Oldest relative of ragworms and earthworms discovered

A reconstruction of Magelonidae living inside the sediment on the sea floor 514

A reconstruction of Magelonidae living inside the sediment on the sea floor 514 million years ago Reconstruction by Robert Nicholls

Scientists at the Universities of Oxford, Exeter, Yunnan and Bristol and have discovered the oldest fossil of the group of animals that contains earthworms, leeches, ragworms and lugworms. This discovery pushes the origin of living groups of these worms (polychaetes) back tens of millions of years, demonstrating that they played an important part in the earliest animal ecosystems.

The fossil specimens are approximately 514 million years old and come from eastern Yunnan Province in China, originating from a geological time known as the early Cambrian. These rocks provide crucial fossil evidence of the dawn of early animal life, known as the "Cambrian Explosion". They are extraordinary because they preserve the soft parts of organisms that do not usually survive fossilisation, such as cuticles (skin) and guts as well as limbs and other appendages. Despite the remarkable preservation of fossils in these rocks, annelid worms are very rare and have not previously been discovered there.

In today’s Nature paper , an international team of scientists, co-led by Dr. Luke Parry from the Department of Earth Sciences at Oxford University, describe a previously unknown species called Dannychaeta tucolus. They show that it belongs to a living group of worms called the Magelonidae (shovel-head worms). Unlike other Cambrian polychaete species, this newly-discovered species lived a sedentary lifestyle inside a tube.

Dr Luke Parry said: ’All of the ancient annelids we knew of previously from the Cambrian were likely crawling around on the seafloor, and what we see in Dannychaeta is quite radically different.

’Other Cambrian fossil annelids have brushy bristles flaring off their body. They clearly scuttled around on the seafloor. Until now, annelids with sedentary modes of life (living in protective tubes or hiding in burrows), weren’t known in the fossil record until many millions of years later.’

Dr. Xiaoya Ma from University of Exeter and co-author, said: ’This is the earliest fossil evidence of a sessile annelid, as well as the first appearance of a living group of annelids in fossil record. Considering how rare any annelid fossils are in the early Cambrian period, we are surprised and delighted by this discovery.’

Hong Chen, the first author of the study from Yunnan University, said: ’We were quite surprised to find a polychaete worm from 514 million years ago that lived in a tube, especially as it is so similar to species that are still alive today.’

Living shovel-head worms can be found in the oceans worldwide, including in coastal areas of the United Kingdom.

Dr Luke Parry said: ’The discovery of Dannychaeta tells us that even in very early animal dominated ecosystems, ancient annelid worms were filling many of the same roles that they perform in the ocean today. This includes animals crawling around that are scavengers and predators, as well as much less active forms gathering food from the safety of protective burrows, just like Dannychaeta’.

The new fossil is named in honour of Danny Eibye-Jacobsen from the Natural History Museum of Denmark in recognition of his work on the origin and evolution of annelid worms.

The paper: ’A Cambrian crown annelid reconciles phylogenomics and the fossil record’ is available online at Nature: www.nature.com/artic­les/s41586-020-2384-8

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