Article: Silurian polyplacophoran molluscs from Gotland, Sweden
A new, unusually diverse and abundant polyplacophoran (chiton) assemblage occurs in the Silurian of Gotland, Sweden, largely as silicified material. The paleoloricate fauna described here includes: Thairoplax pelta gen. et sp. nov., Plectrochiton tegulus gen. et sp. nov., Alastega lira gen. et sp. nov., Heloplax papilla gen. et sp. nov., Enetoplax decora gen. et sp. nov. and Arctoplax ornata gen. et sp. nov.; Gotlandochiton Bergenhayn is revised. One genus represented by an intermediate sclerite, and two unassociated end sclerites, are left under open nomenclature. Most material is from the late Wenlock Halla Formation. Three of the new genera have a holoperipheral growth style and dorsal mucronate apex, normally found only in tail sclerites; plates of these genera display prominent subapical muscle cavities. Patterns of growth banding that associate several such plates as coming from the same individual are important in suggesting that most represent intermediate, and not tail, sclerites. The morphological evidence from sclerites for musculature, and muscle function are assessed for all of the chitons. Recent chiton plates, which have sutural laminae for physical articulation, do not show comparable indications of insertion sites. Several of the Gotland chitons had elongate sclerites, and were elongate animals by comparison with Recent chitons; others had shorter, broader plates and hence a more ovoid body form.