Some upper cretaceous ammonites from southern Nigeria

Journal of Afrwan Earth Sciences, Vol I0, No 3, pp 565-581, 1990 0899-5362/90 $3 00 + 0 00 Printed in Great Britain © 1990 Pergamon Press plc

Some Upper Cretaceous ammonites from southern Nigeria

P M. P ZABORSKI

Dept of Geology mad Mining, Umverslty of Jos, P.M B 2084, Jos, Nigeria

Abstract~Ammomtes ofCenomanmn, Turoman, Conmcmn andMaastnchUan age are descnbed from the Calabar, Lokpanta, Nkzlagu and Lokpauku regions of southern Nigeria. Two species are new Forbestceras vartcostatum sp. nov. from the Lower Cenomanlan and Romamceras (Yubanceras) evolutum sp. nov. from the Upper Turoman. The straagraphical slgmficance of the described faunas is discussed. There are stnlang s~mdantles between the Lower Turoman assemblages of parts of southern Nigeria and South America, parucnlarly Venezuela, which contmn common species of Mamnutes, Fagesta, Vascoceras, Paramammttes and Neoptyclutes

I N T R O D U C T I O N

Ammoni tes have traditionally been of pr ime importance in Nigerian Cretaceous s t ra t igraphy A broad biostrat igraphical f ramework was recognized over 30 years ago (see Reyment 1978 for a review). In recent years new discoveries have allowed detailed biostrat igraphical s tudy of several par ts of the Upper Cretaceous (Zaborski 1982, 1985, 1986, 1987). The presen t contr ibut ion deals with a n u m b e r of Cenomanian , Turonian , Coniacian and Maast r icht ian ammoni t e s mos t of which are new to Nigeria and two previously undescr ibed These f aunas are of considerable b i o s t r a t i g r a p h i c a l a n d p a l a e o g e o g r a p h i c a l interest.

Detailed locality da ta concerning the present mater ial can be found in the works ment ioned above. In s o u t h e m Nigeria the Cenoman ian sedi- m e n t s (north of Calabar) are referred to as the Odukpan i Format ion (Reyment 1955); the Lower Turon ian as the Eze-Aku Formation; later Turon ian and Coniacian beds as the Awgu Shale; and the mar ine Campano-Maas t r ich t ian sha le / l imestone sequence as the Nkporo Shale (see S impson 1954).

S Y S T E M A T I C D E S C R I P T I O N S

R e p o s i t o r i e s The spec imens described here are housed in the

depa r tmen t of Palaeontology, British M u s e u m (Natural History), London Their register n u m b e r s are prefixed by the letter C

Dimens ions (in mm) D = diameter, Wb = whorl breadth , Wh = whorl height; U= umbi l icus Figures in pa ren theses represent percentages of the diameter

Family Phylloceratidae Zittel, 1884 Subfamily Phylloceratinae Zittel, 1884

Genus Phy//oceras Suess , 1865 S u b g e n u s Phy//oceras

(Hypophylloceras) Salfeld, 1924 Type species Phylloceras onoense Stanton,

1895, by original designation. Phylloceras (Hypophylloceras) cf btzonatum

(Fritsch 1872) Figs IA-C, 2A-E

Compare: 1872 Ammonrtes bizonatus Fritsch (in Fri tsch and Schl6nbach), p. 40, pl 14, figs 7a-c

1956 Hyporbulites blzonatus (Fritsch), Collignon, p 17, pl 1, figs. 2, 3

1965a Phylloceras (Hyporbuhtes) bizonatum (Fritsch), Collignon, p. 2, pl 414, fig 1711

M a t e r i a l a n d o c c u r r e n c e - T w e n t y spec imens (C 91350-69) from the Eze-Aku Format lon Lower Turon ian (Mammttes nodosoides Zone), Lokpanta, S E Nigeria.

D i m e n s i o n s - D Wb Wh U

C.91366 16 5.5(34) 9.5(59) 1.8(11) C.91356 23 8.5(37) 12 5(54) 2.5{11) C.91360 30 11(37) 17 5(58) 3(10) C.91359 34 12(35) 20(59) 3(9)

R e m a r k s - The su tu re s in these spec imens (Figs. IA-C) show tetraphylloid to polyphylloid saddle endings, placing t h e m in the group of PhyUoceras (Hypophylloceras) seresitense Pervinquiere (see Kennedy and Klinger 1977). The compressed whorls, f lat tened flanks, relatively open umbi l icus and occasional constr ic t ions with associated

565

566 P M P ZABORSrd

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Fig i A-C, su tu re pa t te rns in PhyUoceras (Hypophylloceras) cf btzonatum (Fritsch), A, spec imen C.91366, B, spec imen C 91363, C, spec imen C 91360 All from the Eze-Aku Format ion (Lower Turonian)- Lokpanta. D, su tu re p a t t e m in Nostoceras ?rotundumHowarth; spec imen C 91321 from the uppe r Nkporo Shale (Lower Maastrichtian), Lokpauku. E, su tu re pa t te rn in Solenoceras aft. bembense Haas; spec imen C.91322 from the uppe r Nkporo shale (Lower Maastrichtian), Lokpauku F, whorl section in Forbesiceras varicostatum sp n o v , based on the holotype, C 91324 from the O d u k p a m Format ion (Lower Cenomanian) , Calabar-Akamkpa road, 1 k m nor th of j unc t ion with Calabar-lkot Ekpene road. G, whorl sect ion in Mammites s p , specimen C 91395 from the Eze-Aku Format ion (Lower Turonian), Lokpanta H-J, whorl sections in Paramammite5 polymorphus (Pervinquiere), H, specimen C 91330, I, spec imen C 91328, J , spec imen C 91329 All from the Eze-Aku Format ion (Lower Turonian), Lokpanta. K, whorl sect ion in Romaniceras (Yubariceras) evolutum sp. nov., spec imen C 91325 from the

Odukpan i Format ion (Upper Turol~lan), 34 k m from Calabar, Calabar-Ikot Ekpene road Bar scales in all cases represent 5 m m

Some Upper Cretaceous ammomtes from southern Nigeria 567

s t rengthened ribs relate this material to P. (H.) blzonatum (Fritsch), previously known from Europe and Madagascar.

Several of the present specimens show diffuse, dark, bilaterally symmetrical radial areas of apparent colour banding (Fig. 2E). Ammonoid colour banding takes the form of both longitudinal stripes and radial pat terns (see, for example, Spath 1935; Arkell 1957; Pinna 1972; Matsumoto and Hirano 1976; Tanabe and Kanie 1978, Mapes and Sneck 1987). In the present material the "colour bands" coincide with the line marked by the saddle endings, visible through the preserved test, and in which the "colour" is concentrated. These bands, therefore, do not appear to reflect original shell pigmentation but are a preservational feature, their association with the septa accounting for their symmetrical nature.

Members of the family PhyUoceratidae are extremely rare in Nigeria. Only one specimen has previously been recorded, from Cenomanian beds north of Calabar (Reyment 1955, p. 12).

