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| Mieridduryn bonniae Pates, Botting, Muir & Wolfe, 2022 |
Abstract
A crucial step in the evolution of Euarthropoda (chelicerates, myriapods, pancrustaceans) was the transition between fossil groups that possessed frontal appendages innervated by the first segment of the brain (protocerebrum), and living groups with a protocerebral labrum and paired appendages innervated by the second brain segment (deutocerebrum). Appendage homologies between the groups are controversial. Here we describe two specimens of opabiniid-like euarthropods, each bearing an anterior proboscis (a fused protocerebral appendage), from the Middle Ordovician Castle Bank Biota, Wales, UK. Phylogenetic analyses support a paraphyletic grade of stem-group euarthropods with fused protocerebral appendages and a posterior-facing mouth, as in the iconic Cambrian panarthropod Opabinia. These results suggest that the labrum may have reduced from an already-fused proboscis, rather than a pair of arthropodized appendages. If some shared features between the Castle Bank specimens and radiodonts are considered convergent rather than homologous, phylogenetic analyses retrieve them as opabiniids, substantially extending the geographic and temporal range of Opabiniidae.
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| Mieridduryn bonniae nov. gen. et sp. from the Castle Bank Biota (NMW.2021.3 G.7). |
Systematic palaeontology
Superphylum PANARTHROPODA Nielsen, 199538
Genus Mieridduryn nov.
Etymology: From Welsh mieri (bramble) and duryn (proboscis, snout), meaning “bramble-snout”. The dd is pronounced as a soft th, and results from mutation following a feminine noun. Gender f.
Diagnosis: Panarthropod with head region bearing dorsal sclerite, annulated proboscis with spiniform dorsal projections and radial mouthparts composed of small, sclerotized plates; gut trace leading to posterior-facing mouth; trunk bears large subrectangular dorsolateral flaps with rounded distal margins; dorsolateral flaps bear setal structures on surface facing body midline; annulated lobopods display triangular outline and possess short triangular spines on posterior margin.
Mieridduryn bonniae nov. gen. et sp.
Etymology: After Bonnie Douel, niece of the site owners and fossil devotee; the family has followed and supported the research extensively since the discovery of the fauna.
Holotype: NMW.2021.3 G.7 known from part and counterpart. Counterpart preserves anterior portion only.
Locality and horizon: Collected from the Darriwilian (Middle Ordovician, Didymograptus murchisoni Biozone) Gilwern Volcanic Formation at Castle Bank, near Llandrindod, Powys (UK)
Material, locality, and horizon: NMW.2021.3 G.8, known from part and counterpart. Collected from the Darriwilian (Middle Ordovician, Didymograptus murchisoni Biozone) Gilwern Volcanic Formation at Castle Bank, near Llandrindod, Powys (UK)
Stephen Pates, Joseph P. Botting, Lucy A. Muir and Joanna M. Wolfe. 2022. Ordovician Opabiniid-like Animals and the Role of the Proboscis in Euarthropod Head Evolution. Nature Communications. 13: 6969. DOI: 10.1038/s41467-022-34204-w
