Ontogeny and microstructure of the enigmatic Cambrian tommotiid Sunnaginia Missarzhevsky, 1969

The tommotiids are a significant component of the earliest skeletal animal remains in the fossil record, occurring in large numbers in the Lower Cambrian. Sclerites of the tommotiid genus Sunnaginia have been implicated as integral to hypotheses regarding the evolution of the brachiopod body plan, with a morphology intermediate between the unspecialized sclerites of the tubular Eccentrotheca and the specialized sclerites of the tannuolinids. Abundant Sunnaginia imbricata sclerites, of a broad ontogenetic spectrum, were recovered from the Comley Limestone, Lower Cambrian (Stages 34), Shropshire, UK and compared to Sunnaginia imbricata from the Aldan River, Siberia (uppermost Tommotian). New microstructural data, collected using synchrotron radiation X-ray tomographic microscopy, reveal a unique microstructure for Sunnaginia ?imbricata sclerites among the tommotiids; interlamellar cavities spanned by a series of continuous pillars, giving a colonnaded appearance contrasting to that of S.imbricata. These data refute the inclusion of Eccentrotheca within the Sunnaginiidae and highlight the need for a revision of suprageneric classification of the tommotiids. Rather, structural similarities between Sunnaginia sclerites and those of the tannuolinids suggest a close affinity to this group. Recent phylogenetic hypotheses place the tannuolinids as stem-linguliform brachiopods, with Paterimitra plus the paterinid (and possibly rhynchonelliform) brachiopods as their sister group. Our new data therefore resolve Sunnaginia as close to the node defining crown-Brachiopoda. However, the characters supporting this phylogenetic scheme cannot be consistently applied to all taxa, nor do they define a series of nested clades. We therefore suggest that a more thorough phylogenetic analysis is required in the light of the data presented here and other recent descriptions.