Family Turrflitidae Gill, 1871 Subfamily NostoceraUnae Hyatt, 1894

Genus Didymoceras Hyatt, 1894 Type species. Ancyloceras nebrascensis Meek and Hayden, 1856; by original designation. Didymoceras navarroense (Shumard, 1862)

Figs 2F, G

1862 Helicoceras Navarroense Shumard, p 190. 1926 Helicoceras navarroense Shumard; Wade,

p. 184, pl. 61, figs. 8-11; pl 62, figs. 1, 2. 1941 Helicoceras navarroense S h u m a r d ;

Stephenson, p. 417, pl. 83, figs 9-13. ? 1969 Didymoceras cf. navarroense (Shumard);

Lewy, p. 115, pl. I, fig. i .

Mater ia l a n d o c c u r r e n c e - A single specimen (C.91320) from the upper Nkporo Shale (Lower Maastrichtian}, Lokpauku, S.E Nigeria.

R e m a r k s - The specimen referred to here is an adult body chamber and is closely comparable with Didymoceras navarroense in its very coarse ornament of strong, curved ribs and two rows of bulbous tubercles each associated with one to three ribs and in what can be seen of its mode of coiling. The neotype, selected by Stephenson ( 194 I, p 418, pl. 83, figs. 9, 10) is especially simJJar in all these respects. The generic assignment adopted here is based primarily on the loosely coiled nature of this species which is a characteristic of Dtdymoceras (see Howarth 1965).

Genus Nostoceras Hyatt, 1894. Type species--- Nostoceras stantoni Hyatt, 1894; by original designation. Nostoceras

rotundwn Howarth, 1965 Figs. ID, 2H, I

1951 Didymoceras angolaense Somay, p 274, pl. 4, figs. 1-3.

1965 Nostoceras rotundum Howarth, p. 381, pl 10,fig. 3.

? 1971 Nostoceras rotundum Howarth, Collignon, p. 8, pl. 643, fig. 2373.

Mater ia l a n d o c c u r r e n c e - - One specimen (C.83505) can be referred here with certainty, a second (C.9132 I) is tentatively included. Both are f rom t h e u p p e r N k p o r o S h a l e (Lower Maastrichtian), Lokpauku, S.E Nigeria

Remarks- The larger of these two specwnens (C 83505, Fig. 2H) consists of the last coil of the spire and first part of the retroversal loop It is a very close match for the specimen figured by Somay (1951, pl. 4, figs. 1-3). The second indivi- dual (C.91321. Figs. ID, 2I) comprises part of a closely coiled sinistral spire. It has two rows of obliquely-aligned tubercles , the upper row centeredJ ust below the siphonal line, the lower row towards the line of contact with the succeeding whorl. Tubercles generally occur on alternate ribs They are large and rounded at first but become more bullate later. Ribbing is effaced between the tubercle rows The suture is shown in Fig ID The spire in N. rotundum is known only from the holotype (see Howarth 1965, pl. 10, fig 3). Its ornament is imperfectly preserved but is similar to that in the present specimen while the degree of coiling is also comparable, though dextral in direction.

Since Howarth (1965) regarded D~Zymoceras angolaense Somay, 1951 as conspecific with Nostoceras rotundum, it was necessary for him to propose the latter name for this material because "N. angolaense (Sornay 1951) is preoccupied by N. angolaense Haughton, 1925" (Howarth 1965, p 383) H a u g h t o n " s spec ies is ac tua l ly N. angolense Haughton (1925, p. 275) but nevertheless, qualifies a senior subjective homonym of N. ango/aense according to Article 58 of the Intemational Code of Zoological Nomenclature.

Both Somay and Howarth's material of N. rotundum is from Barra do Dande, Angola A fragment from Madagascar (Collignon 1971, p. 8, pl 643, fig 2373) can only be doubtfully referred here.

568 P M.P. Z~a3oas~a

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Fig. 2. A-E, P h y l ~ a s ( H y p o p h y l ~ a s ) cf. b/zonatwn (Frltsch); Eze-Aku Formation (Lower Turonlan, Mammites nodoso/des Zone), Lokpanta. A, B, C 91360; C, D, C.91361, E, C.91363 uncoated to show apparent colour banding (all xl 5) F, G, D k i y ~ a s navarroense (Shumard), upper Nkporo Shale (Lower Maastrichtlan), Lokpauku C.91320 (xl). H, Nostocems rotundum Howarth; upper Nkporo Shale (Lower Maastrlchtian), Lokpauku C.83505 (xl) I, Nostoceras?rotun_dtLmHowarth.C.91321 (Xl), same horizonandlocalityas C.83505 J, So/enocerasaff. bembense Haas; upper Nkporo Shale (Lower Maastrichtian), Lokpauku. C.91322 0d.5). K-O, Forbeslc, eras varlcostatwn sp nov, Odukpani Formation (Lower Cenomanian), Calabar-Akamkpa road, I km north of Junction with Calabar-lkot- Ekpene road. K, L, holotype C.91324 0d); M, C.90377 (X1); N, O, C.83497 (xl) P, Q, Acanthoceras rhotomagense (Brongnlart); Odukpani Formation (Middle Cenomanian), 27.9 km from Calabar, Calabar-lkot-Ekpene road C 91323 (xl). R, S, Romantceras (Yubarlceras) evolutum sp. nov., Odukpani Formation (Upper Turonian}, 34 km

from Calabar, Calabar-lkot-Ekpene road C 91325 (xl)

Some Upper Cretaceous ammomtes from southern Nigeria

Genus So/enoceras Conrad, 1860 Type species---Hamltes annultfer Morton,

1842; by original designation. Solenoceras aft. bembense Haas, 1943

Figs IE, 2J

Type species- Ammonites larglUlertianus d'Orblgny, 1841; by the subsequent

designation of Diener, 1925 Forbesiceras tm.Hcostatum sp. nov

Figs. IF, 2K-O

569

Compare" 1943 So/enoceras bembense Haas, p. 11, figs 4, 14.

Materlal and occurrence- A single specimen (C.91322} from the upper Nkporo Shale (Lower Maastrichtian), Lokpauku, S.E. Nigeria

Description- This specimen is relatively large, the first limb is incomplete bu t was probably in excess of 70 m m in length. The first part of the second limb is also preserved, this still being septate. The U-shaped connecting hook is ra ther open but adorally the two limbs are in contact. Initially the whorl section is circular but it becomes ovoid towards the hook. The ornament consists at first of shallow, prorsiradiate constrictions pre- ceded by rounded ribs. Towards the adoral end of the first llmb ribbing becomes dominant. The ribs are irregular in strength, especially adapically, and are prorsiradiate on the first limb and rursiradiate on the second. At the level of the hook each develops a pair of tubercles which are stronger on the more robust ribs On the periphery of the hook each tubercle is subdivided into two longitudinal- ly-aligned nodes but on the second limb they are entire

The simple su ture is shown in Fig. IE. Remarks- This Solenoceras is unusua l in its

large size, ra ther open hook and, especially, in the extension of the phragmocone onto the second llmb The closest previously-described species seems to be S. bembense Haas (1941, p 11, figs. 4, 14) from Angola. It is of comparable size, appears to have a relatively open hook and displays constrictions adapically with ribbing dominant later Tubercles become pronounced adorally and the su tures are similar to those in the Nigerian specimen. Unfortunately, S. bembense is known only from the adoral half of the first shell limb, making full comparison impossible

In some respects the present material resembles Parasolenoceras Collignon, particularly in the unusua l length of its phragrnocone. P. soyaense Matsumoto and Miyauchi (1986, p 9, pl i, figs 1- 4, text-figs 1, 2) is especially similar in its simple suture and subdivided tubercles. In general, however, this genus differs from the Nigerian specimen in its very large size, diverging shell shafts, more complex suture and in its radial ribbing which develops early in ontogeny.

