Lanarkopterus was long seen as a species of the closely related Mixopterus, though more complete specimens discovered in the 1960s determined that it differed in several aspects, enough to warrant a separate genus.

[1] Specimens of Lanarkopterus were first noted by Peach and Horne (1899) in the Ludlowian fish beds of the Lesmahagow and Hagshaw Hills Silurian inliers in Ayrshire and Lanarkshire, Scotland.

Fragments of the genus, then referred to Eurypterus, were recovered from many localities that also yielded Silurian vertebrates such as Birkenia, Lasanius, Thelodus, Lanarkia and Atleleaspis.

An almost complete, but weathered specimen (T. 3357) from the collection of the Geological Survey of Scotland, in Edinburgh, was taken as the holotype of Mixopterus dolichoschelus.

Ritchie (1968)[1] presented evidence that suggested that M. dolichoschelus was not as closely related to the other species of Mixopterus as previously thought and placed it in its own genus within the Mixopteridae, Lanarkopterus.