Australia: The Land Where Time Began

A biography of the Australian continent 


One of the oldest known relatively complete fossil fish has been found in fine-grained Ordovician sandstone, the Stairway Sandstone Formation, which caps Mt Watt near Alice Springs, Northern Territory in central Australia. It is a primitive jawless fish known as Arandaspis prionotolepis. The fossil is in the form of an impression, internal and external, of the body of the fish, in the sandstone and it shows precisely the form of the bony body shields and pattern of the scales and the position of the mouth and eyes. It was 14-16 cm long. A 2-part bony shield, comprised of 2 large plates, completely covered the head and gill region. The ventral plate was deep and curved, the dorsal plate more triangular-shaped. Between these 2 plates there was a row of small, rhombus-shaped plates. It is believed they may indicate where the gill pouches were positioned. The outlet from the gill pouches was probably a single opening at the back of the row of small plates, water entering through the mouth and exiting through this opening. Some of the body scales behind the head were elongate and narrow, with a single row of small projections extending beyond the scale so that the scale appeared fringed. It is not known if it had fins, most of the body is missing from the fossil, so the structure of the body back past the head can only be estimated. In cross-section it was fairly deep and rounded.

It had a pair of eyes on the sides of the head, near the front, and in front of the small plates between the 2 halves of the head shield. There are 2 holes in the dorsal plate between the eyes. They appear to have been the openings of the pineal eye, extensions of the dorsal surface of the brain. Their function is unknown, but possibly light detection in hormone control, as in other vertebrates. The latter is found in humans. There were also  grooves along the sides of the head, suggesting the possibility of a lateral line system. In modern fish these detect pressure changes that warn the fish of movement in the water. A lateral line system can detect the presence of other fish, competitors or predators, in some it is modified to detect electrical currents.

The skeleton of Arandaspis is external, and very thin at 1/10 mm. It is believed that they were deposited after the fish matured, as there is no sign of fusion or resorption, apparently the juveniles lacked armour. No traces of internal bones have been found in association with this fish, so it appears it probably had a notochord as had been thought to be the case with these and other primitive fish. Sharks still survive very well with such a skeletal system.

The feeding method of Arandaspis is uncertain. As it lacks jaws its options would be limited, possibly it was a filter feeder, probably on the sea bed. Its movement was much more confined than modern fish, as it would have lacked the fins modern fish use for fine control of their movements. In its time, Arandaspis was the top of the line in fish architecture, even tough in the seas of the present it would appear disabled. The lampreys and hagfish arose at this time, moving into very specialised niches, eg parasitism, to compete with the advanced forms of fish that were arising to compete with them.

A clue to the feeding of Arandaspis may possibly be found in a related, and much better preserved, species from Bolivia, Sacabambaspis janvieri. There are 30 known specimens of this Bolivian species, all crammed into a very confined area, believed to be the result of a fish kill, such as a sudden inflow of freshwater, such as from a large storm. They were found associated with a large number of lingulid brachiopods, also killed at the same time.

Sacabambaspis had a single large dorsal and a single large ventral plate covering the head and gill region, just like Arandaspis, but more detail is known of the head of this fish than of Arandaspis, or any other fish of the time. Its eyes were protected by a ring of bony plates, and it had a distinct pair of nostrils. Like Arandaspis, it also had a pair of pineal eyes. The feature of Sacabambaspis that could shed light on the feeding mechanism of Arandaspis is the mouth structure. The mouth of the Bolivian species was lined with nearly 60 rows of small bony plates which were possibly moveable, which may have allowed expansion and contraction of the pharyngeal cavity, and this could make possible the sucking action of the "vacuum cleaner" more efficient. But there could have been a totally different function for these plates.

Sacabambaspis had a single caudal fin (tail fin), and its body was covered with 4 rows of elongate, slender scales arranged in chevrons. A well-developed lateral line is present along 2/3 of the body. It is clearly visible on these scales. Isolated scales found in the Horn Creek Siltstone from Central Australia have a very similar ornamentation to the Bolivian scales. More complete fossils of Arandaspis are needed before a definite comparison can be made, but based on what is known at the present it seems the 2 species may have been very similar in many ways.

Bone has been found preserved in the head shields of Sacabambaspis. It is different from that of most vertebrates, being acellular. This bone type links it to the heterostracans (pteraspidomorphs), better known in the Northern Hemisphere.

A number of other agnathans (jawless fish) were also discovered at the site. 

Older fish have now been found in the Chengjiang Beds of China.

See Australian Fossil Fish Beds

Sources & Further reading

  1. Vickers-Rich, Patricia & Rich, Thomas Hewitt, 1993 Wildlife of Gondwana, Reed Australia.
  2. Long, John A., 1995, The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press.


  3. Images of Sacabambaspis


Author: M. H. Monroe
Last updated 30/10/2011 


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