Skip to Main Content

Abstract

The diversity of rudists In the Lower Cretaceous of Arabia offers great potential both for biostratigraphical correlation (particularly of certain key levels, such as the Lower/Upper Aptian boundary) and for facies analysis. With the aim of raising awareness of this potential among geologists working on core and outcrop material, we provide here synoptic diagnoses of the seventeen species (at least) so far recognized, together with comments on their stratigraphical distributions and paleoecology, and a range chart.

Introduction

Rudist bivalves are among the most characteristic and abundant macrofaunal elements in Cretaceous Tethyan carbonate– platform deposits. Growing either as upright forms implanted in sediment (elevators), or lying prone on the surface as substrate–hugging dingers, or free–lying recumbents, vast congregations of their shells formed extensive biostromes, interbedded with related bioclastic units, together constituting rudist formations (Masse and Philip, 1981; Ross and Skelton, 1993; Gili et al., 1995).

Though rudists are present in pre–Hauterivian units (e.g., from boulders contained in debris flow deposits in Wadi Hagil, Ras al Khaimah; Toland et al., 1993), rudist formations are common in the Arabian Lower Cretaceous only from the Hauterivian (Habshan p.p.) upwards. They reach a distinct acme in the Lower Aptian (Shuaiba) (see Witt and Gokdag, 1994; Masse et al., 1998a), but this development is terminated at the Lower/Upper Aptian boundary, at which many rudist taxa disappear. Upper Aptian–Albian rudists are restricted largely to the northeastern part of the Jebel Akhdar tectonic inlier, in northern Oman (in the Nakhl area, e.g., Wadi El Assyi; Masse et al., 1997). Nevertheless, farther to the west there is also some localized evidence in the subsurface of the northeastern Rub’ Al Khali for lowermost Upper Aptian rudist facies succeeding the Shuaiba. Known ranges of the taxa in Arabia are summarized in Figure 1, and brief descriptions and comments on their distributions follow.

Fig. 1

—Known ranges of Early Cretaceous rudists in Arabia.

Fig. 1

—Known ranges of Early Cretaceous rudists in Arabia.

Synoptic Diagnoses, with Comments on Distributions

Major clues to rudist identification are as follows:

  1. Shell structure. Rudists have an outer shell layer of fibrillar prismatic calcite and an inner shell that was originally aragonitic, though now usually either replaced by sparry calcite or leached. The outer layer is normally cream–colored to pale honey brown but may be blackened following deep burial (as in the tectonic irdiers of the Oman Mountains). Such blackening is especially noticeable in those taxa with a relatively thick outer shell (e.g., Figs. 2.1, 5.2). The relative development of the shell layers varies considerably between taxa. In one family (Radiolitidae), the outer shell layer contains distinctive layers of tiny hollow cells (celluloprismatic structure). Other taxa (mostly Caprinidae) have blind-ending hollow canals (pallial canals) in the inner shell.

  2. Shell shape and valve of attachment. In the majority of Cretaceous rudists, the shell has a more or less uncoiled shell form, with a conical to cylindrical attached right valve (RV) and a capuloid to operculiform left valve (LV). A few (Requieniidae) attached by a spirally coiled LV, capped by a flattened RV.

  3. Myocardinal system. One of the most important keys to rudist identification is the arrangement of the hinge teeth and the internal shelly supports for the adductor muscles (myophores). Tilting of the myophores within the shell sometimes separates off distinct accessory cavities from the main body cavity, and these may also be diagnostic (see Skelton and Masse, 1998, for further details). Myocardinal arrangements are often difficult to interpret in random sections, however, such that numerous specimens need to be inspected for a secure identification. It is therefore advisable to inspect, draw, and/or photograph several specimens at each site for identification purposes.

Superfamily Hippuritoidea Gray, 1848 (the group informally known as “rudists”)

The family classification used here is based mainly on Skelton (1978) with further modifications from Skelton and Masse (1998). At present, the family–level taxonomy of Early Cretaceous rudists is in a state of flux, as new approaches to classification (especially cladistics) are applied. Genera, on the other hand, are more firmly established, and so are more likely to remain stable. Many species within genera still present the traditional taxonomic problems of possible synonymy, or of being merely successive chronospecies within single evolutionary lineages. Current morphometric studies (as in Skelton and Masse, 1998) are likely to lead to resolution of some of these problems in the next few years, again possibly resulting in a few changes to the specific names given here. Except where otherwise indicated, the accompanying drawings show specimens at natural size.

[A note on the illustrations. Though all are based on real specimens, some of the drawings shown here are composites from more than one specimen or have been simplified for clarity. A few are based on non-Arabian material (as indicated), to show particular features. These drawings are intended purely as an aid to identification in the field or in core material: they should not be regarded as accurate portrayals of individual specimens for the purposes of descriptive taxonomy.]

Family Requieniidae Douvillé, 1915

Main features of family.—Attached by LV. Ligament external, hence shell growth spirogyrate. Outer shell layer relatively thick. Two teeth in the RV and one in the LV.

Requienia migliorinii (Tavani, 1948). (Fig. 2.1)

What to look for.—LV has extended corkscrew shape, often giving the appearance in section of a stack of rounded–zigzag or kidney–shaped loops of increasing size (Fig. 2.1); RV lid–like. Outer shell relatively thick (at least 2 mm), externally smooth (and typically blackened in sections seen at outcrop in the Oman Mountains); inner shell thin, except around hinge area.

Fig. 2

—Diagrammatic sketches of Arabian Early Cretaceous rudists. All drawings are approximately natural size, except where indicated. Key to abbreviations: ab, anterior radial band; am, anterior myophore; at, anterior tooth; ats, anterior tooth socket; (b), borings; be, body cavity; cp, celluloprismatic shell structure; cs, central tooth socket; ct, central tooth; dd, dorsal depression; Ic, ligamentary crest; o, accessory cavity; pb, posterior radial band; pm, posterior myophore; pt posterior tooth; pts, posterior tooth socket; v, antero–ventral carina.

  • 1. Requienia migliorinii Tavani. Longitudinal section through spirally coiled RV (outer shell, black; inner shell, white).

