ABSTRACT
Twenty taxa are described from the Cretaceous of Oman (Adam Foothills). The genera Puzosia, Placenticeras, Cunningtoniceras, Nigericeras, Metoicoceras, Rubroceras and Hoplitoides and the subgenus C. (Gentoniceras) are recorded for the first time from the Arabian Peninsula. Based on the ammonite ranges, a sequence of nine bioevents of the Albian–Turonian is correlated within the zonation, and some markers allow correlations at a larger scale, at least along the southern Neo-Tethys margin. The ammonite data give new constraints for the correlations of the lithological units along the Adam Foothills West-East transect and they question the definition of the lithostratigraphic units within the Natih Formation, especially the Natih A and B members. From a paleogeographic point of view Oman is a landmark for the distribution of the ammonites between the western Neo-Tethys (Europe, North Africa, Middle East) and the eastern Neo-Tethys (Africa, Madagascar and India).
INTRODUCTION
The present study analyzes the taxonomy and the temporal and spatial distribution of Cretaceous ammonites collected during two missions in the mountains of Oman in 2009 and 2012. This paper follows an important related previous work by Kennedy and Simmons (1991) that has been achieved on the ammonites of Oman. Some biostratigraphic and paleogeographic results have already been discussed in a synthesis of the Cenomanian–Turonian faunas along the southern Neo-Tethys margin from Morocco to Oman (Meister and Piuz, 2013). The microfauna is the object of separate papers (Piuz and Meister, 2013; Piuz et al., 2014). These two fossil groups add greatly to the understanding of the biostratigraphy and paleoecology of the Albian–Turonian deposits of this region.
GEOGRAPHICAL AND GEOLOGICAL SETTING
The mountains of Oman constitute an arcuate range situated on the northeastern margin of the Arabian Peninsula as a result of a compressional up-folding (Figure 1). They may be considered as having been a major mountain range since the Oligocene–Miocene. They are composed of a set of nappes overlying autochthonous and parautochthonous rocks. The mid-Cretaceous deposits have an economic importance and consequently are the subject of extensive studies for geological mapping and petroleum exploration. During the Mesozoic, the eastern part of the Arabian Plate was covered by extensive carbonate platforms globally interpretated as shallow-water areas. The study of Droste and Van Steenwinkel (2004, their figure 15) reveals that the carbonate platform of Oman shows a rather complex structure as illustrated in their geological model.
The Adam Foothills form isolated relative autochthonous outcrops in the southern foreland of Al Jabal al-Akhdar. They constitute a series in relay of abraded anticlines, mainly composed of Cretaceous shelf carbonates. From east to west these are Jabal Madar, Jabal Madmar, Jabal Khaydalah, Jabal Salakh, Jabal Nadah and Jabal Fitri. They represent a transect of about 127 km where several fossiliferous localities have been studied (the distance between each of the localities is given in figure 2 of Piuz et al. (2014).
The geological history of Oman has been extensively discussed by several authors, among them Glennie et al. (1974), Hughes Clarke (1988), Rabu (1987), Robertson et al. (1990), Pillevuit (1993), Rabu et al. (1993), Glennie (1995), Le Métour et al. (1995), Sharland et al. (2001), Droste and Van Steenwinkel (2004), Simmons et al. (2007). For detailled geology, the reader is referred to these papers.
The fossiliferous sediments studied here belong to the Wasia Group, part of the Hajar Supergroup, which is well represented in the northern and eastern Arabian Peninsula. Glennie et al. (1974) synthesized in a precise manner the different lithologic units of the Wasia Group, and subsequently several works have provided greater detail for these units including Simmons and Hart (1987), Hughes Clarke (1988), Smith et al. (1990), Philip et al. (1995), van Buchem et al. (1996, 2011), Homewood et al. (2008).
About 150 specimens of more-or-less well-preserved ammonites have been collected from the Nahr Umr Formation (Albian) and from the Natih Formation covering a period from the Albian to the early Turonian (Appendix). This period corresponds to a series of marine transgressions associated with eustasy (sea-level rise) and tectonic influences (subsidence) that led to the establishment of a wide carbonate platform with intrashelf basins.
The Natih Formation mainly consists of shallow-water, muddy limestone with local rudist occurrences, and intraplatform basin organic-rich calcareous shale. Several disconformities (hard-grounds) punctuate that formation and are commonly used for correlations (e.g. van Buchem et al., 1996, 2002, 2011; Homewood et al., 2008). The Natih Formation was subdivided into seven informal lithostratigraphic units (a to g from top to base) by petroleum geologists (Hughes Clarke, 1988; Scott, 1990). Herein these subdivisions are labelled A to G (as in many previous publications), but should be taken with reservation due to their lack of rock outcrop formal description, and regarding their variation of lithology and thickness along an East-West transect (from Jabal Madar to Jabal Fitri, see Piuz et al., 2014). New and significant constraints for correlations are given by the ammonite data exposed below.
The ammonites of the Wasia Group were studied for the first time in 1991 by Kennedy and Simmons. van Buchem et al. (2011) also listed a set of ammonites. Our study completes these works in describing additional taxa and in ageing the Natih Formation more precisely (Figures 2 to 8). It adds precision to the regional biostratigraphical framework and correlates nine bioevents with the zonation (see Meister and Piuz, 2013).
SYSTEMATIC
Class Cephalopoda Cuvier, 1795
Subclass Nautiloidea de Blainville, 1825
Order Nautilida de Blainville, 1825
Suborder Nautilina de Blainville, 1825
Family Hercoglossidae Spath, 1927
Genus Angulithesde Montfort, 1808
Type species:Angulithes triangularis de Montfort, 1808.
Age: Albian–Oligocene.
Angulithes mermeti (Coquand, 1862)
2010 Angulithes sp.- Cavin et al., p. 403, fig. 7x.
cf. 2012 Angulithes sp.- Benyoucef et al., pl. 1, fig. 1.
2012 Angulithes mermeti (Coquand).- Hannaa and Fürsich, p. 64, text-fig. 7 with synonymy.
Material: 7 specimens.
Discussion: One group of nautilids having a smooth conch and a micro-umbilicus has been found in the Natih Formation. It is characterized by a compressed subogival whorl section associated with a sinuous to falciform suture line.
Age and distribution: This taxon is associated with Neolobites that characterizes the lower part of the late Cenomanian. This association is well represented on the southern Neo-Tethys margin from Morocco to Oman.
Subclass Ammonoidea Zittel, 1884
Order Psiloceratida Housa, 1965
Superfamily Desmoceratoidea Zittel, 1895
Family Desmoceratidae Zittel, 1895
Subfamily Puzosiinae Spath, 1922
Genus PuzosiaBayle, 1878 sensu lato
Type species:Ammonites planulatus J. de C. Sowerby, 1827.
Age: Late Aptian–Campanian.
Puzosia sp. indet.
Material: 1 specimen.
Discussion: Badly preserved, this rather evolute form shows a fine ribbing on the outer part crossing the venter and periodic constrictions. Its whorl section is suboval slightly compressed. This form corresponds to the Puzosia group taken in a wide sense. A second larger specimen has strong constrictions and ribbing.
Age and distribution:Puzosia has a worldwide distribution and in Oman this form has a probable middle late Cenomanian age (M. geslinianum Zone).
Superfamily Hoplitoidea Douvillé, 1890
Family Placenticeratidae Hyatt, 1900
Genus PlacenticerasMeek, 1876
(synonymy: see Klinger and Kennedy, 1989)
Type species:Ammonites placentaDeKay, 1828, p. 278, subsequent designation by Meek, 1876, p. 462.
Age: Late Albian–Maastrichtian. Taken in account the interpretation of Cooper and Owen (2011) the earliest Placenticeras are rather of Cenomanian age.
