RELATIVELY INEXPENSIVE HIGH-RESOLUTION COMPUTED TOMOGRAPHY OF A PERMIAN OSTRACODE SPECIES

Computed tomography (CT) scanning of fossils is not new; a search of the internet reveals numerous examples of the efficacy of this non-destructive technique that has been used for more than a decade to study full three dimensional surface scans of fossils rather than, for instance, 2-D traditional landmark analysis. However, examples of CT scanning of microfossils are rare and seem to be limited largely to the study of foraminifera. We acquired relatively inexpensive CT scans of ostracodes for morphometric study.

We used high-resolution CT scans on a species of hollinellid ostracode in order to perform three dimensional morphometric analysis to better interpret its ontogeny and population dynamics. The major problem we wanted to solve was how to determine accurately the volume of each valve in order to establish its instar group using Brook’s rule. Brook’s rule suggests that with each molt during development crustaceans double their volume, which causes their linear dimensions to increase by 1.26 (the cube root of two). Numerous older ontological studies of ostracodes have attempted to define molt stages two dimensionally by plotting length and height of individuals of the same species and looking for clusters of points that might represent instars and adults. However, the scattergrams that result from such plots more often than not show a scattering of instar sizes with poorly defined cluster boundaries. The lack of discrete clusters has been attributed to time averaging or the use of length and height as proxies for the volumes of shells with complex three dimensional shapes. We printed our 3-D scans and computed their volumes by filling the resulting models with water on an electronic balance. We also computed volumes of scans with software. The use of volume measurements produced better clustering than the use of scatter plots of length versus height on this species. Furthermore, the 3-D printed models revealed details of valve morphology that we had not previously observed under a microscope, including details of valve margins and hingement.