STUDIES IN FOHAMINIFERA 



67 



respectively. The last two are the end forms of the evo- 

 lutionary sequence that began with 0. trinidadensis in 

 the lower Lizard Springs. 0. formosa formosa becomes 

 extinct at the close of the 6. aragonensis zone whereas 

 O. aragonensis continues without noticeable morpoholog- 

 ical changes for a considerable time into the middle 

 Eocene Navet formation. 



Another suite of Globorotalia species closely related 

 morphologically is G. compressa (Plummer) — G. ehren- 

 bergi BoUi, new species — G. pseudomenardii Bolli, new 

 species, and probablj^ G. elongata Glaessner. G. com- 

 pressa appears in the Globorotalia trinidadensis zone and 

 might originate from the same stock as G. trinidadensis. 

 It ranges from the Globorotalia trinidadensis zone into 

 the Globorotalia pusilla pusilla zone where it develops 

 into G. ehrenbergi by increasing its size and becoming 

 more compressed. G. pseudomenardii, the descendant 

 of G. ehrenbergi, becomes stUl more compressed and 

 acquires a peripheral keel. Towards the end of its 

 range this species can become of considerable size and 

 may depart from its usual shape (see pi. 20, fig. 17). 

 G. elongata which probably developed from G. ehren- 

 bergi-G. psevdomenardii at the base of the Globorotalia 

 pseudomenardii zone continues into the Globorotalia 

 velascoensis zone where the suite becomes extinct. 



Globigerina daubjergensis Bronnimann which is re- 

 stricted to the Globorotalia trinidadensis zone shows no 

 apparent morphologic relationship to other species of 

 that zone. It may possibly be regarded as the ancestor 

 of Globigerina spiralis Bolli, new species, which is con- 

 fined to the Globorotalia uncinata zone. Both forms 

 are distinctly trochospiral, however no intermediate 

 forms were observed in the limited number of samples 

 available from these zones. 



No ancestral forms were found in the investigated 

 sections for Globorotalia pusilla pusilla Bolli, new species, 

 new subspecies. This species develops by transitions 

 into G. pusilla laevigata Bolli, new species, new sub- 

 species, of the G. pseudomenardii zone. 



Globorotalia velascoensis (Cushman) is a distinct form 

 characterizing the Globorotalia pseudomenardii and 

 Globorotalia velascoensis zones. The species appears 

 first in the Globorotalia pusilla pusilla zone, where it 

 might have branched off from the Globorotalia angulata 

 (White) ^group. Transitional forms between these 

 species could not be clearly established in the studied 

 sections. 



Globigerina triloeulinoides Plummer which first occm-s 

 in the Globorotalia trinidadensis zone, might have a 

 common ancestor with Globorotalia trinidadensis. 

 Specimens of Globigerina triloeulinoides which show 

 GloborotaliaAiike apertiiral characters are common 

 throughout its range (see pi. 17, figs. 25-26). The 

 triangular shaped Globigerina triloeulinoides seemingly 

 develops into the long-ranging and little-changing G. 

 linaperta Finlay. Before that change, the more triangu- 

 lar shaped G. triangularis White branches off from G. 

 triloeulinoides at the base of the Globorotalia pusilla 

 pusilla zone. Globigerina velascoensis Cushman, a form 

 with a slight lateral compression of the chambers, may 



396818— 57— —6 



be regarded as a fiu-ther evolutionary step from G. 

 triangularis. 



The laterally strongly compressed Globorotalia tortiva 

 Bolli, new name, appears almost contemporaneously 

 with Globigerina velascoensis at the base of the Globoro- 

 talia pseudomenardii zone. This short-lived species is 

 likely to have developed from Globigerina triangularis. 

 It is possible that Globorotalia tortiva BoUi, new name, 

 is the ancestral form of the equally short-lived Globoro- 

 talia mckannai (White) which is found higher in the 

 same zone. 



Globorotalia whitei Weiss which appears in the 

 Globorotalia pseudomenardii zone is another species 

 likely to have developed from the Globigerina tri- 

 angularis-G. velascoensis group. It is regarded as the 

 ancestral form of Globorotalia wilcoxensis Cushman and 

 Ponton and G. quetra Bolli, new species. 



Towards the close of the Globorotalia pseudomenardii 

 zone and during the Globorotalia velascoensis zone the 

 first specimens of the closely related Globigerina 

 primitiva Finlay and G. soldadoensis Bronnimann 

 appear. Similar morphology strongly suggests that 

 G. primitiva developed from G. velascoensis. Several 

 species and subspecies develop in the upper Lizard 

 Springs from G. soldadoensis Bronnimann, which is 

 regarded as related to G. primitiva; in order of first 

 occurrence they are G. gravelli Bronnimann, G. solda- 

 doensis angulosa Bolli, new subspecies, and G. turgida 

 Finlay. G. taroubaensis Bronnimann might also be 

 related to this group, probably most closely to G. 

 turgida. 



Globigerina collactea (Finlay) appears first in the 

 Globorotalia rex zone with no apparent ancestral forms 

 in the underlying Globorotalia velascoensis zone. Such 

 forms might however be expected in beds presumed 

 missing between these two zones. Globigerina prolata 

 Bolli, new species, is likely to have developed from G. 

 collactea at the base of the Globorotalia formosa formosa 

 zone. 



Globorotalia broedermanni Cushman and Bermudez is 

 another form that occiurs first in the Globorotalia rex 

 zone. Some intermediate specimens in the Globorotalia 

 rex zone indicate a possible relationship to Globigerina 

 collactea. 



Direction of Coiling 



Earlier observations on the direction of coiling of a 

 number of planktonic species led to the conclusion that 

 distinct changes in ratios occm- during the evolution of 

 many species (Bolli, 1950, 1951). Diu-ing the early 

 evolutionary stage, such a species or gi-oup of related 

 species normally coils at random. Later, up to 90 to 

 100 percent of the specimens have a preference for 

 either sinistral or dextral coilmg. Once such a prefer- 

 ence has arisen the species does not revert to random 

 coiling any more, except in some possible gerontic 

 stages (Bolli, 1957, p. 54). Very rapid or almost 

 instant changes from one preferred direction of coiling 

 to the opposite can, however, be observed in the later 

 stages of some species, e. g., Globorotalia menardii 



