MORPHOMETRIC ANALYSIS OF FOSSILS USING HEADS-UP DIGITIZING AND GEOGRAPHIC INFORMATION SYSTEM (GIS) SOFTWARE

Typical biometric studies of organisms are based on measurements made on reference points or landmarks. A great deal of effort can go into defining and measuring landmarks, with the hope that they represent the essential morphological characters of the organism. I describe a detailed methodology using heads-up digitizing from photographs and ArcGIS^® software for capturing significantly more information with only minor increased effort. The method is demonstrated by morphometric analysis of sand dollar echinoids. Rather than collect the coordinates of individual landmarks, the sutures between all skeletal plates are digitized, constrained by standard topological rules, and converted into polygons. Although the data are digitized from 2-dimensional photographs, full 3-dimensional models can be generated based on a few key measurements and straight-forward geometric transformations. The data are stored in geodatabase format (which can include the photography as embedded raster datasets, if desired) and can be exported in a variety of common formats, including GIS shapefiles and Excel^® tables. Plate number, centroid, perimeter, area, and juxtaposition are automatically calculated. The method can also be used to capture data regarding ornamentation (tubercle size and location) and associated characters. Examples taken from an ontogenetic series of the Common Sand Dollar, Echinarachnius parma, reveal important and quantifiable aspects of the phenotypic expression of growth that are not apparent from usual morphometric analysis. These new principles can be compared with similar analyses of an extinct clade of sand dollars which includes the well-known Gulf Coast Eocene species Periarchus lyelli.