  • 2–6. Agriopleura sp. cf. A. blumenbachi (Stiider). 2, postero– ventral aspect of whole shell; 3, antero–dorsal view of LV; 4, interior of quadrate RV; 5, section across rounded RV (detail shows compact fibrous prismatic structure of outer shell layer); and 6, radial section through both valves (outer shell, black; inner shell, white).

  • 7. Eoradiolites plicatus (Conrad). Section across RV outer shell (about twice natural size). Note localized development of celluloprismatic structure.

  • 8. Eoradiolites lyratus (Conrad). Section across RV outer shell (about twice natural size). Note development of celluloprismatic structure throughout outer shell layer.

  • 9–12. Horiopleura sp. 9, ventral aspect of whole shell (larger form); 10, section across posterior parts of both valves of the similar H. distefanoi (Parona), seen from behind (outer shell, black; inner shell, white; matrix, stippled) [from Italy (Parona, 1909)]; 11, 12, interiors of LV (11) and RV (12) of larger form.

  • 13. Horiopleura sp. cf. H. lamberti Douville. Section across both valves, viewed from ventral side (outer shell, black; inner shell, white). Reduced; commissural diameter may exceed 10 cms.

  • 14–18. Glossomyophorus costatus Masse, Skelton and SJiskovic. 14, ventral aspect of whole shell; 15, 16, interiorsof LV (15) and RV (16); 17, 18, sections across LV (17) and RV (18) (outer shell, black; inner shell, white; matrix, stippled).

Fig. 2

—Diagrammatic sketches of Arabian Early Cretaceous rudists. All drawings are approximately natural size, except where indicated. Key to abbreviations: ab, anterior radial band; am, anterior myophore; at, anterior tooth; ats, anterior tooth socket; (b), borings; be, body cavity; cp, celluloprismatic shell structure; cs, central tooth socket; ct, central tooth; dd, dorsal depression; Ic, ligamentary crest; o, accessory cavity; pb, posterior radial band; pm, posterior myophore; pt posterior tooth; pts, posterior tooth socket; v, antero–ventral carina.

  • 1. Requienia migliorinii Tavani. Longitudinal section through spirally coiled RV (outer shell, black; inner shell, white).

  • 2–6. Agriopleura sp. cf. A. blumenbachi (Stiider). 2, postero– ventral aspect of whole shell; 3, antero–dorsal view of LV; 4, interior of quadrate RV; 5, section across rounded RV (detail shows compact fibrous prismatic structure of outer shell layer); and 6, radial section through both valves (outer shell, black; inner shell, white).

  • 7. Eoradiolites plicatus (Conrad). Section across RV outer shell (about twice natural size). Note localized development of celluloprismatic structure.

  • 8. Eoradiolites lyratus (Conrad). Section across RV outer shell (about twice natural size). Note development of celluloprismatic structure throughout outer shell layer.

  • 9–12. Horiopleura sp. 9, ventral aspect of whole shell (larger form); 10, section across posterior parts of both valves of the similar H. distefanoi (Parona), seen from behind (outer shell, black; inner shell, white; matrix, stippled) [from Italy (Parona, 1909)]; 11, 12, interiors of LV (11) and RV (12) of larger form.

  • 13. Horiopleura sp. cf. H. lamberti Douville. Section across both valves, viewed from ventral side (outer shell, black; inner shell, white). Reduced; commissural diameter may exceed 10 cms.

  • 14–18. Glossomyophorus costatus Masse, Skelton and SJiskovic. 14, ventral aspect of whole shell; 15, 16, interiorsof LV (15) and RV (16); 17, 18, sections across LV (17) and RV (18) (outer shell, black; inner shell, white; matrix, stippled).

Where found.—Sporadically present from the Barremian to the Albian, though most common in the Upper Aptian–Albian, e.g., in Wadi El Assyi (Nakhl area). Grew as ascending spiral dinger, semi–immersed in platform–top wackestones to packstones, though often found as transported shells in floatstones.

Family Monopleuridae Munier Chalmas, 1873

Main features of family.—Attached by RV. Ligament shallowly invaginated, hence shell growth uncoiled. RV cylindrico–conical; L V low–domed to operculiform. Outer shell variable in thickness. Two subequal teeth in LV and one in RV. Myophores form ledgelike ventral extensions of hinge plates; those of LV may be outwardly inclined, facing down onto their depressed counterparts on the inner walls of the RV.

Although the genus Monopleura, itself, has often been cited from the Arabian Lower Cretaceous, no detailed descriptions have yet been given to confirm its presence. Many citations are evidently misidentifications of other taxa (e.g., the RVs of Glossomyophorus or of the new taxon noted below). However, the presence of primitive Monopleura sp., has been confirmed by one of us (P.W.S.) in sub–Hauterivian Cretaceous limestones in Wadi Hagil, Ras al Khaimah, U.A.E. (see Toland et al., 1993). The only representative of the family common in the Arabian Lower Cretaceous is Agriopleura.

Agriopleura sp. cf. A. blumenbachi (Stüder, 1834) (Fig. 2.2–2.6)

What to look for.—RV conical; LV concave, lid–like (Figs. 2.2, 2.3). Commissural outline quadrate to rounded (Figs. 2.4, 2.5). Outer shell layer thick (a few to several millimeters), with coarse external ribs and steeply inclined flaring growth lamellae in the RV (Fig. 2.6). Two slightly protruding radial bands, externally flat to gently embayed, the anterior one situated ventrally (ab) and the other, postero–ventrally (pb) (Figs. 2.2, 2.4). Outer shell layer of compact fibrillar prismatic microstructure (Fig. 2.5, detail), entirely lacking cells. LV anterior and posterior teeth subequal (at > pt), projecting straight into narrow sockets in RV. Both myophores in LV project as low buttresses adjoining teeth (Fig. 2.3). RV varies in shape from moderately conical, like that supposedly characterizing “A. marticensis(D’Orbigny)”, to slender, asin “A. blumenbachi”, and radial band morphology is like that seen in both of these (cf. Douville, 1918). Apart from a possibly slightly thicker shell, the Arabian forms thus seem indistinguishable from those European forms, which may themselves be synonymous (though further work on this question is needed).

Where found.—Widespread in the Shuaiba Formation, though most common as scattered individuals and small elevator clusters in inner–platform facies (Hughes, 1997).

