Australia: The Land Where Time Began

A biography of the Australian continent 

Osteolepiformes

This is a diverse group of crossopterygians that first appeared in the Middle Devonian. They died out in the Permian. Together with the panderichthyids, they are the only crossopterygians to have a single pair of nasal openings and choana (palatal nostril). The cheek unit usually has a set pattern of 7 bones. As in the Rhizodontiforms, they have strongly ossified paired fins, the pectoral fin has a solid humerus, ulna and radius. Thick rhombic scales are present in the most primitive members of the group, and a cosmine layer on all dermal bones is also present. In some later lineages the scales become thinner and rounded.

The earliest family of lepidoforms are the Osteolepidae. Some well-known forms are in this family - Osteolepis, Gyroptychius, and Thursius from the Old Red Sandstone beds of Scotland. They are less than 50 cm long, and have relatively conservative dermal bone patterns. All have 2 dorsal fins and a simple heterocercal tail.

The last surviving family was the Megalichthyinids, a group of advanced osteolepidids, surviving to the Middle Permian. They had specialised features, such as a special tectal bone that wrapped around the nostril, wide palatal cavities, a long median process that extended from the premaxilliary bone into the mouth. There was also a special articulation on the dermal bones linking the skull roof halves in some Megalichthyinids. They retained a more primitive feature, they retained their cosmine cover. They were highly successful, living in swamps of Gondwana and Euramerica reaching a size of about 1 m.

In the Eusthenopteridae were forms having reduced cosmine cover. Most had lost the extratemporal bone from the skull roof, but 1 form, Marsdenichthys, a primitive genus found in Australia, was an exception. Eusthenopteron foordi is the best-known family member. It was a 1 m long fish found in the Late Devonian Canadian Escuminac Bay fauna.

In the evolutionary transition from fish to amphibian the osteolepiforms, and in particular the panderichthyids, are the most important fish group. They actually have more in common with the most primitive amphibians than with other fish.

Canowindra grossi, Late Devonian, 360 Ma, from Canowindra, New South Wales. Some features were very small eyes, intracranial joint, bone reinforced scales.

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. Canowindra grossi

2. New Onychodontiform (Oeteichthyes; Sarcopterygii) from the lower Devonian of Victoria, Australia

Fish