Australia: The Land Where Time Began

A biography of the Australian continent 

Onychodontiforms - Dagger Toothed Fish

This is a group of Devonian crossopterygians that are not well known. They have large dagger-like tooth whorls on the lower jaws. The group was previously known only from whole specimens of a small fish, Strunius, from Germany, and a number of  jaws, skulls and other bones from a number of other species of the genus Onychodus. It is now known that this genus had a worldwide distribution during the Devonian. Well preserved specimens of Onychodus have now been found in the Gogo Formation of Western Australia. The studies of these Gogo finds is incomplete at the time of writing.

Prior to the discovery of the Gogo specimens that are believed to have reached as much as 4 m, the largest known species of Onychodus reached about 2 m long. It is thought to have been an ambush predator, similar to the modern moray eel. The braincase had a large hinge between the 2 sections, probably to accommodate for the use of the large fangs in the lower jaw. These fangs almost reached the skull roof when the mouth was closed. There are a number of features in skulls of onychodontids that differ from those in other crossopterygians. These include the skull roof and cheek bone patterns, the maxillary, the upper jaw is very similar to that of an actinopterygian in its general shape, having a large postorbital blade. There are 2 cheek bones of almost equal size, the squamosal and preoperculum, with 3 infraorbitals around the eye. They have an opercular mechanism that is typical of crossopterygians, but the sublingual bone can be absent. It had a long, slender body, and the dorsal fins appear to have been situated close to the tail. The limited amount of fossils found so far make it difficult to be certain about the dorsal fin placement. Ossification of the humerus suggests that at least the pectoral fins were powerful, with large muscle attachment sites. Apart from this, the structure of the pectoral and pelvic fins is uncertain because of lack of fossil material.

In the Gogo fish beds a partial skull of Onychodus has been found with the remains of a small placoderm in its throat. The placoderm bones are facing forward, so it seems it was caught by the tail and swallowed whole. The Onychodus was about 60-70 cm long and the placoderm would have been about 30 cm long. So it seems they could swallow prey at least half their own length.

A new species of onychodontiform, Bukkanodus jesseni from the Early Devonian, has been found in the Fairy Formation of Victoria, Australia. This is one of the earliest known occurrences of this group.

Links

1. New Onychodontiform (Oeteichthyes; Sarcopterygii) from the lower Devonian of Victoria, Australia
2. The Late Devonian Gogo Formation Lagerstatte of western Australia: Exception al Early Vertebrate Preservation and Diversity
3. Possible evidence of predation in placoderms (pisces) of the Evlanova basin in central Russia

Sources & Further reading

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

Fish