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Introduction
Tommotiids are an extinct group of metazoans represented by organophosphatic sclerites (individual elements of multicomponent skeletons or scleritomes; Bengtson, 1970, 1985) of various shapes that are often very abundant in lower Cambrian deposits around the world (Rozanov et al., 1969; Landing, 1984; Missarzhevsky, 1989; Bengtson et al., 1990; Conway Morris and Chen, 1990; Esakova and Zhegallo, 1996; Skovsted et al., 2009a, b, 2011; Kouchinsky et al., 2012). The sclerites of tommotiids are generally fairly distinct, but until recently they were exclusively known from disarticulated assemblages, the structure of the skeleton and body plan of the animal remained unknown, and tommotiids were commonly regarded as a phylogenetically problematic group. Reconstructions of the tommotiid animal were largely based on a worm- or slug-like model (e.g., Bengtson, 1970, 1977; Landing, 1984; Evans and Rowell, 1990), and the discovery of the vagrant, slug-like, sclerite-bearing mollusk Halkieria evangelista Conway Morris and Peel, 1995, from the lower Cambrian Buen Formation of North Greenland provided apparent support for this model (Williams and Holmer, 2002; Ushatinskaya, 2002; Demidenko, 2004; Li and Xiao, 2004).
The recent discovery and detailed description of ontogenetically fused specimens of the tommotiid Eccentrotheca helenia Skovsted, Brock, Topper, Paterson and Holmer, 2011 from lower Cambrian carbonates in South Australia by Skovsted et al. (2011) demonstrated that the scleritome of this tommotiid was constructed as a tube composed of vertically stacked sclerite rings. The scleritome exhibited an open-basal aperture that attached to hard substrates and the animal is interpreted to have been a sessile filter feeder related to modern lophophorates (i.e., Phoronida and Brachiopoda; Skovsted et al., 2008, 2011). Subsequently, the tannuolinid tommotiid Micrina etheridgei (Tate, 1892) has been shown to share many characters (shell morphology, ultrastructure, and larval shell morphology) with Cambrian linguliform brachiopods (Holmer et al., 2008, 2011; Balthasar et al., 2009), whereas a third tommotiid taxon, Paterimitra pyramidalis Laurie, 1986 appears to be closely allied with paterinid brachiopods (Skovsted et al., 2009a; Topper et al., 2013; Larsson et al., 2014), thus further strengthening the proposed tommotiid-lophophorate link.
Paterimitra pyramidalis exhibits three different sclerite types: (1) a bilaterally symmetrical, pyramidal sclerite (S1), (2) a triangular and bilaterally symmetrical (S2) sclerite, and (3) laterally compressed asymmetrical (L) sclerites (Skovsted et al., 2009a; Larsson et al., 2014). Ontogenetically...