Psarolepis

|year=2001 |chapter=Something Fishy in the Family Tree |title=Rise of the dragon: Readings from Nature on the Chinese Fossil Record |publisher=University of Chicago Press |pages=pp.64-66||isbn=0226284913] . This new find was the strange fossil fish "Psarolepis".

Most of the "Psarolepis" specimens were found at the same locality of the Yulongsi Formation (Pridoli), the Xishancun Formation (early Lochkovian) and the Xitun Formation (late Lochkovian)] .

Classification

When "Psarolepis" was described for the first time, it was assigned to the group of lobe-finned fishes, the sarcopterygians ] . Palaeontologists were unable to locate "Psarolepis" in the cladogram with certainty, because they did not know if it was the most primitive lobed-finned fish or the most primitive bony fish.

Because "Psarolepis" lacks, and at the same time shares, some characteristics of both lobed-finned fish and ray-finned fish, it is considered to be the missing link [Long, 2001] between the two groups.

Features

Head

The pock-marked head of "Psarolepis" was made of 14 plates containing a layer of porcelain-like cosmine [http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.100.html Palaeos 140.100 Sarcopterygii: Psarolepis] ] . 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.

Fin Spines

[
shoulder girdle and pectoral spine of "Psarolepis".] 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.

Dentition

"Psarolepis" had teeth at the very front of the snout with large fangs on the tooth plate. Outstanding features 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.

Relationships

The fossil of "Psarolepis" from the Upper Silurian to Lower Devonian has shown related features of both actinopterygian and sarcopterygian fishes ] :

* In the skull, "Psarolepis" shows a mix of actinopterygian and sarcopterygian features. In the lower jaw, for example, the teeth are sarcopterygian in showing multiple infoldings ] and in bearing cosmine.

* The cheek bones and the shoulder girdle, however, seem to share features of both groups.

* The pectoral spine extending back from the shoulder girdle, is also found in some placoderms and acanthodians. Furthermore, the median spine, located behind the head, is known in sharks and acanthodians. These two features, however, have not been seen in other bony fish ] .

References

External links

* [http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.100.html#Psarolepis "Psarolepis" at Palaeos]
* [http://www.cas.cn/html/Dir/2002/01/23/8463.htm Searching for the Ancestor of Bony Fishes (chinese)]
* [http://english.cas.ac.cn/Eng2003/page/SRA/D_3.htm Early Evolution of Osteichthyans and Phylogenetic Origin of Tetrapods]
* [http://guide.labanimal.com/nature/journal/v397/n6720/full/397607a0.html A primitive fossil fish sheds light on the origin of bony fishes]