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Middle Devonian acanthodians from Belarus - new data and interregional biostratigraphy

Plax Dmitry P., Newman Michael

Acta Geologica Polonica

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2021

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Vol. 71, no. 4

393--414

EN

The Lower Devonian (Emsian) and Middle Devonian of Belarus contain assemblages of biostratigraphically useful faunal and floral microremains. Surface deposits are few, with most material being derived from borehole cores. Acanthodian scales are particularly numerous and comparison with scales from other regions of the Old Red Sandstone continent (Laurussia), specifically the Orcadian Basin of Scotland, the Baltic Region, Spitsbergen, and Severnaya Zemlya have demonstrated a lot of synonymy of acanthodian species between these areas. This is especially the case between Belarus, the Orcadian Basin and the Baltic Region, which has allowed us to produce an interregional biostratigraphic scheme, as well as to postulate marine connection routes between these areas. The acanthodian biostratigraphy of Belarus is particularly important as it is associated with spores and marine invertebrates, so giving the potential of more detailed correlations across not only the Old Red Sandstone continent, but elsewhere in the Devonian world. We also demonstrate that differences in preservation (e.g., wear and how articulated a specimen is) is one of the main reasons for synonymy.

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The Early Devonian ischnacanthiform acanthodian Ischnacanthus gracilis (Egerton, 1861) from the Midland Valley of Scotland

Burrow C. J., Newman M., den Blaauwen J., Jones R., Davidson R.

Acta Geologica Polonica

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2018

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Vol. 68, no. 3

335--362

EN

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.

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Acanthodians from the Lower Devonian (Emsian) ‘Placoderm Sandstone’, Holy Cross Mountains, Poland

Burrow C. J., Szrek P.

Acta Geologica Polonica

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2018

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Vol. 68, no. 3

307--320

EN

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.