AN ASSESSMENT OF THE LIFE AND DEATH ASSEMBLAGE OF TEREBRATALIA TRANSVERSA: IMPLICATIONS FOR THE RECOGNITION OF MORPHOLOGICAL VARIATION IN THE FOSSIL RECORD

Theoretical models of the effects of time averaging on morphology indicate that under certain taphonomic conditions, variance can be either overestimated or falsely partitioned into seemingly distinct groups, resulting in the designation of several species from one, if only fossil data is available. Recent study of the bivalve genus Mercenaria has shown that morphological variance is consistent from extant population to the sub-fossil record. However, Mercenaria is durable and exhibits a rather low degree of morphological variability.

To fully explore this issue, a geometric morphometric study was conducted on the life and death assemblage of the rhynconelliform brachiopod Terebratalia transversa which is common around the San Juan Islands of the Puget Sound, and exhibits high morphological variability and low durability. Live and dead shells were taken from a range of depths (40-80m), which corresponds to the entire depth range of the species. Collection was accomplished by dredging, which allowed continuous sampling of the benthos for ~45m² parallel to depth contour at each site. Articulated specimens were imaged (n=172 (live n=86; dead n=86)), and x-y coordinates were taken for landmarks from two views (anterior, 7 landmarks; dorsal, 9 landmarks). The results of Procrustes superimposition confirm a high degree of morphological variability without distinct morphogroups that have been postulated for the species. Additionally, no significant difference in average shape and shape variability can be detected between the living, and the sub-fossil assemblage. This trend persists at all depths and sizes of individuals.

The results presented here for T. transversa agree with the trend for Mercenaria. Taken together, these studies imply that morphology is rather consistent between life and death assemblages, even for species of low durability. This could mean that the variability of fossil samples can be used to estimate single-generation variability in some cases. Furthermore, the results of this study indicate that species exhibiting a high degree of morphological variability can be recognized as such in the fossil record, rather than as several less variable species.