Ischnacanthus gracilis (Egerton, 1861), the only ischnacanthiform acanthodian from the Lochkovian Lower

Old Red Sandstone of Scotland, is known from hundreds of specimens in institutional collections worldwide.

Despite this relative abundance, morphology and histology of its skeletal elements have rarely been investigated.

Surface details of spines, dental elements, and scales are often not visible in specimens because they are

usually split through the middle. We have examined a broad size range of fish, from 35 mm to 250 mm long.

Several intact (not split) specimens have been collected in recent years and acid-prepared to show fine details of

the dermal and dental elements. We have also used scanning electron microscopy of scales, jaws and dental elements,

denticles and fin spines, and serial thin sectioning of articulated specimens, to document their structure.

Some of our notable observations include: identification of ventral lateral lines, double-layered subtessellate

calcified cartilage forming the jaws, and the probable occurrence of extraoral tricuspid denticles on the jaws

of most fish. Examination of the size range, body proportions and dentition of institutional specimens gives no

support for recognising more than one species in the Midland Valley localities.

The Lower Devonian ‘Placoderm Sandstone’ in the Holy Cross Mountains (HCM) is filled with abundant impressions

of disarticulated vertebrate remains. The only acanthodian macroremains named to date are fin spines

of Machaeracanthus polonicus Gürich. Fin spine impressions in slabs from the Winna Formation (Emsian) at

Podłazie Hill (near Daleszyce) in the southern HCM, and also the Barcza Formation (?Lochkovian) at Barcza

Quarry, Miedziana Góra Conglomerate (?Lochkovian), Gruchawka, and Zagórze Formation (middle–upper

Emsian) at Bukowa Mountain in the northern HCM, reposited in the University of Warsaw, Polish Geological

Institute-National Research Institute, Warsaw, and Natural History Museum, London collections, have been cast

and studied in order to better document this poorly known taxon. As noted in other Machaeracanthus species,

we have found that M. polonicus has two different morphotypes of spines, which abut lengthwise to form a pair

of spines. Our investigations show that the fin spine assemblage includes Onchus overathensis as well as M.

polonicus, and probably another undetermined acanthodian. The affinities of O. overathensis are reassessed.

It is here considered to be a diplacanthiform, and reassigned to the genus Striacanthus, as S. overathensis.

Acanthodian scapulocoracoids have also been identified, as well as tightly spiralled toothwhorls which could

be from an acanthodian.