Radioporacanthodes scheii, (SPJELDNAES, 1967)

Burrow, Carole Jan, Murphy, Michael & Turner, Susan, 2023, Late Silurian to earliest Devonian vertebrate biostratigraphy of the Birch Creek II section, Roberts Mountains, Nevada, U. S. A., PaleoBios 40 (4), pp. 1-32 : 8

RADIOPORACANTHODES SCHEII ( SPJELDNAES, 1967)

( FIG. 2 CC–EE; TABLE 1 ; SUPPL. 1, FIGS. 2 , 3 , 5 , 9 –13)

Ischnacanthus? Scheii Spjeldnaes 1967 , 807, figs. 1–3, pl. 1.

Radioporacanthodes scheii ; Burrow 2013, 947, figs. 2-6.

Diagnosis —see Burrow (2013).

Type material — The holotype is fin spine CMN55213 , in a block with other fin spines, jaw fragments and scales considered to be conspecific; from the northwest side of Irene Bay , Ellesmere Island , arctic Canada, Devon Island Formation (Přidolí? or early Lochkovian) .

Referred specimens —Rare scales recognized at levels 395’ (120.4 m), 424.9’ (129.5 m), 430.5’ (131.2 m), 456.5′–471′ (139.1-143.6 m) in the BC II section, including UCR 10750-1, -5 (level 456.5’= 139.1 m): Roberts Mountains Formation.

Description —These scales are characterised by regularly spaced short sharp ridges along the anterior edge of the crown, without a median sulcus. Pore openings are relatively small, with a maximum of six subparallel rows running back along the posterior half of the crown ( Fig. 2 CC-EE). The undersurface of the posterior crown has the large canal openings characteristic of this family (not illustrated). The small pores on the crown surface are not always visible with a light microscope, and some scale varieties lack pores.

Comparison — Burrow (2013) reassigned this species from “ Ischnacanthus ?” to Radioporacanthodes based on presence of a pore canal system in the crown of most scales, while also recognising that some variants lack a pore canal system. The BC II scales show some similarity with those of Poracanthodes canadensis in having many short ridges along the anterior edge of the crown, but differ in having a few small pores in subparallel rows on the posterior crown, rather than rows of many pores paralleling the posterolateral crown edges. As noted above, some scales also lack a pore canal system, making it difficult to distinguish these from scales of Gomphonchus volborthi ( Rohon, 1893) , a species resurrected by Vergoossen (1999c), and Gomphonchus sandelensis scales with weakly developed anterior crown ridges. G. volborthi usually has rounded weak anterior ridges, and is probably restricted to older strata ( Vergoossen 1999c). The scales differ from the Gomphonchoporus hoppei variants, which lack a pore canal system in having a smoothly curving anterior crown margin rather than a relatively sharp mid-margin angle ( Vergoossen 1999a, figs. 41–45, 55, 56).

Figures

Figure 2. Poracanthodid acanthodian scales from Upper Silurian levels of the Birch Creek II section, SEM images. A–G. Poracanthodes punctatus: A, B. UCR 930-2 (level 395') in lateral and posterobasal views; C. UCR 930-6 (level 395') in posterobasal view; D, E. UCR 930-5 (level 395') in crown and posterocrown views; F, G. UCR 930-1 (level 395') in lateral and basal views. H–P,?Q–V. Poracanthodes canadensis: H, I. UCR 930-4 (level 395') in crown view; J, K. UCR 10746-6 (level 430.5') in crown view plus closeup of crown pores; L–N. UCR 10750-6 (level 456.5') in anterior and crown views, and closeup of pores near hyphal borings; O. UCR10746-5 (level 430.5') in laterocrown view; P. UCR 540-1 (level 510’) in crown view (from Supplement 1, fig. 17); Q–S. UCR 10768-3 (level 518.3') in crown, anterocrown views, and closeup of posterior crown surface; T. UCR 930-8 (level 395'), horizontal thin section through crown; U,V. UCR 10746-12 (level 430.5'), vertical thin section through anterior half of crown; W–BB. Radioporacanthodes porosus: W, X. UCR 10750-7 (level 456.5') in crown and anterocrown views; Y, Z. UCR 10746-4 (level 430.5') in anterocrown and lateral views; AA. UCR 10746-9 (level 430.5') in crown view; BB. UCR 10750-11 (level 456.5'), horizontal thin section through crown, showing two radial pore canals, viewed with Nomarski optics. CC–EE. Radioporacanthodes scheii: CC. UCR 10750-5 (level 456.5') in crown view; DD, EE. UCR 10750- 1 (level 456.5') in crown view plus closeup of pores and hyphal borings. Abbreviations: arc=arcade canals; b=scale base; c=scale crown; h=hyphal borings; n=neck; p=pore; pc=pore canal; pco=pore canal opening under posterior crown; pr=pore canal opening row on crown surface; rc=radial canal. Scale bars=0.1 mm, anterior of scale is to left or down, otherwise arrows point to anterior. See Table 1 for meterage.

