Gomphonchus sandelensis, (PANDER, 1856)

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 SANDELENSIS ( PANDER, 1856)

( FIG. 3 S-V; TABLE 1 ; SUPPL. 1, FIGS. 1–3 , 11, 12, 13)

Gomphodus sandelensis Pander 1856 , 76, pl. 6 figs. 15-17.

Gomphonchus sandelensis Gross 1971 , 53, figs. 19E–G, 20, 21A, 21B, 22A–D, pl. 5.11, 9.15–9.17.

Diagnosis —Relatively large Gomphonchus scales with a smooth crown that extends beyond the base all round, and a high neck with vertical ribbing posteriorly. Scales have durodentine superficially in some of the youngest crown growth lamellae; they lack radial vascular canals.

Type material —The syntype scales from the Ohesaare Beds, Ohesaare Cliffs ( Pander 1856, pl. 6 figs, 15, 16; Ohesaare Formation) and Sandla ( Pander 1856, pl. 6 fig. 17; lower Kaugatuma or upper Kuressaare Stage), Saaremaa Island, Estonia (Late Silurian, Přidolí) have been long lost.

Referred specimens —Scales assigned to this species are rare compared to the number of poracanthodid scales, occurring at various levels from 395’–545’ (120.4- 166.1 m), including UCR 930-7 (level 395’= 120.4 m) and UCR 10746-11 (level 430.5’= 131.2 m); AMF 97957, 97958 (level 513’= 156.4 m), figured by Parkes (1995, fig. 33.1–33.4) and thin sections AMF 97994, 97995 (level 513’= 156.4 m), figured by Parkes (1995, fig. 42.1, 42.2): Roberts Mountains Formation.

Description —Scales have a smooth flat crown, sometimes with a shallow median sulcus ( Fig. 3S, T ) or very weak short ridges along the anterior margin ( Fig. 3U, V ); the crown is usually wider and longer than the base. The scales can be difficult to distinguish from those Poracanthodes punctatus scales, which also have a smooth flat crown, with pore openings often only detectable in SEM images.

Figures

Figure 1. Birch Creek II section, central Roberts Mountains, Nevada. A. locality map, topographic map showing locations of sections BC II, BC III, WC I, WC IV after Murphy (2016, appendix 1). B. stratigraphic column through the uppermost Silurian– lowermost Devonian (after Klapper and Murphy 1975, fig. 5; scale on left is in feet; X denotes their proposed Silurian–Devo- nian boundary at approx. 567’=172.8 m level); to the right, stratigraphically significant conodont ranges in red, graptolites in blue (see Murphy 2016, fig. 2).

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