Paleoecological and paleobiogeographical significance of the rudist Macgillavryia chubbii sp. n. in the Campanian of Oman

The genus Macgillavryia was recently named by Rojas et al. (1995) for the distinctive Caribbean species widely referred to in the literature as Durania nicholasi (Whitfield). In Oman, Durania aff. nicholasi was cited for the first time, but not described, from the Campanian of the Saiwan area in the Haushi-Huqf massif (Figure 1) by Schumann (1995).

New investigations on the northeastern edge of the uplifted Haushi-Huqf massif (Hayy-Filim area, Figure 1) were made in collaboration with geologists from Elf Aquitaine and the French Bureau de Recherches Géologiques et Minières (BRGM). As a result, well-preserved specimens of large radiolitids that are attributed to Macgillavryia were discovered at various levels in the Campanian Samhan Formation. In this paper, systematic and paleobiologic descriptions of the specimens are made and compared with examples from the Caribbean. In addition, the exceptional exposure of the Samhan Formation in the Hayy-Filim area has enabled many paleoecological observations on the rudist bioconstructions, of which Macgillavryia is an important component. Paleobiogeographic reconstructions are also inferred from this study.

The specimens of Macgillavryia were discovered in carbonate rocks of the Samhan Formation in the vicinity of Filim and Hayy (Figure 1). Platel et al. (1994), Philip and Platel (1995), Philip and Platel (1998a) and Kennedy et al. (2000) established the Campanian age of the Samhan Formation in Oman.

Figure 2 illustrates the paleoenvironments of Oman during middle Santonian times (Le Métour et al., 1995). The sediments were deposited within a compressive foreland basin setting. In Oman during the early Campanian, this compressive tectonic phase led to the overthrusting and emplacement of the Sumeini and Hawasina nappes and the obduction of the Semail Ophiolite onto the eastern margin of the Arabian Plate (Le Métour et al., 1995).

The emplacement of the nappes and ophiolite caused downflexing and the formation of the Muti basin southwest of the newly formed Oman mountain belt. The Muti basin initially underwent rapid subsidence and the deposition of more than 1,000 m of hemipelagic mudstone, chalk and shale of the Campanian to Maastrichtian Fiqa Formation. The Samhan Formation was deposited on an open-marine carbonate shelf adjacent to the Muti basin. The shallow-marine Maastrichtian Simsima Formation conformably overlies the Fiqa Formation in the Muti basin but is unconformable on the Samhan Formation in the Haushi-Huqf area.

A rich, fossiliferous reference section of the Samhan Formation was described by Platel et al. (1995) in the Saiwan area of the Haushi-Huqf massif (Figure 3). Rudists, benthic foraminifera and ostracods provided a Campanian age for the Formation but without precise zonation. However, recent collections of ammonite faunules (e.g., Kitchinites angolaensis Howarth) from the middle part of the Samhan Formation in Saiwan led Kennedy et al. (2000) to attribute a late Campanian age to the middle and upper members of the Formation. This was not corroborated by the planktonic foraminifera association in the overlying Fiqa Formation (Platel et al., 1994, 1995) where Globotruncana ventricosa White and G. elevata (Brotzen) are generally accepted to be characteristic of the middle Campanian Polyplocum Zone (Robaszynski and Caron, 1995; Hardenbol et al., 1998)). Accordingly, Platel et al. (1994) ascribed the Samhan Formation to the early Campanian, an age attribution that is retained here.

In the study area, the most complete section of the Samhan Formation (Figure 4) crops out about 100 m west of the small village of Filim. The section is conformable on the Qitqawt Formation (Turonian? to Santonian), and it is overlain disconformably by the upper Maastrichtian Simsima Formation (Platel et al., 1995; Philip and Platel, 1998a). Four transgressive-regressive sequences consist of marls and siliciclastic sandstones, wackestones-packstones containing echinoderms and large foraminifera (e.g. Orbitoides tissoti Schlumberger), and framestones with rudists, stromatoporoids and corals. Ammonites that are characteristic of the lower Campanian (cf. Praemanambolites hourcqi Collignon, determined by G. Thomel) are present in the upper part of the second sequence.

