Sunday, October 17, 2010

October 17, 2010 : Psarolepis (Extinct)


Psarolepis (pronounced /sæˈrɒlɨpɨs/,
psārolepis, from Greek ψαρός 'speckled' and λεπίς 'scale') is a genuslobe-finned fish which lived around 397 to 418 million years ago (Pridoli to Lochkovian stages). Fossils of of extinct Psarolepis have been found mainly in South China and described by paleontologist Xiaobo Yu in 1998. It is not known certainly in which group Psarolepis belongs, but paleontologists agree that it probably is a basal genus and seems to be close to the common ancestor of lobe-finned and ray-finned fishes. In 2001, paleontologist John A. Long compared Psarolepis with Onychodontiform fishes and refer to their relationships.

Psarolepis had a pair of 'parasymphysical tooth whorls', teeth which extend up at the front of the lower jaw. The head was made of several thick dermal plates and covered with deep pock-marks and large pores. Another trait is a large pectoral spine, just in front of the pectoral fin, extending back from the shoulder girdle, and a dorsal spine located in front of a median fin behind the head, which gives the fish a shark-like form.

The pock-marked head of Psarolepis was made of plates containing a layer of porcelain-like cosmine. Because the cosmine layer obscures the suture lines of the skull, it is difficult to study the exact bone structure. The snout was strangely humped and the nostrils were located above the eyes, which were just above the upper jaw.

The most spectacular findings were the fin spines. Two are known: one extending back from the shoulder girdle and another which is associated with the dorsal fin. These fin spines are found only in primitive jawed fishes and are apparently absent from the most primitive sharks, but present in abundance in more derived forms.

Psarolepis had teeth at the very front of the snout with large fangs on the tooth plate. Outstanding feature are the 'parasymphysical tooth whorls' which place the fish in the order of onychodontida. The premaxilla and the dentary had large inner teeth and irregular array of tiny outer teeth.

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