Re-evaluation of two Early Pennsylvanian (Middle Namurian) ammonoids and their bearing on mid-Carboniferous correlations

Abstract

Reticuloceras (Swintoceras) n. subgen. unites an early group of the mid-Carboniferous goniatite family Reticulocertidae, in which reticulate sculpture is weakly developed or lacking and the typically reticuloceratid suture has slightly expanded ventral prongs. The type species, Reticuloceras (S.) spiraloides (Bisat and Hudson, 1943), is a rare, poorly known species from the British Namurian. Two additional species, R. (S.) wainwrighti (Quinn, 1966) and R. (S.) tiro (Gordon, 1969), are common in the basal type Morrowan Series, Lower Pennsylvanian, of Arkansas. Swintoceras occurs with two distinctive, late forms of the ammonoid, Hudsonoceras: Hd. ornatum (Foord and Crick, 1897) in Britain and Hd. moorei Quinn and Saunders, 1968, in Arkansas. The cooccurrence of these taxa correlates the basal Morrowan in its type region with the Namurian Reticuloceras nodosum (R1b) Zone of Britain and thereby also dates the close of the Mid-Carboniferous Eustatic Event in the North American Midcontinent.