Michael J. Newman

A new species of the antiarch Microbrachius from the Middle Devonian (Givetian) of Estonia

Two Middle Devonian (Givetian) jawless fish, Cornovichthys blaauweni Newman & Trewin, 2001 and Achanarella trewini Newman, 2002 are redescribed based on new material. It can now be demonstrated that C. blaauweni has paired anal fins and a median lobe of the caudal fin. This indicates a close relationship to the Late Devonian (Frasnian) jawless fish Euphanerops longaevus Woodward, 1900. Cornovichthys blaauweni is shown to have possessed a short nasohypophyseal system (olfactory organ) supported by two supporting rings, the anterior of which supported a solitary nasohypophyseal aperture (nostril). Achanarella trewini is shown to be a distinct species to C. blaauweni, which is in the same size range for the largest individuals. Achanarella trewini possessed no anal or dorsal fins with only a caudal fin and possibly paired lateral fin folds. The snout is unusual being place quite far forward of the branchial basket. At the anterior end is a short nasohypophyseal system with a large nasohypophyseal aperture with four barbels surrounding it.

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.

In 1860 Pander described a new species of fish from the Givetian of Scotland using the invalid name Osteolepis microlepidotus , which was a synonym of Osteolepis macrolepidotus . In 1948 Jarvik published a monograph on this fish, giving it the name Osteolepis panderi . However, he did not designate a type specimen or a type locality. This was due to Jarvik's mistake in thinking he had to choose a specimen from one of the specimens used by Pander and being unable to view Pander's collection in Saint Petersburg (then Leningrad) owing to World War II and the subsequent Cold War. Osteolepis microlepidotus first appeared in print in 1829, but Sedgwick and Murchison's description was inadequate for erecting a species. Shortly after, Agassiz described specimens of O. microlepidotus , which were later shown to be specimens of O. macrolepidotus. Jarvik should have chosen one of the specimens he used as he erected the species O. panderi . After a seven-decade delay NMS G.1903.130.35 in the National Museums of Scotland is chosen as the lectotype from the many specimens used by Jarvik in his description of the species. Skinnet Farm Quarry in Caithness, Scotland is also chosen as type locality for this species.

The Middle Devonian acanthodian Orcadacanthus n. gen. from the Orcadian Basin of Scotland

Mesacanthus mitchelli is an important taxon in elucidating relationships amongst stem chondrichthyans, being the best known and most abundant of the oldest known acanthodiform acanthodians. Here we note some newly recognized morphological features and describe the histological structure of the endoskeleton and dermal elements. The jaws are preserved as a single layer of irregularly tessellate bone-like mineralizations. A mandibular bone is not present, contrary to previous descriptions. Fin spines have a wide central cavity that is more than half the width at mid-spine level. Prepelvic spines have a very narrow leading edge ridge and a wide open pulp cavity. Head sensory lines are enclosed by a pair of tesserae with a smooth crown, flat base and concave sides, bordering the sensory line canal. The tiny body scales have a relatively large pulp cavity in the embryonic zone. The large size of the eyes relative to the head and body size in Mesacanthus mitchelli , the structure and small size of the scales relative to those of other acanthodians of a similar size, and the wide central pulp cavity in the spines, all indicate that the species is likely to have undergone paedomorphism.

The ‘acanthodian’ fishes provide key anatomical insights into the deepest branches of the chondrichthyan stem group. We review the anatomy of the acanthodian Vernicomacanthus uncinatus from the Lochkovian (Lower Devonian, 419.2–410.8 Ma) of Scotland based on eight articulated fossils, one of which is newly described. Broadly, the anatomy of V. uncinatus fits with that of contemporaneous acanthodians such as Climatius and Parexus, with a head covered by robust tesserae, an enlarged postorbital scale, an armoured shoulder girdle, and many pairs of ventrolateral spines. However, it departs from this anatomy in key respects. Its pectoral fin spines are obliquely ridged and posteriorly denticulated, similarly to Carboniferous gyracanth stem‐group chondrichthyans. Its scales consist of multiple anteroposteriorly aligned odontodes, similarly to many Palaeozoic ‘sharks’. And its endoskeletal shoulder girdle may have a posterolateral angle, previously observed only in shark‐like chondrichthyans. We propose that the differences between V. uncinatus and its congeneric, V. waynensis, which include potentially phylogenetically significant characters of the shoulder girdle and spines, are sufficient to erect a new genus for V. waynensis: Dobunnacanthus gen. nov. The scales of Vernicomacanthus are identical to those of the ‘shark’ scale genus Altholepis, suggesting that some such scales may instead belong to taxa with acanthodian‐like gross anatomies. Based on these scales we highlight potential patterns in chondrichthyan scale evolution, in particular the axial addition of odontodes. Anatomical similarities between Vernicomacanthus and gyracanths, highlighted by previous authors, may indicate the existence of a grade including these and similar acanthodian‐grade taxa placed relatively crownwards in the chondrichthyan stem‐group.