Specimens of Macgillavryia are abundant in the lower part of the Samhan Formation (Figure 4) and are associated with other rudists, such as Durania sp., Torreites milovanovici Grubic, T. grubici, Philip and Platel, Biradiolites sp. aff. bulgaricus Pamouktchiev, and Vaccinites vesiculosus Woodward. V. vesiculosus is relatively scarce wherever Macgillavryia is present.

Family Radiolitidae d’Orbigny 1847

• Subfamily Biradiolitinae Douvillé 1902

  • GenusMacgillavryia Rojas, Itturalde-Vinent and Skelton 1995

    • Type species Radiolites (Lapeirousia) nicholasi Whitfield 1897 

      • New species Macgillavryia chubbii

• 1994 Durania sp.; Platel et al., p. 155, Plate II, Figure 2.

• 1995 Durania aff. nicholasi (Whitfield); Schumann, Plate 38, Figure 4; Plate 39, Figure 2.

• 1995 ?Durania aff. nicholasi (Whitfield); Schumann; Rojas et al., p. 286, 288.

The new species is named in honor of the American paleontologist L.J. Chubb and in appreciation of his valuable contribution to Jamaican rudist paleontology.

The holotype (Figure 5) is a complete and well-preserved specimen (JP MG 281) having both valves connected. The following three paratypes occur:

1. Specimen JP MG 605a (Figure 6d) has a right valve and a partially broken away left valve;

2. Specimen JP MG 6O5b (Figure 6c) is represented by a right valve, showing the body cavity and the cardinal apparatus; and

3. Specimen JP MG 605c (Figure 7c) shows the meniscus process that separates the cardinal cavity from the body cavity.

All specimens are housed in the Centre de Sédimentologie Paléontologie at the University of Provence, Marseille, France.

The shell is large. The right valve has a much-expanded outer shell layer that spreads directly outward (Figure 6a). The posterior zone shows pronounced folds associated with depressed external radial bands. The ventral band (E) is flat bottomed and unornamented and the dorsal band (S) is slightly excavated and is ornamented with longitudinal costae. A large interband is formed by a downfold in the growth lamellae. The right valve has a cellular structure. The commissural area is large, with well marked bifurcated or polyfurcated vessels (Figure 6a).

The left valve is characteristically hat shaped and the top is hemispheric and caps the body cavity. Its margin is very narrow, covering only the inner part of the commissural area. The outer shell is composed of lamellar calcite whereas the inner layers are of aragonite and are vesicular (Figure 6b).

The holotype (Figure 5) is 350 mm high, the basal attached surface is 220 mm wide, and the diameter of the commissural surface is 450 mm. The right valve (RV) is in the form of a broad truncated cone with the outer shell layer spreading out around the entire valve. The wall structure of the right valve consists of superimposed fine subhorizontal lamellae. The external ornamentation consists of thin radial costae. Bands E and S are excavated and form longitudinally depressed areas on the posterior side of the right valve. The E band (Figures 5 and 7a) is deeper than the S band, is flat bottomed and unornamented except for fine concentric laminae, and is bounded by strong upfoldings of the growth lamellae. The S band (Figures 5 and 7b) is shallower than the E band and is ornamented by three longitudinal costae that are coarser than the regular costae of the right valve. The two bands are separated by a large interband (Figure 5) formed by a downfold of the growth lamellae of the outer shell layer. Cells of the right valve (Figure 7d) are finely celluloprismatic (average diameter 0.25 mm; height 0.50 mm), irregularly denticulate, and amoeboidal on the margins of the commissural area. On certain specimens, such as on the oldest part of the right valve of the holotype, the cellular network appears to be almost totally recrystallized and is generally partially or totally obscured. In this case, the structure of the calcitic shell appears to be compact. The well-marked bifurcated or polyfurcated vessels of the commissural area are characteristic of the genus (Figure 6a).

The left valve (LV) (Figure 5) is characteristically hat shaped. Its top part is hemispheric and caps only the body cavity. The very narrow margin covers only the radial bands area of the inner part of the commissural plate of the right valve. The rest of the commissural plate is uncovered. In the holotype, the commissural area (Figure 6a) has a well-marked growth rim on the flange margin of the left valve, located approximately in its middle part. The left valve (Figure 6b) is composed of two layers: the outer layer is brown, thin, and consists of lamellar calcite, whereas the inner layer is thick and vesicular, with an aragonitic honeycomb texture.

