Crusty old fossils

Paul D Taylor (UK) Many fossil collectors will have been disappointed to discover mollusc and brachiopod shells ‘disfigured’ by crust-like coverings of oysters, serpulid worms, barnacles or bryozoans so firmly cemented to the shells that they cannot be removed. However, rather than discarding these encrusted shells, it is worth considering what they can tell us about the ecology of the host animals and the fate of their shells after death. Furthermore, the surfaces of the shells were battlefields for encrusters fighting for living space, allowing a rare opportunity to observe the effects of biological competition millions of years ago. The key palaeoecological advantage offered by encrusters over most other fossils is that they preserve their original life positions. For example, a fossil barnacle encrusting a fossil bivalve (Fig. 1) is located on the shell exactly where the barnacle larva settled and the adult spent its life. Encrusters are not transported or displaced from where they originally lived, although of course the shells themselves may have been moved. Fig. 1. Field photograph of two barnacles, one large and one small, attached to an articulated bivalve shell. Like other encrusters, the barnacles preserve their original life positions on the bivalve. Pleistocene, Nukumaru Brown Sand, near Whanganui, New Zealand. Encrusters are sclerobionts, a collective term for organisms colonizing all kinds of hard substrates, including shells, bones, wood, rocks and sedimentary hardgrounds (Taylor and Wilson, 2003). Other types of sclerobionts bore into hard substrates, leaving trace fossils as evidence of their former presence, or … Read More

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