66 
0.5 
Fig.12 Shell proportions in Paravascoceras cauvini (Chudeau), Pseudovascoceras nigeriense (Woods), Vascoceras bullatum Schneegans, V. globosum 
costatum (Reyment) and V. globosum globosum (Reyment) from unit O at Ashaka. These individuals are the product of collecting carried out over | 
the course of two hours specifically for the purpose of comparing dimensions. This figure also gives a good indication of the relative abundances of 
the taxa concerned. 
loss of juvenile ornamentation (peramorphosis), simplification 
of suture pattern and development of adult ribbing. Forms from 
unit F at Ashaka and unit H at Pindiga are transitional in nature. 
Schneegans (1943: 119-125) proposed three additional species 
of Nigericeras, N. gignouxi, N.lamberti and N. jacqueti which 
show an increasingly weaker ornament but which all display the 
typically acanthoceratine suture pattern of the genus. It may be 
noted that in north-eastern Nigeria there is a variation in the 
strength of ornament of Nigericeras collected from place to 
place. Individuals from Teli and from north-east of the Biu 
Plateau (see Zaborski 1990a: figs 4, 5) have a weak juvenile 
ornament while those from the Hinna region and from between 
Kanawa and Wayari (see Zaborski 1990a: figs 6, 7) have stronger 
ornament. There is, however, no available evidence of this 
variation having a stratigraphical significance. In Niger, forms of 
N. jacqueti type occur alongside typical N. gadeni (Meister et al. 
1992). 
In north-eastern Nigeria N. gadeni occurs in the equivalent of 
the Geslinianum Zone in north-west Europe and the Gracile 
Zone in the western interior of the United States (Zaborski 
1990a). P. cauvini ranges through the probable equivalents of the 
Juddii Zone in north-west Europe and the Clydense to Scotti 
zones in the western interior (see below). Lewy et al. (1984) 
described P cauvini in association with Metoicoceras 
geslinianum (d’Orbigny) in Israel. Their material, however, 
Wb/Wh 
P.M.P. ZABORSKI 
® P cauvini 
P nigeriense 
9 V. bullatum 
© V.globosum costatum 
o 4 V.globosum globosum 
shows broad flank ribbing and umbilical bulges (Lewy et al. } 
1984: fig. 41) in its middle whorls, features more typical of } 
Nigericeras from which it appears to be transitional. Its suture is } 
unknown. | 
Cooper (1979) and Kennedy & Wright (in press) proposed an 
origin for Nigericeras within Pseudocalycoceras Thomel, 1969. } 
10-16; text-figs 83B, C, 84D-H), a lowest Upper Cenomanian } 
Guerangeri Zone species. Nigericeras may be a peramorphic 
derivative. 
The small Nigerian specimens from low in the Pindiga section!} 
referred to Vascoceras bulbosum (Reyment) and V. depressum sp. 
nov. by Barber (1957) are here regarded as P. cauvini as is the} 
material from Ashaka included in Nigericeras by Meister (1989)./} 
The P. cauvini from Nigeria are compressed forms which fall! 
into a distinct morphological group within the ammonite fauna|} 
from unit O at Ashaka (Fig. 12). Individuals from Niger.) 
however, develop a broader whorl section (see Meister et al.|} 
1992: pl. 6, fig. 2). These specimens seem to be of the same age as! 
those from unit O at Ashaka. The latter are associated with very, 
large numbers of more inflated ammonites referrable to! 
Vascoceras globosum ___costatum, V. bullatum and 
Pseudovascoceras nigeriense. These three forms are of markedly\} 
