UPPER CRETACEOUS AMMONITE 
heterochronic ontogenies (Meister 1989: 34, 36, text-fig. 23). 
Concerned, however, that the faunas from unit O at Ashaka were 
condensed and might contain chronologically successive taxa, 
he refrained from placing them in synonymy. Unit O at Ashaka 
is the product of a ‘slow rate of sediment accumulation 
associated with a marine flooding phase. It contains the most 
diverse marine fauna found at Ashaka including numerous 
bivalves, gastropods and echinoids as well as a large number of 
ammonite species (see also Courville 1992: fig. 2). Encrustations 
of Plicatula occur throughout while many of the ammonites 
show oyster and serpulid overgrowths. There is, however, no 
significant phosphatization or reworking, and glauconite is rare 
or absent. There is no reason to believe that this unit represents 
any greater condensation than several other limestones at 
Ashaka and elsewhere in north-eastern Nigeria. Phosphatic 
matter, glauconite and reworked ammonites are common 
components in the limestone beds of the region, particularly in 
the upper parts of those interbedded with shales. It is the upper 
surface of unit O at Ashaka that marks the most significant 
break in sedimentation, but P nigeriense is found throughout the 
unit below this level. Both Meister (1989) and Courville (1992) 
believed they could differentiate between faunas from different 
levels in unit O (their Niveau 21 and Niveau 22 ), though their 
reported successions differ. No significant stratigraphical 
variation in the nature of the ammonite faunas from this unit 
| has been detected in the present work, apart from the restriction 
of strongly ornamented Thomasites to its upper surface. In the 
cases of the other ammonites present intraspecific variation 1s by 
far the most important factor. No morphometric or ornamental 
evidence has been obtained which allows objective taxonomic 
subdivision of P. nigeriense. There is a complete intergradation 
from smooth to strongly ornamented individuals while the latter 
vary considerably among themselves. In view of these factors all 
_ these morphotypes are regarded as conspecific, despite the great 
differences between end members. Courville (1992: 419-420) 
came to a similar conclusion and favoured the name Vascoceras 
costatum (Barber) which was used by Meister (1989) for 
individuals of intermediate ornamental strength. Priority, 
however, belongs to Vascoceras nigeriense Woods, 1911. The 
| lectotype is a smooth end member of the species, as are the 
individuals referred by Meister (1989) to Paravascoceras 
| nigeriense? (Woods) and Vascoceras ellipticum Barber. 
Smooth examples of Pseudovascoceras nigeriense have in the 
past been compared with Vascoceras gamai (Barber 1957: 15; 
Hancock & Kennedy 1981: 357). Their similarity concerns only 
the outer whorls, however, and is homeomorphic in nature. 
Ornamented examples of P nigeriense share similarities with 
| various genera. Meister (1989) referred forms in which the 
siphonal tubercles disappear early in ontogeny to 
Paramammites, which he regarded as a senior synonym of 
Spathites (Jeanrogericeras) Wiedmann, 1960 (type species 
Ammonites reveliereanus Courtiller, 1860). As mentioned above, 
|\this material cannot be referred to Paramammites or 
Jeanrogericeras as neither shows siphonal tubercles at any 
growth stage (see Choffat 1898: 64; Pervinquiére 1907: 336; 
Wiedmann 1960: 741; Renz 1982: 84; Berthou et al. 1985: 62) 
and resemblances are superficial only. Some of the present 
specimens have the appearance of giant Protacanthoceras 
proteus (compare Fig. 23 and Wright & Kennedy 1980: fig. 5). 
Others with dense, multiple ventral ribbing resemble 
Kamerunoceras Reyment, 1954b (type species Acanthoceras 
eschii Solger, 1904) or Euomphaloceras Spath, 1923 (type species 
| Ammonites euomphalus Sharpe, 1855), though they lack the 
typically euomphaloceratine constrictions upon their early 
71 
whorls. In its style of ribbing and the generally coarse nature of 
the tuberculation, the present material most closely resembles 
Cunningtoniceras Collignon, 1937 (type species Ammonites 
cunningtoni Sharpe, 1855). This is mainly a Middle Cenomanian 
genus (see, for example, Kennedy 1971, Zaborski 1985, Kennedy 
& Cobban 1990a) but it ranges into the Upper Cenomanian 
(Wright & Kennedy 1987, Cobban et al. 1989, Kennedy & 
Cobban 1990h). Pseudovascoceras may be a descendant of 
Cunningtoniceras. Yntroduction into north-eastern Nigeria 
produced peramorphic individuals losing their ornament early 
in ontogeny and coming to resemble Vascoceras. Interestingly 
there is a morphological overlap between P nigeriense and 
Nigericeras ogojaense Reyment (1955: 62, pl. 13, fig. 6; pl. 14, fig. 
3; text-fig. 28); the two are probably conspecific. The latter comes 
from the southern, oceanward, end of the Benue Trough where 
smooth individuals are unknown; the holotype (C.47401) and 
newly collected material (C.93578-61) all show prominent 
ornament. 
Reyment (1979, 1988) has remarked upon the extraordinary 
polymorphism that may be displayed by vascoceratid species. He 
believed that in the changeable environment of the 
Cenomanian-Turonian intracontinental sea in west and 
Saharan Africa selection would have favoured forms with 
genetic or phenotypic flexibility. Such taxa would have been 
capable of responding to environmental fluctuations, each 
morphotype being best suited to a particular kind of 
environment. Meister et a/. (1992) took up this issue in respect of 
the Niger ammonites. They noted that particular stratigraphical 
horizons there commonly yield monospecific faunas or 
assemblages dominated by one species. They speculated that 
taxa able to occupy niches in the exacting environments 
prevailing during the Late Cenomanian and Early Turonian 
faced virtually no competition, the result being a high degree of 
polymorphism. In unit O at Ashaka a number of ammonite taxa 
co-exist, largely as a result of introduction of species during a 
marine flooding episode. While there is some overlap, however, 
each of the four main taxa described here, Paravascoceras 
cauvini, Vascoceras bullatum, V. globosum costatum and 
Pseudovascoceras nigeriense, occupies a particular part of the 
morphological spectrum (Fig. 12). Variation in gross shell 
proportions to the extent of that suggested by Meister er al. 
(1992) for P. cauvini in Niger is not seen in these taxa. On the 
other hand, within P nigeriense there seems to have been 
virtually no selection pressure favouring any particular strength 
of ornamentation. In this respect the polymorphic potential of 
the species was capable of wide expression. Much the same can 
be said of Fikaites varicostatus Zaborski, the other strongly 
ornamented form found in some numbers in unit O at Ashaka. 
This species shows a significant variation in the strength of its 
ribbing and tuberculation (see Zaborski 1993). 
Family VASCOCERATIDAE Douville, 1912 
Subfamily VASCOCERATINAE Douvillé, 1912 
Genus VASCOCERAS Choffat, 1898 
(= Discevascoceras Collignon, 1957; Greenhornoceras Cobban & 
Scott, 1972; Provascoceras Cooper, 1979) 
TYPE SPECIES. Vascoceras gamai Choffat, 
subsequent designation of Roman, 1938. 
1898: by the 
REMARKS. Choffat (1898: 51-53) had a broad concept of 
Vascoceras as encompassing forms basically united by the 
