their relatives), with the latter two constituting Clitellata. Phylogenetic analyses, however, indicate that the Clitellata is nested within the paraphyletic Polychaeta and that several groups previously regarded as separate phyla (Echiura, Sipuncula, Orthonectida, Pogonophora, and Vestimentifera) are actually members of the Annelida.
Several Ediacaran fossils, including Yilingia and cloudinids, have been tentatively interpreted as annelids. However, the widely accepted annelid fossils are sipunculans and polychaetes reported exclusively from Cambrian Burgess Shale-type fossil assemblages (<521 Ma), preserved as flattened macrofossils. Thus, the research team decided to explore Orsten-type fossil localities in the earliest Cambrian Period to fill a fossil gap and to complement the lack of earliest Cambrian Burgess Shale-type fossil localities.
The research team discovered seven phosphatized and millimeter-sized specimens (Fig. 1) from the early Fortunian Kuanchuanpu Formation (ca. 535 Ma) of China. These specimens are preserved as endocasts of the trunk parts, replicating the space surrounded by the integument and lacking information on their heads and tails as well as their integumental structures. The trunk is segmented, and each segment has a pair of lateral or ventrolateral appendages. Appendages may be shorter or longer than width of corresponding segment. Accordingly, two new genera and species are established, i.e., Kuanchuanpivermis brevicruris (Fig. 1A–C; Movie 1) and Zhangjiagoivermis longicruris (Fig. 1D–F; Movie 2). Each appendage terminates in a bifurcation into two lobes of equal, subequal, or even unequal size and morphology.
After careful comparison, the researchers rule out possible affiliations of the current fossils with algae, gut (midgut with cecae), lobopodians, tardigrades, onychophorans, and arthropods. Instead, they suggest that they are most likely to be polychaete annelids. The appendages of the current specimens are comparable with biramous parapodia of polychaete annelids. In particular, the appendages of Zhangjiagoivermis longicruris show striking similarities with the parapodia of living tomopterids (Fig. 2). Accordingly, the two distal lobes on the appendages are comparable with notopodium and neuropodium.
Trunk segments and biramous parapodia may have been present prior to the last common ancestor of living annelid worms. Therefore, Kuanchuanpivermis brevicruris and Zhangjiagoivermis longicruris are interpreted as annelids.
Kuanchuanpivermis brevicruris has relatively short appendages and may be benthic, as modern nereids. Zhangjiagoivermis longicruris has relatively long appendages similar to modern tomopterids (Fig. 2) and may be pelagic. If so, Zhangjiagoivermis longicruris represents the earliest known pelagic annelid. However, they probably moved much slower than modern nereids and tomopterids, given their small body size and the low Reynolds numbers of the surrounding water.
This study reports annelid body fossils for the first time from Cambrian Orsten-type fossil localities. It indicates that early annelids are polychaetes and thus supports that polychaete morphologies are primitive among annelids. It shows that early members of annelids had evolved benthic and pelagic lifestyles in the early Fortunian, extending the fossil record of pelagic annelids to ca. 535 Ma. It supports the phylogenetic analyses that resolve polychaetes as a paraphyletic group and implies that total-group annelids may have had an evolutionary history prior to the Cambrian explosion.
This research was jointly funded by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the National Science Foundation of the United States.
Reference: Xian, X., Zhang, H.*, Xiao, S.*, Waloszek, D., Maas, A., Duan, B., 2026. Polychaete annelids from the earliest Cambrian Period. Proceedings of the National Academy of Sciences of the United States of America. https://doi.org/10.1073/pnas.2538071123.
Fig. 1. Cambrian Fortunian annelids. A–C, Kuanchuanpivermis brevicruris, holotype; B–D, Zhangjiagoivermis longicruris, holotype. (credit to Huaqiao Zhang, NIGPAS)
Fig. 2. Comparison between Zhangjiagoivermis longicruris and Tomopteris. A–D, F, Zhangjiagoivermis longicruris, holotype; E, G, Tomopteris. (credit to Huaqiao Zhang, NIGPAS)
Movie 1. Kuanchuanpivermis brevicruris, holotype. (credit to Huaqiao Zhang, NIGPAS)
Movie 2. Zhangjiagoivermis longicruris, holotype. (credit to Huaqiao Zhang, NIGPAS)
Download：