Carole Jan Burrow

The distinctive isolated elements of Cynopodius , characterised by their deep narrow base and spatulate crown, were first described (as spine-like elements) by Ramsay Heatley Traquair in the late 19th Century, based on specimens from the Burghlee Ironstone (Serpukhovian, Carboniferous) at Loanhead near Edinburgh, Scotland. The type species Cynopodius crenulatus is also known from older Calciferous Sandstone Measures (Viséan) of Fife. Here we provide evidence that the elements are teeth rather than spines or dermal claspers, as hypothesised elsewhere. Thin sections of the teeth, illustrated for the first time, show osteodentine and tubate dentine forming the crown, covered by a thin outer hypermineralised layer on the cusps, and trabecular osteodentine or acellular bone forming the base. In recent decades, teeth of Cynopodius have also been collected from the lower Carboniferous Sainte Genevieve Formation (Mississippian: Viséan) of Iowa, Kentucky, and West Virginia in the USA. We assign these teeth from Iowa to Cynopodius robustus n. sp. They are distinguished from the type species by their relative robustness, with a ratio of maximum crown length to root length of c. 2:3 for C. traquairi and c. 1:2 for C. crenulatus , and lack of longitudinal curvature. The Scottish and American occurrences, though widely separated in the early Carboniferous, are the only known localities for the genus. The similarity between these over 300-million-year-old teeth and those of Recent long-toothed teleost reef fishes like Ctenochaetus , for instance, suggests that the Cynopodius animals might also have been specialised detritivores.

Devonian strata in eastern Canada, to the south of the Restigouche River in New Brunswick and north of Baie des Chaleurs in the Gaspé Peninsula, Quebec, are renowned for their vertebrate fossil faunas. Of the stem chondrichthyans, the taxa originally named as Ctenacanthus latispinosus Whiteaves, 1881, Diplodus problematicus Woodward, 1892, Gyracanthus incurvus Traquair, 1890, and Protodus jexi Woodward, 1892 have been reviewed and reassessed recently. In this work, we reassess ? Cheiracanthus costellatus Traquair, 1893 and Acanthodes semistriatus Woodward, 1892, long known from the Atholville beds, and also found in Gaspé. Specimens from the latter sites comprise fin spines and scales, and the material from the Atholville beds comprises the articulated holotypes for each species plus isolated spines and scales. We reassign ? Cheiracanthus costellatus to the diplacanthid Milesacanthus Young and Burrow, 2004 based on the distinctive structure of the fin spines, and scale morphology and histology. This represents the first record of Milesacanthus from the Americas. Acanthodes semistriatus was reassigned to Mesacanthus Traquair, 1888 in the early 1900s. We reaffirm this assignment based on the morphology and histology of dermal elements, and scapulocoracoid shape. We also describe rarer material from “nostolepid” and undetermined acanthodians, and reassess ? Ctenacanthus ? ornatus from Gaspé.

When first described based on isolated scales, Ligulalepis was assigned to thePalaeoniscoidea, a basal group of actinopterygians (ray-finned fishes). Recent cladisticanalyses, mainly based on skull and neurocranial characters, have mostly recovered thetaxon (or, ‘Ligulalepis’) as a stem osteichthyan. Here we present information on Ligulalepisdermal elements other than scales and skulls, that include a cheek fragment, a premaxilla,other marginal jaw elements and teeth, an accessory vomer, a partial shoulder girdle,incomplete spine-like elements, and a gular plate. The shoulder girdle and premaxilla compareclosely with those of basal actinopterygians, whereas the spine-like element showssome similarity to the distal end of the spines on medial dorsal plates of the Chinese LateSilurian stem osteichthyans Guiyu and Sparalepis, or alternatively to fin rays on the stemosteichthyan Dialipina. One of the jaw elements appears to be a compound jugal plate pluspart of the dentate maxilla, an arrangement not previously known in any Devonian stemosteichthyan, or actinopterygian. Histological structure of dermal plates somewhat resemblesthat of Meemannia, but pore openings in Ligulalepis lead only to the vascular canalnetwork at the base of the ornament layer and not to a pore canal network. Like previousphylogenetic analyses, our analysis incorporating post-cranial dermal skeleton charactersalso recovered Ligulalepis as a stem osteichthyan.

The Early Devonian (Emsian) vertebrate fauna of the Gaspé Peninsula, Quebec (Canada) shows close affinity with the Emsian fauna from the Atholville beds, New Brunswick (Canada). Specimens collected in the early 2000s from several localities in central Gaspé include molds of tooth whorls and isolated teeth that we assign to the stem chondrichthyan Doliodus. Isolated teeth of this taxon were first described from the Atholville beds in the late 1800s by Arthur Smith Woodward, who erected a new species Diplodus problematicus. Whereas the Atholville beds teeth are usually preserved individually or in tooth families, and as hard tissue, the Gaspé teeth are mostly preserved as molds of tooth whorls, with individual teeth sharing a thin bone base. Thin sections of Doliodus teeth from the Atholville beds show parallel close-set vascular canals extending above the base plate and through tooth bases, with teeth formed of osteodentine and meso- or orthodentine. We consider that all the known dental elements assigned to Doliodus are conspecific with the articulated Doliodus specimen and thus all are assigned to the same species D. latispinosus. The teeth are compared with the diplodont teeth of other stem chondrichthyan taxa, in particular the Omalodontiformes.

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.