Modern pinnoideans, or “pen shells,” are shallowly to deeply endobyssate, equivalved, edentulous, ham-shaped bivalves with a single, slightly submerged, opisthodetic ligament. Malacologists have generally regarded pinnids as closely related to the pterioid superfamily Pterioidea, especially its Paleozoic family Pterineidae (e.g., Zittel, 1927, p. 446; Thiele, 1935, p. 803; Pojeta, 1978; Waller, 1978). As early as 1890, Jackson suggested that pinnids evolved from the Silurian-Permian pterineid LeptodesmaHall, 1883. More recently, Pojeta (1978, p. 239) speculated that they evolved from the pterineid PteronitellaBillings, 1874 (a Silurian genus similar to Leptodesma) through...

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