ORIGINAL SHELL COLOURATION

While collecting the brachiopods it became clear that all of the relatively restricted range of taxa present at this locality displayed undoubted preservation of original colouration. The orange-red colouration of Calloria inconspicua (Figures 3, 4, 5) and of the single specimen of Terebratella sanguinea discovered (Figure 6) was particularly distinctive, but the other brachiopod present in some abundance, the rhynchonellid Notosaria nigricans, also displayed preservation of its less obvious black to brownish colouration. Other molluscan taxa displayed original shell colouration (Figure 7), indicating that fossilisation and diagenetic conditions have favoured the survival of colour pigments at this locality.

On the first day of collecting, the brachiopods exposed on the surface of the fossiliferous layer were isolated from those collected from within the sediment, and the state of colour preservation was assessed separately. Although the method of assessment is subjective, depending on a visual assessment under subdued lighting, it reinforced the impression that developed during collection that the specimens exposed at the surface displayed poorer colour preservation than those still entombed in sediment. Of the 54 complete or fragmented specimens of Calloria inconspicua that were exposed at the surface of the sediment, a total of 36 (67%) had obvious signs of colour preservation when washed of sediment. It was more difficult to assess the colour survival in specimens of Notosaria nigricans, but a similar exercise was attempted, and the results suggested that only 22% of specimens exposed at the surface had preserved colouration (5 out of 23) as compared with 41% (17 out of 42) from within the sediment.

The subsurface sampling at this locality yielded a total of 323 complete or partial specimens of Calloria inconspicua, and 261 of these had obvious signs of colouration (81%). The comparable data for Notosaria nigricans were 75 coloured specimens out of a total sample of 179 (42%). The specimens with the strongest colour preservation were predominantly found underneath or at the side of large pebbles, in crevices between pebbles, or inside large mollusc shells. The implication is that these individuals were deposited, and fossilized, in cryptic habitats. (Table 1, and Figures 8, 9).

The indications are that these brachiopods demonstrate the preservation of original shell colouration, rather than post mortem mineralisation or ‘contamination’ of the sample by Recent specimens. The majority of specimens obtained were collected from within the sediment in positions that rule out any possibility of contamination by Recent specimens. Sessile, pedunculate brachiopods living on this part of the coast since the deposition of the Rapanui sediments would not have been able to penetrate the sediment because they are incapable of burrowing. The presence of coloured brachiopods more than 0.3 m beneath the sediment surface, leaving clear imprints on the adjacent sediments, and the occurrence of other specimens within the partially open valves of articulated bivalves, guarantees that these are indigenous specimens.

The fossils are much less strongly coloured than living representatives of the species, the shells of which are a bright red colour (Figure 10). The orange-red colour of the fossils indicates that partial decay of the molecule responsible for shell colouration has occurred. In this species the red shell colouration is a result of the presence of a red caroteno-protein that occurs within the crystal of the shell (Curry et al. 1991). As this molecule decays, its colour will change in much the same way that rotten fruit or leaves pass through a variety of colour stages. An amino acid analysis from the fossil shells demonstrated that partial molecular decay had taken place when compared with an equivalent analysis from living representatives of the species (Figure 11, also includes description of the methodology used). The fossil shells are enriched in Glycine (G) and Alanine (A) relative to Recent specimens, and this is expected as fossil shell are characterized by increases in the proportions of these amino acids (because they are formed post-mortem by the decay of less stable amino acids--Curry et al. 1991). The fossils do display a range of colour intensities and hues, suggesting that variable decay of the carotenoid part of the molecule has also taken place. This range of colours present in the fossils is similar to those in living shells, and there is no indication of post-mortem staining by iron-rich solutions that is known to occur in other brachiopods (Zenger 1967).