There has been considerable debate about the function of colour molecules in the shells of marine organisms. The red colouration of Calloria specimens comes from a caroteno-protein that occurs within the crystal of the shell. Equivalent pigments from mollusc shells have been considered to be the waste by-product of metabolism (Kelley and Swann 1988), although it has been suggested also that such intra-crystalline biomolecules act as a nucleation site for biomineralisation (Cusack et al. 1992). Carotenoids of the type utilized by brachiopods for colouration are not thought to have been synthethized by the animal, but rather to have been obtained from food, in particular marine algae (Cheesman et al. 1967). Any reduction in the availability of food in the environment inhabited by brachiopods would cause a reduction in the quantity of pigment incorporated within the shell. Such a phenomenon would explain the concentration of colour in growth bands deposited at times when shell-growth was slow (and hence the ratio of ingested pigment to shell growth was relatively high).
Red colouration permeates the body tissues of living brachiopods, as well as the shell (James et al. 1991). Such colouration in other taxa is thought to have an important function in protecting the tissue from potential damage caused by solar radiation (Cheesman et al. 1967). In the case of brachiopods, that would be of greatest significance when planktotrophic larvae of articulated stocks swim to the surface during their free-swimming distribution phase. Among living brachiopods, female gonads are almost always orange or red, or brownish in colour even when the shell itself has no colouration (e.g. Terebratulina retusa - James et al. 1991). In a number of living species it is possible to distinguish the sex of mature individuals without separating the valves as the red colouration of the female gonads is visible through the shell and contrasts with the white or cream-coloured testes (e.g., Rokop 1977).
Red pigmentation of animal shells may have a protective role by warning potential predators that the brachiopod tissues may contain toxins (Fox and Veevers 1960). There have been suggestions that the tissue of articulated brachiopods does contain compounds that are poisonous or unpalatable to potential predators (Thayer and Allmon 1991). Feeding experiments have indicated that marine predators such as snails, crabs, and fish prefer mussels and other bivalves to brachiopods (Thayer 1985). The taxonomic distribution of this trait suggests that unpalatability developed prior to the Ordovician divergence of the rhynchonellid and terebratulid (Thayer and Allmon 1991). Alexander (1986) recorded extensive predation damage in many fossil brachiopods, but only rarely in rhynchonellids and terebratulids. This may, as Thayer (1991) suggested, be a reflection of the unpalatability of these groups. The fact that most records of fossil brachiopods with preserved colour or colour patterns are in rhynchonellid and terebratulid stocks may indicate that this unpalatability was often accompanied by bright warning colouration, as is common in nature (Fox and Veevers 1960).
On regions of the seabed colonized by coloured encrusting organisms, as happens in the cave investigated on the Otago Peninsula, the shell colouration could also function as camouflage, allowing the organisms to blend in with their environment. If functioning as camouflage, which is again common in nature (Fox and Veevers 1960), the brachiopod colouration is inflexible compared to the ability of some organisms to use different carotenoids for adaptive colour changes in response to different environmental conditions. A new species of Calloria with a striking colouration pattern composed of red straight or cuneiform dashes was recently described from rocky subtidal habitats in Northern New Zealand (Cooper and Doherty 1993). This species, Calloria variegata, was most abundant in exposed, lower turbidity, clear water environments than the homogeneously coloured Calloria inconspicua (Cooper and Doherty 1993), suggesting that there may well be some adaptive significance for this strikingly different shell colouration pattern.
The fact that it is colouration pattern that is so noticeable, even when pigments have decayed, may mean that important information from the fossil record is being overlooked. Many taxa that do not display colour patterns but are nevertheless strongly but homogeneously coloured will not be so obvious at fossil localities. The fossil shellbeds in the Wanganui Basin are characterized by a wide variety of different hues of greys and browns. These hues may indicate that decayed portions of the original colour molecule survive, even though the great majority of the original shell colouration has disappeared. Studying such degraded colouration is difficult, but some success has been achieved by the use of ultraviolet lighting, because some coloured pigments fluoresce even when partially decayed. In one study, ultraviolet light revealed the presence of radial banding in an Eocene mollusc shell that had revealed no sign of colouration in visible light (Swann and Kelley 1985). The fluorescence of the shells was enhanced by treatment with sodium hypochlorite. Such a technique has not been applied to the specimens in the Wanganui Basin, but may be of use in detecting fossil representatives of Calloria variegata if they are present.
Successful long-term storage of coloured fossils in museums is problematic. Long-term protection of these specimens from the effects of light is relatively straightforward (by storing in light-proof bags and minimising handling), and attempts will be made also to exclude oxygen from a subset of the material stored in the Hunterian Museum by embedding them in a suitable medium such as a resin. Over a period of years it may then be possible to investigate whether other factors apart from the exclusion of oxygen and light are implicated in the survival of colour over geological time spans.