Funicristata

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 : 16-17

FUNICRISTATA NEVADAENSIS N. GEN, N. SP. BURROW, 2023

( FIG. 5 ; TABLE 1 ; SUPPL. 1, FIGS. 2 , 5–7 , 9 –11, 14)

Zoobank LSID — urn:lsid:zoobank.org:act:1BB5B9FC-3E9B-4F4B-B238-A25CA0E9A8D8

Diagnosis —Acanthodian with small scales up to 0.5 mm wide and 0.6 mm long; crown with strong branched ridges with a ‘twisted’ appearance leading back from anterior edge and converging towards posterior corner of crown, or at least extending past lateral corners of crown; neck low or absent anteriorly on most scales, with only small pore openings; base only slightly convex and deepest below anterior margin of crown; crown extends up to a base-length beyond posterior corner of base; four or five superposed crown growth zones with a thin enameloid top; wide-calibre canal extends from primordial crown growth zone to posterior corner of crown, its presence usually marked by longitudinal ridge on crown.

Holotype — Holotype scale UCR 10750-3 ( Fig. 5A–C ); 456.5’ (= 139.1 m) level, BC II section, Roberts Mountains , Eureka county , central Nevada, U.S.A.: Roberts Mountains Formation (Late Silurian, Přidolí) .

Paratypes — Paratypes scales UCR 10750-2, UCR 10746-8, -10, thin sections UCR 10750-10, 12.

Referred specimens —Forty-three scales including the type specimens.

Occurrence —Only known from the type locality at levels 395–492.3’ (120.4–150.1 m).

Etymology — Nevada, for the U.S. state where the type material was found, and ensis, Latin suffix denoting place.

Description —Scales are small, up to 0.5 mm wide, 0.6 mm long and 0.3 mm high. Most scales appear dorsoventrally compressed, with a low or absent neck anteriorly on most scales ( Fig. 5A–E ). The crowns have a distinctive ornament comprising branching ridges with a ‘twisted’ or rope-like appearance leading back from the anterior edge, extending towards the posterior corner of crown ( Fig. 5A–G ), or at least extending past the lateral corners of the crown ( Fig. 5H, I ). The crown extends up to a base-length beyond the posterior corner of the base, with most scales having a marked median ridge running back from the middle of the scale to the posterior tip ( Fig. 5A, E, I ). In many scales, particularly the more ‘flattened’ examples, the crown ornament resembles a branching tree, with the posterior median ridge being the trunk with the radiating, branching ridges extending out towards the anterior edges. The neck has only small pore openings ( Fig. 5I ). The base on all scales is only slightly convex and deepest below the anterior margin of the crown, with a marked rim between the base and neck.

Histological preservation is poor at these levels, with extensive hyphal borings, so that only general features can be discerned. The crown has four or five growth zones with all except the embryonic zone being extremely thin under the crown surface, comprising only enameloid posterior to the apex of the base ( Fig. 5J, L ). A wide calibre canal runs from the embryonic zone up and back through the middle of the posterior crown, aligned with the median ridge on the crown surface, to the posterior corner ( Fig. 5J, K ). Fine branching dentine tubules rise up in the anterior parts of the crown. No bone cell lacunae are detectable in the base.

Comparison —These scales differ from all known taxa, based on their distinctive crown ornament resembling a tree with successively branching ridges. The single large canal extending from the primordial zone back to the posterior corner could indicate that the taxon is a highly derived poracanthodid, but no poracanthodids resemble this species in morphology or position of pore canals.

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 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 6. Chondrichthyan and stem osteichthyan scales from the uppermost Silurian of the Birch Creek II section; A, C–G, light microscope images of whitened specimens; B, ESEM image. A-C. Ellesmereia schultzei. A, scale UCR 10746- 44 (level 430.5’) in crown view (from Suppl. 1, fig. 5); B, UCR 10746-28 (level 430.5’) in anterolateral-crown view; C, scale UCR10746-45 (level 430.5’ in crown and lateral views (from Suppl. 1, fig. 4). D. Polymerolepis sp. scale UCR 10752-1 (level 458.67’) in crown view (from Suppl. 1, fig. 11). E-G. Lophosteus sp. cf. L. superbus. E, dermal bone fragment 10754-1 (level 468’) in external view (from Suppl. 1, fig. 12); F, G, tubercles UCR951-1, 2 (level 457’) in crown views (from Suppl. 1, fig. 10). Scale bars=0.25 mm, anterior of scale is up or to left, otherwise arrow points to anterior. See Table 1 for meterage.

Figure 7. Thelodont scales from the uppermost Silurian of the Birch Creek II section, all level 430.5' except B, C, G, K, L, P, Q. A. Trimerolepis sp. cf. T. tricava. UCR 10746-43 in crown view. B. Trimerolepis sp. AMF97876 (Parkes 1995, fig. 22.19, 20) from level 503’, base view. C.?Lanarkia sp. UCR 944-20 (level 527.25’) in lateral view. D.?Turinia sp. UCR10746-19, showing taphonomic effects and ultrasculpture, in crown view. E-J. Nikolivia spp. E. UCR 10746-13 with cell pattern on surface in crown view; F, UCR 10746-20 in crown view; G, AMF97875 (Parkes 1995, fig. 22.17, 18) from level 402' with possible bite marks, in crown view; H, UCR 10746-27 in ventral basal view; I, UCR 10746-25 showing taphonomic effects in crown view; J, Nikolivia sp. cf. N. auriculata, UCR 10746-18 in crown view. K, L. Apalolepis sp. AMF97872 (level 503’) (Parkes 2005, fig. 22.11, 12), crown and basal views. M-O. Talivalia sp. cf. T. elongata. M, UCR 10746-17 in crown view; N, UCR 10746-37 in crown view; O, UCR 10746-21 in lateral view. P-S. Barlowodus sp. cf. B. tridens. P,Q, AMF97874 (level 875’) in crown and basal views (Parkes 2005, fig. 22.15, 16). R, S, UCR 944-21 (level 527.25') in laterocrown view, closeup in S showing striated ultrasculpture. Scale bars=0.2 mm; anterior to left or top, otherwise arrow points 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.