Tommotiida Missarzhevsky, 1970

Skovsted, Christian B., Balthasar, Uwe, Brock, Glenn A. & Paterson, John R., 2009, The tommotiid Camenella reticulosa from the early Cambrian of South Australia: Morphology, scleritome reconstruction, and phylogeny, Acta Palaeontologica Polonica 54 (3), pp. 525-540 : 528

Order Tommotiida Missarzhevsky, 1970 Family Tommotiidae Bengtson, 1970

Discussion.—Although Landing (1984) assigned Dailyatia and Eccentrotheca to the Tommotiidae , we include only Camenella in this family. Landing (1984) based his concept of the Tommotiidae on his reconstruction of Eccentrotheca as having transverse rows of continually variable sclerites, and his assertion that Camenella and Dailyatia represent reduced versions of the same model where intermediate sclerite morphologies were lost. We now know that Eccentrotheca had a tubular scleritome construction and that sclerite morphology vary vertically with position in the scleritome ( Skovsted et al. 2008). The mode of growth of Eccentrotheca sclerites also seems to be more similar to that of Sunnaginia than to Camenella ( Landing et al. 1980) . We follow Laurie (1986) in placing Dailyatia in the Kennardiidae , based on the presence of three fundamental scleritetypes, including one with bilateral symmetry, compared to the two asymmetrical sclerite types in Camenella . Missarzhevsky (1989) assigned three genera of lapworthellid−like tommotiids to the Tommotiidae ( Bercutia , Geresia , Ninella ), but present knowledge suggests that these genera lack distinct sclerite types and probably belong with Lapworthella (see also Esakova and Zhegallo 1996).

Based on the shared presence of two sclerite types, Bengtson (1970) suggested that Camenella and Tannuolina form a natural group (embraced in the concept of the order Mitrosagophora Bengtson, 1970 ). Later investigations, however, showed that the tannuolinids ( Tannuolina , Micrina ) are characterised by a distinct microstructure including open setal tubes and an alternation of thin compact and thicker spacious layers that were probably originally filled with organic compounds ( Conway Morris and Chen 1990; Williams and Holmer 2002; Li and Xiao 2004). The microstructure of Camenella sclerites is strikingly different with its dense lamination and the lack of setal tubes. The Camenella microstructure is much more similar to Lapworthella (see Landing 1984) and Dailyatia (see Laurie 1986), and for this reason we consider the shared twofold sclerite types of the Tommotiidae and Tannuolinidae to be convergent. As discussed below, the Tommotiidae (e.g., Camenella ) show similarities to members of the Lapworthellidae and Kennardiidae in morphology, ornamentation and mode of shell growth, and these families probably form a monophyletic clade, here termed the camenellans.