188 UPPER CRETACEOUS-LOWER TERTIARY FORAMINIFERA 



both Hedbergella Bronnimann and Brown and Praeglobotruncana Bermudez in 

 lacking umbilical portici ; furthermore Hedbergella has no keel, and the keel in 

 Praeglobotruncana is much weaker than in sharply keeled Globorotalia. It is distin- 

 guished from both Globotruncana Cushman and Abathomphalus Bolli, Loeblich & 

 Tappan by the absence of the umbilical cover-plate, by the occasional presence of 

 a single keel, by its extraumbilical aperture (that of Globotruncana is umbilical), and 

 by the fact that it always has a definite open umbilicus however small, whereas the 

 umbilicus in Abathomphalus is much reduced and is covered by the tegellum. 



Evolutionary Development of globorotalia 



Very little is known about the origin of Globorotalia because of the marked faunal 

 break between the Maestrichtian and the Danian. However, it is possible that the 

 genus evolved from the youngest representatives of Hedbergella (which disappeared 

 completely at the top of the Maestrichtian) by the reduction of the apertural flaps 

 (the umbilical portici) and by the development of the simple umbilicus. Globorotalia, 

 in its turn, evolved into Globigerina by the confinement of the aperture to an interio- 

 marginal, umbilical position. On the other hand, it is not impossible that Rugoglo- 

 bigerina evolved into Globigerina by the loss of the tegilla and the surface meridional 

 costellae, and that the latter evolved into Globorotalia by the development of the 

 extraumbilical, umbilical aperture. Whatever the origin of Globorotalia, its earliest 

 representatives are known to have a rounded, non-carinate test, which is generally 

 smooth surfaced. At stratigraphically higher levels, these rounded, smooth forms 

 show two major tendencies in their evolution : 



i. A general tendency towards the flattening of the dorsal side, followed by the 

 gradual development of a marginal keel through various intermediate 

 stages. 



2. A general tendency towards increasing the surface rugosity. 



These two tendencies develop either separately or in combination, with the result 

 that the early Paleocene rounded, smooth globorotalias evolve gradually into forms 

 with truncated, non-keeled and/or slightly rougher tests which in turn, evolve into 

 the truncated, sharply-keeled and/or very rugose forms which characterize the 

 Upper Paleocene in various parts of the world. In the Lower Eocene, a new tendency 

 towards reduction in the size of test and increase in the surface rugosity is observed. 

 The result is that most of the Lower Eocene forms are smaller than the Paleocene 

 ones and/or have an extremely rough surface. These tendencies are clearly docu- 

 mented in the present study, and are demonstrated by several lineages, summarized 

 on Text-fig. 15. 



Globorotalia acuta Toulmin 



(PI. 19, figs. $a-c ; PL 20, figs. la-d) 



1941 Globorotalia wilcoxensis Cushman & Ponton var. acuta Toulmin : 608, pi. 82, figs. 6-8. 



1942 Globorotalia wilcoxensis Cushman & Ponton var. acuta Toulmin ; Cushman & Renz : 

 12, pi. 3, figs. la-c. 



1957a Globorotalia acuta Toulmin ; Loeblich & Tappan : 185-186, pi. 47, figs. 5«-c ; pi. 55, 

 figs. 4«-5c ; pi. 58, figs. 5«-c. 



