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Polychaete annelids from the earliest Cambrian Period.

Xiaofeng Xian1, Huaqiao Zhang1, Shuhai Xiao2

  • 1State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China.

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Early Cambrian fossils reveal the earliest annelids (ringed worms) with polychaete-like body plans. These 3D microfossils show segmented bodies and paired appendages, indicating early divergence and ecological specialization.

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Area of Science:

  • Paleontology
  • Evolutionary Biology
  • Marine Biology

Background:

  • Annelid (ringed worm) body fossils are known from the Cambrian Period, typically as flattened specimens.
  • Previous discoveries were limited to specific preservation types (e.g., Burgess Shale-type Lagerstätten).
  • The early evolution and body plan diversification of annelids remain incompletely understood.

Purpose of the Study:

  • To report and describe novel phosphatized microfossils from the earliest Cambrian (Fortunian Age).
  • To interpret these microfossils as early polychaete annelids preserved in three-dimensional Orsten-type preservation.
  • To investigate the early diversification and body plan evolution of annelids.

Main Methods:

  • Analysis of millimetric, three-dimensional endocast microfossils from the Zhangjiagou Lagerstätte, South China.
  • Detailed morphological examination of trunk segmentation and lateral outgrowths (parapodia).
  • Comparative analysis with extant and fossil annelids to infer phylogenetic relationships and lifestyles.

Main Results:

  • Discovery of two new species: *Kuanchuanpivermis brevicruris* gen. et sp. nov. and *Zhangjiagoivermis longicruris* gen. et sp. nov.
  • Fossils exhibit clear segmentation and paired lateral outgrowths resembling annelid parapodia with notopodia and neuropodia.
  • *K. brevicruris* possesses short appendages (suggesting benthic lifestyle), while *Z. longicruris* has long appendages (suggesting pelagic lifestyle).

Conclusions:

  • Annelids acquired a polychaete-like body plan by the early Fortunian Age (ca. 535 Ma).
  • Early annelids had already diverged and differentiated into forms with distinct parapodial structures and lifestyles.
  • These findings represent the earliest known semi-pelagic annelid and provide crucial insights into early animal evolution.