Maeandrograptus schmalenseei and its bearing on the origin of the diplograptids

The earliest diplograptids were apparently monophyletic and are characterized by a streptoblastic development and strongly overlapping, markedly sigmoidal thecae. They may be grouped as the genus Undulograptus Bou¯ek 1973. The biramous, reclined Maeandrograprus schmalenseei Moberg 1892 of the upper Arenig Didymograptus hirundo Zone has a striking and detailed resemblance to these early diplograptids in the form of its proximal thecae, the style and degree of prothecal folding, and in the symmetry, form and orientation of the sicula and early thecae. Its appearance coincides with or slightly predates the appearance of diplograptids in the D. hirundo Zone. It is proposed as a representative of the group of biramous graptolites which were directly ancestral to the diplograptids. Derivation of the diplograptids from such a source would have involved some changes in the proximal thecal development, in the attitude of stipes and also in the form of the distal thecae - evolutionary changes which are known to have occurred in other graptolite groups.