Family Radiolitidae D’Orbigny, 1847

Main features of family.—Attached by RV. Ligament invaginated on distinct inward projection of outer shell layer (ligamentary crest). Shell growth uncoiled, with RV conical to cylindrical, and LV more or less operculiform. Outer shell layer thick, with celluloprismatic microstructure in RV, and with characteristic radial bands on the ventral to posterior flanks of the shell. Two straight teeth in LV, flanked by stalked and outward–facing myophores, together projecting as a dorsal arcade into RV; central tooth in RV reduced, and adductor insertions situated on the inner valve walls.

This general diagnosis applies with rare exceptions in the Lower Cretaceous, though there are more exceptions in the Upper Cretaceous, usually involving secondary modification or loss of features, of no concern here. For further details on Upper Aptian taxa, see Masse and Gallo Maresca (1997).

Eoradiolites plicatus (Conrad, 1852) (Fig. 2.7)

What to look for.—Externa] form similar to that of Agriopleura (with sub-quadrate transverse section); but outer surface relatively smooth rather than ribbed, apart from the radial bands (ab and pb). Main difference is the presence of celluloprismatic structure (cp) in the RV, though this is usually limi ted to a meandriform radial network in the thicker parts of the shell (mainly dorsal, but also at the antero–ventral and postero–ventral corners); thinner parts remain compact (Fig. 2.7; see also Douvillé, 1910). Otherwise, a dorsal depression (dd) externally corresponds to a triangular ligamentary crest (lc) within, and the antero–ventral corner (v) projects as a distinct carina.

Where found.—Upper Aptian–Albian of Wadi El Assyi, Wadi Hedeck, and Wadi Hammam (North Oman, Nakhl area), as locally clustered elevators. Probably derived from Agriopleura, its first appearance currently seems to lie just above the Lower/Upper Aptian boundary, for which it may thus serve as an approximate marker.

Eoradiolites lyratus (Conrad, 1852) (Fig. 2.8)

What to look for.—Commissural outline more circular than the previous species, with rounded longitudinal ribs all around shell, and fine rectangular celluloprismatic microstructure (cp) developed throughout the RV wall (Fig. 2.8). Ligamentary crest (Ic) with pedicle. Body cavity may contain transverse tabulae.

Fig. 3

—Diagrammatic sketches of Arabian Early Cretaceous rudists. See caption to Figure 2 for scale and abbreviations.

  • 1-4. Himeraelites douvillei Di Stefano. 1, ventral aspect of whole shell; 2, 3, interiors of LV (2, based on Italian specimen of Di Stefano, 1888) and RV (3, from upper Shuaiba); 4, section across posterior part of both valves, seen frombehind (outer shell, black; inner shell, white; matrix, stippled).

  • 5. Sellaea/“Caprotina” sp. (sensu Di Stefano, 1888). Section across LV (reduced to about half size).

  • 6-12. New genus, new species. 6, interior of slender RV viewed from ventral side; 7, section across slender RV (outer shell, and tooth sockets, black; inner shell, white; matrix, stippled); 8, antero–ventral aspect of whole shell (more robust example); 9, 10, interiors of LV (9) and RV (10); 11, 12, sections across LV (11) and bored RV (12) (outer shell, black; inner shell, white; matrix, stippled).

Fig. 3

—Diagrammatic sketches of Arabian Early Cretaceous rudists. See caption to Figure 2 for scale and abbreviations.

  • 1-4. Himeraelites douvillei Di Stefano. 1, ventral aspect of whole shell; 2, 3, interiors of LV (2, based on Italian specimen of Di Stefano, 1888) and RV (3, from upper Shuaiba); 4, section across posterior part of both valves, seen frombehind (outer shell, black; inner shell, white; matrix, stippled).

  • 5. Sellaea/“Caprotina” sp. (sensu Di Stefano, 1888). Section across LV (reduced to about half size).

  • 6-12. New genus, new species. 6, interior of slender RV viewed from ventral side; 7, section across slender RV (outer shell, and tooth sockets, black; inner shell, white; matrix, stippled); 8, antero–ventral aspect of whole shell (more robust example); 9, 10, interiors of LV (9) and RV (10); 11, 12, sections across LV (11) and bored RV (12) (outer shell, black; inner shell, white; matrix, stippled).

Where found.—Albian of Wad i El Assyi (also continues on into Cenomanian, e.g., Mishrif Formation).

Family Polyconitidae Mac Gillavry, 1937

Main features of family.—Attached by RV. Ligament shallowly invaginated, hence shell growth uncoiled. RV twisted conical; L V low–domed to operculiform. Outer shell relatively thick, especially in RV (Fig. 5.2). Two subequal teeth in LV and one in RV. Myophores form ventral extensions of hinge plates; posterior myophore of LV pedunculate, projecting back over peripheral accessory cavity and facing down onto ledge–like or depressed myophore in RV.

Horiopleura sp.(Fig. 2.9–2.12)

What to look for.~RV stoutly conical to cylindrical; LV low– domed. Both valves fairly smooth externally, except for growth rugae and perhaps some weak ribbing on the RV. Outer shell moderately thick, especially in RV (at least 2 mm) (Figs. 2.9, 2.10). Main diagnostic feature is pedunculate posterior myophore (pm) in LV (Fig. 2.11), which flares back over small posterior accessory cavity (o), and faces down onto distinct myophoral shelf (pm) in RV (Figs. 2.10, 2.12). Small specimens in the Shuaiba Formation are similar to the Barremian–Lower Aptian H. dumortieri (Matheron) from Europe (illustrated in Skelton and Masse, 1998). A slightly larger, probably descendent, form, similar to that described by Baron–Szabo and Steuber (1996) from the Lower Aptian of central Greece, here characterizes the lowermost Upper Aptian.

Where found.—Shuaiba Formation, in the upper part of the Lower Aptian, as well as in overlying limestones of the lowermost Upper Aptian locally encountered in the subsurface.

Horiopleura sp. cf. H. lamberti (Douville, 1889) (Figs. 2.13, 5.2)

What to look for.—Similar in form to the previous species, though larger (commissural diameter may exceed 10 cm), and probably a descendent chronospecies.

