Australia: The Land Where Time Began

A biography of the Australian continent 

Ichthyostegalids, Ichthyostega (Ichthyostegidae) and Acanthostega (Acanthostegidae)- The Earliest Tetrapods

The ichthyostegalids, Ichthyostega and Acanthostega, from the Late Devonian of East Greenland are the first amphibians to be known in great detail. The skull is well ossified, and there is no intracranial joint as in the crossopterygians. The skull roof and cheek pattern is very similar to that found in panderichthyids, except for Ichthyostega, which has an unusual fused median bone in the rear of the skull roof. The head is large compared to the rest of the body, with eyes in the middle of the top of the skull. The single pair of downward facing external nostrils open close to the mouth. There is a large choana (palatal nostril), which is surrounded by the vomer and other toothed palatal bones. The braincase is not hinged and is relatively small. It has been found that the gill arch of Acanthostega shows that in the adult stage it was capable of aquatic respiration, so the early amphibians were very dependent on water.

Their long tails had a well developed tail fin supported by lepidotrichs (bone rods). The ventral surface could be covered with scales appearing like thin slivers of dermal bone. Their front and hind limbs had 7-8 digits, and on each hand or foot the digits could be divided into a series of large digits and a series of much smaller elements.

An amphibian, Tulerpeton, from the Russian Devonian, had 6 digits on its front limbs. The cleithrum is a large, high bone, and external shoulder girdle bones are not seen on the outside of the animal.

Elginerpeton pacheni is slightly older than Acanthostega, known from fragmentary remains, a femur and tibia, a partial shoulder and hip, and jaw fragments, dated to the Upper Devonian, about 375 million years ago (Frasnian) in deposits at Scat Craig of Scotland, was among the earliest well-known amphibians. It was about 5 ft (1.5 m) long. The known jaw fragments indicate a mosaic of  elpisostegalian fish (panderichthyid crossopterygian) and tetrapod characteristics.

The overall structure of these earliest amphibians is very similar to that of panderichthyids. Digits are present only on the limbs of tetrapods, fish having fins and a more complete series of operculogular bones covering the gill chambers.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
2. John A Long, Dinosaurs of Australia and New Zealand, University of New South Wales Press, 1998

Links

1. Ichthyostega
2. Ichthyostega images
3. Devonian Times, Ichthyostega spp
4. Elginerpeton
5. Devonian Times