MINIATURIZATION OF THE TRILOBITE FLEXICALYMENE: IS THERE HETEROCHRONY IN THE CINCINNATIAN SERIES?

The Liberty Formation of the Cincinnatian Series (Upper Ordovician) contains an unusually small morphotype of the trilobite Flexicalymene. The trilobites of the Liberty Formation are particularly important to the evolutionary history of Flexicalymene because these specimens are the last recorded identifiable specimens within the Cincinnatian Series, prior to the maximum regression at the end-Ordovician and a subsequent transgression in the Silurian that reintroduced calymenids into the region. The occurrence of small specimens in both limestones and shales, co-occurrence of specimens with fauna of varying sizes, and the correlation of these beds over 70 km indicates that physical processes and behavioral phenomena are not responsible for this unique occurrence. In the literature, miniaturization of trilobites has been primarily attributed to heterochrony. Two major changes in the rate or timing of development, dwarfism and progenesis, lead to descendants of smaller size. Geometric morphometric analysis shows small-scale changes in the morphology of the cranidia of the Liberty specimens relative to larger specimens in other formations. In addition, size standardization techniques have shown that the ontogenetic trajectories between specimens of the Liberty Formation and the Waynesville Formation are different, indicating that not only has size and shape changed between specimens of these two formations, but patterns of holaspid growth have also changed. The results of morphometric analyses indicate that specimens of Flexicalymene from the Liberty Formation are not simply small versions of the Waynesville Flexicalymene, but rather show changes in both shape and pattern of development. While size may be predominantly being selected for, it is not easily decoupled from shape changes, and selection factors contributing to these patterns are complex.