Polychaete annelids from the earliest Cambrian Period

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.