62 P.M.P. ZABORSKI 
WESTERN INTERIOR ZONES 
20m PINDIGA 
ASHAKA 
Pseudaspidoceras flexuosum 
TURONIAN 15 
CENOMANIAN 
Nigericeras scotti 
| 
10 
he ere aa 
Neocardioceras juddii 
to 
Burroceras clydense ——— 
5 ——— 
a 
—— 
Sciponoceras gracile 
BIMA 
SANDSTONE 
o-: $e a ee 
YOLDE Fm. 
Fig. 1 Stratigraphical sections through the limestone-bearing parts of the Pindiga Formation at Ashaka and Pindiga with letter codes identifying 
limestone units mentioned in the text. Approximate biostratigraphical correlations with the ammonite biozones of the western interior of the 
United States (after Cobban et a/. 1989, Hancock 1991) are also indicated. 
tight stratigraphical control, especially at Ashaka quarry. 
Furthermore, dissection of adults is frequently successful in 
recovering well-preserved inner whorls which in some cases are 
invaluable for identification purposes, as well as for analysing 
ontogenetic development. These attributes have allowed a 
revised taxonomy to be presented here for the Nigerian faunas. 
The family Vascoceratidae as a whole has been discussed by 
Spath (1925), Furon (1935), Schneegans (1943), Reyment 
(19546, 1955, 1956), Barber (1957), Wiedmann (1960), Cooper 
(1978) and Wright & Kennedy (1981). 
From the time of its proposal it has been recognized that the 
genus Nigericeras Schneegans, 1943 is morphologically 
intermediate between the subfamilies Acanthoceratinae and 
Vascoceratinae. More recently, a number of additional genera of 
intermediate character have been decribed, a notable feature 
being the combination of a vascoceratine-type suture pattern 
with an acanthoceratine-type ornament. Such intermediates | 
include Microdiphasoceras Cobban, Hook & Kennedy (1989: 
53), Rubroceras Cobban, Hook & Kennedy (1989: 54), Fikaites | 
Zaborski (1993: 362) and Pseudovascoceras, described herein. It j 
is believed here that the family Vascoceratidae is polyphyletic, | 
including homeomorphic derivatives of various acanthoceratine } 
genera. In Nigeria at least the same is true of forms previously 
referred to the genus Vascoceras. | 
