Two new graptolites from the early Silurian (Llandovery: Aeronian) of central Wales: an origin for monoclimacid thecal morphology

Abstract

AbstractMonograptus chrysalissp. nov. from themagnusZone (Aeronian) of central Wales shows a previously unrecognized combination of thecal types. Distally, the thecae possess marked lappets and short lateral horns, and approximate to those of therevolutusgroup of monograptids and some described examples ofPribylograptus. Proximally, the thecae are elongate and slender, and possess slit-like apertures below a distinct geniculum. They resemble the proximal thecae ofMonograptus imagosp. nov., one of the earliest recognized British monoclimacid monograptids, from the succeedingleptothecaZone of central Wales and the Lake District. It is suggested thatM. chrysalisarose from a graptolite of therevolutusgroup by retraction or suppression of the hooked proximal thecae, while the distal thecae remained constant, and thatM. imagothen evolved fromM. chrysalisby a spreading of the proximal end characteristics distally. The proximal morphology and overall record of the monoclimacids, though, suggests that they may be polyphyletic; in this paper, the generic nameMonoclimacisis reserved for the late Telychian graptolites of thevomerinagroup.