Sclerite fusion, ultrastructure, micro-ornament and homology in early Cambrian eccentrothecimorph tommotiids

Tommotiids are an enigmatic but important group of early Cambrian lophotrochozoans, characterized by a complex external skeleton (or scleritome) comprised of fused or cataphract organophosphatic sclerites produced by basal marginal accretion. Currently, tommotiids are informally subdivided into three main groups: the mobile, benthic camenellans that accreted a complex array of sclerites arranged in serial rows along the length of the body; and two sessile groups that include the tubiform eccentrothecimorphs and bimembrate caniculate tannuolinids, currently recognized as members of the Lophophorata. Here we undertake a detailed study of internal ultrastructure and surface ornamentation of partially fused tubular scleritomes and a wide range of isolated sclerite morphotypes of key early Cambrian eccentrothecimorph taxa from Australia. Comparisons are also made with the shell ultrastructure and ornamentation of the contemporaneous paterinate brachiopod Askepasma, along with other previously described paterinates and enigmatic stem group brachiopods. Mode of fusion between adjacent sclerites in the scleritomes of Kulparina rostrata and Eccentrotheca helenia is shown to be identical, consisting of a series interlocking dome and saddle features. New data also reveal that all Australian eccentrothecimorph sclerites possess first and second order organophosphatic laminations, distinctive penetrative polygonal structures (PPS), and surface ornament textures, which are also shared with Askepasma and other early Cambrian paterinates, reinforcing the close relationship between these taxa. In addition, the newly discovered scleritome of Kulparina rostrata is briefly described for the first time herein, providing further evidence that a tubular scleritome is a shared feature of eccentrothecimorphs.