Climatius, Mesacanthus, Diplacanthus and Brachyacanthus

Mesacanthus mitchelli (renamed with Traquair 1888; Early Devonian, 410 mya; 3cm) is a basal acanthodian transitional to Notopterus and Perleidus. So spiny sharks are not extinct. Mesacanthus has open cheeks exposing the quadrate. Soft tissue preserves membranes posterior to all the spines.

Climatius reticulatus (Agassiz L 1845; Early Devonian; 7.5cm; MS G 1887.35.5E) had several extra small spines between the longer pectoral and pelvic spines. This genus is basal to more recent descendants without spiny fins, like Perleidus and Notopterus (below).

Brachyacanthus scutiger (originally Climatius scutiger, Egerton 1860; Hunterian Museum and Art Gallery 298251; 3.5 cm in length; Early Devonian) is a tiny acanthodian or 'spiny shark' here nesting between Climatius and Acanthodes. The skull bones are subdivided more than any other taxon. The fin rays coalesce to form spines.

Contra Carroll 1988, the small scales/bones that cover the head CAN be directly compared with the dermal skull bones of its sister, Pteronisculus.

Early Triassic Pteronisculus is convergent in overall size and shape.

Diplacanthus horridus (Woodward 1892; Gangnier 1996; Early to Middle Devonian, 380 mya; 15cm long) has some of the longest spines among spiny sharks and nests basal to placoderms and cafish in the LRT. Note the resemblance to Eurynotus. Diplacanthus retained a maxilla prior to loss or fusion with the jugal in higher taxa. It also had a lateral temporal fenestra.

Baron MG 2015. An investigation of the genus Mesacanthus (Chordata: Acanthodii) from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland using traditional morphometrics. PeerJ. 3: e1331. doi:10.7717/peerj.1331

Brazeau M 2009. The braincase and jaws of a Devonian ‘acanthodian’ and modern
gnathostome origins. Nature 457, 305–308.

Carroll RL 1988. Vertebrate paleontology and evolution. W. H. Freeman, New York.

Friedman M and Brazeau 2010. A reappraisal of the origin and basal radiation of the Osteichthyes. Journal of Vertebrate Paleontology 30(1):36–56.

Miller RF, Cloutier R and Turner S 2003. The oldest articulated chondrichthyan from the Early Devonian period. Nature 435:501–504.

Newman M and Davidson B 2010. Early Devonian fish from the Midland Valley of Scotland. National Palaentological Congress London 14–15.

Traquair RH 1888. Notes on the nomenclature of the Fishes of the Old Red Sandstone of Great Britain. Geol. Magazine (3)5:507–517.

Woodward AS 1892. On the Lower Devonian fish-fauna of Campbellton, New Brunswick.. Geol. Mag. 9, 1–6.

Woodward AS 1892. Further contributions to knowledge of the Devonian fishes of Canada. The Geological Magazine, 9 (new series. Decade III):481-485.

wiki/Acanthodii
wiki/Mesacanthus
wiki/Climatius
wiki/Diplacanthus
wiki/Brachyacanthus