Family Forbesiceratidae wright, 1952 Genus Forbesiceras Kossmat, 1897

1986 Acompsoceras cf. essendiense (Schl~ter); Zaborski, p. 377, flgs. 2U, V.

Holotype- C.91324. (Figs. IF, 2K, L) from the Odukpan i Format ion (Lower Cenomanian) , Calabar-Akarnkpa road, i km north of the Junction with the Calabar-lkot Ekpene road, S.E. Nigeria

Paratypes- C 83497, C 83547, C.90377-8, all from the same locality as the holotype.

Description- The shell is compressed and highly involute. The early ornament consists of strong, rounded ribs on the inner flanks which are transverse to slightly prorsiradiate and number about eight per half whorl, with m u c h finer, subequal ribs on the outer flanks, number ing about 70 per half whorl. The flanks are convex and the venter slightly produced and sulcate with a sharp carina either side of the siphonal line. The carinae are at first cont inuous but at a diameter of 40-45 mm they break down into rows of clavate or nodate tubercles marking the ventral terminations of the outer flank ribbing. By a diameter of some 60 m m the carinae have disappeared The flanks and venter are now broadly rounded, max imum whorl breadth being in the mid-flank region The outer flank ribbing persists but fades at the ventrolateral shoulders or terminates in bullate swellings Vague, low, transverse s t ructures may cross the venter At diameters in excess of 60 m m the inner flank ribs remain but become shorter, often no more than elongate bullae The mid-flank is smooth or with only vague ribs The venter becomes more flatte- ned, the flanks converging ventrally with the max imum whorl breadth being on the inner third of the whorl. The outer flank ribbing fades but irregularly-slzed and spaced buUate or nodate s t ructures remain on the ventrolateral shoulder

The suture is not fully displayed and in particular the adventitious lobe in the first lateral saddle, characteristic of Forbesiceras, cannot be clearly made out

Remarks- The Lower Cenomanian Forbesiceras largillierttanum (d'Orbigny) and F. beaumontianum (d'Orbigny) (see Wright and Kennedy 1984: Kennedy and Juignet 1984) have fine outer flank ribbing but it is more persistent t han in F. varicostatum while both lack the coarse inner flank ribbing of the Nigerian species F. baylissi(Wright and Kennedy 1984, p. 92, pl. 13, flgs 4, 5) has a closer ornamental style but its inner flank ribbing is not as strong and this species develops a tricarinate venter F. pseudobtectum Collignon, 1964 may be a microconch of F. baylissi according to Wright

570 P M P ZABORSra

and Kennedy (1984). F. chevOlet (lhctet and Rene- vier) has very weak ivmer f lank ribbing with broad, fiat ribs on the outer f lanks and a t runca ted venter wlth t ransverse ribbing (see Wright and Kennedy 1984; Kennedy and Ju igne t 1984), F nodosum Crick (1907) and F. clarkl Collignon (1964) are possible synonyms, while F. tenuipunctatum Collignon (1964) is a similar species bu t with smooth inner flanks. F. brundretti (Young 1958) has stronger, s ickle-shaped, b ranching ribs and a broader venter t h a n the Nigerian form The F. obtectum group, in which can be included F obtectum (Sharpe 1853), F. subobtectum (Stoliczka 1864), F fl/ck/ Pervinqui~re (1907), F. conhnt Stephenson (1952) and F. mikasaense Matsumoto (1987), is characterized by prorsiradmte ribbing on the inner flanks, mid-lateral nodes and rursi- radmte outer f lank ribbing which is coarser t h a n in the present material The venters are flatter, often tncar ina te and show transverse ribbing F.furnishl Colhgnon (1964) has very weak inner flank o rnament bu t s t ronger ventrolateral nodes and t ransverse ventral ribbing F. blcartnatum Szasz (1976) is a weakly o rnamented species, more compressed t h a n F. vancostatum, with a sharply rounded adul t venter F.falxWright and Kennedy (1984) has strong, s ickle-shaped ribs and mid- lateral nodes

Family Acanthocerat idae Grossouvre, 1894 Subfamily Acanthocerat inae Grossouvre, 1894

Genus Acanthoceras Neumayr, 1875 Type species- Ammonites rhotomagensts

Brongniart 1822, by the subsequen t designat ion of Grossouvre, 1894

Acanthoceras rhotomagense (Brongmart, 1822) Figs 2P, Q

1822 Ammonites Rhotomagensts Brongniart , p 606, pi 6, Fig 2 1987 Acanthoceras rhotomagense (Brongniart), Wright and Kennedy, p 156 (with synonymy}

Mater ia l and occurrence- A single specimen (C 91323) from the Odukpan i Format ion (Middle Cenomanian), 27 9 k m from Calabar, Calabar-lkot Ekpene road, S E Nigeria

Remarks- Large assemblages of Acanthoceras rhotomagense occur at its type locahty of Rouen, France There Is considerable intraspecific variation in this material The present specimen is closest to forms referred to as A. rhotomagense var clavatumby Kennedy and Hancock (1970, p 479, pl 96, Figs 2, 3, pl 97, Fig 6) which show compressed whorls, weak ribbing and clavate outer ventrolateral tubercles

Subfamily Euomphalocera t inae Cooper, 1978 Genus Romaniceras Spath, 1923

Subgenus Romaniceras Orubariceras) Matsumoto, Saito and Fukada , 1957 Type species- Yubanceras yubarense Matsumoto, Saito and Fukada 1957,

by original designat ion

Remarks- Relat ionships within the Romaniceras group have been d iscussed by Matsumoto et al (1957), Matsumoto (1975) and Kennedy et al (1980) Kennedy et oi. t reated both Yubanceras and Obiraceras Matsumoto, 1975 (type species 0 ornatum Matsumoto, 1975) as subgenera of Romaniceras (type species Ammonites deverianus d'Orbigny, 1841) The only reliable dis t inguishing feature they could find within the group was the n u m b e r of tubercle rows, nine in R. (Romantceras), eleven in R. (Yubanceras) and thi r teen in R (Obiraceras) Intermediates between these subgenera are known, however, (see Kennedy et al. 1980, pp 344-348, pl 47,figs 1-4; Reyment 1955, p 46, pi 9, figs 1,2, text-figs 18a, b, 19) whlle the Nigerian species described below may show a subdivision of the outer ventrolateral tubercles, recalling Obiraceras. Kennedy et al. (1980) regarded the Roman/ceras group as monophylet ic and derived from Kamerunoceras. The present Nigerlan form combines a tubercle a r rangement like that in R. (Yubariceras) with an overall shel l -shape hke that in Kamerunoceras, lending suppor t to this phylogenetlc interpretat ion

