Soldier crab paleo-ichnology and its preservation in beach rock along the Pilbara Coast, Western Australia: its geoheritage significance

Abstract

Ichnofossils, although small-scale geological features, can be of geoheritage significance. The Western Australian soldier crab (Mictyris occidentalis Unno 2008) presents an unusual and unique association between the crab and its ichnology because, as the species progresses through life, its behaviour becomes more complex and its traces commensurably more varied. The links between crab ichnology, life stage, behaviour, and environment are so direct that where traces are preserved they can be used to interpret fossil crab sizes, population structures, behaviour and paleo-environment. In this context the ichnofossils comprise important paleo-ecological and paleo-environmental indicators. Middle Holocene soldier crab ichnofossils occur in 5000 year-old beach rock at Port Hedland, Western Australia. The ichnofossils include rosettes, pustules, pellet-roofed tunnels, back-filled burrows and swirl lamination, air cavities and tidal-current-degraded discard pellets. They provide information on soldier crab population sizes and behaviour, and the nature of the environment along middle Holocene shores. The ichnofossils are of geoheritage significance in their own right for the paleontological and paleo-environmental information they present, however, given the rarity of their preservation, and the location of well-developed beach rock largely restricted to the arid Pilbara Coast, these ichnofossils are an uncommon and significant geological tool for use in paleo-environmental, paleo-ecological and paleoclimate reconstructions. They are of geoheritage significance because of their diagnostic signatures and rarity of occurrence. The limitations of current classifications of traces and ichnofossils, owing to restricted definition of terms, are highlighted by these ichnofossils as several ichnological products in the complex range of soldier crab traces are not possible to classify.