Australia: The Land Where Time Began

A biography of the Australian continent 

Antiarchs

The antiarchs arose in the Mid- Late Devonian. They were a diverse group that had retained the heavy armour, and appear to have specialised as bottom feeders, probably extracting organic matter from mud they swallowed. An early form, Pterichthyodes, has a trunk shield with a high dome composed of a small number of large plates. Bone completely encloses the pectoral fin, a complex joint allowing it to move against the trunk shield. There was a second joint about half-way along the fin. Benton suggests the pectoral fins, being of little use in swimming, may have been used to shovel mud over it back when they fish buried itself.

These placoderms had the head and trunk shields of the arthrodires, but also armoured pectoral fins. They appear similar to an armoured catfish of the present. These catfish are not related to the antiarchs, developing their similar appearance by convergent evolution, indicating similar selection pressures on a ray-finned fish.

Most antiarchs are not more than 30 cm long. The head shield is smaller than the trunk shield and the head shield has articular balls which fit into articular sockets on the trunk shield. This is the opposite of the arrangement in the arthrodires. The pectoral fins are heavily armoured and are jointed in the middle, which allows refined movement of the fin, with horizontal and vertical movement as well as rotational movement. In effect, armoured arms. The eye openings in the head shield were on the top of the head, so they were most likely bottom feeders.

The oldest, and most primitive, of the known antiarchs, the yunnanolepidoids, are found only in China, from the Late Silurian to Early Devonian. The sinolepidoids, slightly younger than the yunnanolepidoids, from the Devonian, are found only in China and Australia. Both forms have well-developed jointed pectoral fins.

The asterolepidoids and the bothriolepidoids from the Devonian are found in Australia, but both occur globally. In asterolepidoids the trunk shield is long, the pectoral appendages are short, the head shields are small with large eye openings. Specimens of asterolepidoids from the Cravens Park Beds are some of the oldest in the world.

Head shields are large in the bothriolepidoids, about the same size as the trunk shield, with long pectoral appendages. Bothriolepis is one of the most widely known placoderms being found in many sites around the world, but in the Gogo formation of Western Australia it was not common, apparently being most common on the reef front. This has suggested to some that it probably lived in freshwater but moved between rivers via the sea. It has been suggested that species of Bothriolepis were mostly adapted for freshwater but some had adapted for marine habitats. It has also been suggested that, like modern eels and salmon, they may have lived most of their lives in the ocean, returning to freshwater to breed.

The head and body of Bothriolepis were flattened dorso-ventrally, leading to the belief it was probably a bottom feeder. This was the most successful placoderm, with 100 species known and a worldwide distribution. This fish was slender and had a light covering of scales on its tail. It had paired pectoral and pelvic fins. The slender pelvic fins are armour-plated, some with jagged tooth-like edges. One specimen that was found has structures that some think look like lungs, leading to the suggestion by Benton that they may have fed on detritus in the mud and been capable of surviving for some time in stagnant water by breathing air.  

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

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 19/09/2011 

 

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