Romantceras (Yubanceras) evolutum sp nov Figs IK, 2R, S, 3A-C

1935 Kamerunoceras tinrhertense Collignon, Zaborski, p 51 (pars), fig 58 (only)

Holo type- C 83493 {Figs 3A, B) from the upper Odukpan i Format ion {Upper Turonian), 34 km from Calabar, Calabar-lkot Ekpene road

Paratypes-C 83127, C 83490, C 91325, all from the same locality as the holotype

Dimension D Wb Wh U

C 83493 120 35(29) 45(37 5) 50(42) C 91325 70 20(29) 25(36) -

Description- The shell is highly evolute and the whorls rectangular , being markedly higher t han broad The ribs are rounded , m u c h narrower t han the interspaces and n u m b e r about 12 per half whorl at d iameters between 70-120 m m Nearly all reach the umbilical shoulder bu t occasionaUy shor ter ribs die out on the inner flank. Each major n b carries eleven tubercles, a bulla on the


umbilical shoulder , a p rominen t pointed to bullate inner lateral tubercle about one- third of the way down the flank, a r ounded inner ventrolateral tubercle, a pair of ou te r ventrolateral tubercles the inner of which is larger and rounded to clavate, the outer clavate, and a clavate s iphonal tubercle. The ribs are radial to prorsiradiate be tween the umbilical and inner lateral tubercles, frequently rursiradiate be tween the inner lateral and outer ventrolateral tubercles , and weaken over the venter. In the holotype the two outer ventrolateral tubercles form a single, subdivided, clavate s t ruc ture at a d iameter of 75 m m bu t later there are two discrete tubercles. In C.91325 there are two separate bu t closely spaced tubercles at a diameter of less t h a n 70 m m (Fig. IK).

C 83127 (Fig. 3C) is a f ragment of a larger specimen, having a whorl height of 40 mm. Its ventral o rnamen t is not preserved b u t the f lanks bear a l t emat ing long and shor t ribs. The former show umbilical bul lae and prominent , twisted, bul late inner lateral tuberc les which may fuse into a single s t ructure . The inner ventrolateral tubercles are expanded into large, r ounded to bullate features The shor te r r ibs die out in the mid-f lank region

Evolute, compressed to square-whorled adul t body c h a m b e r s (C.83126, C.83489), one of which was figured by Zaborski (1985, fig. 57), are found alongside th is material . They show massive, alter- nat ing long and shor t ribs with p rominen t ventrolateral ho rns They canno t as yet be m a t ched with certainty to the mater ia l described above bu t C.83127 provides a possible link.

Remarks- - - The overall shel l -shape of R. (Y.) evolutum is tha t o fa Kamerunoceras. K. turoniense (d'Orbigny) (see Kennedy and Wright 1979) is part icularly close, and shows a ribbing style like tha t in the p resen t mater ial as well as the presence of an inner lateral tubercle resul t ing from the migrat ion down the f lank of the umbilical bulla, a process somet imes accompanied by the formation of a new umbilical bulla. The paratype C.91325, however, clearly shows eleven rows of tubercles, revealing tha t the individuals described above are bes t referred to R. (Yubariceras).

Kennedy et al. (1980) recognized only three species of R. (Yubariceras) : R. (Y.) ornatiss~num (Stoliczka), of which Y. y u b a r e n s e w a s regarded as a synonym; R. (Y.) kanei Jones ; and R. (Y.) aequtcostatum Matsumoto. R. (Y.) ornatissimun (see Kennedy et al. 1980 for a review) is generally a markedly more involute species t h a n R. (Y.) evolutum, i ts whorl sect ion is depressed and the ribbing r ema ins s t rong over the venter. The style and densi ty of its lateral ribbing, however, m ay be closely similar to tha t in the present material , while its tubercle a r rangement is also alike. R. (Y.) kanei Jones (1938, p. 120, pl 8, figs 2, 7, 8, pl 9, fig 6)

571

is a n evolute species b u t ha s whorls broader t h a n high and denser, broader r ibbing t h a n R. (Y.) evolutum. R. (Y.) aequicostatum Matsumoto (1975, p. 124, pl. 15, fig. 3; text-fig. 8) is based on a single, imperfectly preserved specimen. It is m u c h more involute and depressed t h a n R. (Y.) evolutum; Kennedy eta/. (1980) t hough t it a probableJ uvenfle R. (Y.) ornatisstmun. Yubarlceras gosavlcum Wiedmann (in Herin etaL 1979, p 46, pl. 6, figs. C, D; text-fig. 71k) is a compressed form with intercalated ribs and eleven rows of tubercles b u t among these is an inconsp icuous outer lateral row. Wright et al. (1983) t hough t it m a y be a Cod aTz~ceras E tayo-Sema

Specimens (C.83491-2, C.83494, Fig. 3D) found alongside R. (Y.) evolutum and previously referred with i t to Kamenawceras (see Zaborski 1985, p. 53) are all f ragmentary and reveal growth stages between 70-110 m m only. They are compressed, have flat tened flanks, dense ribbing, umbilical and inner lateral tubercles , and an umbi l icus represent ing about 45% of the d iameter They c l o s e l y r e s e m b l e a d u l t m a c r o c o n c h s of Codazzlceras ospinae (Karsten 1858) (see Wright et al. 1983) to which they are probably refelTable Codazz/ceras ha s previously been known only from Colombia and, possibly, Austria.

Subfamily Mammit inae Hyatt, 1900 Genus Mammites Laube and Bruder, 1887

Type species- Ammoni tes nodosoa~les Schluter , 1871; by monotypy.

Mammites nodosoldes Schlfiter, 1871 Figs. 3E-R, 4A, B

1871 Ammoni tes nodosoides Schlflter, p 19, pl 8, figs I-4.

1981 Mammites nodosoldes (Schlfiter); Wright and Kennedy, p. 75 (with synonymy)

Material and occurrence-Twenty-five speci- m e n s (C.91370--94) from the Eze-Aku Format ion (Lower Turonian, Mammites nodosoldes Zone), Lokpanta, S.E. Nigeria

Dimensions D Wb Wh U

C.91394 13 5(38) 6.5(50) 4(31) C.91385 18 7.5(42) 8(44) 5(28) C 91374 30 15(50) 14(47) 7 5(25) C 91372 38 19(50) 19(50) 8(21) C 91371 60 25(42) 30(52) 11(18)

Remarks- Mammites nodosoides has previously been described from Lokpanta (Zaborski 1987) bu t the mater ial t reated here conta ins m a n y well- preserved Juveniles which are of interest in assess ing the early ontogeny and variability in the Nigerian material

