Australia: The Land Where Time Began

A biography of the Australian continent 

Acanthothoracii (spiny trunk shield) 

These were the basal placoderms, they are known from Early Devonian marine sediments in Australia. This group is not well known in other parts of the world. There are a number of body shapes found in fishes of this group, suggesting that it may be a polyphyletic group. Some Acanthothoracids are similar to rays, with expanded pectoral fins and a flattened appearance. They had long head shields, very shortened trunk armour, with eyes and nares pointing upwards, suggesting a bottom dweller.

In this group the head armour is not always composed of large interlocking plates, having a few large plates separate by a mosaic of smaller ones. In juvenile specimens the plates haven’t fused together, but are believed to have fused as adults (Benton, 2005) Australian forms such as Murrindalaspis had tall crests on the trunk armour. The head shield was similar to that of some early arthrodires.

Australia has some of the best preserved cranial material of this group in the world, found in limestones from the Early Devonian around Taemas and Wee Jasper, New South Wales, and near Buchan in Victoria. An example of this is Brindabellaspis stensioi. This specimen shows details of the cranial nerves and vessels and impressions of the brain. A complete sclerotic capsule, the bones protecting the soft tissue of the eyeball, has been found in a specimen of Murrindalaspis from Taemas. The pathways of arteries and veins and muscle attachment surfaces for the muscles controlling eye movement are impressed into the bone.

Australian acanthothoracids have been placed mostly into a group with unknown close affinities, or a group unknown elsewhere such as Weejasperaspis and Murrindalaspis, in the family Weejasperaspidae. The Australian forms show closest affinities with the acanthothoracids from the microcontinent Amorica, just north of Gondwana.

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.

Fish