Placenticeras aff. kaffrariumEtheridge, 1904 type 1 sensuKlinger and Kennedy, 1989 Plate 2, fig. 1–3 (Appendix)
cf. 1989 Placenticeras kaffrarium Etheridge.- Klinger and Kennedy, p. 268, only fig. 23C, D, 42, 43A, B, 44, 45A, B.
Material: 5 specimens.
Discussion: The bad preservation of our specimens does not allow a precise determination. They are characterized by a (sub)oxycone coiling, smooth whorls, elliptic strongly compressed whorl section and a complex suture line. The incision of the saddles and lobes differentiates it from the subceratitic suture lines of the Engonoceratidae (Neolobites and Metengonoceras) of the same age (see Klinger and Kennedy, 1989, p. 364). This morphological habitus corresponds well to Placenticeras and especially to Placenticeras kaffrarium Etheridge of type 1 in Klinger and Kennedy (1989) and with P. cumminsi Cragin, a contemporary species. The species P. kaffrarium Etheridge is considered by Klinger and Kennedy (1989) in wide sense, with a very large intraspecific variability. Our specimens are only distinguishable from P. kaffrarium Etheridge sensu Klinger and Kennedy by the venter already pinched rounded in the inner-intermediate whorls and by a smaller umbilicus. Our specimens are more peramorphic for these characters in comparison with the illustration on p. 298, fig. 45B in Klinger and Kennedy (1989). If P. cumminsi Cragin also has large variability, it differs with a wider umbilicus and preserves an ornamentation in all the ontogenetic stages.
Due to the preservation, we put our specimens in affinis with Placenticeras kaffrarium Etheridge of type 1 in Klinger and Kennedy (1989) that represents the closest morphology.
Note that the phylogenetic relationships between the primitive Placenticeratidae and the more derived ones is still under discussion (see Cooper and Owen, 2011, p. 339). In the phylogenetic interpretations of Kennedy and Wright (1983, fig. 5) and Klinger and Kennedy (1989, fig. 8), the evolutive relationships show a lack of information during the Cenomanian exactly corresponding to the position of the Omani Placenticeras.
Age and distribution:P. kaffrarium Etheridge is known in the Turonian–Coniacian of South Africa, Madagascar, India ?, Namibia ? and Angola. P. aff. kaffrarium Etheridge type 1 is only recorded from the late Cenomanian (N. juddii Zone) of Oman.
Family Knemiceratidae Hyatt, 1903
Genus KnemicerasBöhm, 1898
(= Iranoknemiceras Collignon, 1981)
Type species:Ammonites Syriacus von Buch, 1848, p. 20, by original designation.
Age: Albian.
Remark: See Kennedy et al. (2009), Bulot (2010, p. 169), Latil (2011, p. 347) for the discussion of the family.
Knemiceras cf. dubertretiBasse, 1940
cf. 1991 Knemiceras dubertreti Basse.- Kennedy and Simmons, pl. 1, fig. D.
cf. 2009 Knemiceras dubertreti Basse.- Kennedy et al., pl. 6, fig. 1–5; pl. 7, fig. 2–5; pl. 8, fig. 1, 2; pl. 9, fig. 1 with synonymy.
Material: 1 specimen.
Discussion: Two ribbed fragments evoke the morphological habitus of the specimen illustrated by Bulot (2007, fig. 2.3) under the name K. dubertreti Basse, a subinvolute species. The whorls are high and rather compressed for the genus. The whorl section is subtriangular with convergent outer flanks and rounded inner flanks bearing periodic bullae. From the bullae arise two, sometimes three, coarse, blunt, subradiate and spaced ribs; intercalatories are rather developed on the outer flanks. They form a marginal ventro-lateral blunt node. The venter is flat to slightly concave between nodes and low blunt ribs are crossing.
K. gr. uhligi (Choffat) described for Sinai by Geyer et al. (1997, p. 225, fig. 2i) seems to have higher whorls at a similar diameter and a finer and smoother ribbing. Our specimens are more compressed and less coarsely ornamented than K. syriacum (von Buch). K. deserti Mahmoud shows a more rectangular whorl section with a flat, rather wide venter.
Age and distribution: Its age corresponds to the basal middle Albian following Kennedy et al. (2009) and to the lower part of the late Albian (D. cristatum Zone) for Bulot (2007). The species is recorded from Lebanon, Levant, Syria and Oman.
Knemiceras syriacum (von Buch, 1848)
1848 Ammonites Syriacus von Buch, p. 215 (topmost illustration).
1997 Knemiceras sp. ex. gr. syriacum (von Buch).- Geyer et al., p. 225, fig. 2j.
2009 Knemiceras syriacum (von Buch).- Kennedy et al., pl. 1, fig. 1–8; pl. 2, fig. 1–5, 7; pl. 3, fig. 1, 2, 4; pl. 4, fig. 1, 2, 4–6; pl. 5, fig. 1–4 with synonymy.
Material: 3 specimens.
Discussion: With a strong ornamentation, specially with blunt ribs and rather massive whorls, our specimen belong to the coarse inflated Knemiceras like K. syriacum (von Buch), K. arambourgi Basse, K. persicum Collignon or K. iraniense (von Buch), four very close species. Their ornamental habitus is similar to that one of K. dubertreti Basse but with a coarser ornamental development. The whorl section is subrectangular with strongly embracing whorls and the umbilicus narrow. Our specimens are close to the Douvillé specimen (1928, pl. 1, fig. 1a–c).
Age and distribution: Early to middle Albian following Kennedy et al. (2009) and lower part of the late Albian (D. cristatum and M. pricei Zones) (see Bulot 2007, p. 52). It is known from Egypt, Lebanon, Levant, Syria and Oman.
Genus NeolobitesFischer, 1882
Type species:Ammonites Vibrayeanus d’Orbigny, 1841, p. 322, pl. 96, fig. 1–3, by original designation.
This genus includes oxycone (oppelicone to belocone) forms with ceratitic suture line (Figure 9) and with more or less expressed ornamentation according to species.
Age: Cenomanian.
Neolobites vibrayeanus (Orbigny, 1841) s.1.
2005 Neolobites vibrayeanus (Orbigny).- Wiese and Schulze, p. 933, fig. 4A–E, 5A–D, 6A–J, 7A–E, 8A, B, H, 9A, B, D with synonymy.
2005 Neolobites vibrayeanus vibrayeanus (Orbigny).- Meister and Abdallah, pl. 1, fig. 3–6, 8; pl. 2, fig. 2, 3, 6 with synonymy.
2005 Neolobites vibrayeanus brancai Eck.- Meister and Abdallah, pl. 1, fig. 1, 2, 7; pl. 2, fig. 1, 5 with synonymy.
2008 Neolobites vibrayeanus (Orbigny).- Aly et al., pl. 3, fig. 2–3.
2010 Neolobites vibrayeanus (Orbigny).- Cavin et al., p. 403, fig. 7v–w.
2010 Neolobites vibrayeanus (Orbigny).- Nagm et al., p. 478, fig. 5A–F.
2011 Neolobites vibrayeanus (Orbigny).- El-Sabbagh et al., p. 711, fig. 6A–B.
2012 Neolobites vibrayeanus (Orbigny).- Nagm and Wilmsen, p. 70, text-fig. 6A.
2012 Neolobites vibrayeanus (Orbigny).- Hannaa and Fürsich, p. 66, text-fig. 9.
Material: 28 specimens.
