MEASURING UP: HOW DOES THE EARLY CAMBRIAN SMALL SHELLY FOSSIL SALTERELLA FIT IN PHYLOGENETIC SPACE?

Biomineralized shells and agglutinated tests provide inorganic protective coverings that are essential to the success of multiple animal groups. The evolution and radiation of these traits began during the Cambrian explosion. No modern animal is known to make both, but the early small shelly fossil Salterella built an external biomineralized shell with agglutinated layers of sediment inside. This unique set of traits makes the phylogenetic affinity of Salterella uncertain, but critical to our understanding of the early evolution of hard skeletons.

Here, we use light microscopy, scanning electron microscopy, and biometry to quantify the size, shape, microstructures, and growth of Salterella and related cone-shaped shells from the early Cambrian. Specimens were examined from localities in Virginia and Nevada in the United States and in Yukon in Canada. These specimens exhibit various taphonomic conditions, ranging from examples preserving the original fabrics of the organism’s skeletal structure, to those solely represented as casts and molds. Biometric data provide novel information on shell growth, grain agglutination, and taphonomic alteration. Combined, these data will be used to test the hypothesis that Salterella is a basal lophotrochozoan.