Ancient sea spider settles modern-day debate

25 October 2004

A British sea spider fossil from 425 million years ago has helped its modern-day descendents claim their rightful place on the tree of life, according to research conducted by Dr Derek Siveter and Dr Mark Sutton of the University Museum of Natural History and Department of Earth Sciences with colleagues from Yale and the University of Leicester.

For two centuries the relationship of sea spiders to other arthropods has been in debate, partly because the delicate form of sea spiders means they do not fossilise easily, and so they have a very poor fossil record. What’s more, their unusual anatomy, including a long proboscis and specialised egg-carrying legs, has made it difficult to work out who their closest living relatives are.

Just a few confirmed fossil pyconogid (sea spider) species have previously been found globally. The new species, christened Haliestes dasos, was discovered at a site in Herefordshire that yields exceptionally well preserved three-dimensional fossils and is providing unrivalled insights into the palaeobiology of a variety of invertebrates. The research team – whose results were published this month in Nature – digitally reconstructed the 3.5mm long invertebrate from the impression it left in the 425 million-year-old volcanic rock in which it was preserved. In the process they created a picture of an adult sea spider that is some 35 million years older than any previously discovered.

The species was found to have the distinctive characteristics of a sea spider, but it was also found to have the prominent pincers that classify it as a member of the chelicerate group of arthropods. ‘The ancient sea spider’s large pincers place the sea spider group with the chelicerates, a large grouping which includes the true spiders as well as scorpions, mites, ticks and horseshoe crabs,’ said Dr Siveter. ‘Despite its ancient, Silurian age the new species appears to have lived in a very similar way to modern ones.’