Discussion: This well-known species from North Africa and the Middle East has been described in detail by several authors (Kennedy and Juignet, 1981; Meister et al., 1994; Meister and Rhalmi, 2002; Wiese and Schulze, 2005). Most of the Omani Neolobites belong to Neolobites vibrayeanus (Orbigny), a classic species of the southern Neo-Tethys margin. Their variability is low and for the greater part they are characterized by smooth whorls and well developed margino-ventral rows of clavi.
Age and distribution: This species characterizes the lower part of the C. (P.) guerangeri Zone (lowermost late Cenomanian) and is widespread along the southern Neo-Tethys margin (Morocco to Oman, in Europe (France, Spain, Portugal) and South America (Perou, Bolivia and Venezuela). Its repartition is subequatorial (see Meister and Piuz, 2013).
Neolobites fourtauiPervinquière, 1907
1992 Neolobites fourtaui Pervinquière.- Thomel, pl. 22, fig. 1.
Material: 3 specimens.
Discussion: Following Kennedy and Juignet (1981, p. 28) we consider N. fourtaui Pervinquière as a separate species, older than N. vibrayeanus (Orbigny). It is characterized by strong elongated umbilical bullae and strong concave, sometimes biplicate ribs on the upper part of the flanks. The suture line is ceratitic, similar to that of N. vibrayeanus (Orbigny) and rather different from Engonoceras (Figure 9). For the Omani specimens, the umbilical size seems wider than in Orbigny’s species. N. peroni Hyatt is broader, coarser and has more spaced ribs; the whorl section seems more rectangular (see Dominik, 1985, pl. 14, fig. 7).
Age and distribution: This species is present in the middle Cenomanian (A. rhotomagense Zone following Wiese and Schulze [2005: 941, N. fourtaui Pervinquière is associated with A. rhotomagense (Brongniart)] and in the extreme base of the late Cenomanian (Thomel, 1992, Wiese and Schulze, 2005). It is distributed along the southern Neo-Tethys margin from Tunisia to Oman.
Superfamily Acanthoceratoidea de Grossouvre, 1894
Family Acanthoceratidae de Grossouvre, 1894
Subfamily Acanthoceratinae de Grossouvre, 1894
Genus AcompsocerasHyatt, 1903
Type species:Ammonites bochumensisSchlüter, 1871 original designation (= Ammonites renevieri Sharpe, 1854).
Remark: The genus Acompsoceras is characterized by a subdiscocone shell shape and shows a large diversity in ornamentation, umbilical size and in whorl section.
Age:Acompsoceras ranges from early Cenomanian (M. mantelli Zone) to middle Cenomanian (M. dixoni Zone).
Acompsoceras renevieri (Sharpe, 1857)
1987 Acompsoceras renevieri (Sharpe).- Wright and Kennedy, pl. 43, fig. 2, text-fig. 34G, 35D–F, 36A–F, 37–40 with synonymy.
1992 Acompsoceras renevieri (Sharpe).- Thomel, pl. 33, fig. 5.
1993 Acompsoceras renevieri (Sharpe).- Kennedy and Juignet, p. 148, fig. 2b, c, e, f, p. 149, fig. 3a–c, p. 150, fig. 4a, p. 151, fig. 5a, b, p. 152, fig. 6c, d, p. 155, fig. 8d–h, p. 153, fig. 7c, d.
1993 Acompsoceras renevieri (Sharpe).- Robaszynski et al., pl. 11, fig. 4.
2011 Acompsoceras renevieri (Sharpe).- Kennedy et al., p. 223, fig. 12A–C.
Material: 6 specimens.
Discussion: As shown by Wright and Kennedy (1987), the variability of the morphology and the ornamentation of A. renevieri (Sharpe) is wide. A large variability is also observed in the Omani fauna where discocone coarse, massive ornamented forms co-occur with smooth oxycone ones, probably the expression of intraspecific variability. So the smooth forms are close to the specimen illustrated p. 145, fig. 37 (Wright and Kennedy, 1987) whereas the ornamented ones are similar to that of fig. 2, pl. 43 (Wright and Kennedy, 1987) and to the specimen of Thomel (1992, pl. 33, fig. 5).
Two specimens with a diameter of about 150 mm are moderately evolute (umbilicus/diameter = 24%) and are characterized by a weak blunt ribbing, strong umbilico-lateral bullae, small high lateral tubercles and acute ventro-lateral tubercles. The ribs are biplicate from the bullae and end at the ventro-lateral tubercle. The whorl section is suboval with a tabulate smooth venter (Figures 10a, d). Tracks of a siphonal keel are visible. The ornamentation, coarse in the inner whorls, gets weaker on the outer whorls with the loss of the ribbing and of the high lateral tubercles. The ratio whorl width/whorl height = 1.2 at a diameter of 80 mm slightly increases to 1.3 at 135 mm of diameter.
A larger specimen (D = 240 mm) is more involute (U/D = 16%) with smooth, more compressed subelliptical whorls with rather flat flanks. By comparison with the other specimens at the same size, it is already smooth whereas the others still bear umbilico-lateral bullae and ventro-lateral tubercles; moreover their whorls are broader. In term of variability this form corresponds to the peramorphic pole in comparison with the ornamented ones (paedomorphic pole). Some A. renevieri (Sharpe), rather smooth specimens illustrated by Kennedy and Juignet (1993, p. 150, fig. 4, p. 151, fig. 5) or Wright and Kennedy (1987, p. 148, fig. 40) show intermediate morphologies.
Several specimens are also recorded from Jabal Fitri. They are large smooth fragments with part of the body chamber and characterized by high and compressed whorls similar to a specimen of A. renevieri (Sharpe) illustrated by Wright and Kennedy (1987, p. 145, fig. 37) previously known as A. essendiensis (Schlüter).
Some affinities exist with the late Cenomanian Paracompsoceras landisiCobban (1971, pl. 6, pl. 7); but at adult diameters Cobban’s species clearly has a wider umbilicus. For the ornamented stage, the bullae are expressed less than in the Omani specimens. The ornamentation is rather different with less expressed bullae and ventro-lateral tubercles. A. sarthense (Guéranger) in Pervinquière (1907, pl. 17, fig. 4, 6) has a similar morphology.
In A. inconstans (Schlüter) the ribbing is stronger and persist longer and the umbilicus is wider (see lectotype in Wright and Kennedy, 1987, p. 149, fig. 41). Moreover a lateral row of tubercles is well developed until the intermediate whorls.
Age and distribution:A. renevieri (Sharpe) ranges from the M. mantelli Zone (early Cenomanian) to the A. rhotomagense Zone (middle Cenomanian) maybe to the ?A. jukesbrownei Zone if we follow Thomel (1992). Indeed for this author A. rhotomagense (Brongniart), A. jukesbrownei Spath and Acompsoceras are in association. Sharpe’s species occurs in Europe (France, Germany, UK, Poland, Russia), north and west Africa, Madagascar and Arabian Peninsula (Oman).
Genus CunningtonicerasCollignon, 1937
Type species:Ammonites cunningtoniSharpe, 1855 by original designation.
Age: Middle Cenomanian to early late Cenomanian.
Cunningtoniceras sp.
Material: 4 specimens.
Discussion: A quite evolute form with lateral spaced and blunt ribs ending in coarse horns. The venter is flat and smooth and the horns are laterally outspread (Figure 10e). A peri-umbilical reinforcement of the rib is also obvious. These characters evoke those of the holotype of C. cunningtoni (Sharpe) (see Wright and Kennedy, 1987, p. 199, fig. 76). Because of the incomplete preservation of our specimens however, we do not give a specific determination.
There are also close morphological convergences with the genus Euomphaloceras, especially for the outer whorls of the large specimens. This genus formerly included C. cunningtoni (Sharpe).
