Australia: The Land Where Time Began

A biography of the Australian continent 

Petalichthyids

The Petalichthyids are another group of placoderms from the Australian Early Devonian. They are thought to be possibly closely related to the arthrodires. They rarely exceed a metre. They are flattened, bottom-feeding fish, unlike the arthrodires. Long suggests they probably swam slowly above the seafloor looking for prey, though as no mouthparts of the known their food source can only be conjectured. The trunk armour often has elongate spines of the both sides. It had impressive spines on either side the head and the elongate body was covered with small overlapping scales. There were also many small scales around the eyes and nostrils on the anterior part of the head shield. The eye openings in the head shield are on the top.

Their pectoral fins were widely splayed and they had linear rows of small tubercles that were characteristic, ornamenting their dermal bones. There were thick tubes in their bones that carried sensory-line nerves. These tubes can be clearly seen on the inside of the bones of the skull.

There are not many in the known Australian Devonian beds. One of the Australian petalichthyids is Lunaspis. It is also found in Russia, China and Europe. There are several genera endemic to Australia. Some of these were Notopetalichythys and Shearsbyaspis, from the Early Devonian of Tasmania. Wijdeaspis is found in Australia, but also in Siberia and Spitsbergen.

They are known from Europe, North America, South America, Asia and Australia. The Early Devonian was the time of their peak diversity, few species surviving to the Late Devonian. It has been suggested (Long, 2011) that a minor local radiation of the petalichthyids may have occurred on the isolated continental block that was to become South China, as a group of fish, quasi-petalichthyids, that were similar to petalichthyids, have been found in China.

Near Taemas, New South Wales, limestones that have been dated to the Early Devonian have produced some of the best preserved Petalichthyid skulls. The Black Shales of Germany from the Early Devonian have produced what are suggested by Long to probably be the best whole fish fossils of petalichthyids.

Sources & Further reading

  1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
  2. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing
  3. John A. Long, 2011, The Rise of Fishes 500 Million Years of Evolution, University of New South Wales Press
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Last Updated 13/06/2010

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading