22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 48 



moderate- to shallow- water environments with active currents (upper 

 Eagle Ford Shale, Codell Sandstone, parts of the middle Mancos), to 

 shallow-water, inner-sublittoral, high-energy conditions, including 

 wave action, a great deal of reworking, and periods of nondeposition 

 (Juana Lopez Member, "Reworked Eagle Ford zone"). 



Most of the trends thought to be selected for environmental changes 

 concern themselves with improving the hydrodynamic stability, 

 strength, and anchorage of the shell. Hydrodynamic stability of the 

 shell in the face of increased current and wave activity was probably 

 improved by the overall decrease in shell convexity, ventral migration 

 of the high point, reduction of projecting surfaces such as the auricles, 

 increase in relative symmetry of the shell, and evening of the overall 

 shell surface by a great reduction in the prominence of the auricular 

 sulci, interplical sulci, and plicae. Reduction in shell size may also 

 have affected stability of the shell, though it is more likely tied to the 

 restricted amount of available feeding time in active shallow-water 

 environments as compared to that available in deeper, nonagitated 

 waters. 



The development of smaller and more numerous plicae by shallow- 

 water ostreids in this lineage appears to be a twofold adaptation to a 

 near-shore environment. Most obviously this results in strengthening 

 of the shell, necessary for life in the presence of waves, strong 

 currents, and continued buffeting, while at the same time eliminating 

 coarse projections from the shell surface, producing a more even, less 

 resistant surface over which water can flow. Even more important 

 may be the role of plications as an adaptation to feeding in a turbid, 

 high-energy environment. Development of extensively plicate and 

 crenulate commissures in marine bivalves allows the animal to reduce 

 considerably the gape of the valves during feeding over that required 

 by forms with smooth margins, without greatly altering the total 

 amount of open area between the valves for the influx of water. The 

 advantages of reducing the gape while maintaining normal water 

 intake in a turbid environment are apparent, since there is a 

 corresponding reduction in the size and possibly the amount of foreign 

 particles able to filter between the valves, and consequently a greater 

 degree of protection against clogging of the feeding mechanisms and 

 agitation of the mantle. This helps to offset the abundance of mobile 

 detritus found in near-shore, high-energy environments, and allows 

 the oyster to feed for longer periods of time, and tolerate greater 

 turbidity, than nonplicate species subject to the same conditions. 



Progressively younger members of the lineage display a gradual 