The cardinal apparatus (Figures 6c,e) appears as a broad arc-like structure extending around the entire anterior and posterior part of the body cavity. The posterior myophore of the left valve is well developed and its outer side (Figures 6c,d,e) is finely crenulated. Teeth of the left valve (Figures 6d,e) slide into prominently ridged grooves on the inner wall of the right valve. These ridges (Figure 7c) expand into the body cavity and form a meniscus process that delineates a cardinal cavity from the body cavity.

The form described is assigned to the genus Macgillavryia recently established by Rojas et al. (1995), with the type species Radiolites (Lapeirousia) nicholasiWhitfield 1897. M. nicholasi was a species considered until now as characteristic of the Caribbean province but its presence in Oman was suspected I I L I by Schumann (1995). The specimens collected in collaboration with Elf Aquitaine and BRGM confirm the presence of the genus in Oman as they show clearly the distinctive characters of Macgillavryia, especially the domed left valve and the exceptionally large shell.

The Oman species differ from the Caribbean Campanian-Maastrichtian rudist M. nicholasi by the configuration of the band area. In the original specimens described by Whitfield (1897) and Chubb (1971), the band area of M. nicholasi was incomplete because of the poor preservation of the type material. However, a complete description was made by Rojas et al. (1995). In both the Caribbean and Oman forms, the ventralward band (E band) has the same architecture and forms a broad, rounded furrow extending from the base of the right valve to the commissure. But, in the Oman species, this furrow is bounded by two upfolds of the funnel plates, whereas in the Caribbean species the E band is bounded by two steep downfolds (Chubb, 1971). According to Chubb, the S band was matched by a strong upward fold of the margin in the Caribbean form, whereas in the Oman form this band is represented by a depression of the outer layer of the right valve that is shallower than the E band. The interband (IB) is markedly different in the two species—in the Caribbean specimens it is a narrow, centrally concave furrow, but in those from Oman it is a large and convex-downfold protrusion of the funnel plates.

The left valve is low domed in M. nicholasi and high domed in M. chubbii. The vesicular structure of the left valve of M. chubbii has not been observed in M. nicholasi, but Chubb (1971) showed the presence of a vesicular tissue in the deeper part of the body cavity of the right valve of the Caribbean species. D. Schumann (oral communication, 1999) also observed vesicular tissue in the right valve of certain specimens of Macgillavryia from Saiwan (see Figure 1).

With regard to the cellular structure of the right valve, Whitfield (1897) and Chubb (1971) noted an extremely fine polygonal network of meshes in M. nicholasi but did not give the dimensions of the cells. Rojas et al. (1995) illustrated a specimen from Jamaica that had cells of small dimensions—they were relatively thick-walled, and of an irregularly reticulate-amoeboid to denticulate plan. The cells in M. chubbii (Figure 7d) are irregularly polygonal and denticulate to amoeboid.

Rojas et al. (1995) assigned to Macgillavryia nicholasi the Bournonia sp. 4 of Mac Gillavry (1937) from Cuba, in spite of the different shape of the left valve. The Oman species differ from the Cuban ones by the shape of the left valve and by the arrangement of the bands.

The genus Macgillavryia is characterized by an exceptional development of the mantle that allowed a large expansion of the outer calcitic layer. According to Whitfield (1897) and Chubb (1971), the left valve of M. nicholasi covered only a small part of the commissural area, leaving a large part of the mantle of the right valve directly exposed to sea water and sunlight. Kauffmann and Johnson (1988) proposed M. nicholasi as a model for rudists with very reduced left valves, which suggests permanent or long-term exposure of the mantle in life and its expanded role in zooxanthellae symbiosis, possibly promoting both physical and chemical competition. Rojas et al. (1995) rejected this interpretation arguing that the free valve’s compact calcitic outer shell layer on the right valve margin was a preservational relict. They suggested that a thin flange of the left valve rim might originally have covered the entire margin of the right valve.

The exceptionally well-preserved holotype of M. chubbii shows clearly that the free valve completely covered the commissure only in the band area. In the anterior, posterior and dorsal regions of the shell, the right valve was partly covered by a flange of the left valve. In these parts, the commissural area (Figure 6a) has a well-marked growth rim which shows that the position of the free-valve margin was approximately in the middle of the commissure. This probably left the outer part of the mantle exposed during some period in the life of M. chubbii.

