A large arthropod from the Lower Old Red Sandstone (Early Devonian) of Tredomen Quarry, south Wales

: A large arthropod from the Lower Old Red Sandstone of Tredomen Quarry, near Brecon, Powys, southeast Wales is described as Bennettarthra annwnensis gen. et sp. nov. The animal is incomplete, preserved to a length of 30 cm, and displays a trunk of at least ten, non‐diplopodous segments. The trunk may be delineated into two distinct tagmata: the anterior three somites (as preserved in the fossil) appearing ornamented and apparently bearing robust, ornamented, segmented walking legs; the posterior somites appearing relatively smooth, of which at least the anterior five somites bear simple, sclerotized rudimentary appendages. Unfortunately, diagnostic features such as detailed appendage arrangement for the anterior tagma and the head and tail region are largely missing, frustrating detailed classification. However, the delineation of the non‐diplopodous trunk into two tagmata, each displaying apparently distinct appendage morphology, together with the large size of the fossil, is a feature hitherto unknown in any arthropods from terrestrial/freshwater fossil assemblages of this age. In terms of taxonomic placement, based on the currently limited morphological evidence, a malacostracan or hexapodan affinity for Bennettarthra appears most likely. The animal is preserved in laminated sandstone with plant debris, osteostracan fish and abundant thin irregularly shaped carbonaceous sheets. The sandstone was deposited from a river flood that buried and preserved articulated fish and Bennettarthra. These animals were probably buried alive and suffocated in sediment. Trace fossils including Diplichnites trackways and Beaconites burrows occur in close stratigraphic proximity to the flood deposit, and it is possible that this arthropod was the maker of one or both of these traces.