Gomphonchus undetermined

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 : 12

GOMPHONCHUS SP. AFF. G. MEDIOCOSTATUS

( Fig. 3W–3Z ; Table 1 ; Suppl. 1, figs. 20, 21)

Referred specimens —Rare scales assigned to this species occur in a very restricted range, at levels 526’ (160.3 m), 527.25’ (160.7 m), and possibly 527’ (160.6 m), and include UCR 944-6, -8, -17, -19 (level 527.25’= 160.7 m): Roberts Mountains Formation.

Description —Scales from BC II with crowns resembling the type scales of this species have a marked median sulcus extending over half the length of the crown, usually edged by sharp ridges. Smaller scales have only these two ridges ( Fig. 3W, X ), larger scales have oblique lateral ridges directed towards the median ridges ( Fig. 3Y, Z ). Most scales have a short- to medium-height neck. The base is strongly convex. Histological structure was not investigated, due to the lack of material.

Comparison —The type scales of Gomphonchus mediocostatus are from an upper Přidolí East Baltic-derived erratic limestone; the species is also recorded from Lithuania, Timan–Pechora (Valiukevičius 2005), Latvia ( Valiukevičius 2004b), the Welsh Borderlands ( Vergoossen 1999a) and Novaya Zemlya ( Burrow et al. 2018). In Latvia, Valiukevičius (2004b) nominated G. mediocostatus as the acanthodian index species for his second highest fine-scale biostratigraphic interval in the Přidolí. A crown ornament similar to that of G. mediocostatus has been illustrated in a number of other taxa, including Pechoralepis zinaidae ( Valiukevičius 2003b, fig. 3K) from the Lochkovian of Timan–Pechora, and one of the morphotypes of the older taxon Gomphonchus ? turnerae from the Ludlow of north Queensland, Australia ( Burrow and Simpson 1995, fig. 2A–C). The type scales of G. mediocostatus all have deep necks with relatively shallow bases, thus differing from the BC II scales with their short necks and deeply convex bases.

Figures

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