In conclusion, it seems that M. chubbii could be a potential candidate for an association with zooxanthellae. Kauffmann and Sohl (1974) and Skelton (1979) had previously suggested such a relationship for M. nicholasi.

Figure 8 is a reconstruction of Macgillavryia chubbii sp. n. in growth position. Macgillavryia is a typical epifaunal rudist that probably lived in very shallow, clear seas. The following evidence supports this interpretation:

• Firstly, the upper valve did not fully cover the lower one (Figures 6a and 8); therefore, a large part of the commissure of the right valve was probably exposed to the light, as for instance with Torreites (Skelton and Wright, 1987). Only the commissural area corresponding to the radial bands was possibly protected by an expansion of the calcitic layer of the left valve (Figure 8).

• Secondly, scarce, encrusting rudists are present on the upper-outer part of both valves (Figure 8). Borings (Figure 7b) increase in density from the top to the apex of the right valve. They reach a greater abundance in the lower part of the right valve and have caused intense corrosion of the cellular calcitic outer shell (Figure 8).

These facts indicate that the shell was not buried during its lifetime but was exposed to epibionts and bioeroders. According to Schumann (1995), the life span of such forms might be about six years. The specimen of Macgillavryia was probably buried abruptly due to a sudden depositional event such as storm-induced sedimentation, and this led to its exceptional preservation.

In the Filim and Hayy area, the excellent exposures allow reconstruction of relationships between Macgillavryia and other biotic or abiotic components. In an outcrop of the Samhan Formation 4 km northwest of Hayy on the road to Muscat (Figure 1), Macgillavryia together with Durania, Biradiolites, Vaccinites, and others, constitute a densely packed association with spaces filled by a very coarse bioclastic material (Figure 9). These individuals formed a rudist bank that grew on a firm ground. It can be interpreted as a superstratal growth fabric according to the classification of Gili et al. (1995). This rudistid framework was probably exposed to periodic, high-energy events (storms) as recorded by the presence of overturned right valves, coarse angular bioclastic grains of rudists, and the paucity of upper valves. In the Hayy area, the superpositioning of two or more layers with superstratal growth fabrics created a rudist bank more than 1 m thick and with an area of several hundred square meters.

Other types of Macgillavryia associations are present in the Samhan Formation. In the Filim area, Macgillavryia is associated with Torreites, tabular corals, and branched stromatoporoids. In the Saiwan area in the northwestern part of the Huqf massif, patches of Macgillavryia developed in the upper part of the Samhan Formation in association with scarce Torreites, large massive stromatoporoids, and cerioid corals. These beds were formerly designated as ‘large’ Durania beds by Platel et al. (1994), or as Durania-dominated associations by Schumann (1995).

In summary, the abundance of Macgillavryia is highly variable. Its occurrence depended on the ecological niches that were available and on the competition from other rudists and associated organisms that occupied the various substrates of the Campanian carbonate platform.

The shallow-marine rudist-rich carbonates and sands of the Samhan Formation were deposited during the Campanian marine transgression in the Haushi-Huqf area that followed the emplacement of the Semail Ophiolite and the Hawasina nappes. The transgression culminated in a glauconite hardground at the base of the Fiqa Formation (Figure 3) represented by Maximum Flooding Surface MFS K170 (Sharland et al., 2001) dated by planktonic foraminifera and nannofossils as middle Campanian (Globotruncana ventricosa Zone; 78 Ma).

Accumulations of Macgillavryia appear to have developed during high-stand phases of small-scale transgressive-regressive cycles within the major transgression (Figure 4), when high-energy conditions and the scarcity of siliciclastic input were realized. In this context, the firm ground on which the rudist banks developed (Figure 9), could be interpreted as representing the maximum flooding surface of the individual cycles. Interruptions in the rudist buildup were probably due to a regressive event or a stillstand that resulted in a decrease in the amount of accommodation space, so preventing new rudist generations from adding to the buildup. Although some corrosion or leaching of the shells has been recorded, no trace of clear emergence of the rudist banks was found. The banks are generally overlain by sandy wackestones (Figure 4) that correspond to the transgressive part of the next small-scale cycle.