572 P. M P. ZABORSKI

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G

Q

Fig 3 A-C, Romaniceras (Yubariceras) evolutum sp. nov., Odukpani Formation (Upper Tumnian), 34 km from Calabar, Calabar-lkot-Ekpene road A, B, holotype C.83493 (xl}; C, C.83127 (xl). D, Codazziceras ?osplnae Karsten); Odukpani Formation (Upper Turonian), 34 km from Calabar, Calabar- Ikot-Ekpene road C.83494 (xl). E-R, Mammites ~ s (SchlOter); Eze-Aku Formation (Lower Turonian Mammites nodoso/des Zone), Lok-panta. E, F, C.91374 (xl}; G, H, C.91389 (x3); I, J , C.91394

(x3); K, L, C.91371 (xl); M, N, C 91387 (x3); O, P, C.91372 (xl); Q, C.91375 (xl); R, C.91378 (xl)

Some Upper Cretaceous ammom~ from southern Nigeria

While the initial whorls are uniformly smooth and ovoid in section, the development of ornament is variable in its na ture and timing. In C.91394 (Figs. 3I, J) weak constrictions, swept forwards ventrally, appear at a diameter of 7.5 mm. A pair of outer ventrolateral tubercles occur on the adoral side which, by a diameter of 10 m m are associated with ribs which gradually extend across the flanks. Inner ventrolateral tubercles appear at a diameter of 17 m m and shortly after the ribs reach the umbilical shoulders where tubercles develop. In C.91387 (Figs. 3M, N) inner and outer ventrolateral tubercles appear almost s imultaneously at a diameter of 7 ram. Constrictions are absent. Umbilical tubercles are already present at a diameter of 10 mm. In C.91390 (Figs. 4A, B) the ventrolateral tubercles have appeared by a diameter of 8 m m and umbilical tubercles by 11 mm. Finally in C.91380 outer ventrolateral tubercles appear first at a diameter of 7 ram, inner ventro- laterals about one-thlrd of a whorl later and, a little over ha l fa whorl later at a diameter of some 9 ram, umbilical tubercles develop. In these early stages 8-15 umbilical tubercles and 19-25 ventrolateral tubercles are present in a complete whorl.

During the middle growth stages the whorls may be higher t han broad or a little broader than high. Between diameters of 30-90 m m the umbilical tubercles vary from spinose to bullate. The ribs are frequently bent back over the umbilical shoulder and are convex over the flanks. Forms with stronger, spinose umbilical tubercles generally show less well defined ribbing but stronger ventral tuberculation. Their o rnament is relatively evenly developed with 6-7 umbilical and 15-17 ventrolateral tubercles per whorl. Forms with weaker, more bullate umbilical tubercles tend to show a more irregular o rnament with 12 or more major ribs per whorl and weaker intercalatories also present. In all forms there is a strong tendency for the umbilical tubercles to be twisted backwards.

A comparable, large population ofM. nodosoides was described from the Lower Turonian of Venezuela by Renz (1982). He identified a typical form, a variety with seven high, bullate umbilical tubercles twisted backwards, a variety with irregular umbilical buUae, and a small form tran- sitional towards Benueltes Reyment. Individuals similar to all these types are present in the Nigerian association. In addition, Renz (1982, p. 90, pl. 27, fig. 11; pl. 28, figs. 1-5, 26; text-figs. 68b, 69a, c, d) referred a n u m b e r of associated specimens to Mammites spinosus Basse. This form was considered as a variety of M. nodosol__d_es with exaggerated tuberculat ion and weak ribbing by Basse (1940, p. 458) and was treated as such by Collignon (1966). Individuals of this type also occur in the Nigerian material While there are

573

generally fewer umbilical tubercles in Renz's M. splnosus, individuals referred to M. nodosoldes may show a simfl_ax number (see Renz 1982, pl. 27, figs. 5, 6). Renz noted that the more spinose Mammltes in Venezuela are concentrated in the upper part of the range of M. nodoso/des. No such straUgraphical separa t ion occurs with the Nigerian forms, the spinose and non-spinose forms vary continuously within the same assemblage, indeed both types of umbilical tubercles may co- exist on the same specimen (Fig. 3Q). All the Nigerian individuals are therefore treated here as examples of intraspecific variation within M. nodosoides

One specimen (C.91395, Figs. IG, 4C, D), however, has very large, sharp, spinose umbilical tubercles, practically no ribs, markedly spinose inner ventrolateral and very high, clavate outer ventrolateral tubercles. This form is more coarsely ornamented than anything else in the Nigerian fauna and nothing from Venezuela seems to be as massively tuberculated. Mammites nodosoidesappelatus Etayo-Serna (1979, p. 85, pl 13, flg. l) from Colombia is similar but, as only a single specimen is available, it is here left in open nomenclature as Mammites sp.

Family Vascoceratidae Douvi116, 1912 Subfamily Vascoceratinae Douvill6, 1912

Genus Vascoceras Choffat, 1898 Type species- Vascoceras gamai Choffat 1898, by the subsequent designation of Roman, 1938

Vascoceras venezolanum Renz, 1982 Figs. 4.E-O

1982 Vascoceras venezolanum Renz, p 80, pl 23, figs. 5-11; pl 24, figs. 1-10; pl. 25, figs. 1-8.

Materia l and o c c u r r e n c e - - Twenty-seven specimens (C.90323, C.90325, C.91416-40)from the Eze-Aku Forma t ion (Lower Turon ian , Mammites nodosoides Zone), Lok-panta, S.E Nigeria.

D e s c r i p t i o n - - T h e collection is dominated by Juvenile specimens The flanks and venter are generally broadly rounded in the early stages but sometimes the whorl section is squarer. Whorl breadth is equal to or distinctly greater than the whorl height The umbilical diameter varies between 23-33% of the diameter The earliest whorls are smooth Ornament appears at a diameter of some 8 m m and consists of four to nlne variably pronounced constrictions per whorl bounded adorally or adapically by a strong, rounded rib bent forwards over the venter Some individuals show, In addition, only a few vague intercalated ribs. Others display three or more strong minor ribs, arising on the inner flank or at

574 P M P ZABORSm

D E

G

J

M

R

~V W

Fig. 4 A, B, Mammltes nodosokles (Schltlter), Eze-Aku Formation Lower Turonlan, Mamm~tes nodosoides Zone, Lokpanta. C.91390 (x3). C, D, Mammltes s p , Eze-Aku Formation (Lower Turonian, Mammltes nodosoldes Zone), Lokpanta. C 91395 (xl). E-O, Vascxx~ras venezolanum Renz, Eze-Aku Formation (Lower Turonian, Mammttes nodosokles Zone), Lokpanta E, C 91435 (x2); F, C 91423 (xl.5), G, C 91425 [xl.5); H, I, C 91434 (x2); J , C.91424 (xl), K, L, C.91432 (x2); M, C.91426 (xl 5}, N, O, C.91430 (x2) P-S, Paramammltespolymorphus (Pervinqulere); Eze- Aku Formation (Lower Turonian, Mammites nodoso/des Zone), Lokpanta P, Q, C 91328 (x2), R, S, C 91329 (x2) T-X, Prkmocycloceras mu/tk-ostatum Collignon; Awgu Shale (Conlacian), Nkalagu quarries. T, U, C.85301 (xl), V, W,

C.85302 (xl), X, C 85303 (xl)


a weak umbilical buna, being swept forwards over the venter and increasing in number during ontogeny. Others still have no pronounced con- strictionsbut showmore regularly developed, sharp ribs first appearing over the venter and gradually extending across the flanks to the umbilical shoulders.