Age and distribution: A cosmopolitan genus of the middle Cenomanian–early late Cenomanian. C. cunningtoni (Sharpe) has a large distribution, but its age precisely corresponds to the A. rhotomagense Zone. The Omani specimens characterize the base of the late Cenomanian (C. (P.) guerangeri Zone).
Genus CalycocerasHyatt, 1900
Type species:Ammonites navicularisMantell, 1822 (ICZN opinion 557 and ICZN specific name 1633).
Age: Middle to early late Cenomanian.
Subgenus ProeucalycocerasThomel, 1972
(syn. HaugicerasThomel, 1972)
Type species:Calycoceras (Eucalycoceras) besaireiCollignon, 1937 original designation by Thomel, 1972.
Age: Middle to early late Cenomanian.
Calycoceras (Proeucalycoceras) cf. canitaurinum (Haas, 1949)
Plate 10, fig. 1–4; Plate 11, fig. 1–2; Plate 12, fig. 2; Plate 13, fig. 1; Plate 14, fig. 1 (Appendix)
1990 Calycoceras (Proeucalycoceras) canitaurinum (Haas).- Cobban and Kennedy, pl. 1, fig. 1–12; pl. 2, fig. 1–12; pl. 3, fig. 1–7; pl. 4, fig. 1–2 with synonymy.
Material: 18 specimens.
Discussion: These inflated moderately evolute (planorbicone-platycone) forms are characterized by rather strong ornamentation. The whorl section is subcircular and weakly depressed, varying from the juvenile to the adult stage with a ratio Ww/Wh = 1 to 1.25 (Figures 10f–h). The venter is flat rounded. The embracing whorl overlap is moderate to strong. The ribbing is annular and regular, bearing strong peri-umbilical bullae and ventro-lateral spiny tubercles. One to three ribs originate from the bullae and some intercalatories appear on the third part of the flank. In the inner whorls, the ribs are subradiate slightly flexuous becoming rigider by the adult. A variability in rib-density can be observed [Plate 10, fig. 2 and Plate 11, fig. 2 (Appendix)]. In some specimens, two rows of discrete remaining lateral tubercles, one margino-ventral and another siphonal, are obvious in the inner whorls.
In the adult stage the bullae are still well expressed; but the ornamentation varies in the outer part with a tendency of a smoothing; the tuberculation is reduced and the ventro-lateral tubercles more or less disappear [Plate 11, fig. 1 (Appendix)] but still remain in some specimens [Plate 11, fig. 1 (Appendix)]. The ventral ribbing clearly becomes evanescent [Plate 10, fig. 3 (Appendix)]. Some specimens [Plate 11, fig. 1 (Appendix)] are more inflated with stronger, blunt and spaced ornamentation, expressing a rather wide variability in this species.
The Omani C. (Proeucalycoceras) are closely related to C. (P.) guerangeri (Spath), a rather cosmopolitan species and to C. (P.) canitaurinum (Haas) a species only recorded from the Western Interior (USA). These two contemporary species have a large, partly overlapping variability. In the inner-intermediate whorls Haas’ species (see Cobban and Kennedy, 1990, pl. 1, fig. 2–9) is more paedomorphic than C. (P.) guerangeri (Spath) for the persistence of a strong multituberculation (see Wright and Kennedy, 1990, pl. 73, fig. 1, 2, 4). At this ontogenic stage, our specimens better correspond to C. (P.) canitaurinum (Haas) with the rib density and the flexuosity of the ribs but they are distinguishable with the early loss of the multituberculation. At similar size, C. (P.) guerangeri (Spath) has a coarser and more spaced ribbing (see Ahmad et al., 2013, p. 26, fig. 3a–c, 4a-b). At the adult size some of our specimens [Plate 14, fig. 1 (Appendix)] are similar to the holotype of C. (P.) canitaurinum (Haas, 1949, pl. 1, fig. 1, 2), others [Plate 10, fig. 3; Plate 11, fig. 1 (Appendix)] have more inflated and less ornamented whorls closer to the specimen illustrated by Haas (1949, pl. 3, fig. 1, 2). At this size, some C. (P.) guerangeri (Spath) are also close to our specimens (e.g. Reyment and Bengtson, 1986, pl. 9, fig. 6, 7 or Wright and Kennedy, 1990, pl. 70, fig. 1) but this species always keeps a coaser and more spaced ribbing.
C. (P.) besairiei (Collignon) shows more developed ventral tuberculation (ventro-lateral and siphonal) at comparable size. C. (P.) haugi (Pervinquière) is more compressed with stronger spaced ribs.
Although very close to C. (C.) naviculare (Mantell), Haas’ species is classically put in the subgenus Proeucalycoceras mainly based on the inner whorls which show a more rounded venter, more depressed and broader whorls and a persisting ribbing in the adult, mainly on the venter. Moreover the ventro-lateral tubercle remains clearly more prominent in C. (Proeucalycoceras) during the ontogeny, and the ventral part remains more flat. The subgenus Gentoniceras clearly differs with a wider umbilicus. Some C. (Newboldiceras) like C. (N.) planecostatum (Kossmat) also shows some affinities with the ribbing.
Age and distribution: This is the index species of the C. (P.) canitaurinum Subzone (lower part of the C. (P) guerangeri Zone), the first subzone of the late Cenomanian. It is recorded from USA (Western Interior) and Oman.
Calycoceras (Proeucalycoceras) sp. indet
Material: 2 specimens.
Discussion: Our two C. (Proeucalycoceras) sp. indet. share rather compressed whorls; the second one has only the body chamber preserved. They are characterized by coarse spaced annular ribs. On the flank, the ribs are taut and subradiate with intercalatories at the mid-flank. A reinforcement of the ribs (tubercles) is obvious at the peri-umbilical part and the ribs become rather acute and prominent on the venter. The whorl section with a ratio Ww/Wh of about 0.83 is subrounded with flat parallel flanks. The morphology of the whorls (Figure 10b) and a wider umbilicus distinguish them from C. (P.) guerangeri (Sharpe). The small specimen evokes the C. (P.) haugi (Pervinquière) in Kennedy and Juignet (1994b, p. 479, fig. 6d, e) and is similar to the specimen described by Kennedy and Simmons (1991, pl. 2, fig. H) for Oman.
Age and distribution: Probably the same age than the Neolobites vibrayeanus beds (early late Cenomanian).
Subgenus GentonicerasThomel, 1972
(syn: Subeucalycoceras,Thomel, 1972)
Type species:Ammonites gentoniBrongniart, 1822 by original designation.
Age: Middle to late Cenomanian.
Calycoceras (Gentoniceras) aff. gentoni (Brongniart, 1822)
aff. 1990 Calycoceras (Gentoniceras) gentoni (Brongniart).- Wright and Kennedy, pl. 56, fig. 1–3, 6–8; pl. 57, fig. 2, 3, 8; pl. 58, fig. 7; pl. 66, fig. 1, 2; text- fig. 88a, c; 89a, b; 90a–c, with synonymy.
aff. 1990 Calycoceras (Gentoniceras) subgentoni (Spath).- Wright and Kennedy, pl. 56, fig. 4, 5; pl. 57, fig. 4; pl. 58, fig. 5, 6; pl. 59, fig. 1–4; text- fig. 88k; 90d–f, with synonymy.
aff. 1994a Calycoceras (Gentoniceras) gentoni (Brongniart).- Kennedy and Juignet, p. 30, fig. 1a; 2d, e; 6d, e, j, k; 7a–l; 8a–e; 22a, b.
aff. 2010 Calycoceras (Gentoniceras) gentoni (Brongniart).- Kennedy and Klinger, p. 9, fig 31, with synonymy.
Material: 1 specimen.
