Sciency Thoughts: Luoping Biota

Monday, 23 April 2012

An early Ray-Finned fish from the Middle Triassic Luoping Biota of Yunnan Province, China.

Neopterygian (Ray-Finned) Fish first appeared in the Late Permian, and became the most abundant group of fish during the Mesozoic; they remain such today. Neopterygians cane be divided into three groups, the Teleosts, fish with expandable mouthparts, which are the most abundant fish today, and two other groups, the Semiontids (which include the modern Gars) and Halecomorphs (which include the modern Bowfin), which were abundant during the Mesozoic but are now represented by only a few species. The relationships of the early Neoptergyians are unclear, but the Semiontids and Halecomorphs are generally thought to be more closely related to each other than either is to the Teleosts; together they are collectively referred to as Holosteians.

A fisherman with an Alligator Gar, Atractosteus spatula, one of North America's largest fish and a surviving Semiontid Fish. The Megafishes Project.

In a paper in the April edition of the journal Acta Palaeonotologica Polonica a team of palaeontologists led by Wen Wen of the Chengdu Institute of Geology and Mineral Resources announce the discovery of an early Neopterygian Fish that does not fit clearly into any of these groups, and the implications of this.

The new species is named as Luoxiongichthys hyperdorsalis, meaning 'The humpbacked fish from Luoxing'. It has a distinctive forward curving hump on its back that Wen Wen et al. compare to a 'reverse shark’s dorsal fin'. This is unlikely to have been advantageous to the fish when swimming, and was probably used for display.

Luoxiongichthys hyperdorsalis. Top, photograph of original specimen. Middle, interpretive drawing based upon photograph. Bottom, reconstruction of living fish. From Wen Wen et al. (2012).

Luoxiongichthys hyperdorsalis was described from a single specimen found in the Daaozi Section of the Guanling Formation, at Daaozi Village in Luoping County, about 25 km northeast of the town of Luoxiong. This is a muddy limestone that has produced a large number of excellently preserved vertebrates, invertebrates and plants from the Middle Triassic, collectively refered to as the Luoping Biota.

The location of the section that produces the Luoping Biota. From Wen Wen et al. (2012).

Luoxiongichthys hyperdorsalis does not fit easily into any group of Neopterygian Fish, but appears to be more closely related to the Semiontids and Halecomorphs than to the Teleosts, supporting the theory that these groups are related, and probably more closely related to the the Semiontids than to the Halecomorphs. Several early Semiontids have also been shown to have humped backs, supporting this relationship.

Diagram showing the probable relationships of Luoxiongichthys hyperdorsalis to other groups of fish. From Wen Wen et al. (2012).

Follow Sciency Thoughts on Facebook.

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.