Maikhanellidae Missarzhevsky, 1989

Kouchinsky, Artem, Bengtson, Stefan, Landing, Ed, Steiner, Michael, Vendrasco, Michael & Ziegler, Karen, 2017, Terreneuvian stratigraphy and faunas from the Anabar Uplift, Siberia, Acta Palaeontologica Polonica 62 (2), pp. 311-440 : 359-364

Family Maikhanellidae Missarzhevsky, 1989

Remarks.—The family Maikhanellidae was initially assigned by Missarzhevsky (1989: 24, 179) to the monoplacophorans. Ponder et al. (2007) supported that assignment

Fig. 29. Molluscs Salanyella cf. costulata Missarzhevsky, 1981 (A–C, E–H) and Salanyella sp. (D), internal moulds, from early Cambrian Medvezhya → Formation, Kotuj River, western Anabar Uplift, Siberia, Russia; samples 3/12.2 (A–D), 3/12 (E), and K2/25 (F–H) from section 3. A–H. SMNH Mo182314–182318 (A–E, respectively), 160414 (F; see also Kouchinsky 2000a: fig. 7H–J), 182319 (G), 182320 (H). A 1, B 2, C 3, D 2, E1, supra-apical;

A 2, B 1, C 2, D 1, E 2, F 2, G, H, lateral; A 3, B 3, C 1, D 3, subapical views; F 1, apex with spiny structures enlarged; D 4, apertural area in D 3 enlarged. Scale bar 125 μm (D 4, E 2, F 1), 250 μm (A 1, B 1, D 1, E 1), 500 μm (A 2, A 3, B 2, B 3, C, D 2, D 3, F 2, G, H).

but also discussed the similarity of the cap-like shell and its scaly ornamentation to those of some patelliform gastropods see also Parkhaev and Demidenko 2010). An alternative phylogenetic interpretation by Bengtson (1992) is based on evidence for maikhanellids merging siphogonuchitid sclerites into their shells. Based on younger Wiwaxia -like organisms with non-mineralized scleritomes and on complete scleritomes of halkieriids, which have two terminal shells and rows of intermediate sclerites ( Conway Morris and Peel 1995), maikhanellids were assigned to a phylum-level,lophotrochozoan stem-group called Halwaxiida Conway Morris and Caron, 2007. Butterfield (2006) suggested, however, that Wiwaxia and halkieriids have markedly different sclerite construction and composition and cannot be assigned to one phyletic grouping. This conclusion was also supported by comparison of ontogenetic series of wiwaxiid scleritomes from Cambrian Stage 3 deposits of the Chengjiang Lagerstätte with the Halkieria scleritome (Zhang et al. 2015).

An alternative concept of a new molluscan class, Diplacophora Vinther and Nielsen, 2005, a sister group to

Fig. 31. Mollusc Purella antiqua ( Abaimova, 1976) , calcium phosphate coatings of the shells, from early Cambrian Nemakit-Daldyn (D, E) and basal Emyaksin (A–C) formations, Anabar Uplift, Siberia, Russia; samples 4/27.5 (A), 5/0 (B, C), and K1a/47 (D, E) from sections 96-4, 96-5, and section 2, respectively. A–E. SMNH Mo160403 (A; see also Kouchinsky 2000a: fig. 3), 182323 (B), SMNH Mo182324 (C), SMNH Mo160406 (D; see also Kouchinsky → 2000a: fig. 4G), SMNH Mo182325 (E). A 1, A 4, views of inner shell surface; A 2, close-up of inner shell surface in A 1; A 3, microtubules of the initial shell enlarged; B 1, lateral; B 2, apical; B 3, subapical views; C, oblique subapical view showing apically smooth external shell surface and distal shell composed of circumferentially elongated scales; external (D) and inner (E) surfaces of shell fragment. Scale bar 50 μm (A 3), 100 μm (A 2), 250 μm (A 1, A 4, B–E).

the Polyplacophora De Blainville, 1816, was advanced by Vinther and Nielsen (2005). Parkhaev and Demidenko (2010: 955) further discussed the affinity of sachitids (halkieriids and siphogonuchitids) to chitons and placed them in a subclass Diplacophora of the class Polyplacophora . The presence of two shell-plates (anterior and posterior) may however not characterize all members of the halkieriid stock, since only a single shell plate is present in the probably closely related Orthrozanclus Conway Morris and Caron, 2007 . In this report, we consider an assignement of the maikhanellids and sachitids to the stem-lineage of the Polyplacophora , although persuasive evidence is still lacking.

New evidence on the widespread occurrence of multiunit, shell-bearing scleritomes in the Terreneuvian Series emerged from studies of the Ocruranus – Eohalobia group of fossils (Qian and Bengtson 1989; Siegmund 1997), which were interpreted to represent components of the same organism from the chiton stem lineage (Vendrasco et al. 2009). Similarly, halkieriids allow comparisons with the shells of Ocruranus Liu, 1979 , and Eohalobia Jiang in Luo et al., 1982 synonymized under Ocruranus by Vendrasco et al. 2009). However, there is no evidence of merged sclerites in the latter genera, and their shells have prismatic and lamello-fibrillar microstructures similar to those known from early Cambrian helcionellid molluscs (Vendrasco et al. 2009).