Discussion: With a more opened umbilicus, slender whorls and regular rather dense ribbing, this form differs from the other Omani Acanthoceratinae. The single specimen is of moderate size and has a half a whorl of the body chamber. It is characterized by annular ribs, a subcircular whorl section with flat flanks and moderately embracing whorl overlap (Figure 10c). The conch is planorbicorne-platycone evolute with a rather open umbilicus (U/D = 33%). The ribbing is subradiate with peri-umbilical bullae sometimes subdivided into two ribs on the flank, and with intercalatories appearing near the base of the flank. At this ontogenetic stage, siphonal and latero- ventral tubercles are already absent. If we follow the dimorphic hypothesis, our form represents the microconch (see Wright and Kennedy, 1990).
The specimen illustrated by Crick (1907) under the name C. (G.) paucinodatum Crick is close to our specimen with its ribbing habitus but has a slightly smaller umbilicus; this ‘species’ now is included in the C. (G.) gentoni (Brongniart). C. (G.) sarthense (Bayle) shows a similar evolute coiling but differs with its more spaced and coarser ribbing.
Age and distribution: The genus is well represented in Europe, in South Africa, Madagascar, India, Japan, USA, ? Iran and now from the Arabian Peninsula; the species C. (G.) gentoni (Brongniart) occurs in Europe, South Africa, South India, Oman and ? Iran. The species ranges from middle Cenomanian to early late Cenomanian. In Oman, C. (G.) gentoni (Brongniart) is associated with M. geslinianum (Orbigny) and has a younger age (middle late Cenomanian).
Genus NigericerasSchneegans, 1943
Type species:Nigericeras gignouxiSchneegans, 1943, subsequent designation by Reyment, 1955, p. 62.
Age: Late Cenomanian.
Remark: Several Omani specimens are attributed to the genus Nigericeras because of a) the ontogeny of the ornamentation [it is well developed in the inner whorls and smoothing or evanescent as in the paedomorphic N. gadeni (Chudeau) or disappears completely as in the peramorphic N. jacqueti Schneegans]; b) the habitus of the suture line; although corroded, the tracks of the suture line show very large shallow saddles and lobes like in Nigericeras (see fig. 2c in Wright et al., 1996, p. 161); c) their platycone evolute coiling with suboval more or less compressed whorl section; d) their close similarities with the Nigericeras described in Niger (Meister et al., 1992, 1994); and e) the morphological variability of our forms is close to that described for Nigericeras by Meister et al. (1992, fig. 15).
There are also some similarities with Vascoceras like V. cauvini Chudeau and there are some morphological convergences with Paracompsoceras Cobban, a genus of the middle (or late for Wright et al., 1996) Cenomanian that shows a similar ontogeny with the loss of the ornamentation relatively early during growth, especially the specimen illustrated by Kennedy et al. (1988, p. 39, fig. 2c, e). Nevertheless the ornamentation persists longer in Paracompsoceras. Moreover Cobban’s genus shows narrower and deeper suture lines.
The convergences with V. cauvini Chudeau mainly rest on smooth evolute adult forms like those in Meister et al. (1992, pl. 6, fig. 1). A strong ornamentation in the inner whorls and the suture line habitus distinguish the Nigericeras group (Meister et al., 1992, pl. 3, fig. 1).
The Omani forms come from a rich level in Jabal Fitri and with rare specimens from Jabal Salakh.
Nigericeras gadeni (Chudeau, 1909)
1989 Nigericeras gadeni (Chudeau).- Kennedy et al., fig. 9L, M; fig. 11O, P, with synonymy.
1992 Nigericeras gadeni (Chudeau).- Meister et al., pl. 3, fig. 1–3, 5, 7; pl. 4, fig. 1, with synonymy.
2012 Nigericeras gadeni (Chudeau).- Meister and Abdallah, pl. 2, fig. 1, 2; pl. 3, fig 1; pl. 12, fig 9, with synonymy.
Material: 17 specimens.
Discussion:Nigericeras is considered in the very wide sense, and the Omani specimens correspond well to the concept discussed in detail by Meister et al. (1992, fig. 13). They are close to the N. gadeni (Chudeau) characterized by a morphological habitus somewhere between the strong ornamented N. scotti Cobban and the smooth forms like N. jacqueti Schneegans and N. jacqueti involutus Meister et al.
The inner-intermediate whorls are characterized by strong umbilical bullae extending in ribs that diminish in intensity on the upper part of the flank. In rare cases, ribs crossing the venter are obvious at small diameters. In adults, some tracks of ornamentation are still present towards the outer part of the body chamber. One specimen develops rather fine ribs near the aperture [Plate 15, fig. 1 (Appendix)]. The whorl section is suboval, weakly compressed in the inner-intermediate whorls and a little more depressed in the body chamber.
Our specimen also evokes the rather badly preserved Nigericeras from Texas (Kennedy et al., 1989, fig. 9L, M; 11O, P). In N. scotti Cobban the ornamentation persists throughout the ontogeny (see Cobban, 1971, pl. 19, fig. 1–4) and represents the paedomorphic pole in this genus.
Age and distribution:N. gadeni (Chudeau) ranges from the upper part of M. geslinianum Zone to probably the lowermost N. juddii Zone. It is present in Europe, Tukestan, USA, Morocco, Levant, Oman and especially along the Trans-Saharan seaway from Tunisia to Northern Nigeria where its morphological variability is very high.
Nigericeras jacquetiSchneegans, 1943
Material: 1 specimen.
Discussion: Following Meister et al. (1992) these forms represent the platycone involute and smooth morphologies of Nigericeras, compared with N. gadeni (Chudeau). Their whorl sections are suboval slightly more compressed and their umbilicus are narrower.
Age and distribution: It co-occurs with N. gadeni (Chudeau) in the upper part of the M. geslinianum Zone and maybe in the lowermost part of the N. juddii Zone. It is recorded from Tunisia, Niger, Northern Nigeria, in Oman and probably southern France.
Subfamily Mammitinae Hyatt, 1900
Genus Metoicoceras,Hyatt, 1903
Type species:Ammonites swalloviiShumard, 1861, p. 591, subsequent designation by Shimer and Shrock, 1944, p. 56.
Age: Late Cenomanian.
Metoicoceras gr. geslinianum (Orbigny, 1850)
1850 Ammonites geslinianus Orbigny, p. 146.
2005 Metoicoceras geslinianum (Orbigny).- Meister and Abdallah, pl. 8, fig. 1, with synonymy.
? 2009 Metoicoceras gr. geslinianum (Orbigny).- Lehmann and Herbig, pl. 1, fig. T–U.
2010 Metoicoceras geslinianum (Orbigny).- Nagm et al., p. 482, fig. 8A–C.
Material: 3 specimens.
Discussion: Our poorly preserved specimens are characterized by an involute coiling (suboxycone form) and high compressed subrectangular whorls with a flat external part. The ribbing is broad, blunt and spaced, well developed on the outer flanks. Primary ribs are slightly prorsiradiate starting from the umbilicus, evanescent on the lower half of the flank and strong on the upper half where they alternate with secondaries. Despite the preservation, one specimen shows small periumbilical bullae. The ventro-lateral edge is marked by a reinforcement of the ribs that become blunt and cross the venter.
Our specimens are close to the M. geslinianum (Orbigny) illustrated by Meister et al. (1992, p. 65, fig. 1, 3).
Age and distribution: A cosmopolitan species and the index species of the M. geslinianum Zone (middle late Cenomanian). Along the southern margin of the Neo-Tethys, this species allows acute correlations from Morocco to Oman, and to Nigeria in the South.
Subfamily Euomphaloceratinae, Cooper, 1978
Genus PseudaspidocerasHyatt, 1903
Type species:Ammonites footeanusStoliczka, 1864
Age: Late Cenomanian–early Turonian.
