. Bulletin of the British Museum (Natural History), Geology. 22 SHELL STRUCTURE their ability to fix to marine algae (Bergstrom 1968). Within this stock posterior 'hold fasts' retained the ability to secrete shell material, so evolving tissue-filled posteriorly directed spines. Like their ancestors the spat would have been attached by their pedicle to hard material on the sea-floor, or perhaps to seaweeds. However, at an early age the pedicle atrophied and the development of the spines would have helped stabilize benthonic specimens residing in areas subject to marine currents, particularly th
. Bulletin of the British Museum (Natural History), Geology. 22 SHELL STRUCTURE their ability to fix to marine algae (Bergstrom 1968). Within this stock posterior 'hold fasts' retained the ability to secrete shell material, so evolving tissue-filled posteriorly directed spines. Like their ancestors the spat would have been attached by their pedicle to hard material on the sea-floor, or perhaps to seaweeds. However, at an early age the pedicle atrophied and the development of the spines would have helped stabilize benthonic specimens residing in areas subject to marine currents, particularly those specimens facing into the current which were consequently more susceptible to being overturned posteriorly when the shell opened. In a low velocity unidirectional flow from front to back a 'dead water' zone behind the raised dorsal valve might have prevented the burial or erosion of the spines spread out more or less at the sediment to water interface. These adaptations contributed to the evolution of the Chonetacea in the Lower Palaeozoic fine-grained sedimentary environments in which they are commonly found. During Upper Palaeozoic times chonetaceans spread into regions of coarse shelly detritus as well as living in silt and mud Fig. 8. Hypothetical chonetacean adult community on a soft-bottomed sea floor. The two shells at the top right are dead; one overturned (seen in transverse section), the other part buried. The other three specimens are living (with marginal setae). The two front specimens are cut in longitudinal section; on the left parallel to and on the right along the median line. In these specimens musculature, body wall and lophophore are represented and the arrows indicate the possible flow of water within the brachial 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 work.
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