As growth proceeds, ribbing becomes dominant over constrictions. C.91423 (Fig. 4F) has approximately 14 umbilical bullae per whorl, each giving off one or two sharp, rounded ribs which are convex over the venter. Numerous intercalated ribs are also present, some arising at the umbilical shoulder, others on the inner third of the flanks Periodic constrictions, especially pronounced ventrally and associated with s t rengthened ribs, also occur.

The later growth stages are shown by C.91424 (Fig 4J) which has a diameter of 80 ram. The shell is moderately involute with convexly rounded flanks There are approximately eight umbilical bullae in the last whorl which give rise to indistinct, broad ribs on the inner flanks.

The su tures are only partially displayed but conform closely with those figured by Renz (1982, text-fig. 61).

Remarks--- Renz (1982) identified a typical form of V. venezo/anum and five other groups which he believed might ultimately be considered as separate species. A high degree of intraspecific variation is, however, present in other species of Vascoceras (see Scht~be11975; Berthou etal. 1985, Kennedy etal. 1987; Reyment 1988) The Nigerian collection, albeit smal le r t h a n tha t from Venezuela, shows a good deal of the variation in ornament and whorl shape described by Renz Boundaries between the various morphotypes would be entirely artificial and these, along with the Venezuelan specimens, are here regarded as members of a single species.

The ornament in the early whorls of the present material is very similar to that in Nannovascoceras Renz andAlvarez, 1979, which occurs alongside V. venezolanum in the Lower Turonian of Venezuela Nannovascoceras is a dwarf genus, reaching a max imum diameter of about 34 m m according to Renz and Alvarez (1979) It is more compressed and evolute than the present material but its range of ornament is almost identical. Renz and Alvarez (1979) proposed tha t three co-existent species were present in Venezuela, N. constr/ctum, N. intermedium and N. costatum, characterized by a progressive dominance of ribbing over constrictions Since intermediates occur, however, they alternatively suggested that all these forms might be treated as the single species N. interrnedium. This latter procedure would appear to be more appropriate in view of the matching variation in the

575

inner whorls of the Nigerian V. venezolanwn. Renz (1982, p. 77) considered Leweslceras ? uUoai Etayo- S e m a (1979, p 30, pl. 13, fig. 7; text-flg. 4D) from Colombia to be a possible synonym of N. constnc- turn. While this form shares with Nannovascoceras a relatively wide umbilicus, its comparatively depressed whorls recall those in V. venezolanum. L.? ulloai is, however, known only from two small specimens. If it should prove conspecffic with V. venezolanum, Etayo-Serna's name would have priority

Genus Paramammites Furon, 1935 Type speciesVascoceras polymorphum Pervinquiere, 1907; by the subsequent

designation of Reyment, 1954. Paramammites polymorphus (Pervinqui~re,

1907) Figs IH-J, 4P-S

1907 Vascoceras polymorphum Pervinqui~re, p 336, pl 21, Figs 2-6, text-fig 126

1982 Paramamm~tes polymorphus p~nguts (Pervinqui~re); Renz, p 84, pl. 25, figs 10-12, text- fig 64

Mater ia l and occurrence - Four specimens (C 91328-31) from the Eze-Aku Formation (Lower Turonian, Mammites nodosoides Zone), Lokpanta, S E. Nigeria.

Descr ipt ion- Only juvenile material is available In its earliest stages, the shell is evolute, smooth and has a rounded whorl section Umbilical bullae appear at a diameter of some 10 m m and shortly after both inner and outer ventrolateral tubercles develop (Fig. IH) At diameters in excess of 15 m m the umbilical buUae enlarge into spinose s t ructures while the ventrolateral tubercles, especially the outer row, also extend. The whorl section may now be as high as wide but is generally markedly depressed with a pentagonal costal outline (Figs II, J) There are seven umbilical spines per whorl, each giving rise to a pair of broad, rounded ribs which bear inner and outer ventrolateral tubercles In the largest specimen (C 91329), wlth a diameter of 25 mm, the umbilicus represents 40% of the diameter The inner and outer ventrolateral tubercles are partially fused in this indivi- dual

The su ture is not displayed. Remarks--- Furon (1935) proposed Paramammi-

tes as a subgenus of Vascoceras for evolute, non- globular, strongly ornamented members of that genus, citing V. polymorphum Pervinquiere and Ammonites subconciliatus Stoliczka as typical members Reyment (1954) subsequent ly raised Paramammttes to generic status, designating the former as type species Wiedmann (1960, p 752)

576 P M P ZABORSKI

pointed out difficulties in the recognition of the genus It is questionable how much of the material previously referred to Paramammites actually belongs there The inner whorls In most of these forms are unknown, precluding comparision with both the early growth stages of the type species and the present material For Nigeria, Barber's (1957) P. tuberculatus, P. rarlcostatus and P. Inflatus, all probably conspecffic, show siphonal tubercles in their early whorls and cannot be included in Paramammites The Algerian species P laflttei Collignon, 1965b andP. subtuberculatusCollignon 1965b are similar to these Nigerian forms in their later whorls. Venezuelan specimens referred to P polymorphus by Leanza (1967, p 200, pl I, figs. 1-4 ) belong in Mtton/a venezo/ana Renz and Alvarez, 1979 The Paramammites aff polymorphusofAm6dro (1982, p 21, Figs 2a-c ) is a large adult, impossible to compare with the present material Juvenile and middle whorls from Venezuela described by Renz (1982, p 84, pl 25, figs 10-12), however, are closely slmilar to those in P polymorphus (see Pervinqui6re 1907, pl 21, figs 2, 4-6) and match the present material Pervinquiere (1907) identified three varieties of this species, apart from the typical form, a variety gracilus, a variety semt-p~nguis and a variety pinguts, distinguished by increasingly strong ornamentat ion and depressed whorls Renz ( 1982. p 85, pl 25. figs 13, 14) also recorded a densely ribbed form with weak tuberculation as P polymorphus gracilis. Its ornament is more delicate than that in any of the Nigerian specimens and it is not regarded here as synonymous with them P colombianusEtayo-Sema (1979. p 90, pl 13. figs 5, 13) is similar to this Venezuelan form and may be conspecific though its inner ventrolateral tubercles are weak. recalling Mitonio.

Family Collignoniceratidae Wright and Wright, 1951

Subfamily Collignoniceratinae Wright and Wright, 1951

Genus Pnononcycloceras Spath, 1926 Type species Ammonites guayabanus

Steinmann, 1881, by original designation lhnononcycloceras multicostatumCollignon, 1965a

Figs 4T-X

1965aPnononcycloceras multtcostatumCollignon, p 45, pl. 433, figs 1794, 1795.

