30 UPPER CRETACEOUS LOWER TERTIARY FORAMINIFERA 



gerina-like, rounded, non-keeled Globorotalia. These globorotalias are generally 

 smooth-surfaced, with a tendency towards the development of a slightly roughened 

 surface and a gently compressed test upward in the section. This zone is character- 

 ized by the presence of Globorotalia pseudobidloides (Plummer), G. trinidadensis 

 Bolli, G. compressa (Plummer), Globigerina triloculinoides Plummer and G. daub- 

 jergensis Bronnimann among other species, but as both Globorotalia compressa and 

 Globigerina daubjergensis are found to die out completely at its top, they are taken as 

 the index species of the zone, which is known as the Globorotalia compressa I Globi- 

 gerina daubjergensis Zone. The planktonic Foraminifera of this zone characterizes 

 the Danian in its type section and the known Danian deposits elsewhere, thus it is 

 taken to represent the Danian, or the oldest stage of the Paleocene series. The 

 lower limit of this zone marks the Cretaceous-Tertiary contact, and its upper limit is 

 marked by the disappearance of its index species and the first appearance of the 

 truncated Globorotalia which characterize the following zone. 



2. A middle zone characterized by an assemblage of Globigerina and truncated, 

 non-keeled Globorotalia, in addition to the rounded form which first appeared in the 

 underlying zone. At the base of this zone, most representatives of the genus 

 Globorotalia (generally rounded or slightly compressed in the underlying zone) start 

 to become flattened on the dorsal side and strongly protruding on the ventral, with 

 the development of an acute axial periphery, but with no keel, or only a partially 

 developed, incipient one. Again the tendency towards the development of a rough- 

 ened surface, flattening of the dorsal side and the development of a sharply acute 

 axial periphery and a partial keel, increases gradually upwards in the section. An 

 excellent example of these truncated globorotalias is G. angulata angulata (White) 

 which is taken as the index species of this zone, although it continues in the over- 

 lying zone. The development of G. angulata angulata from the typically rounded 

 form G. pseudobidloides (Plummer) through G. trinidadensis Bolli and G. uncinata 

 uncinata Bolli is clearly documented (Text-fig. 15), and demonstrates the tendency 

 of Globorotalia to develop from rounded to truncated forms upwards in the section. 

 On the other hand, G. angulata angulata also demonstrates the evolutionary develop- 

 ment of the truncated, non-keeled Globorotalia into the sharply-keeled ones by the 

 development, in the overlying zone, into G. velascoensis velascoensis (Cushman) 

 through G. angulata abundocamerata Bolli. This, added to the fact that the first 

 appearance of G. angulata angulata coincides with the disappearance of the index 

 species of the underlying zone which represents the lowest Paleocene or the Danian, 

 and that G. angulata angulata was never recorded from the type Danian, justifies the 

 position of the G. angulata Zone at the Middle of the Paleocene series. From an 

 evolutionary point of view, this zone with truncated Globorotalia is regarded as a 

 transitional stage between the underlying rounded Globorotalia Zone and the over- 

 lying zone with sharply-keeled Globorotalia. This is also substantiated by the fact 

 that this intermediate zone is represented by a relatively smaller thickness of strata. 

 However, as its characteristic assemblage of planktonic Foraminifera can not be 

 assigned to either the overlying or the underlying zones, it is advisable to treat 

 it separately. The lower boundary of this zone is marked by the first appearance 



