Vesicular microfossils on middle Cambrian shells: insights into early substrate colonization in North Greenland (Laurentia)

Biomineralization around the Ediacaran–Cambrian boundary enabled new ecological strategies, including encrustation, boring and cavity‐dwelling, across various lineages. Here we describe problematic vesicular fossils that are attached to, or embedded within, the calcium phosphatic shells of the tommotiid genus Tesella from the middle Cambrian Henson Gletscher Formation of North Greenland. Despite their simple morphology, the combination of characteristics such as size, shape, presence of an invagination, habitat, and clustering behaviour, suggests that these fossils are best interpreted as colonizing microbes in a biological relationship with the tommotiid. We explore potential affinities of these vesicular microfossils, including comparisons to parasitic fungi such as Chytridiomycota (‘chytrids’). We describe a new genus and species, Avannacystis polaris , which provides new insights into early biological interactions, and substrate colonization in Cambrian ecosystems.