Material and o c c u r r e n c e - Three spec imens (C 85301-3) from the Agwu Shale (Coniacian), Nkalagu quarries, S.E Nigeria

Descr ip t ion - The shell is moderately evolute The whorls are subquadrate with angular ventrolateral shoulders at first but become more ovoid in

the adult stages At a diameter of 30 m m there are strong, rectiradiate, evenly developed ribs, each with a high, bullate umbilical tubercle and a spinose ventrolateral tubercle obliquely projected forwards over the venter At a diameter of 65 mm, there are 19 ribs in the last whorl, now weaker and unevenly developed, the umbilical bullae often being effaced There is a pronounced keel, entire to coarsely crenulated on internal moulds Rib den- sit] increases at larger diameters when the ventrolateral tubercles also become unevenly developed In general, ornament weakens towards adulthood, but every third to fourth rib is s t rengthened with a hypemodose ventrolateral tubercle. The largest specimen (C.85303) is still septate at a diameter of 120 mm There are only weak, broad ribs at this stage and no marked tubercles. The keel is broad and entire

The suture is typical for the genus The first lateral saddle may be symmetrically or asym- metrically bifid

Remarks- In its early and middle growth stages, this material matches very closely Pnonocycloceras multtcostatum Collignon from Madagascar In addition, both show a reduction in ornamentat ion and a rounding of the venter in their later stages while Collignon (1965a, p 46) mentioned the development of large ventrolateral tubercles in the Malagasy material P. maarfiense Sornay ( 1957, p 191, pl 16, figs 8, 11) from Algeria is known only from its juvenile stages but its robust ribbing recalls that in P. multicostatum. The two may be s y n o n y m s Also of in teres t is a juvenile Prionocycloceras from Venezuela (see Renz 1982, p 108, pl 34, fig 7) which shows a comparable ornament but is, again, unknown in the adult stages

STRATIGRAPHICAL AND PALAEOGEOGP~.PHICAL CONCLUSIONS

As concerns the Cenomanian species described here, Forbestceras vancostatum occurs alongside Mariella oehlerti (Pervinqui~re), Pachydesmoceras cf radaodyt Collignon and Acompsoceras subrenevien Zaborskl in beds referrable to the upper part of the Lower Cenomanian (Zaborski 1986) Acanthoceras rhotomagense is a Middle Cenomanian species In N W Europe, its epony- mous zone can be subdivided into a lower subzone of Turrilites costatus Lamarck and an upper subzone of 71trrilites acutus Passy (Kennedy 1971, Kennedy and Hancock 1978, Wright et al. 1984) The bed from which the Nigerian specimen of A. rhotomangense came has been correlated with the Costatus Subzone (Zaborski 1985) on the presence of Turrihtes scheuchzenanus Bosc, T. costatus, Cunningtoniceras inerme (Pervinquiere)


and C. cunningtont merldlonale (Stoliczka). It is of interest to note, however, tha t Cunningtonlceras is of far less impor tance t h a n A. rhotomagense in Europe at th i s level. The opposite in t rue of Nigeria t hough cer ta in m e m b e r s of the former genus are intergradat ional with Acanthoceras.

The Ear lyTuron ian (Mammites nodosoides Zone) age of the mater ia l described from Lokpanta ha s been es tabl ished (Zaborski 1987). It is associated with Pachydesmoceras denisonianum (Stoliczka), Kamerunoceras puebloense (Cobban and Scott), Fagesla catinus (Mantell) (=F. levis Renz), Neoptychites cephalotus (Courtiller), Hoplitoides latefund~t_us Zaborski and Herricklceras(?) sp. Fur the r nor th in Nigeria beds of the same age are domina ted by Pseudotissotia nigeriensis (Woods), Thomasites gongilensis (Woods), Wrightoceras wa/ /s lReyment and Eottssotla simplexBarber The presen t mater ia l emphas i ses both this variat ion in biofacies wi th in Nigeria and the remarkable similarity be tween the Lower Turon ian ammoni t e f aunas of Venezuela (see Renz 1982) and Lokpanta. Mammttes nodosoides, Neoptychites cephalotus, Fagesla catinus, Paramammites polymorphus and Vascoceras venezolanum are all c o m m o n to the two areas while examples of Hoplltoides showing a s imi l a r o n t o g e n e t i c d e v e l o p m e n t to H. I atefundatus are also known from Venezuela (Renz 1982, p. 99, pl. 30, fig. 6). The differences in the Early Turon ian ammoni t e f aunas of Nigeria were in terpre ted as environmental ly-control led by Zaborski (1987) Younger Turon ian f aunas are m o r e h o m o g e n e o u s , however , Neoptychites cephalotus, Hoplitoldes and Coilopoceras, all known from the south , occur above Lower Turon ian beds domina ted by vascocerat id ammoni t e s in nor th- east Nigeria.

Romaniceras (Yubarlceras) evolutum occurs in associat ion with Co//opoceras and spec imens probably referrable to Codo~z/ceras ospmae in beds ass igned to the middle par t of the Tu ron i anby Zaborski ( 1985, 1986). On the basis of foraminifers and calcareous nannofossfls , Perch-Nielsen and Pet ters (1981) dated the same horizons as Turon ian to Coniacian. C. osp /naewas assigned an Early Coniacian or, pe rhaps Late Turon ian age by Wright et al (1983). Co//opoceras is a dominant ly Upper Turo]alan genus in the wes tern interior of the United Sta tes b u t begins its range in the highest Middle Turon ian Zone ofPrionocyclus hyat- t/ (Stanton) (Cobban and Hook 1980, Cobban 1984). In Europe, it has a similar strat igraphical dis t r ibut ion (Am~dro et al 1982, Robaszynski 1984, Kennedy and Wright 1984). On the basis of its associated ammoni tes , therefore, a Late Turon ian age is mos t probable for R. (Yubarlceras) evoluturra

Various ages have been proposed for beds

exposed in one par t or ano the r of the Nkalagu quarries: on the basis of ammoni tes , Early Turon ian (Arua and Rao 1978); on ammoni t e s and i n o c e r a m i d s , Middle T u r o n i a n a n d Ear ly Coniacian, at least in par t and with a h ia tus between the two (Offodfle and Reyment 1976); on ostracods, Early Turon ian (Neufvflle 1973); on feraminffers and ostracods, Ear lyTuronian (Fayose and de Klasz 1976); on feraminifers, Early Coniacian (Petters 1980); and on foramlnifers and calcareous nannofossfls , Early Coniacian (Perch- Nielsen and Pet ters 1981). Only one ammoni te from Nkalagu h a s previously been figured, Forresteria (F) nwalii (Reyment) (see Offodfle and Reyment 1976, p 61, text-figs. 14a, b; Kennedy et o3. 1983, text-figs. 39A, B) This species is allied to F. (F.) madagascariensis (Collignon 1965a, p 42, pl. 432, figs 1784-1787), known from the lower par t of the Coniacian in Zu lu land (Kennedy et al 1983) bu t reported from Collignon's (1965a, 1978) highest of three subdivis ions of the Malagasy Con iac i an , t he "Zone of Prionocycloceras guayabanum a n d Gauthiericeras margae". Prionocycloceras mult icostatum is a l so character is t ic of th is zone in Madagascar Unfortunately, the exact horizon of the spec imens described from Nkalagu is u n k n o w n , t hough frag- m e n t s have been seen from the shale- l imestone sequence in the upper par t of the section exposed there. The ammoni te evidence, however, clearly demons t ra tes the presence of Coniacian beds in the Nkalagu quarries which may include horizons as young as Late Coniacian.

