Results of the Puritan-American Museum of Natural History Expedition to Western Mexico. 14, A zoogeographic analysis of some shallow-water Foraminifera in the Gulf of California. Bulletin of the AMNH ; v. 123, article 5

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New York : [American Museum of Natural History]
"The Carmen Island area appears to be of significant ecologic importance with respect to the Foraminifera in the Gulf of California. Both northern and southern faunas show increased rates of change in this region. Squires (1959, p. 395) shows that a gradual diminution of hermatypic coral fauna occurs in the Gulf from El Pulmo (latitude 23 26' N.) to South San Lorenzo (latitude 28 35' N.) north of Salinas Bay, Carmen Island. At South San Lorenzo kelp replaces Porites, and Pavona and Pocillopora were not found. This area also marks the approximate northern limit of several tropical forms that were found in the present collection, of which the frequencies range as high as 65 per cent, and average 15 per cent of those of the southern populations. The distinctive character of the northern populations is due not only to the absence of these significant tropical species, but also to the greater abundance of several previously mentioned subtropical species, such as Eponides hannai, Elphidium gunteri, and Cibicides concentricus. A graphic analysis of the southern and northern populations shows a greater decrease in the number of species in the Carmen Island area (samples 41-49). With respect to the Foraminifera, the Gulf, therefore, is believed to support a tropical fauna as far north as the twenty-sixth parallel (Carmen Island area). To the north the Gulf supports a more or less thinned-out, warm-water fauna which has most of its species in common with the southern population, but with a greater frequency of some typical subtropical forms. Colder minimum temperatures to the north are presumed to be the main factor influencing this distribution. In general the Buliminidae and Cassidulinidae dominated the deeper-water samples; Cassidulina subglobosa and Bulimina denudata are typical examples. A study of the geographical affinities of the foraminiferal population of the Gulf indicates that the number (27.3%) of species that are endemic to the eastern Pacific warm water is larger than the number that are Amphi-American (20.2%), although the difference is not so great as that shown in a similar study made on crabs and echinoderms of the eastern Pacific (Eckman, 1953, p. 40). The percentage of circumtropical species in the Foraminifera (30.9%) was larger than in the megafauna (2%). This fact may be attributed to the greater capacity of the Foraminifera to be carried by ocean currents for great distances. Only a small percentage (4.3%) of the Foraminifera show affinities with the Indo-Pacific region. Depth is considered to be the main factor that influences the foraminiferal number of the samples. The deeper samples usually contain the highest F.N. values, except for a few bioclastic areas supporting prolific populations. The writer wishes to suggest further directions of study that might lead to the solution of important zoogeographical problems in the area. Foraminiferal populations can be studied in Recent samples collected between San Diego, California, and Cape San Lucas, at the tip of Baja California. With methods similar to those of this study, it may be possible to delineate more accurately the northern limits of the Panamic (tropical) Province. Late Tertiary samples taken throughout the extent of Baja California can be statistically analyzed, and the geographic ranges of the Foraminifera can be compared with those in this study. Such studies might indicate significant shifts in the tropical and subtropical fauna and aid in the interpretation of broad paleoecological problems"--P. 290.
p. 253-297, [1] leaf of plates : ill., map ; 27 cm.
Includes bibliographical references (p. 297).