Pseudaspidoceras sp.
Material: 3 specimens.
Discussion: Three badly preserved specimens evoke the morphology of the Pseudaspidoceras with their rather evolute coiling with massive whorls, latero-ventral and peri-siphonal tubercles. One of the specimens [Plate 17, fig. 2ab (Appendix)] shows some affinities with some Nigerian P. pseudonodosoides (Choffat) (Zaborski 1990) or with P. tassaraensis Meister, Alzouma, Lang and Mathey from Niger. Another [Plate 16, fig. 2ab (Appendix)] has a narrower umbilicus that evokes rather P. flexuosum Powell-P. barberi Meister group.
This genus was already recorded from Jabal Salakh by Kennedy and Simmons (1991, pl. 1, fig. C and pl. 4, fig. A, B).
Age and distribution: A rather cosmopolitan genus ranging from the late Cenomanian (N. juddii Zone or maybe even M. geslinianum Zone) to early Turonian (W. coloradoense Zone).
Family Vascoceratidae Douvillé, 1912
Subfamily Vascoceratinae Douvillé, 1912
Genus RubrocerasCobban, Hook and Kennedy, 1989
Type species:Rubroceras alatumCobban, Hook and Kennedy, 1989 by original designation.
Age: Late Cenomanian.
Remark: Originally placed in the Pseudotissotinae by Cobban et al. (1989), following Wright et al. (1996) this genus is now included in the Vascoceratidae.
Rubroceras aff. burroenseCobban, Hook and Kennedy, 1989
? 1991 Paramammites sp. Kennedy and Simmons, pl. 3, fig. A–C.
aff. 1989 Rubroceras burroense Cobban, Hook and Kennedy.- Cobban et al., p. 55, fig. 93A–C, 94Q–S.
aff. 2012 Rubroceras gr. burroense Cobban, Hook and Kennedy.- Meister and Abdallah, pl. 10, fig. 1, 3-5, 7, 9; pl. 11, fig. 1, 3, 5.
Material: 1 specimen.
Discussion: A weakly depressed whorl section with strongly embracing whorl overlap, a coarse ornamentation and a subdiscocone shell shape (type 8–9 in Meister and Piuz, 2013, fig. 13) characterize this specimen. The ornamentation is represented by coarse peri-umbilical nodes, blunt lateral subradiate ribs crossing the venter and periodically alternating with weaker ribs.
The Omani Rubroceras is distinguishable from R. burroense Cobban, Hook and Kennedy s.s. by the more compressed whorls, On the other hand it is close to the Tunisian specimens, having a rather open umbilicus. It is especially close to the coarse blunt ornamented Rubroceras sp. of Meister and Abdallah (2012).
Our specimen also evokes the Paramammites sp. of Kennedy and Simmons (1991), but that has a more open umbilicus.
Age and distribution: This species characterizes the N. juddii Zone and is known in New Mexico (USA), Tunisia, Oman and maybe in Nigeria and Ibericas (Spain). Another Rubroceras, R. alatum Cobban, Hook and Kennedy is recorded from Sinai and Portugal as well as the USA.
Genus VascocerasChoffat, 1898
Type species:Vascoceras gamaiChoffat, 1898, p. 54, pl. 7, fig. 1–4, pl. 8, fig. 1, pl. 10, fig. 2, pl. 21, fig. 1–5, subsequent designation from Diener, 1925, p. 182.
Age: Late Cenomanian–early to ? middle Turonian.
Vascoceras cauviniChudeau, 1909
1992 V. (Paravascoceras) cauvini Chudeau.- Meister et al, pl. 4, fig. 6; pl. 5, fig. 1; pl. 6, fig. 2, with synonymy.
? 2004 Vascoceras cauvini Chudeau.- Abdel-Gawad et al., pl. 4, fig. 2, 3, 5.
2009 Vascoceras cauvini Chudeau.- Lehmann and Herbig, pl. 2, fig. A–B.
2010 Vascoceras cauvini Chudeau.- Nagm et al., p. 484, fig. 9A–B.
? 2011 Vascoceras cauvini Chudeau.- El-Sabbagh et al., p. 711, fig. 6C–D.
2012 Vascoceras cauvini Chudeau.- Nagm and Wilmsen, p. 73, text-fig. 9A.
Material: 1 specimen.
Discussion: This specimen is characterized by strong ornamentation on the phragmocone mainly on the outer part of the whorl. There is an alternation of blunt ribs starting from the base of the flanks with some intercalatories appearing at the mid flank and crossing the venter. The coiling is planorbicone-platycone and the whorl section is suboval, weakly depressed and with a moderately embracing whorl overlap.
The body chamber is smooth (preservation?) or similar to the variety semiglabra of Furon (1935, pl. 4, fig. 3). Our specimen corresponds well to the ornamentation of V. cauvini Chudeau s.s. (see Chudeau, 1909 or Furon, 1935) and to the wide interpretation of this species by Meister et al. (1992, p. 76, fig. 17).
Age and distribution: This species (sensu stricto) is known in Nigeria, Niger, Algeria, Egypt, Levant, Oman, and possibly in Angola and Texas. It indicates the late Cenomanian (M. geslinianum and N. juddii Zones).
Vascoceras aff. barcoicensis exileCobban, Kennedy and Hook, 1989
aff. 1989 Vascoceras barcoicensis exile Cobban, Kennedy and Hook, pl. 87, fig. Q–S; pl. 89, fig. M–GG. 2012 Vascoceras cf. barcoicensis exile Cobban, Kennedy and Hook.- Meister and Abdallah, pl. 412, fig. 4.
Material: 1 specimen.
Discussion: One specimen with discocone involute smooth shell shape (type 5 in Meister and Piuz, 2013), weakly compressed whorls and very strongly embracing whorl overlap is near the group of V barcoicensis exile Cobban, Kennedy and Hook. Only its narrower umbilicus (U/D = ca. 10%) is distinguishable from the holotype. It is also similar to the small Tunisian specimen (Meister and Abdallah, 2012, pl. 12, fig. 4).
Age and distribution: Known from New Mexico, Tunisia and Oman, this taxon is present in the N. juddii Zone.
Vascoceras sp.
Material: 3 specimens.
Discussion: Under this name we group several badly preserved Vascoceras. Among them, one specimen [Plate 19, fig. 2 (Appendix)] with a rather opened umbilicus and relatively compressed whorls, shows large similarities with the specimen illustrated by Kennedy and Simmons (1991, pl. 5, fig. A, B) under the name Vascoceras durandi (Peron) or with the specimens of Barroso-Barcellina and Goy (2010, pl. 3, fig. A–G). Due to the bad preservation we give only the generic determination.
Another specimen of Salakh [Plate 19, fig. 3 (Appendix)] corresponds to subsphaerocone Vascoceras with small umbilicus. Its habitus evokes several forms like V. kossmati Choffat or to a lesser extend V. proprium (Reyment), V. tectiforme Barber or V. crassum Furon.
The third, [Plate 19, fig. 5 (Appendix)] with its narrow umbilicus and compressed whorls, evokes the V. barcoicensis exile of Cobban, Kennedy and Hook or maybe the broader V. arnesensis Choffat.
Age and distribution: All these form are from the late Cenomanian–? early Turonian of Jabal Salakh.
Genus FagesiaPervinquière, 1907
Type species:Olcostephanus superstesKossmat, 1897 original designation by Pervinquière, 1907, p. 322.
Age: Early Turonian.