Where found.—Upper Aptian–Albian succession in NE Jebel Akhdar (Nakhl area) (Fig. 5.2), most typically in outer–platform facies (Masse et al., 1997).

Family Caprotinidae Gray, 1848

Main features of family.—Attached by RV. Ligament shallowly invaginated, hence shell growth uncoiled (see Skelton 1978 for explanation); RV twisted conical to cylindrical; LV gently domed to enrolled–capuloid. Outer shell variable in thickness. Two subequal teeth in LV and one solid tooth in RV. Myophores attached to hinge plates, with differing orientations, sometimes creating various accessory cavities.

Despite removal of the monopleurids and polyconitids from the family as previously defined by Skelton (1978), the “caprotinids” described here are still merely a residual grouping of various primitive uncoiled rudists whose exact relationships remain to be resolved. For example, although Himeraelites and the Sellaea/“Caprotina” group (sensu Di Stefano, 1888) appear to be closely related (Masse et al, 1998b), their evolution was probably independent from that of Glossomyophorus, and, for that matter, from that of the Cenomanian type species of Caprotina, C. striata d’Orbigny.

Glossomyophorus costatus (Masse, Skelton, and Sliskovic, 1984) (Figs. 2.14–2.18, 5.1)

What to look for.—RV elongate, straight, slightly twisted; L V tall enrolled capuloid. Outer shell moderately thick (at least 1 mm), with strongly developed, broad but salient ribs, and two prominent radial bands (anterior, ab and posterior, pb) on the ventral flanks of both valves (Fig. 2.14). Outer shell layer characteristically honey brown in core material from areas west of the Oman Mountains. Tongue–shaped posterior myophore (pm) projects across commissural plane from LV (Fig. 2.15) into posterior accessory (myophoral) cavity (o) in the RV (Fig. 2.16). Hence sections of the RV (Fig. 2.18) typically show a row of three holes of similar size, i.e., two sockets for the LV teeth and the posterior accessory cavity. See Masse et al. (1984) for further illustrations and discussion.

Where found.—Most abundant rudist in Lower Aptian of Arabia, though the relatively recent date of recognition as a distinct taxon means that it was frequently mistaken for other rudist taxa in older literature. Limited to the uppermost Barremian to Lower Aptian (middle Kharaib to Shuaiba formations; see Masse et al., 1998a). Widely found in biostromes ranging from more or less in situ elevator congregations (Fig. 5.1), with a packstone to wackestone matrix, to transported drifts of shell debris (floatstones to rudstones); most commonly associated with outer platform–top facies, immediately behind caprinid– dominated platform–margin facies (Hughes, 1997; Masse et al., 1998a).

Fig. 4

—Diagrammatic sketches of Arabian Early Cretaceous rudists (caprinids). See caption to Figure 2 for scale and abbreviations.

  • 1-6. Pachytraga tubiconcha Astre [1–4 based on material from Portugal and France (Skelton and Masse, 1998)]. 1, antero– ventral aspect of whole shell; 2, 3, interiors of LV and RV (slightly magnified); 4–6, sections across LV (4) and RV (5, 6) (outer shell, black; inner shell, white; matrix, stippled).

  • 7, 8. Pachytraga paradoxa (Pictet and Campiche) [based on material from France (Skelton and Masse, 1998)]. Sections across LV (7) and RV (8) (outer shell, black; inner shell, white; matrix, stippled).

  • 9. Praecaprina sp. cf. varians Paquier (from Masse and others, 1998a). Section across LV (note: viewed from apical side; inner shell here shown in black).

  • 10. Offneria sp. cf. O. interrupta Paquier (from Masse and others, 1998a). Section across LV (note: viewed from apical side; inner shell here shown in black).

  • 11-13. Offiieria sp. cf. O. nicolinae (Mainelli). 11, antero–ventral aspect of whole shell; 12, 13, sections across LV and RV.

  • 14-16. Offneria murgensis Masse. 14, 15, sections across LV and RV (some obscuring of features through recrystallization suggested in dorsal part of 15; a typical occurrence); 16, longitudinal section of a shell fragment, showing typical frequency of transverse partitions in pallia) canals.

  • 17. Offneria italica Masse (from Masse and others, 1998a). Section across LV (note: viewed towards apex; inner shell here shown in black).

Fig. 4

—Diagrammatic sketches of Arabian Early Cretaceous rudists (caprinids). See caption to Figure 2 for scale and abbreviations.

  • 1-6. Pachytraga tubiconcha Astre [1–4 based on material from Portugal and France (Skelton and Masse, 1998)]. 1, antero– ventral aspect of whole shell; 2, 3, interiors of LV and RV (slightly magnified); 4–6, sections across LV (4) and RV (5, 6) (outer shell, black; inner shell, white; matrix, stippled).

  • 7, 8. Pachytraga paradoxa (Pictet and Campiche) [based on material from France (Skelton and Masse, 1998)]. Sections across LV (7) and RV (8) (outer shell, black; inner shell, white; matrix, stippled).

  • 9. Praecaprina sp. cf. varians Paquier (from Masse and others, 1998a). Section across LV (note: viewed from apical side; inner shell here shown in black).

  • 10. Offneria sp. cf. O. interrupta Paquier (from Masse and others, 1998a). Section across LV (note: viewed from apical side; inner shell here shown in black).

  • 11-13. Offiieria sp. cf. O. nicolinae (Mainelli). 11, antero–ventral aspect of whole shell; 12, 13, sections across LV and RV.

  • 14-16. Offneria murgensis Masse. 14, 15, sections across LV and RV (some obscuring of features through recrystallization suggested in dorsal part of 15; a typical occurrence); 16, longitudinal section of a shell fragment, showing typical frequency of transverse partitions in pallia) canals.

  • 17. Offneria italica Masse (from Masse and others, 1998a). Section across LV (note: viewed towards apex; inner shell here shown in black).

