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.
Palaeopycnogonides gracilis. Scale bars – 5 mm. Image credit: Sabroux et al., doi: 10.1002/spp2.1515.
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 rare, but we know a few of them from different periods.”
“One of the most remarkable fauna, by its diversity and its abundance, is the one of La Voulte-sur-Rhône that dates back to the Jurassic, some 160 million years ago.”
Unlike older sea spider fossils, the La Voulte pycnogonids are morphologically similar — but not identical — to living species, and previous studies suggested they could be closely related to living sea spider families.
But these hypotheses were restricted by the limitation of their observation means.
“We used two methods to reinvestigate the morphology of the fossils: X-ray microtomography, to ‘look inside’ the rock, find morphological features hidden inside and reconstruct a 3D model of the fossilized specimen; and reflectance transformation imaging, a picture technique that relies on varied orientation of the light around the fossil to enhance the visibility of inconspicuous features on their surface,” Dr. Sabroux said.
“From these new insights, we drew new morphological information to compare them with extant species.”
This confirmed that these fossils are close relatives to surviving pycnogonids.
Two of these fossils belong to two living pycnogonid families: Colossopantopodus boissinensis was a Colossendeidae while another, Palaeoendeis elmii was an Endeidae.
The third species, Palaeopycnogonides gracilis, seems to belong to a family that has disappeared today.
“Today, by calculating the difference between the DNA sequences of a sample of species, and using DNA evolution models, we are able to estimate the timing of the evolution that bind these species together,” Dr. Sabroux said.
“This is what we call a molecular clock analysis. But quite like a real clock, it needs to be calibrated.”
“Basically, we need to tell the clock: ‘we know that at that time, that group was already there.’ Thanks to our work, we now know that Colossendeidae, and Endeidae were already ’there’ by the Jurassic.”
The results were published in the journal Papers in Palaeontology.
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Romain Sabroux et al. 2023. New insights into the sea spider fauna (Arthropoda, pycnogonida) of La Voulte-sur-Rhône, France (Jurassic, Callovian). Papers in Palaeontology 9 (4): e1515; doi: 10.1002/spp2.1515