Fagesia cf. catinus (Mantell, 1822)
cf. 1822 Ammonites catinus Mantell, pl. 22, fig. 10 only.
cf. 1981 Fagesia catinus (Mantell).- Wright and Kennedy, pl. 26, fig. 2; text- fig. 31-36, with synonymy. cf. 1987 Fagesia catinus (Mantell).- Kennedy et al., pl. 7, fig. 1–13; pl. 8, fig. 1–4, 6–9; text- fig. 2j, k, m, n; 10, with synonymy.
cf. 1991 Fagesia catinus (Mantell).- Kennedy and Simmons, pl. 4, fig. C-E.
cf. 1992 Fagesia catinus catinus (Mantell).- Thomel, pl. 84, fig. 1; pl. 85; pl. 86, fig. 1; pl. 89, fig. 3; pl. 90, fig. 1, 2.
1998 Fagesia catinus (Mantell).- Callapez Tornicher, pl. 13, fig. 5, 6.
2009 Fagesia catinus (Mantell).- Barroso-Barcenilla and Goy, p. 23, fig. 4.1-3.
Material: 2 specimens.
Discussion: Two fragments of Fagesia are characterized by very broad, less high, subtrapezoidal whorls, a smooth less convex ventral part, narrow flanks ornamented with large tubercles and by a large umbilicus. The smoothing of the ribbing begins to develop at small diameters. This form belongs to the smooth Fagesia that include F. peroni Pervinquière, F. levis Renz, F. fleuryi Pervinquière and F. catinus (Mantell). Our specimens are attributed to the Mantell’s species because of their large umbilicus that distinguish this species from all other Fagesia. However, F. fleuryi Pervinquière also seems to have sporadic large ribs at larger sizes.
Age and distribution: According to Kennedy and Simmons (1991) this species characteristic of the lower part of the Turonian. It occurs in Europe (UK, France), in America (USA, Venezuela and ? Mexico) and Arabian Peninsula (Oman).
Fagesia tevesthensis (Peron, 1896)
2009 Fagesia tevesthensis (Peron).- Barroso-Barcenilla and Goy, p. 25, fig. 4(4), 5(1), with synonymy.
Material: 1 specimen.
Discussion: This rather cadicone form is characterized by a strong ventral ribbing and more or less expressed tubercules on the low part of the ‘flank’. The ventral part is convex and the whorl thickness is weak in comparison with the other Fagesia. F. superstes (Kossmat) is broader, massive, more depressed and more coarsely ornamented in ribbing and tubercles. This may represent the expression of a wide intraspecific variability for a F. superstes (Kossmat)-F. tevesthensis (Peron) group.
Age and distribution:F. tevesthensis (Peron) is known from early Turonian (upper part of the W. coloradoense Zone and M. nodosoides Zone). It is recorded from South Europe (France, Portugal, Spain), North Africa (Tunisia, Algeria), Egypt, Levant, Oman and Japan.
Family Coilopoceratidae Hyatt, 1903
Genus Hoplitoidesvon Koenen, 1898
Type species:Hoplitoides latesellatus von Koenen, 1898, p. 56, pl. 6, fig. 1–3, subsequent designation by Solger, 1904, p. 127.
Age: Early to middle Turonian.
Hoplitoides wohltmanni (von Koenen, 1897)
2005 Hoplitoides gr. wohltmanni (von Koenen).- Meister and Abdallah, pl. 14, fig. 2; pl. 25, fig. 1, with synonymy.
Material: 1 specimen.
Discussion: Typical smooth oxycone form of the early Turonian that belongs to the H. wohltmanni (von Koenen) because of a pinched rounded ventral part, only the beginning of the last whorl shows a narrow tabular venter. This character is longer developed during the ontogeny (paedomorphic) in H. mirabilis Pervinquière or in H. latefundatus Zaborski. H. ingens (von Koenen) has broader whorls.
Age and distribution: Recorded from America (Trinidad, Venezuela, Colombia and USA), western and northern Africa (Nigeria, Cameroon and Tunisia) and Oman, this species indicates the early Turonian (W. coloradoense and M. nodosoides Zones).
Suborder Ancyloceratina Wiedmann, 1966
Superfamily Turrilitoidea Gill, 1871
Family Turrilitidae Gill, 1871
Genus TurrilitesLamarck, 1801
Type species:Turrilites costatusLamarck, 1801 by original designation. Subsequent designation by Kennedy (1971) for a lectotype (Douvillé, 1904, fig. 1, fiche 54a).
Age: Cenomanian.
Turrilites costatusLamarck, 1801
1801 Turrilites costata Lamarck, p. 102.
1904 Turrilites costata Lamarck.- Douvillé, p. 54, 54a, 54b.
1983 Turrilites (Turrilites) costatus Lamarck.- Kennedy and Juignet, p. 47, fig. 25a-o, 26a, b, 27a-i, 28a with synonymy.
1996 Turrilites (Turrilites) costatus Lamarck.- Wright and Kennedy, pl. 103, fig. 1, 2, 5; pl.104, fig. 1–4, 6, 8–10; pl. 105, fig. 1, 5, 6, 10, 12, 13, 16, 17, 19; pl. 106, fig. 1–6, 9, 10; text-fig. 137C, 139A-C, 142A, F, G, 143A-G, I–P with synonymy.
2013 Turrilites costatus Lamarck.- Reboulet et al., p. 177, fig. 5D.
Material: 37 specimens.
Discussion: The Omani specimens range in size from 2 cm to 20 cm. They correspond well to the detailed description given by Wright and Kennedy (1996, p. 357). As demonstrated by these authors, the variability in ornamentation intensity is rather high. The coarse ornamentation is dominant among the Omani specimens, with well-developed and coarse ribs on the upper half part of the exposed whorl face ending in an elongated tubercle nearby the mid-flank. A spiral groove obliterating the ornamentation is situated above a row of rounded tubercles on the lower part of the exposed whorl face and separates them from the overlying row of elongated tubercles. A third row of smaller spirally tubercles just marks the inter whorl junction and is separated from the second row of tubercles by a second groove.
Some of these specimens show a finer, closer ribbing and more tiny tubercles. These forms have superficial affinities with T. scheuchzerianus Bosc (eg. Atabekian, 1985, pl. 31, fig. 7, 10) but the grooves and tubercles are less developed.
T. acutus Passy differs by the more depressed whorls, with only two well expressed rows of tubercles and a less developed ribbing in the upper part of the exposed whorl face.
Age and distribution: A cosmopolitan species rather common in the middle Cenomanian and rarer in the lower part of the late Cenomanian (C. (P.) guerangeri Zone). The Omani fauna belongs to the second period.
BIOSTRATIGRAPHY AND PALEOBIOGEOGRAPHY
A range chart for the Cenomano–Turonian ammonites (Figure 11) adds precision to the biostratigraphical framework proposed for Oman by Meister and Piuz (2013, their figure 6).
The succession of the Albian–Turonian ammonites in the Adam Foothills allows us to propose a set of nine bioevents for this period (Figure 12).
Age of the Fauna
Albian
The age of the Knemiceras beds with K. syriacum and K. cf. dubertreti is poorly constrained and still under discussion. Moreover in the recent literature, there is no consensus. Indeed K. syriacum indicates the early–middle Albian for Kennedy et al. (2009), most probably of middle Albian age if we follow Lewy and Raab (1978) and the early late Albian for Bulot (2007). Whereas K. cf. dubertreti corresponds to the basal middle Albian sensuKennedy et al. (2009), and to the base of the late Albian for Bulot (2007) (D. cristatum and M. pricei Zones), respectively hypothesis A and B in Figure 12. For the moment without better local biostratigraphical constraints, we cannot determine the precise age of this fauna: either middle Albian (hypothesis A) or late Albian (hypothesis B).
