The genus was named byLouis Agassiz in 1843.[1] It was formerly regarded as belonging to theClimatiformes but recently reassigned to theDiplacanthiformes, in which it is united with, amongst others,Rhadinacanthus,Uraniacanthus, andCulmacanthus. Diplacanthiforms were widespread during the Middle and earlyLate Devonian. They are best represented in theMiddle Devonian, by articulated fossils, fin spines, and abundant scales, the latter particularly from northern Europe.[2]
In a latest revision of the genusDiplacanthus, a large number of species from Europe were synonymized with earlier Scottish species, and these too were redefined.D. crassisimus was taken to have precedence overD. striatus as the name of the type specimen.Diplacanthus longispinus was reassigned toRhadinacanthus longispinus, within which were also includedDiplacanthus horridus (Woodward, 1892) andDiplacanthus ellsi.Diplacanthus tenuistriatus andDiplacanthus kleesmentae were however retained. Non-Laurussian species such asDiplacanthus acus from South Africa were not considered in this review.[2]
Diplacanthus acus is described from a near complete whole-bodied impression discovered in 1999 during roadworks cutting theWaterloo Farm lagerstätte in South Africa.[3] The type specimen ofDiplacanthus acus is approximately 100 mm long and has exceptionally long and thin ribbed spines. The intermediate spines are, conversely, extremely reduced.[4] Unusually it preserves complete outlines of many of the fins.
Diplacanthus is most commonly associated with deposits traditionally interpreted as fresh water. However, Waterloo Farm is interpreted asestuarine in origin, as is the CanadianMiguashaia lagerstätte from which two species ofDiplacanthus have been described.[5] The description ofDiplacanthus acus provided the first record of a diplacanthid from theFamennian, with diplacanthids having previously been thought to have gone extinct by the end of theFrasnian.[6]
^Gess, Robert W. and Whitfield Alan K. (2020). "Estuarine fish and tetrapod evolution: Insights from a Late Devonian (Famennian) Gondwanan estuarine lake and a southern African Holocene equivalent". Biological Reviews. doi:10.1111/brv.12590.PMID32059074
^Gess, R. and Coates M. (2008). Vertebrate Diversity of the Late Devonian (Famennian) Deposit near Grahamstown, South Africa. Journal of Vertebrate Paleontology: 28 (3) conference supplement p 83.
^Janvier, P.(1996). Early Vertebrates. Oxford University Press, Oxford.
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