EARLY EVOLUTION OF SPONGE ANIMALS: SPICULOGENESIS AND BIOMINERALIZATION

Most modern sponges have biomineralized skeletons in the form of siliceous or calcareous spicules. Siliceous sponge spicules consist of a thin axial filament surrounded by concentric layers of amorphous silica. The axial filament serves as a template for silica deposition, contains the biosilicification enzyme silicatein, and has either a triangular or tetragonal in cross section. Sponge spicules are unsurprisingly common in the rock record, but fossil axial filaments are rare. Since sponges are some of the earliest animals to have developed biomineralization, it is critical to understand the role of axial filaments in the early evolution of sponge spiculogenesis and biomineralization. Here we report well-preserved axial filaments in taphonomically demineralized spicules from the early Cambrian Hetang Formation in South China. The axial filaments were examined using secondary electron microscopy, backscattered electron microscopy, energy dispersive X-ray spectroscopy, element mapping, and focused iron beam electron microscopy techniques. These axial filaments are micrometric in diameter and monaxonal or triaxonal in shape, consistent with the shape of their hosting spicules. Thus, the axial filament must have played an important role in guiding and regulating sponge spiculogenesis and biomineralization. However, unlike their modern counterparts, the Cambrian axial filaments are proportionally large relative to spicule size, suggesting that the earliest sponge spicules were composed of a significant amount of organic material and represent a case of weak mineralization or early ontogenetic development. In addition, these Cambrian axial filaments are neither triangular nor tetragonal in cross section; they are cylindrical instead, suggesting a greater diversity of axial filament morphologies among early sponges. This discovery has significant implications on spiculogenesis and biomineralization of early sponges.