Micro-ornamentation, ultrastructure, composition and homology among early Cambrian tommotiids

Tommotiids are an important but enigmatic early Cambrian animal group, characterized by accretion of organophosphatic sclerites as part of a complex external skeleton called a scleritome. Previous research suggests micro-ornament similarities occur among eccentrothecimorphs, tannuolinids, camenellans, and the earliest brachiopod groups. Their taxonomic and phylogenetic relationships have remained uncertain for decades, impeding understanding of the origin and early evolution of lophotrochozoan animals. Currently, tommotiids are informally subdivided into two groups: mobile, slug-like camenellans and sessile, tubiform eccentrothecimorphs. Tommotiids are found in hyperabundance in lower Cambrian carbonate deposits across the Arrowie Basin, South Australia, which provided raw material for novel and advanced investigations. 

This project undertakes a detailed comparison of micro-ornamentation, ultrastructure and elemental composition of key tommotiids, primarily eccentrothecimorphs, at different scales and across sclerite types. Novel data reveal that expression of 1st and 2nd order lamination, penetrative polygonal ultrastructures (PPS), and ornamental textures are shared among eccentrothecimorph tommotiids and the paterinate brachiopod Askepasma. It suggests that the bulbous ornament and the internal PPS structures provide a strong base for sclerite articulation for eccentrothecimorphs, while these ultrastructures are weakly developed in early brachiopods. By contrast, both tannuolinids and camenellans lack these shared traits, potentially suggesting they are more distantly related.