RADIOPORACANTHODES POROSUS (BROTZEN, 1934)

(Fig. 2W –AA,?2BB; Table 1; Suppl. 1, figs. 3–13)

Poracanthodes porosus Brotzen 1934, 44, pl. 3 fig. 2.

Diagnosis —see Vergoossen (1999a).

Type material —The holotype scale MB.f.11989a (Brotzen 1934, pl. 3 fig. 2, Gross 1947, text-fig. 11M) is from an erratic limestone boulder in northern Germany (Early Devonian, Lochkovian).

Referred specimens —Scales probably occur at most levels from 395.01′–491.3’=M48A (120.4–149.7 m), and possibly up to 510.90′ (155.7 m) in the BC II section (Table 1), and include UCR 10750-7 (456.5’= 139.1 m), UCR 10746-3, -4, -9 (430.5’= 131.2 m) and thin section UCR 10750-11 (456.5’= 139.1 m): Roberts Mountains Formation.

Description —The most common scale form of this species in the BC II section has a flat relatively smooth crown with a shallow median sulcus and up to six low weak ridges on the anterior margin (Fig. 2W–Y). These scales are smaller on average than those of Poracanthodes, being less than 0.3 mm wide and long. Pore openings in the posterior half of the crown surface are round, or short slits, and arranged in two or four subparallel rows leading back from the widest area of the crown to its posterolateral edges, although on most scales the posterior end of the crown has broken off (Fig. 2Y, BB). A horizontal thin section of the crown of one of the smaller scales (Fig. 2 AA) shows two radial canals of the pore canal system, without any interconnecting arcade canals. The posterior crown is missing on one scale (Fig. 2 BB), but the anterior crown and exposed canals in the posterior fracture surface indicate that this is also probably R. porosus.

Comparison —Scales of Radioporacanthodes porosus sensu stricto (i.e. conforming to the type illustrated by Brotzen 1934, pl. 3.2) can be distinguished morphologically from other poracanthodid taxa by their median sulcus on the anterior rim of the crown, their lack of, or only weakly developed, anterior crown ridges, and the subparallel to radial alignment of large pores or small slits on the posterior half of the crown. The taxon is widely distributed, being recorded in the Lower Devonian erratic boulders of the north German lowlands, Přidolí–Emsian of the Baltic countries (Valiukevičius 2000), upper Přidolí–lowermost Lochkovian of the Welsh Borderlands (Vergoossen 2000, pl. 3.28, 3.29, Turner et al. 2017), Lochkovian Cookeys Plains Formation, New South Wales, Australia (Burrow 2003b), Lochkovian Borschchov and Ivane Regional Stages, Ukraine (Plax 2011) and possibly the lowermost Devonian of Tewo and Zoige counties, West Qinling Mountains, China (Wang et al. 1998: Poracanthodes cf. P. porosus and Poracanthodes zoigensis). The scale varieties with much more ornate crowns from the Baltic and central Urals that Märss (1986, 1997) and Valiukevičius (1998) assigned to Poracanthodes porosus have subsequently been reassigned to Poracanthodes? lehmani Vergoossen (1999b), and Radioporacanthodes biblicus (Lehman 1937) by Vergoossen (2002a); neither of these species are found in the BC II section.