Pelagiella exigua: A Stem Gastropod with chitinous chaetae from the Early Cambrian Kinzers Formation of Pennsylvania.
[sciencythoughts.blogspot.com]
Dextrally coiled Pelagiellids, globally distributed in Early to Mid-Cambrian assemblages of small shelly fossils, are among the best known of the early Conchiferans (one of two subphyla of the Molluscs, which includes the Monoplacophorans, Bivalves, Scaphopods, Cephalopods, and Gastropods). Their tiny shells (mostly 1–2 mm across, but Pelagiella atlantoides reached 9 mm) expand asymmetrically away from the plane of the first whorl, down the spiral axis, then back upward to establish an adult form that is almost planispiral, with a depressed apex. They have been treated implicitly or explicitly as stem group Conchiferans, as untorted Helcionelloids (an extinct branch of the Conchiferans known from the Cambrian and Early Ordovician), as Monoplacophorans, and explicitly as Gastropods presumed to have undergone torsion during their development (torsion, rotation of the visceral mass by 180° relative to the head and foot during larval development, is considered to be a key feature of Gastropods). Recently, together with the Aldanellids, they have been linked erroneously to the Hyolitha.
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