Himeraelites douvillei (Di Stefano, 1888) (Figs. 3.1–3.4)

What to look for.—Large solid shell; RV slightly twisted conical; LV low–domed. Commissure dorso–ventrally compressed, giving broad ventral aspect (Fig. 3.1). Outer shell quite thin (around 1 mm); inner layer thick and compact. LV posterior myophore (pm) a raised, inwardly tilted platform (Fig. 3.2), lacking a well– marked peripheral accessory cavity (Fig. 3.4), in contrast to Horiopleura (see above). LV pm faces down onto abroad myophoral shelf (pm) in RV (Fig. 3.3).

Where found.—So far known in Arabia only from the subsurface, around the Lower–Upper Aptian boundary, as rare elevator in inner–platform wackestones (see also Hughes, 1997, Fig. 6a).

Sellaea/Caprotina sp. (sensu Di Stefano, 1888) (Figs. 3.5, 5.4)

What to look for.—Elongate, robust shell with high capuloid LV. Prominent LV posterior myophore (pm) connects posterior tooth {pt) to lamina running from anterior tooth (at) to postero– ventral margin (Fig. 3.5). Large accessory cavity (o) behind LV pm. LV anterior myophore (am) also separated from margin by accessory cavity (o). These myocardinal arrangements are common to the various species assigned to Sellaea and“Caprotina” by Di Stefano (1888), which is an assemblage of forms in need of further taxonomic revision.

Where found.—So far known only from scattered specimens in the Aibian inner platform–top facies of NE Jebel Akhdar (Nakhl area) (Masse et al., 1997).

Family Uncertain

New genus, new species (Figs. 3.6–3.12)

What to look for.—RV elongate, slightly twisted cylindrico– conical; LV tall, spirally enrolled. Outer shell thin (less than 0.5 mm), with fine moderately spaced ribbing on RV, though LV is smooth (Figs. 3.8, 3.11). Commissural outline sub–quadrate, with blunt antero–ventral and postero–ventral carinae. Posterior tooth (pt) much smaller than anterior tooth (at) (Fig. 3.9). Myophores extended ventrally as raised platforms oriented subparallel to the commissural plane in both valves or facing outwards somewhat in LV and depressed in RV (Figs. 3.9, 3.10). Inner shell thick but compact without any pallial canals Fig311 note however that frequent borings b may be mistaken for canals although their irregular outlines and distribution rule them out as such (Fig. 3.12)].

Some more slender shellswith the myophores sloping down more steeply into the RV interior (Figs. 3.6, 3.7) also belong with this new taxon [in the draft field guide on which this paper is based (Skelton and Masse, 1997), a RV of this slender form was attributed to Monopleura sp., but new material, including spirally enrolled LVs, indicate assignment instead to the new taxon.] Naming of this new taxon awaits a formal published description (Skelton, in prep.).

Where found.—Locally accompanying Glossomyophorus in the Shuaiba Formation, in which it may be quite common, especially in the more external platform facies around the Abu Dhabi intra- platform basin. A robust elevator, usually found transported in floatstones to rudstones. Because this taxon has only recently been recognized, further finds of it will be of particular value prior to formal publication; in the past, thick-shelled specimens with abundant borings have sometimes been erroneously identified as canaliculate caprinids.

Family Caprinidae d’Orbigny (emend. Skelton and Masse, 1998)

Main features of family.—Attached by RV. Ligament invagi- nated, hence valves uncoiled. RV slightly twisted, straight to curved, cylindrico-conical; LV more or less extended and dorsally curved or enrolled. Outer shell layer thin; inner shell thick, with pallial canals in most taxa. Two unequal teeth in LV (at > pt), and robust central tooth in RV. In both valves, myocardinal lamina separates posterior accessory cavity, containing inclined posterior myophore, from body cavity.

A recent revision of this family by Skelton and Masse (1998) has identified the myocardinal system as the key diagnostic character, instead of the presence of pallial canals, because certain other rudist groups may show canals, too, and a few caprinids lack them.

Pachytraga tubiconcha (Astre, 1961) (Figs. 4.1–4.6)

What to look for.—Elongate shell with slender, tubular RV and somewha t erect capuloid LV.LV posterior myophore (pm) situated on the posterior valve wall, connecting dorsally with small posterior tooth, and inclined steeply into posterior accessory cavity (o), which itself adjoins central tooth socket (Figs. 4.2, 4.4). RV posterior myophore (pm) an erect, outward-facing plate on myocardinal lamina, separated from posterior valve wall by a long, narrow posterior accessory cavity (o) dorsally connected to the posterior tooth socket (pts) (Figs. 4.3, 4.6). Bottom of R V posterior accessory cavity may be subdivided by radial laminae, especially in larger specimens (Fig. 4.5). A few small pallial canals may be present around the antero–dorsal margin of the RV (Fig. 4.6).

Fig. 5

—Rudists at outcrop in the Oman Mountains.

  1. Detail of Glossomyophorus biostrome, in vertical section, with specimen in life position to right of lens cap, surrounded by displaced valves. Shuaiba Formation (Lower Aptian) of WadiMi’aidin, Oman Mountains. Lens cap is 5.5 cms across.

  2. Displaced articulated Horiopleura in rudist floatstone, in vertical section. AlHassanat Formation (Aibian) ofWadiEl Assyi, Oman Mountains. Large specimen is at least 10 cm .

  3. Transported caprinid assemblage, dominated by Offneria, in vertical section. Kharaib Formation (Lower Aptian) of Wadi Hedeck, Oman Mountains. Larger specimens up to c. 10 cm across.

  4. Displaced Sellaea/“Caprotina” floatstone in vertical section. All Hassanat Formation (Aibian) of Wadi El Assyi, Oman Mountains. Specimens up to ca. 8 cm across

Fig. 5

—Rudists at outcrop in the Oman Mountains.

  1. Detail of Glossomyophorus biostrome, in vertical section, with specimen in life position to right of lens cap, surrounded by displaced valves. Shuaiba Formation (Lower Aptian) of WadiMi’aidin, Oman Mountains. Lens cap is 5.5 cms across.

  2. Displaced articulated Horiopleura in rudist floatstone, in vertical section. AlHassanat Formation (Aibian) ofWadiEl Assyi, Oman Mountains. Large specimen is at least 10 cm .

  3. Transported caprinid assemblage, dominated by Offneria, in vertical section. Kharaib Formation (Lower Aptian) of Wadi Hedeck, Oman Mountains. Larger specimens up to c. 10 cm across.

