Machaeracanthus goujeti n. sp. (Acanthodii) from the Lower Devonian of Spain and northwest France, with special reference to spine histology

We describe here a new machaeracanthid acanthodian (Machaeracanthus goujeti n. sp.), based on isolated spines, scales and scapulocoracoids from the Lower Devonian (Lochkovian-Pragian) of the Nogueras Formation, Celtiberia, Spain. The new taxon also includes a fragmentary spine and isolated scales from the Lower Devonian of northern Spain (Palencia and Cantabrian Mountains) and western France (Saint-Céneré) originally assigned to Machaeracanthus sp. The spines of M. goujeti n. sp. comprise two morphotypes in agreement with the morphofunctional model of a pair of pectoral spines articulating with the pectoral girdle already indicated for M. hunsrueckianum Südkamp & Burrow, 2007, M. longaevus Eastman, 1907, and M. sulcatus Newberry, 1857. The morphology and size of the spines distinguish M. goujeti n. sp. from the coeval species M. bohemicus Barrande, 1872; the new species most closely resembles the younger species M. peracutus Newberry, 1857. The spines of M. goujeti n. sp. consist of trabecular and lamellar dentine layers which form the wall of the central axis (pierced by a longitudinal pulp cavity) and lateral expansions. The most superficial layer of dentine is centrifugally deposited in the complete spine; this condition is found in fin spines of some chondrichthyans and contrasts with that observed in typical acanthodian fin spines where the exserted portion is ornamented with ribs of centripetally growing dentine. Very small spines and scapulocoracoids of M. goujeti n. sp. described here, are the first report of juvenile specimens of a species of Machaeracanthus Newberry, 1857. The distal part of the juvenile spine lacks lateral expansions (keel and wing) and demonstrates the first stage in the development of the spine.