Numerous studies have been made on the Samhan Formation of the Haushi-Huqf massif over nearly a decade (Platel et al., 1994; Philip and Platel, 1994; Philip and Platel, 1995; Schumann, 1995; Philip and Platel 1998a,b). As a result, it has been possible to reconstruct the environment of the carbonate platforms that carried the Macgillavryia associations, together with their relationships with the other rudist beds and the surrounding carbonate biofacies.

The setting of the rudist carbonate facies developed through the gradual progression of the ?late Santonian-Campanian transgression onto the Haushi-Huqf massif (Figure 10). The oldest rudist bank occurs in Wadi Khuraysifah (Figure 1) in the southwest of the Haushi-Huqf massif (Philip and Platel, 1994). It developed during the first brief transgressive-regressive cycle, the age of which was probably ?late Santonian to earliest Campanian. It is characterized by a predominance of Praetorreites omanensis (Philip and Platel) associated with Vaccinites, Praeradiolites, branched corals, scarce red algae, among others (see details in Philip and Platel, 1995). This rudist bank is capped by large individual radiolitids attributed previously to Durania but which are probably Macgillavryia. The development of this first rudist bank recorded in the Samhan Formation was abruptly buried by coarse siliciclastic sediment and is only 1.5 m thick.

A second rudist bank that formed during a second transgressive regressive cycle is recognized in the Filim area (Figure 4, first sequence). In this bank, Macgillavryia is associated with Torreites, Biradiolites, branched stromatoporoids, tabular corals, and others (see details in Philip and Platel, 1998a).

A true carbonate platform with diverse biofacies and a distinct biozonation developed during the second sedimentary sequence (Figure 4). Typical outcrops of this sequence can be seen in the Hayy-Filim, Nafun, and Saiwan areas (Platel et al., 1994; Schumann, 1995) and the western part of the Haushi-Huqf massif (Abu Tan). The more distal biofacies are represented in the Nafun area (Philip and Platel, 1998b) where the Samhan Formation consists of echinoid-rich chalky wackestones bearing ammonites. Rudists are totally absent.

In other areas, the carbonate platform contains rudist-rich or stromatoporoid-rich associations that alternate vertically or pass laterally into echinoid and benthic-foraminiferal wackestones. Massive domal stromatoporoids associated with Durania and Macgillavryia appear to occur preferentially in the outer part of the carbonate platform, so making a sort of rimmed bank; rudist-dominated bioconstructions—including Macgillavryia—seem to thrive in these more protected setting. The Vaccinites-dominated associations with massive corals grew closest to the coastal (locally siliciclastic) deposits. None of the rudist or stromatoporoid bioconstructions are thicker than 1 or 2 m and are some hundred meters square. Most are formed by one or two generations of benthic building organisms. In general, all of the buildup units pass laterally into echinoid-rich wackestones.

The Samhan Formation carbonate platform was a carbonate ramp as defined by Read (1984) that gently dips toward the southeast and southwestward and passes laterally into the Samhan outershelf of echinoid-rich wackestones. The Samhan platform was drowned during the middle to late Campanian, and was replaced by the outer shelfal deposits of the Fiqa Formation (Platel et al., 1994).

The disjunct endemism of the genus Macgillavryia, both in Oman and in the Caribbean, implies oceanic connections between these two domains. This probably first occurred in the ?late Santonian-early Campanian in order to allow the dissemination of Macgillavryia larvae across the Pacific Ocean. Skelton and Wright (1987) and Skelton (1988) had proposed this seaway for the dispersion of other rudist taxa (i.e. Torreites), albeit in the opposite direction to that inferred from the present study. Figure 11 is a reconstruction of the Campanian-Maastrichtian paleogeography (from Philip, 1998), indicating where rudist have been recorded between Oman and the Caribbean. It also shows disjunt rudist provinces that were probably connected by ocean surface currents during ?late Santonian to early Campanian times.

As discussed by Skelton and Wright (1987) and by Skelton (1988), the migration of rudists through the Pacific during the Late Cretaceous would have required ‘stepping stones’ such as seamounts or volcanic islands. This would have provided the epiplanktonic larvae of rudists with the optimum conditions for a broad dispersion of the species, and allowed for the possibility of population exchanges between distant regions.

