Traces of marine nematodes from 470 million years old Early Ordovician rocks in China

Cylindrical, mostly horizontal, burrows of 20-60 μm diam. and sinusoidal course, found in the middle part of the Early Ordovician (early Floian) Fenxiang Formation in the Hubei Province of China, represent the oldest record of activity by marine nematodes, preceding known nematode body fossils by 70 million years. The burrows are filled with secondarily oxidised pyrite framboids and clay mineral flakes, indicating low oxygen content in the mud and proving that the animals lined their burrows with organic matter, being bacteriovores and mud-eaters. The marine bottom environment enabling such a mode of life originated no earlier than the mid Early Cambrian (approximately 535 million years ago) owing to peristaltic bioturbation, mostly by nemathelminthans of priapulid affinities. Before the so-called ‘Agricultural Revolution’, the bottoms of shallow seas were covered with microbial mats preventing within-sediment animal life. This event imposes the lower time limit on the possible date of origin of nematodes.