Poracanthodes punctatus, BROTZEN, 1934

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 : 5

PORACANTHODES PUNCTATUS BROTZEN, 1934

( FIG. 2A–2G ; TABLE 1 ; SUPPL. 1, FIGS. 1 ?, 2-18)

Poracanthodes punctatus Brotzen 1934 , 43, pl. 3 figs. 1, 8.

“scales ... were assigned by Parkes (1995) to P. punctatus ” in part; Burrow et al. 1999, 355, 358.

“ P. punctatus ... scales ... figured ... are from P. canadensis ” in part; Burrow 2003a, 490.

Type material —The syntype scales MB.f.11979a and b figured by Brotzen (1934, pl. 3 figs. 1, 8), were from an erratic limestone boulder Bey. 36 in northern Germany. Gross (1947, 1971) used Brotzen’s material in his redescription of P. punctatus and other taxa, and captioned scale MB.f.11979b ( Gross 1947, text-fig. 11A) as the holotype, now designated as the lectotype by Burrow & Märss (2022).

Referred specimens — Poracanthodes punctatus scales are found at most levels from 395’–513’ (120.4– 156.4 m) in the BC II section ( Table 1 ), and include UCR 930-1, -2, -5, -6 (all level 395’= 120.4 m), and AMF 97948, 97950 (level 402’= 122.5 m), figured by Parkes (1995, fig. 32.5, 32.6, 32.9, 32.10): Roberts Mountains Formation.

Description —These scales are the most common form in the lower levels of the section. They have a relatively smooth crown with a shallow sulcus leading back from the anteromedian corner, and close-set evenly spaced pores along the margin between growth zones paralleling the posterolateral edges of the crown ( Fig. 2A, 2D–2F ). Ridges are absent or only very weakly developed along the anterior margin of the crown ( Fig. 2A ). Each side of the undersurface of the posterior crown on most scales has three large openings to the pore canal system at the crown–neck junction ( Fig. 2A–2C, 2F, 2G ). The area of the neck below the openings has strong buttresses aligned with the openings, separated by deep grooves. A strong rim separates the neck and base, which is convex and of a similar height to the neck, with the deepest area usually forward of the centre of the scale and with the front of the base protruding in front of the anterior edge of the crown.

Despite the poor preservation and extensive hyphal borings through the scales, the scale crowns are observed to have the pore canal layout typical of Poracanthodes punctatus , with c. six radial pore canals interconnected by arcade canals, from which short canals rise up to the pore openings aligned along the growth zones on the crown.

Comparison —These scales conform to the morphology of the lectotype scale ( Brotzen 1934, pl. 3.1; see Burrow and Märss 2022) from an earliest Devonian (Lochkovian) erratic boulder (Bey. 36) of the north German lowlands. Gross (1971) noted that the scales from this level were morphologically the same as those from the Beyrichienkalk and the Kaugatuma and Ohesaare Beds (Přidolí, Upper Silurian) of the Baltic countries, but the latter differed in having many more radial canals (4–10) in their internal pore canal system, and should probably be assigned to a new species – to date, this has not been done. The BC II scales resemble the older forms, but with no morphological features to distinguish them (other than the number of large canal openings under the posterior crown), we prefer to keep the present nomenclature, whilst recognizing that the number of radial canals decreases between the upper Přidolí and the lowermost Devonian. Poracanthodes punctatus is a zonal vertebrate for the Baltic in the upper Přidolí ( Märss et al. 1995; Märss and Mannik 2013), although its range is greater in shallow shelf than in deep shelf environments ( Märss 1997, fig. 6). As well as the Baltic region and the north German lowlands, P. punctatus is recorded from the Přidolí of the Barlow Inlet Formation ( Märss et al. 1998) and Cape Phillips Formation (Burrow et al. 1999) in the Arctic Canadian Archipelago, upper Přidolí erratics of the northern Netherlands ( Vergoossen 1999a), uppermost Přidolí and lowermost Lochkovian of the Welsh Borderlands ( Vergoossen 2000, table 1, Turner et al. 2017), Ludlow–Přidolí of October Revolution Island and Komsomolets Island and Lochkovian–Pragian of Severnaya Zemlya ( Valiukevičius 2003a), and the S–D boundary beds in the Eastport Formation, Maine ( Turner and Burrow 2018). It has also been recorded from another area in the Roberts Mountains Formation, in the Přidolí of Pete Hanson Creek ( Burrow 2003a).

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.