Australia: The Land Where Time Began

A biography of the Australian continent 

Rhizodontiforms - "Root Tooth"

These had long fangs extending deep into the jaws. They were the largest of the known crossopterygian fish, growing to an estimated 6-7 m. The group is characterised by stiff fins, the main part of which is supported by long unbranched bony rods (lepidotrichia). A robust humerus and strong ulna and radius bones supported very strong pectoral fins. This pattern of arm bones is found in osteolepiforms and all higher land vertebrates.

The structure of the shoulder joint suggests they may have been capable of powerful rotational movements, using its large stiff fins to twist around in the water to tear off chunks of flesh, possibly as seen in feeding crocodiles, the "death roll". Some large rhizodontiforms have another characteristic feature of the group, laterally compressed teeth that have razor sharp blade edges.

The first rhizodontiforms appeared in the Middle Devonian. They reached a peak of diversity and size in the Carboniferous. In some forms the fangs, the largest teeth in the mouth, in the front of the lower jaw, reached 22 cm in length. The Barameda, from Mansfield, south-eastern Australia, demonstrates that the head of rhizodontiforms resembled that of some osteolepiforms, and that the cranial joint allowed a large gape when they opened their mouth. The rhizodontiforms are believed to have preyed on large amphibians and fish living in coal swamps and lakes of the time.

The only complete rhizodontiform fossil, Strepsodus, was relatively elongated, with small pelvic, anal and dorsal fins, and large paddle-like pectoral fins. The body is designed for a slow-swimming ambush predator, capable of occasional rapid bursts of movement. The fish had powerful jaws to hold struggling prey, and powerful pectoral fins that could be used in crocodile-like feeding process involving spinning or thrashing powerfully, tearing off large chunks of flesh that was then swallowed without chewing. By the start of the Permian the rhizodontiforms became extinct, possibly outcompeted by the increasing numbers of aquatic amphibians in their habitats.

Sources & Further reading

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

Links

1. Devonian macrovertebrate assemblages and biogeography of East Gondwana (Australasia, Antarctica)
2. The Age of Fishes Museum, Canowindra, New South Wales

Fish