Paleontologists have examined a collection of 160-million-year-old sea spider fossils from Southern France. The rare specimens show that the diversity of sea spiders that still exist today had already started to form by the Jurassic period.
Sea spiders (Pycnogonida) are an enigmatic class of living marine arthropods.
Their body plan includes a proboscis; internal organs such as the digestive tract and gonads extend into the legs; median eyes are carried on an ocular tubercle while lateral eyes are absent; and earliest life stages typically take the form of a protonymphon larva.
Sea spiders are morpho-anatomically unique among arthropods and in the past this has led to their interpretation as the sister group of all other living arthropods.
But phylogenetic studies now confidently support a position of pycnogonids within the subphylum Chelicerata, as sister to the Euchelicerata, showing that the body plan of sea spiders has undergone radical transformations from the last common ancestor of the crown group Arthropoda.
This evolutionary transformation cannot be understood solely through the study of living sea spiders given that they largely share the same body plan.
Fossils are integral to elucidating the evolutionary origin of the sea spider’s morpho-anatomy, as well as in calibrating its evolution against geological time.
This has been attempted on a few occasions, but the known fossil record of sea spiders is too poorly characterized to obtain material insights into their evolution.
“Sea spiders are a group of marine animals that is overall very poorly studied,” said Dr. Romain Sabroux, a paleontologist at the University of Bristol.
“However, they are very interesting to understand the evolution of arthropods — the group that includes insects, arachnids, crustaceans, centipedes and millipedes — as they appeared relatively early in the arthropod tree of life. That’s why we are interested in their evolution.”
“Sea spider fossils are very…
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