This could have been the strategy for the distribution of Macgillavryia between Oman and the Caribbean during the Campanian. However, the assumption of early Campanian stepping stones is not corroborated from the evidence of carbonate platforms of this age in the Pacific realm. As confirmed by recent results of the Ocean Drilling Program (Camoin et al., 1998), rudist carbonates from the northwest Pacific region (Marshall Islands) are either of Aptian-Cenomanian age or late Campanian-Maastrichtian (Figure 12). If the hypothesis of staging posts through the Pacific (Skelton, 1988) is correct, this implies that early Campanian rudist carbonates such as Macgillavryia have yet to be discovered on Pacific seamounts. Alternatively, the epiplanktonic larval stage of Macgillavryia may have been long enough to survive the several weeks or months needed for crossing the Pacific area without recourse to staging posts.

According to the biostratigraphic data recently established for Oman and the Caribbean (Figure 12), Macgillavryia seems to appear earlier in the Arabian region than in the New World (late Santonian-earliest Campanian versus early late Campanian). If this chronology were correct, it would imply a dispersion of Macgillavryia from west to east across the Pacific (Figure 11) as the expanding Atlantic Ocean and barriers created by the collision of the Adriatic Block with Eurasia would have prevented dispersion westward. Philip and Platel (1994) and Philip (1998) had already proposed this route for the distribution of Torreites.

It is important to note that the early Campanian transgression probably played a prominent role in the dispersion of Macgillavryia by expanding the oceanic connection between Arabia and the Caribbean. It facilitated the distribution of ubiquitous rudist taxa such as Macgillavryia into the shallow-water biotopes of the two domains.

A new investigation of the Campanian Samhan Formation that is exposed in the eastern part of the Haushi-Huqf massif, resulted in the discovery of well-preserved specimens of Macgillavryia, a genus previously known only in the Caribbean Province. A new taxon, M. chubbii, has been established that differs from the Caribbean species mainly by the morphology and arrangement of the radial bands. M. chubbii is a typical epifaunal rudist that probably lived in a very shallow and clear sea. It could be a potential candidate for an association with zooxanthellae. It was frequently associated with other rudists and stromatoporoids in building banks or other biostromal bodies having superstratal growth fabrics.

The development of the carbonate facies that contain Macgillavryia was realized by the gradual progression of the ?late Santonian to Campanian transgression across the Haushi-Huqf massif. Rudist banks were formed during the first stages of the transgression, and evolved progressively to a true carbonate platform with a diversity of biofacies during the late stages. In a paleobiogeographic context, the disjunct endemism of Macgillavryia implies oceanic connections between Oman and Caribbean domains, probably through the Pacific. According to recent biostratigraphic data, Macgillavryia appeared earlier in Oman than in the New World. This would imply a dispersion of Macgillavryia from west to east through the Pacific. Additional systematic and biostratigraphic studies are needed to establish the origin of the genus Macgillavryia and the accurate reconstruction of its paleobiogeographic dispersion.

G. Massonnat and H. Soudet (Elf-Aquitaine Exploration) invited me to participate in the field study carried out in February 1998 as a contribution to the project ARTEP: Quantification of Carbonate Reservoirs. J. Beltramo, S. Chuine, X. Dellamonica, R. Lions, J.-P. Platel and M. Rebelle are thanked for their kind assistance in collecting the specimens of Macgillavryia. I thank Robert (Bob) Scott for improving the English of the submitted manuscript. Photography of Macgillavryia was by J.J. Roccabianca. Technical assistance was provided by L. Boiroux and A. Arnoux, and N. Pelegrin typed the manuscript. Referees Richard Höfling and Robert Scott are thanked for valuable scientific suggestions. I gratefully acknowledge GeoArabia editors David Grainger and Moujahed Al-Husseini for their constructive comments. The design and drafting of the final figures were by Gulf PetroLink.

Jean Philip is Professor of Geology at the University of Provence in Marseille, France. He received his Doctorate of Sciences in 1970. His main fields of research are the Upper Cretaceous carbonate platforms of the Tethyan region, including stratigraphic, paleontologic (rudists) and sedimentological aspects. He has collaborated with geologists of petroleum companies such as Total, Elf, Institut Français du Pétrole, and Petroleum Development Oman, and also the French Bureau de Recherches Géologiques et Minières for field investigations in Tunisia, Algeria, Italy, Oman, and Saudi Arabia.

jphilip@newsup.univ-mrs.fr