Maeandrograptus schmalenseei and its bearing on the origin of the diplograptids

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Abstract
The earliest diplograptids [Ordovician] were apparently monophyletic and are characterized by a steptoblastic development and strongly overlapping, markedly sigmoidal thecae. They may be grouped as the genus Undolograptus Boucek 1973. The biramous, reclined M. 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 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 in the form of the distal thecae.sbd.evolutionary changes which occurred in other graptolite groups.

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