Barlowodus 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 : 24-25

BARLOWODUS SP. CF. B. TRIDENS

( FIG. 7 P- S; TABLE 1 ; SUPPL. 1, FIG. 4 )

Referred specimens—one scale from BC II level 430.5’ (131.2 m); one from level 527.25’ (160.7 m); one scale AMF 97874 from BC II level 875’ (266.7 m); this last specimen is missing: Roberts Mountains Formation.

Description and comparison—A scale figured by Parkes (1995, fig. 22.15, 16, captioned as?nikoliviid gen. et sp. indet.; Fig. 7P, Q ) is c. 0.4 mm long with a broken posterior crown, which rises quite steeply posteriad. The dorsal crown is elongated and subdivided into three with a raised central section with two high flat-topped ridges with a deep central groove anteriorly that flattens posteriad. Laterally there are expanding lappets,one anteriorly seen to the left, also with a bifurcate dorsal ridge that extends about one-third of the crown, and on the left a series of two broken and one complete ridge that extends about two-thirds of crown length. The posterior points of the lappets tend to be rounded but are also broken. A flattened lateral ridge then extends from the base of the lappets towards the broken posterior tip. The ventral crown is covered with an ultrasculpture of fine parallel striae extending from the anterior crown/neck interface to the posterior tips of the lateral lappets and posterior section of the broken crown. The neck itself is relatively shallow and unornamented. The base is oval, extending about one-quarter of the estimated length of the scale. It is relatively mature with a thick torus surrounding the large central oval pulp opening and with a slight anterior thickening. One small, rounded scale has a strong tripartite crown with deep grooves between the high flat ridges. It is most like the simpler cephalopectoral or trunk of B. tridens (e.g., Märss et al. 2006, text-fig. 65) from the Přidolí Barlow Inlet Formation of Cornwallis Island. The scale from level 527.25’ is tricuspid with clear striated ridges on the sides of the cusps ( Fig. 7R, S ).

Remarks— Märss et al. (2002, 2006) discovered the thelodont genus Barlowodus in the southern section eastern Cornwallis Island at Read Bay (RBBI and RBBI*), which became the stratotype for the Přidolí Barlow Inlet Formation that is formed from argillaceous carbonate marine rocks. At 59.0 m from only one sample, Märss et al. (2002) identified three taxa, Barlowodus tridens , B. excelsus , and B. floralis , the latter doubtfully. Barlowodus tridens sensu stricto is only known elsewhere from Cornwallis Island. However, scales possibly from this, as well as the other two species of this genus, were recorded from the late Přidolí Barlow Inlet Formation by Märss et al. (2006, pp. 124-126).

The fine microornament on the scale figured by Parkes (1995, fig. 22.15,16; Fig. 7P, Q ) is similar to that in other Devonian taxa such as Neoturinia spp., Barlowodus florialis and even Boothialepis thorsteinssoni ( Märss et al. 2006, pl. 16). Such fine ridging on the scales might have contributed to drag reduction (e.g., Reif 1985). It is unusual in the Nevadan case being on the ventral surface of the crown; we tentatively refer it to B. sp. cf. tridens . However, it is at a level much higher than the other thelodont scales, in the lower to middle Lochkovian.

Figures

Figure 4. Nostovicina and Nostolepis sp. acanthodian scales from the Siluro-Devonian boundary levels of the Birch Creek II section. A. Nostovicina sp. cf. N. elegans: UCR 10755-1 (level 470.2’) in crown view (from Supplement 1, fig. 13). B–D, J. Nostovicina laticristata: B. UCR 10768-10 (level 518.3') in crown view; C, D. UCR 10768-6 (level 518.3') in anterocrown and crown views; J. UCR 10768-11 (level 518.3') horizontal thin section through crown. E–I, K, L. Nostovicina sp. cf. N. guangxiensis; E. UCR 5443-1 (level 562.6') in anterocrown view; F, G. UCR 5443-3 (level 562.6') in crown and lateral views; H. UCR 5443-4 (level 562.6') in posterocrown view; I. UCR 5443-6 (level 562.6') in crown view; K, L. UCR 5443-7 (level 562.6') vertical transverse section, magnified view of crown structure in L. M–P. Nostovicina sp. cf. N. lacrima: M. UCR 5439-8 in lateral view; N. UCR 5439-6 in posterior view; O. UCR 5439-4 (level 549') in lateral view; P. UCR 10768-1 (level 518.3') in anterior view. Q–T. Nostolepis matukhini: Q, R. UCR 5443-2 (level 562.6') in crown and laterocrown views; S, T. UCR 5443-5 (level 562.6') in crown and anterocrown views. Abbreviations: cgz=crown growth zones; h=hyphal borings; Sf=Sharpey’s fiber bundles. Scale bars=0.1 mm, anterior of scale is to right or up, otherwise arrows point 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.