Thursday, 1 November 2012

A Flying Fish from the Middle Triassic of Guizhou Province, China.

Modern Flying Fish, Exocoetidae, are known in the fossil record as far back as the Eocene, and may have originated slightly earlier, though it is unlikely that they are much older than this, since they are known to be related to other groups considered to be quite modern. However a second group of Flying Fish, the Thoracopteridae, are known from the fossil record, occurring in deposits from the Late Triassic of Austria and Italy.

In a paper published in the Proceedings of the Royal Society Series B: Biological Sciences on 31 October 2012, Guang-Hui Xu of the Key Laboratory of Evolutionary Systematics of Vertebrates at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of SciencesLi-Jun Zhao of the Zhejiang Museum of Natural History, Ke-Qin Gao of the School of Earth and Space Sciences at Peking University, and Fei-Xiang Wu of the Key Laboratory of Evolutionary Systematics of Vertebrates at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences describe the discovery of two specimens of a new Thoracopterid Flying Fish from the Middle Triassic Falang Formation of Guizhou Province, China.

The new species is named Potanichthys xingyiensis, meaning the 'Winged-Fish from Xingyi'; the nearest city to the site where the fossils were discovered. It is a 153 mm fish with both pectoral (front) and pelvic (back) fins enlarged to give it a four winged flying capacity, typical of Thoracopterids but unlike the enlarged pectoral fin only arrangement seen in modern Exocoetid Flying Fish. It also has a deeply forked caudal fin (tail) with a greatly enlarged upper lobe, a feature seen in both Thoracopterid and Exocoetid Flying Fish, but not in other putative modern and fossil 'Flying Fish'.

(Top) Photograph of the first specimen of Potanichthys xingyiensis. (Upper Middle) Interpretive drawing of the first specimen. (Lower Middle) Photograph of the second specimen. (Bottom) Interpretive Drawing of the second specimen. Abbreviations: (af) anal fin; (ao) antorbital; (boc) basioccipital; (br) branchiostegal ray; (cb) ceratobranchials; (cl) cleithrum; (df) dorsal fin; (dls) dense lepidotrichial segments; (dpt) dermopterotic; (dsp) dermosphenotic; (fr) frontal; (hh) hypohyal; (hym) hyomandibular; (ju) jugal; (lac) lachrymal; (llcf) lower lobe of caudal fin; (md) mandible; (mx) maxilla; (na) nasal; (op) opercle; (pfr) pectoral fin rays; (pmx) premaxilla; (pop) preopercle; (pq) palatoquadrate; (psp) parasphenoid; (pvf) pelvic fin; (pvg) pelvic girdle; (ro) rostral; (sc) scales; (scl) supracleithrum; (sop) subopercle; (spo) supraorbital; (suo) suborbital; (ulcf) upper lobe of caudal fin. Xu et al. (2012).

Xu et al. note that a number of other fossils have been described as 'Flying Fish' due to enlarged pectoral fins, but that these Fish lack the distinctive tails seen in both Thoracopterids and Exocoetids. They do not believe that any of these Fish would have been able to glide in the way a modern Flying Fish does, just as a number of modern Fish have enlarged pectoral fins but do not fly (such as the Butterfly Fish, Pantodon sp.); enlarged pectoral fins can clearly be used for other purposes that arial flight.

They also note that while a large number of terrestrial Vertebrates have mastered the art of gliding, this is far less common in Fish, with apparently only two groups of Fish ever mastering the trick. They consider this is unsurprising, since most terrestrial gliders are tree-dwellers, and glide as a low-energy method of getting from one tree to another. For a marine animal, however, gliding is not a low-energy option, as lift must be generated by rapid movement in the water before the Fish leaves the water for a short (but rapid) flight. This is an effective method of avoiding predators, and Flying Fish have a lot of predators, but costly in terms of energy. Xu et al. suggest that if the Thoracopterid Flying Fish had evolved gliding flight by the Middle Triassic, then this implies that they suffered sufficient threat from large predators to make this worthwhile, which has implications for our understanding of how rapidly marine ecosystems recovered after the End-Permian Extinction.

Artists impression of Potanichthys xingyiensis Xu et al. (2012).

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