Euthacanthus Powrie, 1864

Newman, Michael J., Burrow, Carole J., Den Blaauwen, Jan L. & Davidson, Robert G., 2014, The Early Devonian acanthodian Euthacanthus macnicoli Powrie, 1864 from the Midland Valley of Scotland, Geodiversitas 36 (3), pp. 321-348 : 325-326

Genus Euthacanthus Powrie, 1864

TYPE SPECIES. — Euthacanthus macnicoli Powrie, 1864 by original designation.

INCLUDED SPECIES. — Euthacanthus macnicoli Powrie, 1864 ; E. gracilis Powrie, 1870 .

GEOLOGIC AND GEOGRAPHIC DISTRIBUTION. — Early Devonian (Early Lochkovian) of the Midland Valley of Scotland.

REVISED DIAGNOSIS. — Medium-sized, fusiform fish with maximum length to depth ratio5:1; jaws formed of calcified cartilage; slender closeset branchiostegal plates; fin spines relatively slender and straight with smooth longitudinal ridges separated by deep smooth grooves; four or five pairs of prepelvic spines; posterior dorsal fin spine longer than anterior dorsal fin spine; scales ornamented with subparallel rounded ridges leading back from the anterior edge of the crown, with a slightly convex base.

REMARKS

When Powrie (1864) first described Euthacanthus macnicoli he had only one complete specimen at his disposal, the holotype NMS G.1891.92.231 from Turin Hill (although he mentioned no locality at this time). Powrie (1864) also stated that he had disarticulated remains which indicated that the species grew quite large and that some of the remains showed that another species was present. Later, Powrie (1870) stated that his first specimens of E. macnicoli came from Farnell and Tealing, though no remains from these localities in his collection can be said with confidence to belong to this species. Articulated Euthacanthus specimens from these localities are now assigned to E. gracilis ( Newman et al. 2011) . Powrie (1864) believed that all the fish beds at the different localities constituted one continuous lake deposit, hence he assigned any disarticulated remains to his first named species, E. macnicoli . However, Armstrong & Paterson (1970) demonstrated that the fossil fish localities are at various stratigraphic horizons.

Powrie (1870) stated that he had two reasonably complete specimens of E. macnicoli and that these came from Turin Hill. In the same publication, he raised another species, Euthacanthus grandis , which he said only occurred at Turin Hill. Powrie (1870) differentiated E. grandis from E. macnicoli by the relative size of the fin spines, with the former having shorter fin spines relative to body size. However, the specimens Powrie (1870) assigned to E. grandis were much larger than the specimens he assigned to E. macnicoli . Very few studies have been made in acanthodian species that compare the ratio of fin spine length to total length of the fish. As far as we can determine, Acanthodes Agassiz, 1833 and Lodeacanthus Upeniece, 1996 are the only genera in which such work has been undertaken. Zidek (1976: table 2C-E; 1985: table 1d-g) showed that in Acanthodes bridgei Zidek, 1976 and also A. lundi Zidek, 1980 , the ratio of fin spine length to total specimen length showed no allometry, i.e. the ratio was approximately the same in fish of all sizes. In contrast, Upeniece (2011: table 5.26) showed that relative fin spine lengths in Lodeacanthus vary remarkably during ontogeny, but the variation between spine length and fish length appears random not allometric ( Upeniece 2011: fig. 5.4). In at least some of the species of LORS acanthodians including E. macnicoli , the fin spines grew at a slower rate than the body, so that proportionally, larger specimens of a given species have smaller fin spines. This phenomenon was first recognised, as far as we know, by W. Graham-Smith (unpublished correspondence held in the Perth Museum and Art Gallery, Scotland) and confirmed by the authors in many species. The allometric growth of the fin spines relative to body length in E. macnicoli (using the posterior dorsal fin spine, which is most often the best preserved spine) is illustrated in Fig. 2 .

Powrie (1870) also differentiated between the two species by E. grandis having larger scales with more grooves and ridges. Because acanthodian “onionskin-type growth” scales continued to increase in size while the fish itself grew (e.g., Zidek 1976, 1985), it is hardly surprising to see larger scales in larger individuals. Like Acanthodes fin spines, and unlike Euthacanthus fin spines, “scale size relative to the total specimen size is remarkably similar in most… Acanthodes species” ( Zidek 1985: 164). We consider E. grandis to be an advanced growth stage of E. macnicoli , and can recognize no specific differences between the two forms.

The head and branchial region is not well preserved (if at all) in specimens of Euthacanthus gracilis . However, NMS G.1891.92.243 ( Newman et al. 2011: fig. 7A) shows several subparallel thick rods behind the jaws that are most likely to be branchiostegal plates; this specimen also has displaced sclerotic bones preserved above the jaws. We have thus revised the family diagnosis.

Figures

FIG. 2. — Euthacanthus macnicoli Powrie, 1864: comparison of length of fish compared with length of posterior dorsal fin spine, for 12 near complete specimens. Symbols: •, pdfs length (mm); ¢, fish length (mm).