. Bulletin. Natural history; Natuurlijke historie. 100 150- 200 250- 300-. Gastrochaenidae Hiatellidae Permophoridae Fig. 50. Generic diversity through time for the Gastrochaenidae, Permophoridae and Hiatellidae. The Gastrochaenidae are regarded as comprising three extant genera (Spengleria, Gastrochaena, and Eufistulana) and one extinct genus (Kummelia). The genus Gastrochaena includes the subgenera Gas- trochaena ,Rocellaria and Cucurbitula. Following Runnegarand Newell (1971) the"hiatellid"/?, a possible ancestor of the Hiatellidae, is classified with the Pholadomyidae. T
. Bulletin. Natural history; Natuurlijke historie. 100 150- 200 250- 300-. Gastrochaenidae Hiatellidae Permophoridae Fig. 50. Generic diversity through time for the Gastrochaenidae, Permophoridae and Hiatellidae. The Gastrochaenidae are regarded as comprising three extant genera (Spengleria, Gastrochaena, and Eufistulana) and one extinct genus (Kummelia). The genus Gastrochaena includes the subgenera Gas- trochaena ,Rocellaria and Cucurbitula. Following Runnegarand Newell (1971) the"hiatellid"/?, a possible ancestor of the Hiatellidae, is classified with the Pholadomyidae. The data are otherwise abstracted from Keen (1969a, 1969b) and Chavan (1969). and many modern shallow-burrowing bivalves, it is not unreasonable to assume that this genus evolved more or less directly from shallow- burrowing ancestors, , from Permian or Triassic forms morphologi- cally similar to Sanguinolites? sp. of Chronic (1952). By this hypothesis, the immediate gastrochaenid ancestors had not become morphologically specialized for epibyssate nestling prior to their assumption of the en- dolithic habit. Instead, forms like Sanguinolites? sp. may have evolved through a semiendolithic nesding stage in protected microhabitats. This would brmg their mantle epithelium in contact with coral or shell substrates, thereby permitting evolution of chemical boring. Unlike mechanical bor- ing, chemical boring would require little change in the ancestral shell and anatomical features except for elaboration of glandular tissues involved in the process of calcium carbonate erosion. Gastrochaenids retaining a primarily chemical boring mechanism (, Spengleria) would keep the lateral shell profile, separated siphons, and diffuse pedal musculature of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Peabody Museum of
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