A Comparative Study of the Australian Fossil Shark Egg-Case <i>Palaeoxyris duni</i>, with Comments on Affinities and Structure

Abstract

The enigmatic fossil noted by Dun in 1913 as Spirangium and named by Crookall in 1930 as Palaeoxyris duni is described in detail in the light of the discovery of other Palaeoxyris specimens, which are now accepted by most workers to be shark egg-cases. Palaeoxyris duni is the only Australian shark egg-case yet described and is one of the largest Palaeoxyris species so far discovered. Comparison of the macro morphology of P. duni with other described Palaeoxyris specimens confirms that it is a separate form species. The palaeoenvironment in which P. duni was deposited was a Triassic low lying fluvial and lacustrine coastal floodplain. One of the Triassic species of hybodontid sharks was the possible egg producer as these fishes have been shown to penetrate freshwater environments. The process of egg production in the nidamental gland of modern sharks is applied to conjecture about the egg-case structure of ancient sharks. The egg-cases of Heterodontus have a helical structure broadly similar to that of Palaeoxyris except that Palaeoxyris have four or six bands in their construction compared to two for the modern Heterodontus. Evidence of shark nurseries, clustered egg-cases and tendril attachment of Palaeoxyris egg-cases indicates ancient shark breeding behaviour was similar to that of modern oviparous sharks.