GROWTH SERIES AND POSSIBLE MOLT INCREMENTS IN FOSSIL CRABS

As paleontologists, when we examine fossil specimens for classification, we are usually dealing with a small quantity, few morphological attributes, and often with species that no longer exist today. Systematics of fossil crabs is often based upon subtle differences in the dorsal carapace, which is the most often preserved portion of the fossil crab. In addition, new species of fossil crabs are often based upon as little as one specimen. Therefore, it is significant to understand growth stages and the changes that are reflected from juvenile to adult specimens. Morphological growth from juvenile to adult and molt increments is poorly known among fossil crabs. This paucity of information has mainly been attributed to lack of availability of large numbers of fossil specimens and preservation. Fifteen individual specimens of Mursia sp. from the Miocene Astoria Formation of Oregon and twenty-seven individual specimens of Orbitoplax weaveri from the Eocene Llajas Formation provide a unique opportunity for analysis of growth changes among fossil crab individuals. This 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 are needed to further develop this idea.