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| ?Carcinosoma aurorae Van Roy, Richards & Ortega-Hernández, 2025 |
Abstract
Eurypterida were a diverse clade of aquatic euchelicerates that occupied environments ranging from freshwater to fully marine and included several of the largest euarthropods on record. Although a Middle Ordovician megalograptid hitherto represented the oldest evidence of this clade, its phylogenetic position suggested an earlier history for the origin and main diversification within Eurypterida. Here, we report unequivocal eurypterid fragments from the Early Ordovician Fezouata Biota of Morocco, pre-dating the previously oldest record of this group by 12–15 million years. We describe ?Carcinosoma aurorae n. sp. based on several distinctively spinose isolated appendages diagnostic of the eurypterine clade Carcinosomatidae. This discovery demonstrates that the major morphological and ecological diversifications within Eurypterida between swimming Eurypterina and benthic crawling Stylonurina had taken place by the Early Ordovician. Furthermore, the derived phylogenetic position of carcinosomatids implies that most eurypterine clades had already diversified by that time. A cuticle patch with dense scales, reminiscent of pterygotids, likely belongs to a second eurypterid species. The remarkable diversity of euchelicerates in the Fezouata Biota indicates undocumented Cambrian origins and provides further evidence for an early eurypterid radiation centred off Gondwana. Significantly, the sister-group relationship between Eurypterida and Arachnida entails equally early arachnid origins.
Keywords: Arachnida, Cambrian explosion, Carcinosomatidae, Chelicerata, Eurypterida, Fezouata Biota, Great Ordovician Biodiversification Event, Konservat-Lagerstätte, Pterygotidae, Tremadocian
Euarthropoda Lankester, 1904
Euchelicerata Weygoldt & Paulus, 1979
Prosomapoda Lamsdell, 2013
Planaterga Lamsdell, 2013
Dekatriata Lamsdell, 2013
Eurypterida Huxley, 1857
Eurypterina Novojilov, 1962
Diploperculata Lamsdell Hoşgör & Selden, 2013
Carcinosomatoidea Størmer, 1934
Carcinosomatidae Størmer, 1934
Genus: ?Carcinosoma Claypole, 1890
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| Isolated appendages of ?Carcinosoma aurorae n. sp. from the Early Ordovician (upper Tremadocian) Fezouata Biota of Morocco. (A–D) Holotype NM S 5974 consisting of a walking limb, gnathobase and four fragmentary tergites, part. (A) Overview, dry. (B) Overview, under a mixture of ethanol and propanol. (C) Details of the limb and gnathobase, dry. (D) Details of the limb and gnathobase, under a mixture of ethanol and propanol. (E,F) Paratype MCZ.IP. 202866, consisting of nearly complete walking limb, part. (E) Dry. (F) Under demineralized water. All scale bars represent 10 mm. |
?Carcinosoma aurorae n. sp.
Derivation of name. Genitive, from Latin, aurora, -ae, meaning ‘of the dawn’, referring to the early occurrence of this animal, making it the oldest eurypterid currently on record.
Diagnosis. Species of ?Carcinosoma with articulating spine pairs of unequal length on podomeres of limbs II–V , with positions of long and short spines alternating between podomeres. Gnathobases with long and slender teeth.
Peter Van Roy, Jared C. Richards and Javier Ortega-Hernández. 2025. Early Ordovician Sea Scorpions from Morocco suggest Cambrian Origins and main Diversification of Eurypterida. Proc. R. Soc. B. 292: 20252061. DOI: doi.org/10.1098/rspb.2025.2061 [12 November 2025]
