134 



ments. A diagram illustrating the relationship of these assemblages to the 

 geomorphology of the sea bottom and surrounding land masses is shown 

 in fig. 27. 



The average standing crop of benthic invertebrates for the inshore 

 assemblage appears low as compared to that found for quantitative studies 

 carried out in temperate or boreal regions. Since so few quantitative 

 studies have been made elsewhere in the tropics and the area covered 

 quantitatively in this study is so small, a comparison between the Gulf 

 of California and other tropical regions is difficult. 



Evidence was produced from a comparison of shelf assemblages found 

 off the Pacific coast off Mexico and in the Gulf of Mexico that a large 

 number of twin or allopatric species of mollusks are present at certain 

 preferred depths and on specific sediment types. These almost indis- 

 tinguishable "twin" species are presumed to have descended from single 

 common species which moved freely between the Atlantic and Pacific 

 Oceans across submerged portions of Middle and South America during 

 Miocene and Pliocene times. Since the "twin" species now occupy exactly 

 the same environment on each side of the Middle American land mass, it is 

 hypothesized that the original Mio-Pliocene species also lived in similar 

 environments. This phenomenon is most evident in the intermediate shelf 

 sand bottom, 27 to 65 meters, and outer shelf, clay bottom, 66 to 126 

 meters environments. For this reason, it is suspected that at some time 

 the marine connections across Middle America must have been in depths 

 of at least 27 to 120 meters, with certain areas covered continuously with 

 sand and others with silty clay. Few "twin" species were found below 

 120 meters, although a number of cosmopolitan deep-water species were 

 taken at depths below 1,000 meters. 



An analysis of feeding types (mostly of mollusks) for the various assem- 

 blages indicated a close relationship between feeding type and bottom 

 character, as well as other factors in the marine environment. In general, 

 a larger number of suspension feeders, along with an equally large number 

 of predators, are found on well-sorted sands and silts, especially in the 

 shallower waters. The deposit feeders and scavengers usually outnumbered 

 other types on poorly-sorted sandy mud and deeper silty clay sediments. 

 Certain areas below the euphotic zone were still occupied by fairly large 

 numbers of suspension feeders. These areas are also characterized by very 

 high surface plankton production and at times considerable turbulence. 

 These factors may allow suspension feeders to utilize as food, the suspended 

 organic matter resulting from the high surface production. 



Finally, this study has produced a few more parameters for paleoeco- 



