A NEW METHOD FOR ESTIMATING ORIGINAL SIZE OF MONOTIS FROM COMMISSURAL SHELL FRAGMENTS AND APPLICATION TO NORIAN POPULATIONS FROM KIRITEHERE BEACH, NEW ZEALAND

Kiritehere Beach, North Island, New Zealand represents a marine mid-ramp siliciclastic (fine-grained sandstone) depositional sequence containing shell beds dominated by the late Norian bivalve, Monotis. Seven species of Monotis are documented from this locality including M. subcircularis gigantea. Using complete individuals from collections and monographs, we developed a method for reconstructing original size of fragmented Monotis comprising the shell beds at this locality. While each species of Monotis exhibited strong correlation between height and width of individuals, we selected M. subcircularis gigantea to develop this method because individuals of this species had the strongest r² correlation value. Monotis primary ribs originate at the umbo and terminate at the line of commissure−the total number of primary ribs is conserved throughout the individual’s lifetime, thus they are a useful measure of the percent of the complete shell represented by the fragment. These ribs can also be projected back to point of origin to determine height of the fragmented individual. A grid system based on percent total body size was superimposed on complete individuals of M. subcircularis gigantea to determine the minimum commissural fragment size needed to estimate the original size of the individual. Using this method, there was no significant difference between the actual size of the individual and the estimated size, using fragments that were >40% of the original body size. This method was applied to the Monotis assemblages at Kiritehere, resulting in accurate estimation of original body size for those shell fragment greater than 40% of the complete shell (>5 primary ribs) present. Individuals with less than 30% shell (<5 primary ribs) present had greater variation in projected height, while Monotis fragments of >40% body size produced estimated sizes well-within the observed size ranges of complete individuals.