Figure 3. Poracanthodid and ischnacanthid acanthodian scales from Upper Silurian levels of the Birch Creek II section. A–E,?F,?G. Gomphonchoporus hoppei: A. UCR 949-1 (level 567.3') in crown view (from Supplement 1, fig. 24); B. UCR 944-11 (level 527.25') in laterocrown view; C, D. UCR 5439-2 (level 549') in anterocrown and laterocrown views; E. UCR 5439-3 (level 549') in basal view; F. UCR 5439-7 (level 549') in crown view; G. UCR 5439-1 (level 549') in anterocrown view. H–J. Zemlyacanthus menneri: UCR 10815-1, 2, 3 (level 556.5'), in crown views (from Supplement 1, fig. 22). M–P. Trundlelepis cervicostulata: K, L. UCR 10768-4 (level 518.3') in anterior and lateral views; M. UCR 944-17 (level 527.25') in crown view; N. UCR 944-3 (level 527.25') in crown view; O. UCR 944-23 (level 527.25'), vertical longitudinal thin section; P. UCR 944-22 (level 527.25'), horizontal thin section through crown. Q, R. Poracanthodidae gen., sp. indet. sensory line scale UCR 930-3 (level 395') in crown and posterior views. S-V. Gomphonchus sandelensis: S, T. UCR 10746-11 (level 430.5') in crown and lateral views; U,V. UCR 930-7 (level 395') in antero- and posterocrown views. W–Z. Gomphonchus mediocostatus: W. UCR 944-6 (level 527.25') in crown view; X. UCR 944-16 (level 527.25') in crown view; Y. UCR 942-1 (level 526') in crown view (from Supplement 1, fig. 20); Z. UCR 944-19 (level 527.25') in laterocrown view. Abbreviations: cgz=crown growth zones; dt=dentine tubules; h=hyphal borings; vc=vertical canals. Scale bars=0.1 mm, anterior of scale

Figure 5. Funicristata nevadaensis n. gen. n. sp. from the uppermost Silurian of the Birch Creek II section. A–C. UCR 10750-3 (level 456.5') in crown views and magnified view of the crown ridges; D, E. UCR 10750-2 (level 456.5') in crown and lateral views; F, G. UCR 10746-10 (level 430.5') in anterocrown and crown views; H, I. UCR 10746-8 (level 430.5') in anterocrown and laterocrown views; J, K. UCR 10750-10 (level 456.5') vertical longitudinal section; L. UCR 10750-12 (level 456.5') horizontal section through crown. Abbreviations: c=canal; en=enameloid; h=hyphal boring; po=pore openings. Scale bars=0.1 mm in A, B, D–L, 0.01 mm in C; anterior is to left or down, otherwise arrows point to anterior. See Table 1 for meterage.

Figure 9. Paleogeographic reconstruction for the late Silurian, base map from Scotese (2014, map 73), showing possible vertebrate dispersal route from the Baltic to the circum-Arctic and thence south to western Laurentia.