86 
cauvini in unit O at Ashaka is found in the described Niger 
horizons. Higher in the sequence Thomasites gongilensis, so 
abundant in unit R at Ashaka and unit O at Pindiga, and with an 
overall range from unit O to unit T2 at the former locality, is not 
found in any of the Niger sections. Pseudotissotia nigeriensis, on 
the other hand, occurs in large numbers at Tanout Aviation but 
none of the associated taxa in Nigeria accompany it there. 
Biostratigraphical comparison between Niger and Nigeria is 
complicated by the fact that ammonites are restricted to 
limestone horizons which are, in the main, thin units within 
dominantly argillaceous sequences. The presence or absence of 
particular faunas, therefore, may in some cases be related to the 
occurrence of calcareous beds (see also Meister et al. 1992: 91). 
The possibility of control over ammonite distributions by 
transgressive pulses of the trans-Saharan sea during the Late 
Cenomanian and Early Turonian has long been discussed, most 
recently by Courville et a/. (1991). Meister et al. (1992: 94-95), 
however, have speculated that local palaeoenvironments were a 
strong influence on ‘vascoceratid’ distributions, their 
morphological polymorphism and _ their consequent 
evolutionary potential. In support of the latter hypothesis it may 
be noted that members of evolutionary lineages ‘indigenous’ to 
north-eastern Nigeria (Nigericeras, Paravascoceras cauvini and 
Pseudotissotia nigeriensis) extend into Niger. Introduced taxa do 
not. Among these may be mentioned Burroceras? from unit F at 
Ashaka; Vascoceras woodsi from units K and M; the greater part 
of the fauna from unit O including Pseudaspidoceras footeanum 
(Stoliczka), Fikaites, Rubroceras, Pseudovascoceras nigeriense 
and, probably, Vascoceras globosum costatum; Pseudaspidoceras 
paganum from unit R; and Pseudaspidoceras flexuosum, 
Watinoceras, Choffaticeras and Wrightoceras munieri from unit 
T2. The appearances of these taxa are probably related to 
transgressive pulses, the influences of which did not fully extend 
into Niger. As suggested by Meister et al. (1992) the absence of 
these forms and consequent lack of competition may have 
permitted local intraspecific variants and evolutionary lineages 
to develop in Niger. Examples would be the inflated variants of 
Paravascoceras cauvini and the lineage leading to Nigericeras 
jacqueti involutum. Rather than the overall extent of the 
trans-Saharan sea as such, more localized influences such as 
water depth and temperature may have controlled the 
distribution of taxa. If these factors did apply they would place 
important constraints on the use of ‘vascoceratid’ species in long 
distance correlation. 
Associated with the above matter is the probability that a 
number of acanthoceratine taxa independently gave rise to 
vascoceratid-like forms during Late Cenomanian times. 
Reyment (1979: 111) suggested that the family Vascoceratidae 
was polyphyletic, the morphological similarities between its 
members being due to adaptation to the same kind of 
epicontinental palaeoenvironments rather than to close 
phylogenetic affinities. The Late Cenomanian transgression 
brought several forms into north-eastern Nigeria which show 
elements of the ‘vascoceratid’ morphology, notably simplified 
sutures. Rubroceras and Fikaites, the latter probably derived 
from Eucalycoceras Spath, are examples, as is Pseudovascoceras 
which, as mentioned above, may be a descendent of 
Cunningtoniceras. Reyment (1955: 62, text-fig. 27) regarded 
Nigericeras as the root stock of the entire family Vascoceratidae 
while Cooper (1979) suggested that Vascoceras diartianum gave 
rise to both Paravascoceras and the younger Vascoceras. It is 
suggested here that Paravascoceras is derived from Nigericeras 
and belongs to a lineage separate to that leading to Vascoceras. 
The earliest Vascoceras in north-eastern Nigeria, V. woodsi, 
P.M.P. ZABORSKI 
appears to be a peramorphic derivative of V. diartianum. The 
immediate origins of V. bullatum, V. globosum and V. harttii are 
obscure. It may be mentioned, however, that V. globosum 
costatum probably contains the progenitors of Thomasites 
gongilensis. This species in turn gave rise to Pseudotissotia 
nigeriensis in terminal Cenomanian times (see also Barber 1957, 
Cooper 1979, Meister 1989) from which Eotissotia simplex 
originated as a paedomorph during the Early Turonian 
(Zaborski 1993). The youngest member of the V. globosum 
group, IY, globosum proprium, straddles the 
Cenomanian-Turonian boundary. It gave rise to V. obscurum — 
and probably also Neoptychites. Kennedy & Wright (1979: 681) 
believed the latter genus to have been derived from 
Paravascoceras but used this name in the sense of Vascoceras 
without umbilical tubercles. 
