SciNews has an article about the discovery of a horseshoe crab fossil in the Silurian.  Horseshoe crabs are known from the late Ordovician, but there was a gap of 80 million years from the Devonian.  This animal, Ciurcalimulus discobolus, lived about 424 million years ago.  It was collected by Samuel J. Ciurca in 1975 from the Kokomo Member of the Wabash Formation in Indiana.  The fossil was described by Dr. James Lamdell in “The first Silurian horseshoe crab reveals details of the xiphosuran ground plan”, which was published in the journal Proceedings of the Royal Society B. In 2020, Mr. Lamdell did a presentation for ESCONI about the Big Hill Lagerstatte known for its Ordovician arthropod fauna.

“Horseshoe crabs (order Xiphosura) are aquatic chelicerate arthropods defined by the fusion of their body segments into a thoracetron,” Dr. James Lamsdell of West Virginia University wrote in his new paper.

“Four living species are known and exhibit a disjunct geographical distribution, with one species occurring in the western Atlantic (ranging from the east coast of Canada to the Gulf of Mexico) and three in the western Pacific and northeast Indian Oceans (extending from the south of Japan to the east coast of India).”

“The group is famous as an example of an evolutionary conservative lineage and is considered to comprise archetypal ‘living fossils,’ although more recent work has demonstrated repeated ecological transitions within the group to be associated with the development of extreme morphologies.”

“Horseshoe crabs have a long evolutionary history stretching back to the Late Ordovician (450 million years ago) with two species described from North America and another, slightly older (Early Ordovician, 480 million years ago) species reported but awaiting formal description from Morocco.”

“The origins and early evolution of horseshoe crabs are poorly known, however, with an 80-million-year gap between these Ordovician species and the first record of Xiphosurida (horseshoe crabs that have reduced their postabdomen to a single segment) in the Late Devonian (370 million years ago).”

“This lack of a fossil record for horseshoe crabs in the Silurian, a time during which other aquatic chelicerate groups were rapidly diversifying, makes it difficult to determine the timing of the origin of xiphosurids and to what extent the end Ordovician mass extinction and Silurian ecosystem recovery influenced horseshoe crab evolution.”