  4. Displaced Sellaea/“Caprotina” floatstone in vertical section. All Hassanat Formation (Aibian) of Wadi El Assyi, Oman Mountains. Specimens up to ca. 8 cm across

Where found.—Hauterivian, Habshan Formation of Wadi Mi‘Aidin and basal Lekhwair Formation equivalent of Wadi Bani Kharus (central Jebal Akhdar), as common transported shells in outer platform–top floatstones. In Europe, this species is known to form elevator clusters in platform–top packstones to wackestones (Skelton and Masse, 1998).

Pachytraga paradoxa (Pictet and Campiche, 1869) (Figs. 4.7, 4.8)

What to look for.—Morphology essentially similar to that of the previous species, though reaching significantly greater size, with more pronounced antero–ventral and postero–ventral carinae. Also, more distinct accessory cavity in front of anterior myophore (am) in LV, which may be subdivided to form pallial canals (Fig. 4.7); canals may also develop in front of am in RV (Fig. 4.8). Small internal ridge common on the mid-ventral wall.

Where found.—So far only dubiously noted in Arabia, in the Shuaiba Formation (Wadi Hedeck, Nakhl area). In Europe the species is known predominantly as a robust elevator, but also as a facultative recumbent, in outer platform–top packstones to rudstone/grainstones.

These two species of Pachytraga appear to represent a chronospecies lineage, spanning the Hauterivian to Lower Aptian and showing phyletic size increase (Skelton and Masse, 1998).

Praecaprina sp. cf. P. varians (Paquier, 1905) (Fig. 4.9)

What to look for.—Similar to Pachytraga, though broader antero– posteriorly, with pallial canals well developed outside both myophores in the LV only (Fig. 4.9).

Where found.—Only recently recorded from Arabia (Masse et al., 1998a), from the middle Kharaib Formation equivalent of Wadi Hedeck and adjacent sections (NE Jebel Akhdar, Nakhl area).

Offneria sp. cf. O. interrupta (Paquier, 1905) (Fig. 4.10)

What to look for.—Pallial canals in the margins of both valves, containing transverse partitions, concave towards the commissure. In this relatively primitive Offneria species, the canals do not extend all the way around the ventral margin (Fig. 4.10).

Where found.—Again, recently recorded from Arabia (Masse et al., 1998a), from the middle Kharaib Formation equivalent of Wadi Hedeck. Also recorded here was another closely related Offneria species, O. rhodanica Paquier, 1905, again previously known only from Europe.

Offneria sp. cf. O. nicolinae (Mainelli, 1983) (Figs. 4.11–4.13)

What to lookfor.—Shell small, with long slender RV and capuloid LV (Fig. 4.11). Commissural outline rounded. Row of simple canals extends around entire margins of both valves, with some transverse partitions on the anterior and posterior sides (Figs. 4.12, 4.13). For further details of this and the next two species of Offneria, see Masse (1992).

Where found.—From the middle Kharaib Formation equivalent of Wadi Hedeck (Masse et al., 1998a); also scattered, though not common, in the Shuaiba Formation of several areas. Elevator, locally clustered in platform–top wackestones to packstones.

Offneria murgensis (Masse, 1992) (Figs. 4.14—4.16, 5.3)

What to look for.—Medium–sized shell, also with elongate RV and capuloid LV‘. Commissural outline develops blunt angulations around the antero–ventral and postero–ventral corners, with a slight mid–ventral depression. Row of simple canals on the ventral margin of each valve expands to multiple rows around the remaining margins (Figs. 4.14, 4.15), with abundant transverse partitions (Fig. 4.16). Tabulae, concave towards the commissure, are also well developed in the body cavity.

Where found.—Most abundant of the Offneria species in Arabia. Scattered in the middle Kharaib Formation and equivalents in the Jebel Akhdar region (Masse et al., 1998a), but also widespread in the Shuaiba Formation (e.g., Hughes, 1997). Usually found only as transported specimens in outer platform–top floatstones to rudstones (Fig. 5.3), it appears to have been a facultative elevator to recumbent.

Offneria italica (Masse, 1992) (Fig. 4.17)

What to look for.—Larger, again, than the previous species, with numerous canals of polygonal section within the marginal row of pyriform canals (Fig. 4.17). Tabulae well developed in the body cavity.

Where found.—So far, positively identified in Arabia only from the Shuaiba Formation of northern Oman (Masse et al., 1998a). Like O. murgensis, it appears to have been a facultative elevator to recumbent.

Conclusions

The diversity of rudists in the Lower Cretaceous carbonate platform deposits of Arabia offer much potential both for bio– stratigraphy and for paleoenvironmental analysis, and hence for the development of fades models for production development.

In relation to biostratigraphy (Fig. 1), the uppermost Barremian to Lower Aptian interval is especially well marked by a diverse fauna, amongst which Glossomyophorus is a notable and widespread element, together with a variety of caprinids. Few lineages from this fauna survive into the Upper Aptian, which can nevertheless be identified by the appearance of the first radiolitids, showing celluloprismatic structure. The Albian is, in turn, marked by the appearance of the Sellaea/“Caprotina” (sensu Di Stefano, 1888) group.

With respect to paleoecology, distinct biofacies become especially evident in the Shuaiba Formation, reflecting the diversity of rudists present. Robust, predominantly recumbent caprinids tend to dominate the coarse bioclastic outer platform-margin facies, while elevator Glossomyophorus, locally accompanied by the new taxon, form extensive biostromes behind, on the outer platform top (see, for example, Hughes, 1997; Masse et al., 1998a). In more internal platform facies, Agriopleura becomes dominant. Less well marked patterns of this kind can also be seen at other levels. In the Habshan Formation, for example, the robust Pachytraga tends to be most common in the outer–platform facies. In the Albian, the open platform–margin bioclastic facies is dominated by Eoradiolites and Horiopleura, together with corals, while the Sellaea/“Caprotina” assemblage occupies a more internal setting (e.g., A1 Hassanat Formation of Jebel Akhdar; Masse et al., 1997).

