TAPHONOMY OF MOPALIA MUSCOSA AND KATHARINA TUNICATA (PHYLUM MOLLUSCA, CLASS POLYPLACOPHORA) FROM CATTLE POINT AND FALSE BAY, SAN JUAN ISLAND, WASHINGTON, USA

The fossil record for chitons (Phylum Mollusca, Class Polyplacophora) spans most of the Phanerozoic yet comprises only about 350 accepted species. The sporadic record implies low diversity and low preservation potential when compared to the more than 850 known extant chiton species. Although some excellently preserved plates and fully articulated specimens have been reported, poorly preserved and isolated plates are more common, suggesting that the fossil record is biased against chitons. Taphonomic filters that induce bias can be due to extrinsic or intrinsic factors. Results from field experiments focusing on Mopalia muscosa and Katharina tunicata suggest that extrinsic characters (e.g., community structure, locality and life habit) affect the preservation potential of chitons.

Since more abundant members are more likely to become fossils, community structure can affect the preservation potential of a particular chiton species. The number of chiton plates collected in the field during surface surveys on San Juan Island, Washington agrees with species abundance by locality. At Cattle Point, where K. tunicata was more common, the ratio of K. tunicata to M. muscosa was 78:41, but at False Bay, where M. muscosa was more abundant, the ratio was 23:171. However, when individual plates were scored for taphonomic grade (i.e., the higher the score the less pristine the plate), the total score for all collected K. tunicata plates tended to be higher than M. muscosa with mean values of 11.04 and 5.05 respectively out of a maximum score of 20. There was a significant difference in median values, based on the results of a Wilcoxon test (Z=-2.5635, Two-sided Pr > /Z/=0.0104).

Locality and life habit can affect preservation potential since species living in lower energy environments are more likely to be fossilized. When mean taphonomic scores were analyzed by parameter and locality, the higher energy levels at Cattle Point resulted in generally higher scores for K. tunicata relative to those at lower energy False Bay. However, habitat preferences within a locality can result in mitigated energy levels. Whereas K. tunicata prefers areas exposed to the strongest wave action, M. muscosa favors protected tide pools. There was little difference in mean parameter scores by locality for M. muscosa, apparently due to this preference.