Fish from the freshwater Lower Cretaceous, near Koonwarra, Victoria, Australia : with comments on the palaeo-environment

Freshwater Lower Cretaceous sediments near Koonwarra, Victoria, Australia, and the vertebrate fauna contained in them are described. The evolutionary implications of the fish fauna are considered in detail. It is considered that the palaeo-environment was lacustrine or paludal, and that periodic mass mortality occurred due to anoxic winter conditions beneath an ice-cover ( "winterkill"). The graded laminations present in the fish-bed are thought to be, at least in part, of fairly regular origin and to be due to influxes of silt and clay-laden floodwaters in successive spring seasons. Although such laminae are generally only preserved in deep, cold lakes and wide- spread aquatic arthropod fauna is characteristic of a shallow-water environment today. The number and variety of terrestrial insects, and the small size of the fish also indicates the proximity of shallow waters. The vertebrate fauna consists mainly of fish, although bird feathers are known from the deposit. The discovery of fleas indicates the probable presence of mammals. Five families of fish are known from Koonwarra: Ceratodidae, Coccolepididae, Archaeomaenidae, Koonwarriidae nov. and Leptolepididae. The third and fourth families appear to be limited to Australia. The Ceratodidae are known at Koonwarra from one very poorly preserved specimen of parts of the axial skeleton and an associated skull fragment of Ceratodus sp. The Coccolepididae are represented by Coccolepis woodwardi sp. nov. and are generally small although one fragmentary specimen indicates a ?coccolepid of large size. Coccolepids are rare at Koonwarra. Only one archaeomaenid is known from this site, Wadeichthys oxyops n. gen. et sp. nov. This seems to be the most primitive archaeomaenid known, possessing enamelled scales overall, no ossification in the notochord, an uncompacted caudal skeleton, and most significantly, a pattern of preopercular pit-lines and a sensory canal similar to those of some parasemionotids. Koonwarria manifrons n. gen. et sp. nov. is derived from the archaeomaenids, and is considered to have reached the teleostean grade of organization. The caudal skeleton is compact and homoheterocercal, with elongate urodermals and well formed hypurals. Its scales are completely free of enamel and pit-lines have disappeared from the preopercular and are greatly reduced on the parietal. A new species of Leptolepis, L. koonwarri, is described and found to be closely related to the Jurassic L. talbragarensis from New South Wales. It may also have affinities with the Cretaceous L. diasii from Brazil and Clupavus brodiei from the English Upper Jurassic. L. 'koonwarri is a mosaic form with "advanced" skull features (e.g. loss of postorbital, few preopercular canal branches) whereas the axial and caudal skeleton resemble L. coryphaenoides from the Lower Jurassic of England.