References

Baron–Szabo
,
R.C.
and
Steuber
,
T.
1996
,
Korallen und rudisten aus dem Apt im tertiaren Flysch des Parnass–Gebirges bei Delphi– Arachowa (Mittelgriechenland)
:
Berliner Geowissenschaftliche Abhandlungen
 , v.
FJ8
, p.
3
75
.
Sterano
,
G. Di
1888
,
Studi stratigrafici e paleontologici sul sistema Cretaceo della Sicilia. 1. Gli strati con Caprotina di Termini– Imerese
:
Reale Accademia di Scienze, Lettere e Belle Arti, Atti (N. S.)
 , v.
10
,
44
p.
Douville
,
H.
1910
,
Etudes sur les Rudistes: Rudistes de Sicile, d’Algerie, d’Egypte, du Liban et de la Perse
:
Societe Geologique de France, Memoires, Paleontologie
 , no.
41
,
84
p.
Douvilll
,
H.
1918
,
Le Barremien superieur de Brouzet—Troisieme Partie
:
les Rudistes: Societe Geologique de France, Memoires, Paleontologie
 , no.
52
, p.
5
19
.
Gili
,
E.
Massl
,
J.–P.
and
Skllton
,
P.W.
1995
,
Rudists as gregarious sediment–dwellers, not reef–builders, on Cretaceous carbonate platforms
:
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
118
, p.
245
267
.
Hughes
,
G.W.
1997
,
The Great Pearl Bank Barrier of the Arabian Gulf as a possible Shuaiba analogue
:
GeoArabia
 , v.
2
, p.
279
304
.
Masse
,
J.–P.
1992
,
Les rudistes del’Aptieninferieurd’Italie continental: aspects systematiques, stratigraphiques et paleo– biogeographiques
:
Geologica Romana
 , v.
28
, p.
243
260
.
Amsse
,
J.–P.
and
Maresca
,
M. Gallo
1997
,
Late Aptian Radiolitidae (rudist bivalves) from the Mediterranean and Southwest Asiatic regions: taxonomic, biostratigraphic and palaeobiogeographic aspects
:
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
128
, p.
101
110
.
Masse
,
J.–P.
and
Philip
,
J.
1981
,
Cretaceous coral–rudistid buildups of France
, in
Toomey
,
D.F.
ed.,
European Fossil Reef Models: Society of Economic Paleontologists and Mineralogists
 , Special Publication
30
, p.
399
426
.
Masse
,
J.–P.
Skflton
,
P.W.
and
Si.iskovic,
,
T.
1984
,
Glossomyophorus costatus nov. gen. nov. sp., rudiste (Caprotinidae) nouveau de l’Aptien du domaine mediterraneen central et oriental
:
Geobios
 , v.
17
, p.
723
732
.
Masse
,
J.–P.
Borcomano
,
J.
and
Maskiry
,
S. Al
1997
,
Stratigraphy and tectonosedimentary evolution of a late Aptian–Albian carbonate margin: the northeastern Jebel Akhdar (Sultanate of Oman)
:
Sedimentary Geology
 , v.
113
, p.
269
280
.
Masse
,
I.–P.
CuARTROussr
,
A.
and
Borcomano
,
J.
1998a
,
The Lower Cretaceous (Upper Barremian–Lower Aptian) caprinid rudists from northern Oman
:
Geobios, Memoire Special
 , no.
22
, p.
211
223
.
Masse
,
J.–P.
Maresca
,
M. Gallo
and
Sinni
,
E. Luperto
1998b
,
Albian Rudist faunas from Southern Italy: taxonomic, biostratigraphic and palaeobiogeographic aspects
:
Geobios
 , v.
31
, p.
47
59
.
Parona
,
C.F.
1909
,
La fauna coralligena del Cretaceo dei Monti d’Ocre nell’Abruzzo Aquilano
:
Memorie per servire alia Descrizione della Carta geologica d’ltalia
 , no.
5
.
233
p.
Ross
,
D.J.
and
Skelton
,
P.W.
1993
,
Rudist formations of the Cretaceous: a palaeoecological, sedimentological and stratigraphical review
:
Sedimentology Review
 , v.
1
, p.
73
91
.
Skelton
,
P.W.
1978
,
The evolution of functional design in rudists (Hippuritacea) and its taxonomic implications
:
Royal Society [London], Philosophical Transactions
  v.
B284
, p.
305
318
.
Skeliton,
,
P. W.
and
Masse
,
J.–P.
1997
,
Biostratigraphy and palaeoecology of rudists in the Lower Cretaceous of Arabia, in Alsharhan, A.S., compiler, Mesozoic Carbonate Platform–Basin systems, United Arab Emirates; Guidebook prepared for the research conference on Jurassic/Cretaceous carbonate platform–basin systems—Middle East models
,
4–12
December
,
1997
:
Unpublished field–guide, U.A.E. University, Al Ain, U.A.E.
 , p.
24
36
.
Skelton
,
P.W.
and
Masse
,
J.–P.
1998
,
Revision of the Lower Cretaceous rudist genera Pachytraga Paquier and Retha Cox (Bivalvia: Hippuritacea), and the origins of the Caprinidae
:
Geobios, Memoire Special
 , no.
22
, p.
331
370
.
Toland
,
C.
dPeebles
,
R.G.
and
Walkden
,
G.M.
1993
,
Upper Jurassic and basal Cretaceous outcrop sequence stratigraphy of Wadi Hagil, Ras al Khaimah: Society of Petroleum Engineers
,
8th Middle East Oil Show and Conference, Bahrain, Proceedings, SPF paper
 
025581
, p.
533
0–
543
.
Witt
,
W.
and
Gokdag
,
H.
1994
,
Orbitolinid biostratigraphy of the Shuaiba Formation (Aptian), Oman—implications for reservoir development
, in
Simmons
,
M.D.
ed.,
Micropalaeontology and Hydrocarbon Exploration in the Middle East
 :
London
,
Chapman & Hall
, p.
221
241
.

Acknowledgments

We are grateful to numerous colleagues with whom we have worked on Arabian rudists over many years, and to Prof. A. S. Alsharhan and Dr. R. W. Scott for inviting us to submit this paper, originally as a field guide for the Jurassic/Cretaceous platform– basin systems conference in Al Ain, in 1997, and to Prof. J. Philip and Dra. E. Gili for their constructive reviews of the paper. Than ks to Janet Dryden and Ashea Tambe for formatting of the manuscript and to Andrew Lloyd for d igitally composing the specimen figures (drawn by PWS).

