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UNITED STATES NATIONAL MUSEUM BULLETIN 215 



2. By development of elongate true spines as in living 

 Glohigerina, Hastigerina, Olobigerinoides, and Hastiger- 

 inella. 



Coiling ratios. This is one apparent evolutionary 

 trend which does not directly affect the pelagic nature 

 of the organism. 



As shown by Bolli (1950, p. 82 and 1951, p. 139) the 

 trochospiral genera may develop a preference for sin- 

 istral or dextral coiling. Early representatives of a 

 species or group of closely related species may show 

 random coiling, with sinistral and dextral specimens in 

 approximately equal numbers. The stratigraphically 

 younger specimens studied prefer a single direction 

 almost to the exclusion of the other, and this may be 

 either sinistral or dextral, according to the species con- 

 cerned. These results have been obtained with species 

 of Rotalipora, Globorotalia and Globotruncana. Similar 

 tendencies, though less distinct, have also been observed 

 in several species of Globigerina, Globigerinoides and 

 Oatapsydrax. Once a preferred direction of coUing is 

 estabhshed, it is generally persistent as in all species of 

 Globotruncana and in the Globorotalia fohsi group, or a 

 rapid change to the opposite direction may take place, 

 indicating possible ecological changes, as in Globorotalia 

 menardii (BoUi, 1951) and Globorotalia truncatulinoides 

 (Ericson, G. WoUin and J. Wollin, 1954). In rare cases 

 a return to random coUing has been observed in late 

 evolutionary stages, shortly before the extinction of 

 the genus, as in certain related groups of species of 

 Rotalipora. All known species of Globotruncana and 

 Rugoglobigerina tend to develop an almost exclusive 

 preference for dextral coUing. On the other hand, 

 many species of Globorotalia develop predominantly 

 sinistral coiling in their later stages, as do some mid- 

 Tertiary species of Globigerina, Globoquadrina and 

 Globigerinoides. However, the type species of Catapsy- 

 drax, originally named Globigerina dissimilis, prefers a 

 dextral coiling (text-figs. 6, 7). 



Bolli (1951, p. 142) fm-ther stated that "it appears 

 unlikely that a species with random coiling in its early 

 phylogenetic stage can be genetically related to a strati- 

 graphically older species which shows a distinct prefer- 

 ential direction of coiling in its later stages". 



An interesting study in the coiling direction of living 

 and subrecent Globorotalia truncatulinoides (d'Orbigny) 

 was made by Ericson, G. Wollin, and J. Wollin (1954) 

 from a study of specimens foimd in deep sea cores. 

 They foimd three great provinces of the north Atlantic 

 defined by populations with a dominance of one or the 

 other coiling direction. The northeast quadrant of the 

 north Atlantic shows a dominance of dextral coiling. 

 A central zone of sinistral coiling extends from north- 

 west Africa to North America. The third province is 

 equatorial, extending through the Caribbean and Gulf 

 of Mexico and around the Florida Straits, and contains 

 again dominantly dextraUy coiled specimens (text-fig. 8), 

 Near the boundaries of these zones, coiling is almost 

 random. 



An examination of a niunber of cores showed that 

 there was also a variation in coiling direction with time. 



Figure 6. — Percentage of sinistral specimens of the Globorotalia fohsi 

 group in the Miocene Cipero formation of Trinidad, showing evolution 

 from random coiling to a sinistral dominance with time. (Modified 

 after Bolli, 1950). 



Two cores from the area with a present day dominant 

 sinistral coiling, showed that sinistral coiling is excep- 

 tional in this region, for during deposition dextral coiling 

 was dominant 80 percent of the time. Two swings to 

 the left of short duration were noted in both cores, 

 affording an excellent possibility for correlation. 



Core evidence shows that the Recent province of 

 sinistral coiling dominance has been in existence for 

 at least 2,000 years and probably much longer. Equa- 

 torial cores show that the southern province of dextral 

 coiling has existed continuously for at least some tens 

 of thousands of years. There is no physical barrier 

 between these provinces and the species distribution is 

 continuous. Therefore, some imknown environmental 

 factor or selective process must favor the coiling direc- 

 tion dominance in these provinces. 



Vasicek (1953) also made a study of coiling ratios, 

 based on the species Globorotalia scitida, both in time 

 and regionally. He also concluded that the change in 

 ratio was due to an unknown change in life environ- 

 ment, but that the coiling ratios were extremely useful 

 in correlations within the Moravian Tortonian, where 

 no species suitable for zoning had been found. 



Other genera, namely Spirillina and Discorbis, were 

 shown by Myers to have the direction of coifing related 

 to the alternation of generations, one generation being 

 dextral, the other sinistral. However, other species 

 show a variation of coiling in the megalospheric forms. 



It was suggested by Vasicek (p. 413) that the coiling 

 might be related to the reproductive process of "plas- 

 togamy." Myers' work on life cycles showed the 



