Coelacanths are Sarcopterygian (lobe finned) Fish, related to Lungfish and Tetrapods (terrestrial vertebrates). Modern Coelacanths are deepwater ambush predators, able to survive in waters with low oxygen levels. They move primarily using their fins, with the tail being used for short busts of movement in pursuit of prey. Coelacanths evolved this basic body-plan before the Permian, and have changed little since. They were at there most diverse in terms of species numbers in the Early Triassic, having apparently survived the Permian extinction, which is widely thought to have been associated with low ocean oxygen levels, relatively unscathed and prospered in the absence of other large marine predators.
In a paper published in the Journal of Vertebrate Paleontology on 3 May 2012, Andrew Wendruff of the Department of Biological Sciences and Laboratory for Vertebrate Paleontology at the University of Alberta and the School of Earth Sciences at The Ohio State University and Mark Wilson of the Department of Biological Sciences and Laboratory for Vertebrate Paleontology at the University of Alberta describe a new species of Coelacanth from the Early Triassic Sulphur Mountain Formation in the Wapiti Lake Provincial Park of British Columbia.
The new species is described as Rebellatrix divaricerca, the 'rebel-finned fork-tail'. It has an elongate, apparently fairly rigid body, with a large, forked tail, similar to that of a modern tuna. This body-plan is associated with rapid pursuit predators in open water today, a very different lifestyle to that generally associated with Coelacanths.
Rebellatrix divaricerca, (A) Photograph of specimen. (B) Close up of lateral line system. (C) Close up of a scale, dotted line indicating scale margin. (D) Line drawing of (A). (E) Cast made of skull in ammonium chloride dusted silicone peel. (F) Line drawing of skull. Abbreviations: Acl, anocleithrum; A.f, anal fin; Ang, angular; Art, articular; Cl, cleithrum; Cla, clavicle; D1.b, anterior dorsal basal plate; D1.f, anterior dorsal fin; D2.b, posterior dorsal basal plate; D2.f, posterior dorsal fin; Ecl, extracleithrum; lPG, left pectoral girdle; occ.n.a, occipital neural arches; Op, operculum; o.p.l, oral pit line; Rart, retroarticular. Scale bars: (A) 10 cm. (B) 2 mm. (C) 2 mm. (D) 10 cm. (E) 2 cm. (F) 2 cm. Wendruff & Wilson (2012).
Rebellatrix divaricerca, second specimen. (A) Photograph. (B) Lobed base of posterior dorsal fin. (C) Tapered anal fin. (D) Posterior dorsal basal plate (E) Anal basal plate. (F) Pelvic girdle. (G) Line drawing of (A). Abbreviations: A.b, anal basal plate; A.f, anal fin; D2.b, posterior dorsal basal plate; D2.f, posterior dorsal fin; P.b, pelvic bone; P.f, pelvic fins; sb, swim bladder; s.l, supplementary lobe. Scale bars (A) 10 cm. (B-F) 2 cm. (G) 10 cm. Wendruff & Wilson (2012).
Rebellatrix divaricerca does not appear to have been distantly related to other post-Palaeozoic Coelacanths; it is unlikely to have been a relic of some more ancient lineage that survived into Triassic times. Rather it appears to have been a unique Early Triassic innovation, a Coelocanth that evolved to fill a niche that had been filled by Sharks in the Permian, after this group had been badly hit in the end Permian extinction. While this may have been a temporary success, the niche was soon reclaimed by other groups, notably Ray-Finned Fish and Marine Reptiles, suggesting that the slow Coelacanth metabolism was not ultimately well suited to this lifestyle.
Reconstruction of Rebellatrix divaricerca as a living animal. Michael Skrepnick.
See also Live birth in a Middle Triassic Coelacanth, The pelvic girdles of two primitive boney fish from China, New Tetrapodomorph Fish from the Devonian of Nevada and Boney Fish on Sciency Thoughts YouTube.
Follow Sciency Thoughts on Facebook.