The systematic relationship of the monograptid species acinaces Tornquist, 1899 and rheidolensis Jones, 1909

The relationship between the early Silurian (Rhuddanian) monograptid species rheidolensis Jones, 1909 and acinaces Tornquist, 1899 has been contentious, with debate as regards both their possible synonymy and their generic assignation. We have re-examined the type and associated material of rheidolensis, and conclude that this taxon is almost certainly a junior synonym of acinaces. Lacking ventral apertural processes, acinaces does not belong within Lagarograptus, as previously thought, but may be placed within Huttagraptus, as proposed by Koren' & Bjerreskov (1997). The confusion between these two taxa arose because the material that Jones assigned to rheidolensis includes two taxa, one representing non-topotype (and probably mis-localized) specimens of an undoubted Neolagarograptus. This is morphologically very similar to, and probably referable to, the late Aeronian species Neolagarograptus tenuis, the type material of which we show here to be tectonically distorted.