Supplementary data for: A Cnidarian affinity for Salterella and Volborthella: Implications for the evolution of shells

This is the supplementary material for the publication "A Cnidarian
Affinity for Salterella and Volborthella: Implications for the Evolution
of Shells" in the Journal of Paleontology. It includes 22 figures and
3 data tables of material used in the analyses in the publication. The
Cambrian Explosion saw the widespread development of mineralized
skeletons. At this time, nearly every major animal phylum independently
evolved strategies to build skeletons either through agglutination or
biomineralization. Although most organisms settled on a single strategy,
Salterella Billings, 1865 employed both strategies by secreting a
biocalcitic exterior shell that is lined with layers of agglutinated
sediments surrounding a central hollow tube. The slightly older fossil,
Volborthella Schmidt, 1888, shares a similar construction with
agglutinated grains encompassing a central tube, but lacks a
biomineralized exterior shell. Together these fossils have been grouped in
the phylum Agmata Yochelson, 1977, although no phylogenetic relationship
has been suggested to link them with the broader metazoan tree, which
limits their contribution to our understanding of the evolution of shells
in early animals. To understand their ecology and place them in a
phylogenetic context, we investigated Salterella and Volborthella fossils
from the Wood Canyon and Harkless formations of Nevada, USA, the Illtyd
Formation of Yukon, Canada, and the Shady Formation of Virginia, USA.
Thin-section petrography, acid maceration, scanning electron microscopy,
energy dispersive X-ray spectroscopy, X-ray diffraction, and X-ray
tomographic microscopy were used to provide new insights into these
enigmatic faunas. First, morphological similarities in the aperture
divergence angle and ratio of central tube diameter to agglutinated layer
thickness suggest Salterella and Volborthella are related. Second, both
fossils exhibit agglutinated grain compositions that are distinctive from
their surrounding environments and demonstrate selectivity on the part of
their producers. Finally, the calcitic shell composition and simple layers
of blocky prismatic shell microstructure in Salterella suggest a possible
cnidarian affinity. Together these data point to these organisms being
sessile, semi-infaunal filter or deposit feeders, and an early
experimentation in cnidarian biomineralization chronicling a hypothesized
transition from an organic sheath in Volborthella to a biomineralized
shell in Salterella.