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

Monograptus chrysalis sp. nov. from the magnus Zone (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 the revolutus group of monograptids and some described examples of Pribylograptus. Proximally, the thecae are elongate and slender, and possess slit-like apertures below a distinct geniculum. They resemble the proximal thecae of Monograptus imago sp. nov., one of the earliest recognized British monoclimacid monograptids, from the succeeding leptotheca Zone of central Wales and the Lake District. It is suggested that M. chrysalis arose from a graptolite of the revolutus group by retraction or suppression of the hooked proximal thecae, while the distal thecae remained constant, and that M. imago then evolved from M. chrysalis by 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 name Monoclimacis is reserved for the late Telychian graptolites of the vomerina group.