Wednesday 11 April 2012

Live birth in a Middle Triassic Coelacanth.

Coelacanths are Sarcopterygian (lobe finned) Fish, members of the same group as Lungfish and Tetrapods, which makes them of interest to palaeontologists studying the colonization of land by vertebrates. They were thought to have been extinct until the 1950s, when a surviving species of Coelacanth was discovered in the deep waters of the Mozambique Channel. A second species of modern Coelacanth was discovered in Indonesia in the 1990s.

Diver Arnaz Mehta Erdmann with an Indonesian Coelacanth. Mark Erdmann/Smithsonian National Museum of Natural History.

In a forthcoming paper in the journal Acta Palaeontologica Polonica, a team of scientists led by Wen Wen of the Chengdu Institute of Geology and Mineral Resources at Chengdu University of Technology describe two new species of Coelacanth from the Middle Triassic Luoping Biota of Yunnan Province.

The first of these, Luopingocoelacanthus eurylacrimalis (Coelacanth from Louping with a triangular, broad lachrymojugal bone) is described from four specimens, one of which has two well developed embryos within it.

The specimen of Luopingocoelacanthus eurylacrimalis with two embryos within (A), and closer images of the embryos (B & C). Wen et al. (2012).

The discovery of embryos within L. eurylacrimalis is not entirely surprising. While only 1-2% of modern boney fish produce live young (compared to 55% of sharks) the modern Coelacanth, Latimaria, does, practicing a form of live bearing called Ovoviviparity, in which eggs are retained within the mother until they hatch. This has also been found in the Cretaceous Coelacanth Axelrodichthys and the Jurassic Coelacanth Undina, whereas egg-laying has been determined for the Carboniferous Coelacanth Rhabdoderma. The eggs of Rhabdoderma were up to 53 mm in diameter, compared to a 600 mm fish, suggesting the animal invested heavily in a small number of eggs. For such an animal retaining the eggs within the adult until they hatch would be distinctly advantageous.

The second new species is named as Yunnancoelacanthus acrotuberculatus (Coelacanth from Yunnan with sharp dermal tubercles), named from a single 255 mm specimen.

Yunnancoelacanthus acrotuberculatus. Wen et al. (2012).

Wen et al. note that Coelacanths underwent a dramatic increase in species number worldwide in the Early Triassic, having been somewhat rare in the Permian, and subsequently suffered from falling numebers throughout the Mesozoic, apparently becoming extinct at the end of the Cretaceous (without the living species we would conclude that they had done just that, since we have no Cenozoic fossils). They suggest that Coelacanths may have been well adapted to conditions during the End Permian Extinction, as they have low metabolisms, a slow reproductive cycle and can tolerate low oxygen levels (if the modern species are typical of the group). Since it is generally believed that the during End Permian there was widespread ocean anoxia and food was scarce, conditions to which Coelacanths were pre-adapted, providing them with the opportunity for an adaptive radiation at a time when many other groups were in decline. As other groups recovered after the extinction event Coelacanths again became rarer.

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