Late Silurian to earliest Devonian vertebrate biostratigraphy of the Birch Creek II section, Roberts Mountains, Nevada, U.S.A.

Abstract

Intensive sampling of uppermost Silurian to lowermost Devonian levels of the marine sequence of the Roberts Mountains Formation exposed in the Birch Creek II section, Nevada has yielded assemblages of vertebrate microremains dominated by acanthodian scales. Taxonomic assessment of the vertebrates represented shows a suite and succession of taxa most similar to those recorded from contemporary circum-Arctic assemblages, with scales of the poracanthodid acanthodians Poracanthodes punctatus, P. canadensis, Radioporacanthodes porosus and ischnacanthid Gomphonchus sandelensis most abundant below the Silurian–Devonian (S–D) boundary (as previously determined by conodont and graptolite occurrences), and stem chondrichthyan ‘acanthodians’ Nostovicina laticristata and other Nostovicina spp. (order, family indet.) the most abundant above the boundary. Just one endemic acanthodian taxon, Funicristata nevadaensis nov. gen. nov. sp., was found, in the uppermost Silurian. Agnathan vertebrate remains are rare relative to acanthodian scales, with dermal fragments of heterostracans at fewer than 10 levels and scales of thelodonts from 12 levels scattered throughout the whole section. Thelodonts Apalolepis, Barlowodus spp., ?Boreania, Gonioporus alatus, Lanarkia, Loganellia, Nikolivia, Talivalia, Thelodus, ?Turinia sp. and Trimerolepis spp. are found below the purported S–D boundary. Rare thelodont scales including Boreania sp. cf. minima, N. gutta and a single scale of ?Amaltheolepis sp. are found above the S–D boundary level. The only identifiable gnathostome remains of groups other than acanthodians are from the putative osteichthyan Lophosteus sp. and chondrichthyans Ellesmereia schultzei and Polymerolepis sp., in the uppermost Silurian levels.