The suggested phylogenetic relationships of forms from 
north-eastern Nigeria are shown in Fig. 65. Several converging 
lineages are believed to exist, their frequently homeomorphic 
similarities being due to colonization of the same 
palaeoenvironment. 
ACKNOWLEDGEMENTS. Thanks are due to Drs M. K. Howarth and H. G. 
Owen, and Mr. S. Baker for help in many ways. Dr. N. Morris kindly 
allowed access to a collection of ammonites from Niger. Mr. M. Baku, 
Quarry Manager, Ashaka Cement Co. kindly allowed easy access to the 
Ashaka quarry. Photographs were provided by the Natural History 
Museum (London) Photographic Unit. Dr. W. J. Kennedy kindly made 
useful suggestions concerning the manuscript. 
REFERENCES 
Adkins, W. S. 1931. Some Upper Cretaceous ammonites in western Texas. Bulletin 
of the University of Texas Bureau of Economic Geology and Technology, Austin, | 
3101: 35-72, pls 2-5. 
Amard, B., Collignon, M. & Roman, J. 1981. Etude stratigraphique et | 
paléontologique du Crétacé supérieure et Paléocene du Tinrhert-W et | 
Tademait-E (Sahara Algérien). Documents du Laboratoire de Géologie de la 
Faculté des Sciences de Lyon, (H.S.) 6: 15-173, pls 1-17. 
Barber, W. 1957. Lower Turonian ammonites from north-eastern Nigeria. Bulletin 
of the Geological Survey of Nigeria, Kaduna, 26: 1-86, pls 1-35. 
— 1960. Notes on Upper Cretaceous Ammonoidea from north-eastern Nigeria. | 
Records of the Geological Survey of Nigeria, Kaduna, 1957: 60-67, pls 13-14. | 
Benavides-Caceres, V. E. 1956. Cretaceous system in northern Peru. Bulletin of the 
American Museum of Natural History, New York, 108: 353-494, pls 31-66. 
Bengtson, P. 1983. The Cenomanian-Coniacian of the Sergipe Basin, Brazil. Fossils ) 
and Strata, Oslo, 12: 1-78, | map. | 
Berthou, P.-Y., Chancellor, G. R. & Lauverjat, J. 1985. Revision of the 
Cenomanian-Turonian ammonite Vascoceras Choffat, 1898 from Portugal. ) 
Comunicagoes dos Servicos Geologicos de Portugal, Lisbon, 71: 55-79, pls 1-6. | 
Bose, E. 1920. On a new ammonite fauna of the Lower Turonian of Mexico. | 
Bulletin of the University of Texas Bureau of Economic Geology and Technology, | 
Austin, 1856: 173-252, pls 12-20. ! 
Carter, J. D., Barber, W., Tait, E. A. & Jones, G. P. 1963. The geology of parts of 
Adamawa, Bauchi and Bornu provinces in north-eastern Nigeria. Bulletin of the) 
Geological Survey of Nigeria, Kaduna, 30: 1-108, 6 pls., 3 maps. 
Chancellor, G. R. 1982. Cenomanian-Turonian ammonites from Coahuila, 
Mexico. Bulletin of the Geological Institution of the University of Uppsala, (NS)j 
9: 77-129. 
——, Kennedy, W. J. & Hancock, J. M. In press. Turonian ammonite faunas from 
central Tunisia. Special Papers in Palaeontology, London. 
, Reyment, R. A. & Tait, E. A. 1977. Notes on Lower Turonian ammonite 
from Loma el Macho, Coahuila, Mexico. Bulletin of the Geological Institution of 
the University of Uppsala, (NS) 7: 85-101. 
Choffat, P. 1898. Recueil d’études paléontologiques sur la faune crétacique du 
Portugal. 1. Espéces nouvelles ou peu connus. 2. Les ammonées du Belliasien, 
des Couches a Neolobites Vibrayeanus, du Turonien et du Senonien. Memorias 
da Commissdo dos Trabalhos Geologicos de Portugal, Lisbon, 1898: 41-86, pls 
3-22. 
