WHERE HAVE ALL THE YOUNG RAFINESQUINA GONE? GONE TO TAPHONOMIC PROCESSES EVERY ONE (MOSTLY)

Stratinomy of the mixed carbonate/mudstone system of the Cincinnati Ordovician suggests that shell beds are condensed horizons that grew in place over decades to millennia of low siliciclastic input. In contrast, mud- and siltstones were deposited rapidly. Time between blanketing mud deposits varied from days to years. This range of time richness has implications for paleoecological studies. Given a range of taphonomic sensitivities, the taxonomic composition of many fossil assemblages likely resulted from a mixture of ecological and taphonomic signal.

Shell growth offers an opportunity to test this hypothesis. Juveniles break easily, while larger mature individuals are more durable. Most shelled organisms have high juvenile mortality rates so an accumulated death assemblage should contain at least as many juveniles as adults. A paucity of juveniles suggests their preferential taphonomic destruction. Over time, exceptionally large individuals accumulate as smaller ones are destroyed. Assemblages subjected to more time averaging will be characterized by fewer to no juveniles, but more gerontic individuals.

Taphonomy may explain a rarity of juvenile Rafinesquina. Rafinesquina has a very thin, flat shell, strengthened through geniculation and thickening with ontogeny. Rapid juvenile growth rates have been suggested to explain the paucity of juveniles, but examination of growth lines reveals nothing unusual. Rarity of small individuals likely results from taphonomic selectivity.

We measured the hinge width of 1500+ valves on limestone surfaces through 14 meters of section, carefully scanning surfaces for small individuals. Hinge lengths in the data set range ~4–30+ mm. As an initial test, we used 1 meter and ½ meter running averages for lithologies and plotted valve size against shale percentages for the stratigraphic interval around each sample. In aggregate, samples are progressively skewed toward smaller individuals as shale percentage increases, and for low shale percentages, distributions are skewed toward larger individuals.

Thus differential taphonomic survival appears to alter average size by destroying the smallest individuals while concentrating the largest individuals. As a result of varying time richness, taphonomic signals may leave complex signatures.