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Yet another spine-fin > ray-fin transition: Brachyacanthus > Paratarrasius

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Spiny sharks are getting more interesting.
Today eel-like and previously enigmatic Paratarrasius (Figs 1, 2) enters the large reptile tree (LRT, 2223 taxa) alongside the spiny shark (acanthodian) Brachyacanthus (Fig 1), rather than alongside a traditional member of the Tarasiidae (Tarrasiiformes), Tarrasius (Fig 1), a relative of the extant moray eel, Gymnothorax in the LRT.

Earlier we looked at several other clades of spiny shark descendants evolving ray fins.

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’. The skull bones are subdivided more than in any other tested taxon in the LRT. Several thin and parallel fin spines coalesce to form sharp, thick combined spines. Contra Carroll 1988, the small scales/bones that cover the head CAN be directly compared with the dermal skull bones of other taxa, including its LRT sister, Paratarrasius. If incorrect, let me know.

Paratarrasius hibbardi
(Lund and Melton 1982, UM5557, CM 35231, Mississippian, Bear Gulch Fm, 325 mya, up to 13.6 cm long, Figs 1, 2) nests as a sister to the older and smaller Brachyacanthus in the LRT. Lateral spines have disappeared. Only fan-like pectoral fins on a short lobe are present. The dorsal and anal fins are long and continuous, merging with the caudal fin. A segmented notochord with neural spines is present, rather than individual vertebrae (with centra) and ribs. Paratarrasius is not related to
Tarrasius, a traditional sister with vertebrae + ribs and nests with the moray eel, Gymnothorax, in the LRT. Diagrams above from Lund and Poplin 2002, who used suprageneric taxa in analysis and did not test Brachyacanthus. Nevertheless, that study nearly matched the LRT in indicating Paratarrasius was close to stem actinopterygians. As in Brachyacanthus cheek bones may include scales not tightly associated in a plate. The covered hyomandibular is short and T-shaped.

Carroll RL 1988. Vertebrate paleontology and evolution. W. H. Freeman, New York.
Egerton P de MG 1860. Report of the British Association for Science for 1859.
Transactions of the Sections. 116.
Lund R and Melton WG Jr. 1982. A new actinopterygian fish from the Mississippian Bear Gulch limestone of Montana. Palaeontology 25(3):485-498.
Lund R and Poplin C 2002. Cladistic Analysis of the Relationships of the Tarrasiids (Lower Carboniferous Actinopterygians). Journal of Vertebrate Paleontology 22(3):480–486.
Leidy J 1870. [Remarks on ichthyodorulites and on certain fossil Mammalia]. Proceedings of the Academy of Natural Sciences, Philadelphia 22:12–13.
Newman M and Davidson B 2010. Early Devonian fish from the Midland Valley of Scotland. National Palaentological Congress London.14–15.

wiki/Paratarrasius – not yet posted


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