At the present day brachiopods are not common in inter-tidal habitats, but Calloria inconspicua is found in a few very shallow water localities around New Zealand. The colouration of living and subfossil specimens of one of these intertidal populations has been studied to investigate the pattern of colour decay. Although the Portobello locality differs in some respects from that inferred for the Wanganui Basin fossils, it does provide a rare and invaluable opportunity to investigate brachiopod colour survival in an enclosed shallow water environment.
Large numbers of living Calloria have been recorded from the Otago Peninsula on the South Island of New Zealand. One important locality close to the Portobello Marine Laboratory is in a long, narrow cave excavated in volcanic rocks. This cave has a prominent lip at the seaward entrance, which restricts access to the open sea and indeed isolates the cave at low tides. Calloria inconspicua is abundant in this cave, living at the base of a sloping rock wall and on boulders scattered on the floor of the cave. Significant numbers of these brachiopods are exposed at low tide, although the great majority occurs at and below low tide level. While not directly comparable to the soft-sediment coastline of the Wanganui Basin, the water depths…
In life, most of the cave brachiopods are situated
in cryptic habitats out of direct sunlight or are covered by encrusting
organisms. However, dead specimens fall onto the cave floor, which is exposed to
direct sunlight at the mouth of the cave. The sediment of the cave floor is a
very fine-grained greenish silt that is easily disturbed. To investigate the
colour survival of dead specimens of Calloria inconspicua, all
individuals (either complete specimens or separated dorsal or ventral valves)
visible on a 2 m2 section of the cave floor
were carefully collected using a snorkel and mask at low tide.
This surface collection yielded a total of 281 specimens of which 57% displayed
colouration. The upper layers of the sediment were then bulk sampled and sieved
to a depth of approximately 50 mm using a 2-mm sieve, and the resulting sample
of an additional 321 Calloria shells contained only 46% of individuals
with discernible colouration (Figures 11,
12).
In
the interest of conservation of this remarkable locality it was not possible to
harvest and sample the living population, but an underwater survey of
approximately 100 specimens living attached to the cave wall suggested that at
least 95% had strong colouration (Figure 13).
A survey of specimens living in the open sea close to the mouth of the cave
indicated that virtually all shells displayed strong red colouration, although
assessing colouration in living specimens is problematic because of the
prevalence of encrusting organisms.
Sedimentation rates in the cave are likely to be low, and the shells exposed at the surface do become exposed to full sunlight. However these results confirm that brachiopods tend to lose their shell colouration when exposed at sediment surfaces. This decay continues after burial to the extent that more than 50% of the specimens in the top 50 mm of sediment lose their colour. The fact that around 80% of the fossil specimens of Calloria inconspicua displayed some traces of colour after 80 ka, as compared with less than 50% in the top 50 mm of sediment in the Portobello cave, confirms that the taphonomic conditions at the former locality are unusual.
There is no doubt that the majority of fossil brachiopods lose whatever colour they may have possessed. Virtually nothing is known about the length of time over which brachiopod shell colouration will disappear. Specimens of Calloria inconspicua exposed to daylight for five years show no obvious signs of colour decay. It seems probable that the timing of colour decay will vary depending on taphonomic conditions, and that other factors, such as the nature and effect of percolating groundwater, will also have an effect. Calloria inconspicua itself occurs extensively throughout the Wanganui Basin in 35 horizons ranging in age from approximately 2.98 Ma to 40 ka (see the "Interrograte the Wanganui Database" web site), but this is the first record of any fossil representatives displaying original shell colouration from the Basin. Extensive field investigation of many localities in the Wanganui Basin has failed to reveal any other coloured representatives of Calloria inconspicua, and specimens from the 40 ka locality do not display any colour. It seems likely that rapid burial, presumably associated with the resultant exclusion of light and oxygen, are at least factors in colour survival in the fossil record. Rapid burial is a realistic inference for the fossil Calloria shells in any beach gravel deposited on a marine terrace that is likely to have been formed rapidly in unconsolidated sediments.