Gyracanthus sp. (acanthodian fish)

reconstruction of Gyracanthoides Side and bottom views of Gyracanthides murrayi,
a Lower Carboniferous relative of Gyracanthus ©

The large and distinctive fossil pectoral spines of Gyracanthus sp. are common in the shallow channel margin facies at Red Hill. An acanthodian tail fragment has also been found at Red Hill, but since this specimen lacks diagnostic fin spines, it remains unidentified.

Gyracanthus belongs to the Gyracanthidae, a family of acanthodians first recorded from Middle Devonian deposits in Antarctica (once part of eastern Gondwana). Fossils of this family can be found in Upper Devonian and Carboniferous deposits from around the world, but most of these were very fragmentary. Indeed, the genus Gyracanthus is known only from isolated spines and shoulder bones. The only published account of articulated gyracanthid specimens belong to an Australian species, Gyrancanthides murrayi. This species was also recovered from alluvial (river) deposits, but these were somewhat younger (Tournaisian, Lower Carboniferous) than those at Red Hill.

Gyracanthides murrayi was first described by A. S. Woodward in 1902. One conspicous feature was that the pectoral spines measured almost half the total body length. Some authorities have speculated that these spines may have served as a deterrent to predators, but a recent redescription and reinterpretation of Gyracanthides by Warren et. al. (2000) suggest an alternative function. Their reconstruction presents a dorsoventrally flattened fish with an approximately triangular cross-section. The pectoral spine are positioned low and laterally to the body. These spines, together with their attached fin webs and the wide ventral surface of the body, may have served to counteract the downward force imparted by the heterocercal tail. The long and deep insertion of the pectoral spine indicate that they were capable of little if any movement.

Gyracanthides murrayi is also the only gyracanthid with any material from the head region. The head, which was poorly preserved, is proportionately smaller than in other acanthodians. There was no evidence of an ossified endoskeleton. Nor was there any indication of the dermal bones (i.e., sclerotic bones encircling the eyes, branchiostegal rays and cheek plates) commonly found on the heads of other acanthodians. Finally, no teeth were found in this nor any other gyracanthid.

The nine Gyracanthides pectoral spines examined by Warren et. al. (2000) ranged from 11-58 cm in length. Since these spines are about half the length of the body, the fishes ranged from 22 cm to 1.2 m long. The fin spines for Gyracanthus sp. at Red Hill indicate that they had a similar size range.

Gyracanthus fossil spine Fossil pectoral spine of Gyracanthus sp., approximately 25 cm in length;
original photo courtesy of Ted Daeschler, ANS.

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Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
Scientific Papers:
Warren, A., B. P. Currie, C. Burrow and S. Turner. 2000. "A Redescription and Reinterpretation of Gyracanthides murrayi Woodward 1906 (Acanthodii, Gyracanthidae) from the Lower Carboniferous of the Mansfield Basin, Victoria, Australia." J. Vert. Paleo. 20(2): 225-242.
Image Credits:
The reconstruction of Gyracanthides is copyrighted © 2002, Dennis C. Murphy. (See Terms of Use.) It's based on Warren et. al. (2000).

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