Ancient fossil gives clues to spider past

A team of scientists, including Yale geology professor Derek Briggs, has digitally reconstructed the fossil of Haliestes dasos, a 425 million-year-old “marine soothsayer with a hairy back-side.”

This newly discovered sea spider species was uncovered in Herefordshire, England. It has provided insight which may end the centuries-old debate among scientists concerning the relationship of modern sea spiders to chelicerates, a subphylum of arthropods that includes “true” spiders, scorpions, mites, ticks and horseshoe crabs.

Pycnogonids, commonly known as sea spiders, are soft-bodied marine arthropods found all over the world. Scientists are familiar with over a thousand species of modern sea spiders. However, because of their unique body structure and lack of significant fossil record, they remain difficult to categorize within the diverse arthropod phylum, which includes insects, crustaceans and arachnids.

Sea spiders feed with a unique straw-like organ composed of three fused, elongated plates, and their legs are disproportionately large in comparison to their small bodies.

“Pycnogonids are frankly a bit weird,” said Mark Sutton, a team member from the Department of Earth Sciences at Oxford University. “They do have pincers, but are otherwise rather unlike spiders, scorpions et cetera. Whether or not they are related to these ‘true’ chelicerates has long been a matter of debate.”

According to scientists from Yale, Oxford and the University of Leicester, there are competing theories concerning the phylogenetic position of sea spiders. However, the discovery of Haliestes dasos supports the relationship between sea spiders and chelicerates.

The well-preserved specimen is 35 million years older than any other adult form of sea spider, said Derek Siveter, curator of the geological collections of the Oxford University Museum of Natural History.

“The presence in Haliestes of well-developed pincers argues for the sea-spider group to be placed with the chelicerates,” Siveter said. “Modern sea spiders have pincers, but the earlier we can see this feature in ancient sea spiders, the more certain it is that the sea spiders and the chelicerates have a common ancestry. Thus, sea spiders are not best regarded as a stand-alone group, independent from all the other arthropods.”

The fossil record of ancient sea spiders is extremely sparse due to the delicate nature of the animal, making this discovery exceptionally important. According to scientists, the quality of preservation observed in the Herefordshire specimen is extraordinarily rare, and the factors that led to its preservation are unique. Haliestes dasos lived 100 to 200 meters below the ocean’s surface. Millions of years ago, volcanic ash quickly solidified around this specimen, preserving the animal’s three-dimensional form. Crystalline calcite filled the imprint left by the spider’s body, and round nodules grew around the calcite, maintaining the imprint’s form.

“The nodules have acted as time capsules, preserving the fossil casts almost perfectly for 425 million years,” Sutton said. “This type of fossil is unique. To the best of our knowledge, there is no other site in the world that produces concretions containing three-dimensional calcite molds of soft-bodied animals.”

The digital reconstruction of the fossil was a lengthy process that involved slicing the specimen into portions approximately 30 microns thick. The slices were digitally photographed and then combined into three-dimensional images on a computer.

“These [fossils] are very tiny mineral in-fills in the rock,” Briggs said. “They have to be reconstructed using digital imaging and computer graphics. It takes several months to produce one reconstruction.”

Although the reconstruction process destroyed the fossil, it was necessary because normal paleontological approaches to fossil recovery do not work with the delicate Herefordshire material, Siveter said.

“We didn’t know that it was a sea spider until we digitally reconstructed it as a virtual fossil because initially there was not too much of the fossil visible in the rock,” Siveter said.

The research team has been working with Herefordshire fauna since 1994.

This digital reconstruction of the fossil of Haliestes dasos may provide clues regarding the relationship between today’s sea spiders and chelicerates.
Courtesy OPA
This digital reconstruction of the fossil of Haliestes dasos may provide clues regarding the relationship between today’s sea spiders and chelicerates.

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