ACANTHODII Owen, 1846

Newman, Michael J., Davidson, Robert G., Blaauwen, Jan L. Den & Burrow, Carole J., 2012, The Early Devonian Acanthodian Uraniacanthus curtus (Powrie, 1870) n. comb. from the Midland Valley of Scotland, Geodiversitas 34 (4), pp. 739-759 : 742

Class ACANTHODII Owen, 1846 Order DIPLACANTHIFORMES Berg, 1940

REVISED DIAGNOSIS. — Acanthodians with a short mouth and cheek region (<¹⁄₅ body length), and a deep body (> ⅓ body length); inserted portion of the median fin spines with narrow,closely spaced parallel ridges;anterior dorsal fin spine with long inserted portion and fin supported by large basals with short radials; pair of admedian spines; paired pinnal plates with or without spines, or prepectoral spines without plates; scapulocoracoid with a prominent ridge separating the postbranchial lamina from the posterior flange; large postorbital plate; circumorbital ring of small plates plus large anterior and/or posterior plates; plate-like hyoidean gill covers; no true teeth; ossified dental plates on lower jaws;scales with an acellular bone base pierced by branching canals of Williamson, and long branching ascending vascular and dentine canals in the crown (i.e. Diplacanthus - type histological structure sensu Valiukevičius 1995).

REMARKS

As noted by Young & Burrow (2004), few characters have been recognised as uniquely diagnostic for the order. Hanke & Davis (2008) assigned Gladiobranchus probaton to the suborder Diplacanthoidei Miles, 1966 within the order Climatiiformes Berg, 1940 but did not revise the last published diagnosis by Denison (1979), who assigned all diplacanthiforms/diplacanthoids to the family Diplacanthidae (also within the Climatiiformes ). Only a few of the characters listed by Denison are probably diagnostic: lack of teeth, and scale structure ( Diplacanthus - type sensu Valiukevičius 1995).We propose two previously unrecognised diagnostic characters, one of which is a smooth ossified prearticular plate on the Meckel’s cartilage. Before Hanke & Davis’s (2008) description of the lower jaw ossifications in Uraniacanthus probaton n. comb., smooth bones in the lower jaw of many other diplacanthiforms were interpreted as mandibular splints, homologous to the bones reinforcing the ventral margins of the Meckel’s cartilages in acanthodiforms. The latter bones wrap around the posterior end of the cartilage and taper to a point anteriorly.In diplacanthiforms other than Uraniacanthus , however, the bones in the lower jaw are blade-like (e.g., Milesacanthus antarctica Young& Burrow, 2004 ) or spatulate (e.g., Diplacanthus horridus Woodward, 1892 and Diplacanthus ellsi Gagnier, 1996 [ Gagnier 1996: figs1, 7]). Ossified dental plates have also been recognised by the authors in the type species Diplacanthus crassisimus Duff, 1842 (e.g., NMS G. 1892.8.5 from Gamrie, Banffshire, Scotland), and Diplacanthus longispinus Agassiz,1844 (e.g., NMS G.1891.92.338 from Gamrie; Fig. 2A, B ). Hanke & Davis’s (2008) description of deep thin ossified plates on the lower jaws in U.probaton n. comb., without similar plates on the occluding upper jaws, encouraged our reinterpretation of the bones associated with lower jaws in other diplacanthiforms as homologous to those of U. probaton n. comb. The other recently recognized diagnostic character for the diplacanthiforms is the narrow, closely spaced parallel ridges on the deep bases of insertion on the median fin spines ( Burrow 2007: 835), seen for example on the posterior dorsal fin spine of Diplacanthus longispinus ( Fig. 2C ).

Figures

FIG. 2. — Diplacanthus longispinus Agassiz, 1844, NMS G.1891.92.338, a complete specimen from Gamrie, Banffshire, Scotland: A, detail of the head showing the position of the jaw plates; B, close up of the tooth plate positioned on the upper surface of the lower jaw; C, detail of the posterior dorsal fin spine showing the deep base of insertion with fine, parallel ridges Scale bars:A, 10 mm; B, 1 mm; C, 5 mm.