Australia: The Land Where Time Began

A biography of the Australian continent 

Basic Structure - Crossopterygians

Crossopterygians are characterised by having relatively long bodies. In the more primitive forms the skull has a hinged braincase. There is a front section (ethmosphenoid), and a rear section (ottico-occipital). The braincase flexure is seen in the skull roof bones of each crossopterygian group. An intracranial joint separates the frontal and parietal shields. In some advanced forms the intracranial joint is immobilised by bone fusion between the frontal and parietal shields, such as in panderichthyids. They had large eyes and paired external nostrils in all except the osteolepiforms, where there is a single external nostril pair and a choana, a palatal nostril opening.

They have a regular bone pattern in the cheeks, 1 or more large squamosal bones, large dentary bones in the jaws, supported by a series of infradentaries. On the toothed bones there are regular marginal teeth and large fangs. Coelacanths differed in having a smaller area of toothed biting jaws and a double-tandem jaw joint, which is unusual, obviously a specialised feeding mechanism. The teeth have enamel over dentine. The osteolepiforms, porolepiforms and rhizodontiforms, among others, the enamel and dentine are infolded in a complex way. These teeth appear highly complex in cross-section, called labyrinthodont teeth. The gill arches are well-ossified and have large ventral gill bones (basibranchials), in some cases with a forward-pointing sublingual bone.

The shoulder girdle and the internal shoulder girdles (scapulocoracoid) are well-ossified. The pectoral and pelvic fin skeletons are well ossified in all crossopterygians, but especially so in the osteolepiforms and rhizodontiforms. These forms have a powerful humerus articulating with the ulna and radius, as in all higher vertebrates, except the limbless forms. The bodies of crossopterygians seem to have retained a rather conservative shape, there are no known specimens that became flattened or unusually deep-bodied.

Cosmine is present in the scales and dermal bones of all primitive crossopterygians. This is often lost in more advanced forms. The scales of most crossopterygian groups are characteristic. Primitive members of the osteolepiforms and porolepiforms have thick, cosmine-covered rhombic scales. The scales of both groups changed from rhombic to round, lacking cosmine, in their radiations. The rhizodontiforms, onychodontiforms and actinistians have thinner, rounded scales. Crossopterygians have well developed sensory line systems, visible as a series of pores and pit-line canals on the dermal bones and scales.

The coelacanth Latimeria chalumnae is the only known living crossopterygian fish. They have been filmed in the natural habitat, between 119 and 198 m down in the Indian Ocean. It has been found that they range between 100 and 700 m below the surface. It is a slow-moving fish that often drifts in the current, adjusting its position using its mobile pectoral and mobile fins. The fins can move in a similar fashion to that of quadrupeds, both the pectoral and pelvic fins can move in opposite directions. They eat fish and cuttlefish.

The first coelacanth described was Coelacanthus granulatus from the  Late Permian of Germany. The coelacanths were thought to have gone extinct at the end of the Cretaceous, after having been found in the rocks from the Devonian to the Cretaceous. The coelacanth was the first fish to be suggested as a missing link between fish and quadrupeds. Fish in this group from the Devonian are believed to be the closest fish to the first amphibians.

There are 5 major groups of crossopterygian, the extinct groups, osteolepiforms, porolepiforms, onychodontiforms and rhizodontiforms. The 5th group is the Order Actinistia, the group to which the coelacanths belong, represented by the single known living species. There are also some stem groups that are intermediate between some of the main known groups, representing the primitive forms of the groups.

Sources & Further reading

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