Cenomanian
The association of the long-ranging Acompsoceras renevieri and Neolobites fourtaui indicates an age situated in the middle part of the middle Cenomanian (Acanthoceras rhotomagense Zone). A. renevieri ranges from early Cenomanian to the middle Cenomanian although doubtfully until the Acanthoceras jukesbrownei Zone, whereas N. fourtaui ranges from the middle Cenomanian (A. rhotomagense Zone) to the base of late Cenomanian.
The presence of N. fourtaui alone above the previous bioevent corresponds to the upper part of the middle Cenomanian (Acanthoceras jukesbrownei Zone)–early late Cenomanian (base of C. (P.) guerangeri Zone). Neolobites vibrayeanus as well as C. (Proeucalycoceras) cf. canitaurinum (subzonal index species) are restricted to the base of late Cenomanian (C. (P) guerangeri Zone). They are associated with Turrilites costatus and Cunningtoniceras sp. Metoicoceras geslinianum is the index species of the eponymous zone (middle part of the late Cenomanian). C. (Gentoniceras) aff. gentoni is also present in this zone.
The presence of Nigericeras gadeni and N. jacqueti indicates a period situated in the upper part of the Metoicoceras geslinianum Zone and the lowermost part of the Neocardioceras juddii Zone. Puzosia sp. is also present. Vascoceras barcoicense exile and V. cauvini are known from Neocardioceras juddii Zone, rather the lower part; but may already appear in the Metoicoceras geslinianum Zone. The Placenticeras aff. kaffrarium correspond to a period in the Neocardioceras juddii Zone (upper part of the late Cenomanian). Rubroceras aff. burroense also characterizes this zone.
The presence of some undeterminate Vascoceras and Pseudaspidoceras does not allow attributing a precise age near the Cenomanian/Turonian boundary (topmost Cenomanian–basal Turonian). In some part of the southern Neo-Tethys margin (Egypt, Jordan) Vascoceras proprium is used to delineate this boundary (see discussion in Meister and Piuz, 2013).
Turonian
Fagesia (F. catinus, F. tevesthensis) as well as Hoplitoides wohltmanni attest to the presence of the early Turonian. F. tevesthensis could be a little younger (upper part of the early Turonian). Kennedy and Simmons (1991) also cited some early Turonian taxa like Vascoceras durandi, ? Thomasites cf. gongilensis, ? Eotissotia simplex and Wrightoceras sp. which we have not rediscovered and that could represent three other bioevents (Figures 11 and 12). All these bioevents are correlated with the standard zonation. For regional correlations along the southern Neo-Tethys margin we refer to the detailed discussion recently published by Meister and Piuz (2013).
Paleobiogeographical Remark
Ammonite distributions along the southern Neo-Tethys margin from Morocco to Oman have recently been presented and discussed by Meister and Piuz (2013). Only worldwide paleogeographical distributions of some characteristic taxa present in Oman are shown from the middle Cenomanian to the early Turonian (Figure 13).
For the middle–early late Cenomanian A. renevieri has a distribution restricted to Europe and western and eastern part of Africa, N. vibrayeanus shows a rather subequatorial/tropical repartition as does C. (P.) canitaurinum. If the latter is not present in Central and North America, it is recorded from South America, Peru and Bolivia, and possibly following Wiese and Schulze (2005) from Venezuela and Columbia. N. fourtaui is endemic to the southern Neo-Tethys margin.
For the late Cenomanian, T. costatus is a cosmopolitan species whereas C. (G.) gentoni is distributed in Europe and eastern and southern part of Africa. P. kaffrarium is only known around Africa. For the early Turonian Fagesia is a cosmopolitan genus. The species F. tevesthensis and F. catinus are also widely distributed, but they are not known in the South Atlantic. H. wohltmanni is present along the southern Neo-Tethys margin, in the Gulf of Guinea and in Central America more or less according to a subequatorial trend.
CONCLUSIONS
New discoveries of ammonites in Oman (Adam Foothills) allow further precision of their taxonomy and provide better chronostratigraphical constraints for the Albo-Turonian period. Regionally the sequence of nine bioevents gives greater precision for dating the Nahr Umr and Natih formations, especially for the Natih A and B members. In the Adam Foothills, the ammonite correlations along a West-East transect raises questions regarding the definitions of the lithostratigraphic units of the Natih Formation. Obviously the facies of Natih A and B differs from the western part (Jabals Fitri, Nadah and Salakh NW) to the eastern part (Jabals Salakh, Khaydalah and Madmar) (see Piuz et al., 2014, their figure 2).
A sequence of nine bioevents is correlated with the zonation. Moreover, some ammonite are good temporal markers, among them Neolobites vivrayeanus (Orbigny), Calycoceras (Proeucalycoceras) cf. canitaurinum (Spath), Metoicoceras geslinianum (Orbigny) and Vascoceras cauvini Chudeau. They allow good correlations at a large scale, at least along the southern Neo-Tethys margin. Several taxa including Puzosia, Placenticeras, Cunningtoniceras, P. (Gentoniceras), Nigericeras, Metoicoceras, Rubroceras and Hoplitoides are recorded from Oman for the first time.
From a paleogeographic point of view Oman is a landmark for correlation between western Neo-Tethys (Europe, North Africa, Middle East) and southeast Neo-Tethys (Africa, Madagascar, India. Moreover the presence of Placenticeras in a key period for understanding its evolution (still in discussion) is to be underlined.
APPENDIX
All the material is figured with accompanying scales and is whitened with ammonium chloride. After study, the material will be stored in Geological Survey of Oman, moreover, a collection of casts will be deposited in the Natural History Museum of Geneva (Switzerland).
ACKNOWLEDGEMENTS
We thank John Hollier (Natural History Museum of Geneva) for linguistic assistance, and GeoArabia’s reviewers for their constructive comments. Salim Omar Al-Ibrahim, Director General of Minerals (Ministry of Commerce and Industry of the Sultanate of Oman) and Ali Al Rajhi, Director of Survey and Research are thanked for their logistic support. We also are very grateful to Edwin Gnos (Natural History Museum of Geneva) for his expertise of Oman. Ingrid Rosset and Thibaud Meister are also thanked for their help in the field for sample collecting. GeoArabia’s Assistant Editor Kathy Breining is thanked for proofreading the manuscript, and GeoArabia’s Production Co-manager, Arnold Egdane, for designing the paper for press.
ABOUT THE AUTHORS
Christian Meister is a researcher at the Natural History Museum of Geneva since 1992, convenor of the GSSP Pliensbachian working group since 1996 and voting member of the International Subcommission on Jurassic Stratigraphy (ISJS). He has published more than 140 papers. His research focuses on paleontology, biostratigraphy, evolution and paleogeography of the Early Jurassic ammonites in Europe, North Africa, Central and South America and Asia. His interest concerns also the Cretaceous (Cenomanian–Turonian) ammonites of Africa (Morocco, Algeria, Tunisia, Niger, Nigeria, Gabon, Angola) and Arabian Peninsula.
André Piuz is researcher in the department of Geology and Paleontology of the Natural History Museum of Geneva, Switzerland, and scientific advisor in the council of the Fondation Paul Broennimann. His MSc about the Upper Jurassic of the Vocontian Basin (France) was followed by a PhD regarding micropaleontology of the Bajocian bioclastic echinodermic shelfs of the Jura Mountains and Burgundy (France). Both were carried out at the University of Geneva (1997 and 2004). Working at the Museum since 2005, he shares his time between geology-paleontology and managing the Scanning Electron Microscope for the imagery lab. Since 2008 his research focuses mainly on the southern Tethys Cenomanian and Turonian in Morocco and Oman. His principal interests concern micropaleontology, stratigraphy, taxonomy, paleogeography and field geology.
