The synchronous and widespread appearance of the ability to biomineralize was a defining event in the early evolution of the Metazoa. However our understanding of how animals first evolved this capacity, and the degree of genetic conservation in the metazoan biomineralization toolkit, is extremely poor. Because sponges are thought to be the earliest branching phylum of biomineralizing metazoans, we have studied the molecular basis of skeleton formation in the marine sponge Astrosclera willeyana. I will present several key insights we have made into how A. willeyana forms its skeleton using a variety of techniques, and how this process may have originated. I will also present some results of our work on molluscs regarding their shell-forming molecular toolkits, the ontogeny of the shell from the gastropod model Lymnaea stagnalis, and recent insights we have gained into how this phylum-level novelty may have evolved.