Unlike true Palaeozoic gastropods, but similar to some coeval hyoliths, the cup-like hemispherical embryonic shell of Aldanella attleborensis (Shaler and Foerste, 1888) from the earliest Cambrian (early Tommotian) Erkeket Formation of northern Siberia bears a mucro. Also, the pattern of mortality, with right-skewed distribution and a peak at about 1.0 mm diameter, is not similar to that of early Palaeozoic gastropods; there is no evidence of metamorphosis that would end the pelagic larval stage of ontogeny. Specimens of larger size are rare in samples of phosphatized “small shelly fossils” but are known in related species of the genus, of up to 3–5 mm diameter. A phosphatized soft body is preserved in a few specimens of A. attleborensis, one bearing possible chaetae of about 5 μm diameter. Such bunches of chaetae arming locomotory organs were earlier identified in the genus Pelagiella Matthew, 1895, a more derived member of the same lineage. It shares with the genus Aldanella Vostokova, 1962 also the mucronate embryonic shell and acicular aragonitic shell wall microstructure. The presence of chaetae-bearing organs suggests pelagic mode of life of pelagiellids at maturity. Middle Cambrian Pelagiella shells reached 7 mm in diameter, suggesting evolutionary increase in mature size. Embryonic shell morphology, wall microstructure, and the presence of locomotory organs with a fan of chaetae contradicts gastropod, and even conchiferan affinity of the pelagiellids, but together with the pattern of ontogeny conforms to the enigmatic Palaeozoic hyoliths. They differ in having opercula closing the shell apertures and in lacking evidence of chaetae. The helens, paired apertural appendages of possible locomotory function occurring in apertures of some of them, do not reveal any similarity to chaetae in their development. We propose classifying the order Pelagiellida in the class Hyolitha rather than in the class Gastropoda, until its phylogenetic position is clarified. Such understood hyoliths may represent the earliest stage in evolution of molluscs, immediately following initial diversification of the spiralians (lophotrochozoans) into phyla.
Magnetic reversal frequency and apparent polar wander of the Siberian platform in the earliest Palaeozoic, inferred from the Khorbusuonka river section (northeastern Siberia)