Figures & Tables

Contents

GeoRef

References

References

Baron–Szabo
,
R.C.
and
Steuber
,
T.
1996
,
Korallen und rudisten aus dem Apt im tertiaren Flysch des Parnass–Gebirges bei Delphi– Arachowa (Mittelgriechenland)
:
Berliner Geowissenschaftliche Abhandlungen
 , v.
FJ8
, p.
3
75
.
Sterano
,
G. Di
1888
,
Studi stratigrafici e paleontologici sul sistema Cretaceo della Sicilia. 1. Gli strati con Caprotina di Termini– Imerese
:
Reale Accademia di Scienze, Lettere e Belle Arti, Atti (N. S.)
 , v.
10
,
44
p.
Douville
,
H.
1910
,
Etudes sur les Rudistes: Rudistes de Sicile, d’Algerie, d’Egypte, du Liban et de la Perse
:
Societe Geologique de France, Memoires, Paleontologie
 , no.
41
,
84
p.
Douvilll
,
H.
1918
,
Le Barremien superieur de Brouzet—Troisieme Partie
:
les Rudistes: Societe Geologique de France, Memoires, Paleontologie
 , no.
52
, p.
5
19
.
Gili
,
E.
Massl
,
J.–P.
and
Skllton
,
P.W.
1995
,
Rudists as gregarious sediment–dwellers, not reef–builders, on Cretaceous carbonate platforms
:
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
118
, p.
245
267
.
Hughes
,
G.W.
1997
,
The Great Pearl Bank Barrier of the Arabian Gulf as a possible Shuaiba analogue
:
GeoArabia
 , v.
2
, p.
279
304
.
Masse
,
J.–P.
1992
,
Les rudistes del’Aptieninferieurd’Italie continental: aspects systematiques, stratigraphiques et paleo– biogeographiques
:
Geologica Romana
 , v.
28
, p.
243
260
.
Amsse
,
J.–P.
and
Maresca
,
M. Gallo
1997
,
Late Aptian Radiolitidae (rudist bivalves) from the Mediterranean and Southwest Asiatic regions: taxonomic, biostratigraphic and palaeobiogeographic aspects
:
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
128
, p.
101
110
.
Masse
,
J.–P.
and
Philip
,
J.
1981
,
Cretaceous coral–rudistid buildups of France
, in
Toomey
,
D.F.
ed.,
European Fossil Reef Models: Society of Economic Paleontologists and Mineralogists
 , Special Publication
30
, p.
399
426
.
Masse
,
J.–P.
Skflton
,
P.W.
and
Si.iskovic,
,
T.
1984
,
Glossomyophorus costatus nov. gen. nov. sp., rudiste (Caprotinidae) nouveau de l’Aptien du domaine mediterraneen central et oriental
:
Geobios
 , v.
17
, p.
723
732
.
Masse
,
J.–P.
Borcomano
,
J.
and
Maskiry
,
S. Al
1997
,
Stratigraphy and tectonosedimentary evolution of a late Aptian–Albian carbonate margin: the northeastern Jebel Akhdar (Sultanate of Oman)
:
Sedimentary Geology
 , v.
113
, p.
269
280
.
Masse
,
I.–P.
CuARTROussr
,
A.
and
Borcomano
,
J.
1998a
,
The Lower Cretaceous (Upper Barremian–Lower Aptian) caprinid rudists from northern Oman
:
Geobios, Memoire Special
 , no.
22
, p.
211
223
.
Masse
,
J.–P.
Maresca
,
M. Gallo
and
Sinni
,
E. Luperto
1998b
,
Albian Rudist faunas from Southern Italy: taxonomic, biostratigraphic and palaeobiogeographic aspects
:
Geobios
 , v.
31
, p.
47
59
.
Parona
,
C.F.
1909
,
La fauna coralligena del Cretaceo dei Monti d’Ocre nell’Abruzzo Aquilano
:
Memorie per servire alia Descrizione della Carta geologica d’ltalia
 , no.
5
.
233
p.
Ross
,
D.J.
and
Skelton
,
P.W.
1993
,
Rudist formations of the Cretaceous: a palaeoecological, sedimentological and stratigraphical review
:
Sedimentology Review
 , v.
1
, p.
73
91
.
Skelton
,
P.W.
1978
,
The evolution of functional design in rudists (Hippuritacea) and its taxonomic implications
:
Royal Society [London], Philosophical Transactions
  v.
B284
, p.
305
318
.
Skeliton,
,
P. W.
and
Masse
,
J.–P.
1997
,
Biostratigraphy and palaeoecology of rudists in the Lower Cretaceous of Arabia, in Alsharhan, A.S., compiler, Mesozoic Carbonate Platform–Basin systems, United Arab Emirates; Guidebook prepared for the research conference on Jurassic/Cretaceous carbonate platform–basin systems—Middle East models
,
4–12
December
,
1997
:
Unpublished field–guide, U.A.E. University, Al Ain, U.A.E.
 , p.
24
36
.
Skelton
,
P.W.
and
Masse
,
J.–P.
1998
,
Revision of the Lower Cretaceous rudist genera Pachytraga Paquier and Retha Cox (Bivalvia: Hippuritacea), and the origins of the Caprinidae
:
Geobios, Memoire Special
 , no.
22
, p.
331
370
.
Toland
,
C.
dPeebles
,
R.G.
and
Walkden
,
G.M.
1993
,
Upper Jurassic and basal Cretaceous outcrop sequence stratigraphy of Wadi Hagil, Ras al Khaimah: Society of Petroleum Engineers
,
8th Middle East Oil Show and Conference, Bahrain, Proceedings, SPF paper
 
025581
, p.
533
0–
543
.
Witt
,
W.
and
Gokdag
,
H.
1994
,
Orbitolinid biostratigraphy of the Shuaiba Formation (Aptian), Oman—implications for reservoir development
, in
Simmons
,
M.D.
ed.,
Micropalaeontology and Hydrocarbon Exploration in the Middle East
 :
London
,
Chapman & Hall
, p.
221
241
.

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal