IN THE ESNA-IDFU REGION, NILE VALLEY, EGYPT 73 



Evolutionary Development of GLOBOTRUNCANA 

 Although the evolutionary development of the genus Globotruncana has been 

 discussed by several authors e.g. Reichel (1950), Hagn & Zeil (1954), Gandolfi (1955), 

 Bronnimann & Brown (1956) and Cita (1963), its origin remains uncertain. However, 

 the fact that the early part of the test in all representatives of the genus is reminiscent 

 of Globigerina, led to the belief that the genus had probably evolved from a general- 

 ized " Globigerina-like " stock. On the other hand, Globigerina, as fixed by the 

 original designation of its type species (Globigerina bulloides d'Orbigny 1826) is 

 known to have appeared first at the base of the Danian (i.e. after the disappearance 

 of the genus Globotruncana), a fact previously recognized by various authors and 

 confirmed by the present study. This throws doubt on the validity of the previous 

 records of Globigerina species in Cretaceous and Upper Jurassic rocks. A restudy of 

 these forms may prove them to be species of Hedbergella, Rugoglobigerina, Prae- 

 globotruncana, Globotruncana, or other genera. Nevertheless, with the limits of our 

 present knowledge, the genus Globotruncana may have originated in one of the 

 following ways : 



1. Praeglobotruncana evolved into Globotruncana by the confinement of the 



aperture to an interiomarginal, umbilical position, and by the development 

 of the umbilical cover-plate ; and Globotruncana in its turn evolved into 

 Rugoglobigerina by the loss of the keel or keels and by the development of 

 distinct surface rugosity. 



2. Hedbergella evolved in one direction into Praeglobotruncana which continued 



its evolution as mentioned above, and in another direction, into Globo- 

 truncana by the confinement of the aperture to an interiomarginal, umbilical 

 position, and by the development of both the cover-plate and the keel 

 (or keels) : Globotruncana, in its turn evolved into Rugoglobigerina by the 

 loss of the keel or keels and by the development of surface rugosity as 

 mentioned above. 



3. Some of the so-called " Globigerina " species in the lower part of the Upper 



Cretaceous and even in the Lower Cretaceous may possibly belong to 

 Rugoglobigerina (although the genus has, up till now, been recorded from the 

 Campanian and Maestrichtian only), but the cover-plate is either broken or 

 has been lost during the process of fossilization ; hence it can be suggested 

 that a hypothetical " Rugoglobigerina " stock has probably evolved into 

 Globotruncana by the flattening of the dorsal side and the development of 

 keel or keels, although Gandolfi (1955) strongly emphasized the fact that 

 most Globotruncana species had undergone a process of " globigerinization " 

 to develop into Rugoglobogerina. 



However, nothing can be decided about the origin of Globotruncana until the 

 earliest known globigerinid forms have been carefully examined and traced to the 

 first known Globotruncana species, either directly or indirectly through Hedbergella, 

 Praeglobotruncana, or Rugoglobigerina. 



Several evolutionary trends demonstrated by one or more lineages of the genus 

 Globotruncana, were suggested by various authors, (e.g. Gandolfi 1955 ; Bronnimann 