The L o k p a u k u h e t e r o m o r p h s Didymoceras navarroense, Nostoceras rotundum a n d Solenoceras aft bembense occur, alongside numerous Libycoceras dandense (Howarth), close to the top of the Nkporo Shale (see Zaborski 1982). This fauna is mos t obviously comparable with tha t from Barra do Dande, Angola, d i scussed by Howarth (1965). This assemblage also conta in N. rotundum and L. dandense and was dated Late Campan ian to Early Maastr icht ian Didymoceras navarroense occurs in the Neylandville Marl and overlying Nacatoch Sand in Texas (Stephenson 1941) for which Young (1960) sugges ted Late Campan ian and Early Maastr icht ian ages respectively In Colorado, D. navarroenseis known from the zones of Bacuhtes cuneatus and B. reesidei of the Pierre Shale (Cobban 1973), considered to be Upper Campan ian by Izett et al. (1971) and Gill and Cobban (1973). The upper Mishash Format ion of Israel, again dated Late Campanian , also conta ins forms close to D. navarroertse (Lewy 1969) In Colombia, however, the species ha s been reported from the Cimarrona Format ion which has been ass igned to the Maast r icht ian (Petters 1955, Porta 1966) as it conta ins Sphenodiscus Meek and

AES 10/3~-L

578 P. M P ZABORSKI

Coahuilites B6se, forms character is t ic of th is stage in Texas and n o r t h e r n Mexico (see Cooper 1971). The above evidence indicates ei ther a Late C a m p a n i a n or a Maas t r ich t ian age for the Lokpauku ammoni tes . At Akanu , some 60 k m to the sou th-eas t o fLokpauku , Libycoceras dandense also occurs, high in the Nkporo Shale, alongside Nostoceras s p , Gaudryceras b e a n t a l y e n s e Collignon, Sphenodiscus sp. and forms identical to the Pachy_dLccus dossantosi (Maury) of Zaborski (1985). The last three of these are present nor th of C a l a b a r , s t i l l f u r t h e r s o u t h - e a s t , w h e r e Sphenodiscus occurs in very large n u m b e r s (Zaborski 1982, 1985). L/bycoceras d a n d e n s e is u n k n o w n from the Calabar assemblage which was considered a little younger t h a n tha t from Lokpauku by Zaborski (1982, 1983). Following their revision of Pachydiscus neubergicus (yon Hauer), Kennedy a n d Summesberge r (1986) regarded the Nigerian P. dossantosi as a synonym P neubergicus begins its range in the Lower Maastr lcht ian, Kennedy (I 984), indeed, suggested tha t i ts appearance might be used to m a r k t h e base of the stage On th is basis, the Calabar and Akanu f au n as can be da ted Ear ly Maastr icht ian. While the Calabar a n d Lokpauku f a u n a s have a different composit ion, t ha t from Akanu, geographically intermediate , con ta ins e lements of both Either the Lokpauku f a u n a is older t h a n the o ther two, bu t all are wi th in the s t ra t igraphical range of Libycoceras dandense or all three f a u n a s are con temporaneous , occurr ing in beds deposited dur ing the same Early Maas t r ich t ian mar ine incurs ion into wha t were largely margina l mar ine env i ronments at th is t ime in sou the rn Nigeria. Interestingly, the Barra do Dande f a u n a also c o n t a i n s a s p e c i m e n of S p h e n o d i s c u s (Howarth1965, p 398) a n d examples of the genus were reported from the Nacatoch Sand by S tephenson (1941, p 20) It is possible, however, tha t the Angolan specimen came from a higher s t ra t lgraphical level t h a n the o ther Barra do Dande ammoni tes , while Young (1960, 1963} believed tha t the Nacatoch Sand mater ia l might be more closely related to the C a m p a n i a n Manambolites ncens/s Young, 1963. Nevertheless, in Nigeria, Sphenodiscus appears in the basa l Nkporo Shale nor th of Calabar at a s t ra t igraphical level well below tha t conta in ing the Lokpauku f a u n a (Za- borski 1982, 1983)

Acknow/edgeme~ts - Thanks are due to Drs M. IC Howarth and H G Owen for help in several respects. Dr K M Nair collected the Nkalagu ammonites and gene- rously passed them on for study. I am grateful to Mrs I Arua for help in locating the Akanu locality. Photographs were provided by the British Museum (Natural History) Photographic Unit Field work was made possible through a University of Maiduguri Senate Research Grant

Am6dro, F 1982. PUzosia (Anapttzos~ et Paranmmm#es. deux genres d'ammonites rares dana le Cr6tac~ moyen du nord de la France BUl/. trim. Soc. g~o/ Normandie Amls Mus Havre 69, 19-27

AmL~dro, F , BadilIet, G and Devalgne, C. 1982 Biostra- tlgraphie et biozonatlon des ammonites du Turonien fran~ais M~,m. Mus. natn. Hist. nat. Paris (n s ) (C) 49, 167-173

Arkell, W J. 1957 Introduction to Mesozoic Ammonoidea In: Treatise on Invertebrate Paleontology, Part L, Mollusca 4. Cephalopcx:la, A ~ (Edited by Moore, R. C ), pp. L81-L129. Lawrence, Kansas.

Arua, I. and Rao, V I~ 1978 Ammonite evidence for the age of the Nkalagu limestone, Anambra State. J. Mira GeoL 15, 47-48

Barber, W 1957 Lower Turonian ammonites from north-eastern Nigeria Bull. geol Surv. N~jeria, 28, 1- 86

Basse, E 1940 Les cephalopodes cr~taces des massifs c6tiers syriens Pt 1 Notes Mbm. Ht-Comm. Syrte Liban, 3, 412-490

Berthou, P.-Y., Chancellor, G R. and Lauverjat, J. 1985 Revision of the Cenomanian-Turonian ammonite Vascoc~as Choffat, 1898 from Portugal. Comun. Serv. geoL Port 71, 55-79

Brongniart, A 1822 Sur quelques terrains de Craie hors du bassln de Paris. In Description gg~Ioglque des env/rons de Parts (Cuvier, G and Brongniart, A ), pp 80-106 Nouv ed, Paris

Choffat, P 1898 Recuefl d'etudes paleontologiques sur la faune cr~tacique du Portugal 1 Esp~ces nouvelles ou peu connues. 2, Les ammon~es du BelIasien, des Couches & Neolobltes Vibrayeanus, du Turonien et du Senonien., Merra Comura gg, ol. Port 1898, 41-86

Cobban, W A~ 1973 The Late Cretaceous ammonite Baculltes unclatus Stephenson in Colorado and New Mexico J. Res U S geol Surv 1, 459-465

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Some upper cretaceous ammonites from southern Nigeria

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