GROWTH SERIES AND POSSIBLE MOLT INCREMENTS IN FOSSIL CRABS: A PRELIMINARY REPORT

Morphological growth from juvenile to adult and molt increments is poorly known amongst fossil crabs. This paucity of information has mainly been attributed to availability of large numbers of fossil specimens and preservation. Fifteen individual specimens of Mursia sp. from the Miocene Astoria Formation of Oregon, twenty-seven individual specimens of Orbitoplax weaveri from the Eocene Llajas Formation and twenty-seven specimens of Longusorbis cuniculosus from the Cretaceous Northumberland Formation of Vancouver Island, Canada provide a unique opportunity for analysis of growth changes amongst fossil crab individuals. The unusual abundance of fossil juvenile and adult crabs within the same stratigraphic units is unique. Dorsal carapace comparisons of maximum length (L1) and width (W1) depict a strong coefficient of correlation R² > 0.97. When plotted, the two variables are linearly related, suggesting a relatively continuous growth series with possible groupings corresponding to molt stages. In addition, dorsal carapace morphological characteristics such as fronto-orbital percentages to maximum carapace width and differences in shape and size of regions and tubercles between juveniles and adult members suggest a relatively continuous growth series. Morphological differences in the two genera studied fit a general trend in external form, namely the distinction of dorsal carapace regions from moderately defined in juveniles to weakly defined in adults. These ontogenetic changes are attributable to allometric relationships between juvenile and adult forms. Larger sample groups of fossil crabs and dorsal carapace measurements on extant crabs is needed to further develop this idea.