Australia: The Land Where Time Began

A biography of the Australian continent 

Placoderms-Basic Structure

Placoderms are characterised by overlapping bony plates of armour that form a head shield and a trunk shield. The 2 parts of the armour usually articulate by a knob and groove mechanism, but it may be fused into a single composite shield. There are 7 known groups of placoderms that are distinguished from each other by a unique pattern of armour plates. each plate having a specialised shape, overlapping regions, surface texture - ornamentation - and the presence of possible sensory line grooves. Primitive placoderms had tubercles - wart-like protuberances, the more advanced forms had linear ridges and network patterns.

All primitive placoderms had braincases that were well ossified with layers of perichondral bone, in later species it is sometimes entirely made of cartilage, that is thought to have had a weight-saving function. The jaws are always simple bone rods that could have pointed tooth-like structures for gripping prey. Those that specialised in hard-shelled prey the jaws can have areas of thickened tubercles. They had a simple jaw structure - the lower jaws articulates against a knob-like quadrate bone that is often fused to the cheek bones. The eyes are surrounded of a bone ring composed of 3-5 parts called sclerotic plates, and are connected to the braincase by an eyestalk.

The head shield can have a variety of bone patterns, some with eye notches to the sides of the head, while others had eyes and nostrils towards the centre of the skull.

As in sharks, the body is usually torpedo-like, but some groups were flattened - the phyllolepids and the rhenanids. It was originally thought that there were always 2 dorsal fins, but it is now known that there was a single dorsal fin. They had paired pectoral and pelvic fins that shaped like those of sharks and had an internal structure similar to those of sharks. In many species there is a single anal fin. In the more more primitive forms the body is covered with bony platelets resembling miniature dermal bones, and often each scale had similar ornamentation. Advanced lineages tended to have reduced or absent scales. Probably a weight-saving mechanism as took place with the rest of the skeleton. They were one of the most successful fish groups of all time, but by the Devonian, about 355 million years ago, they were extinct, having dominated seas, rivers and lakes for about 60 million years.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995