UPPER CRETACEOUS AMMONITE 
levels of 
immigrant faunas 
especially inter-regionally. A number of problems complicate 
‘their use including: the variable taxonomic treatment authors 
have employed; the difficulty of identifying poorly preserved 
‘material, especially if the inner whorls are not available; the lack 
of an exact stratigraphical provenance for many species; and the 
problem of polymorphism. In regard to the last of these factors 
Meister (1989) and Meister et al. (1992) have made the 
important point that in certain regions only a portion of the 
potential morphological range of a species may be expressed on 
account of palaeoecological factors. 
It has long been appreciated that ‘vascoceratid’ ammonites are 
predominantly a Tethyan group. Less attention has been paid to 
the potential palaeoenvironmental influences on their regional 
distribution. In this regard it is of interest to compare the faunas 
lof north-eastern Nigeria and Niger, the  stratigraphical 
distributions of which are now well understood. 
Despite their geographical proximity correlation between 
these two areas is not as straightforward as might be expected. 
Ther is little problem with the horizons of Geslinianum Zone 
85 
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2 Vascoceras Cunningtoniceras? 
diartianum 
Fig.65 Stratigraphical ranges in the Ashaka section of taxa mentioned in the text and their suggested phylogenetic relationships. 
age; in both places assemblages of Nigericeras, Metoicoceras 
and Metengonoceras are found. Similarly, in the upper part of 
the Turonian Coilopoceras is present in both areas. There are, 
however, faunal differences in the intervening sequences. 
Vascoceras woodsi is unknown in Niger. In Nigeria it occurs 
alongside Pseudaspidoceras pseudonodosoides which was 
compared with P tassaraense Meister, Alzouma, Lang & 
Mathey (1992) from the Monts Iguelela area of Niger by 
Zaborski (1995). The two may be of comparable age. P 
tassaraense occurs alongside Nigericeras jacqueti involutum, a 
form unknown in Nigeria. Slightly lower in the same section 
there occur large numbers of Cibolaites(?) africaensis Meister, 
Alzouma, Lang & Mathey (1992) which has not been found 
further to the south. At Tanout Aviation and Birgimari there are 
horizons dominated by compressed individuals of 
Paravascoceras cauvini. In their ornament or lack of it they 
match the variation shown by the species in unit O at Ashaka. 
The Niger faunas, however, include more inflated individuals of 
Vascoceras crassum type. None of the species associated with P 
