WAS EARTH’S OLDEST MINERAL COLLECTOR A CNIDARIAN? MORPHOLOGICAL, MINERALOGICAL, AND MICROSTRUCTURAL ANALYSIS OF SALTERELLA

The early Cambrian was the scene of the evolution and first radiation of nearly all major clades of animals. During this time, many groups experimented with new biological traits and ecological modes to survive in a rapidly changing ecosystem. Multiple groups independently developed hard skeletons for protection and support using biomineralization or agglutination. While many animals today are known to use one or the other, the Cambrian fossil Salterella is exceptional in that it used both to construct an external biomineralized shell with internal agglutinated grains. This unique characteristic makes Salterella difficult to place in a phylogenetic framework, but significant to our understand of the early evolution of animal skeletons.

Here, we present new morphological, mineralogical and microstructural evidence that supports Salterella as a stem cnidarian. Specimens have been collected from multiple localities across the Laurentian paleocontinent representing a range of depositional environments and taphonomic modes. The funnel-shaped body plan of Salterella is shared with other Cambrian organisms of cnidarian affinity including Cambrorhytium, Glossolites, and Sphenothallus. Salterella was selective in the grains it incorporated into its shell and must have had a morphology conducive to interacting with sediments in its environment. The tentacle structure of cnidarians would allow Salterella to pick grains and move them into its shell. The shell microstructures of Salterella consist of small lath-shaped prisms arranged in inclined lamellae, similar to the microstructures observed in Conularia and Sphenothallus. There are modern cnidarians known to create biomineralized shells and others that agglutinate grains. This study indicates that the capacity to use both mechanisms of skeleton construction is deeply rooted in Cnidarian biology.