A study of Jurassic sea spiders offers insight into their evolutionary history

Palaeopycnogonides gracilis fossil (Photo courtesy Dr Romain Sabroux)
Palaeopycnogonides gracilis fossil (Photo courtesy Dr Romain Sabroux)

An article published in the journal “Papers in Palaeontology” reports the results of an examination of rare fossils of so-called sea spiders dating back to about 160 million years ago, during the Jurassic period. A team of researchers examined these fossils of marine arthropods which technically belong to the class of Pycnogonids (Pycnogonida). Fossils of these arthropods are rare and among them are some of the so-called fauna of La Voulte-sur-Rhône, a deposit in southwestern France known for its excellent conservation and abundance of fossils. This study led to the identification of three species of sea spiders which were named Palaeopycnogonides gracilis, Colossopantopodus boissinensis, and Palaeoendeis elmii. Their resemblance to the current species of Pycnogonids leads to the conclusion that their diversification began right in the Jurassic.

Studies of sea spiders are limited and in several cases, concern the examples of polar gigantism, a phenomenon linked to the characteristics of those seas, discovered in Arctic and Antarctic waters. However, Dr. Romain Sabroux of the British University of Bristol, lead author of this new study, explained that they’re interesting to understand arthropod evolution because the fossils indicate they appeared quite early in the history of this phylum.

Fossils of sea spiders are rare but those available date from several periods. The ones discovered in the site of La Voulte-sur-Rhône have a conservation that in some cases is excellent for a total of 14 specimens. The problem here was to examine fossils that were only partially visible because various parts were hidden in the rock. For this reason, Dr. Romain Sabroux’s team set up advanced examinations using an imaging technique called Reflectance Transformation Imaging (RTI) to interpret the exposed parts of the fossils and an X-ray microtomography to probe into hidden details.

The results allowed to obtain new information on the morphological characteristics of the fossils and to compare them with those of today’s sea spider species. In the past, exams that could not benefit from modern technologies suggested that these Jurassic species were closely related to present-day families. The new study confirmed that hypothesis by identifying three species and their relations.

The species named Colossopantopodus boissinensis was assigned to the Colossendeidae family. The species Palaeoendeis elmii was assigned to the Endeidae family. The species Palaeopycnogonides gracilis is the only one that was assigned to no family of modern sea spiders and appears to belong to an extinct taxonomic family. According to the researchers, these findings indicate that the diversification within the sea spider group that led to today’s species began in the Jurassic.

Actually, previous fossils show that a diversification had already occurred in the Paleozoic era but the scarcity of fossils makes it difficult to reconstruct their history. Mass extinctions or isolated extinctions may have profoundly marked the history of Pycnogonids. For this reason, the researchers intend to continue their studies focusing this time on fossils found among the so-called fauna of Hunsrück Slate, in Germany, dating back to about 400 million years ago.

An in-depth study of sea spiders from the Devonian period could help resolve classification problems that still lead to discussions because it’s difficult to understand the exact relationships within the arthropod phylum. Generally, Pycnogonids are considered a class within the subphylum Chelicerata but according to various scientists, they have anatomical and physiological characteristics that make them different from other arthropods and therefore they should constitute a separate subphylum. The use of modern technologies helped to better understand the relationships between Jurassic and modern sea spiders and could offer new information on relationships with other arthropods as well.

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