Graptolites from three shallow core holes penetrating lower Silurian strata in central Saudi Arabia identify rocks of mid Aeronian and early Telychian age. The graptolites comprise Campograptus undulatus?, C. lobiferus?, Lituigraptus convolutus, Metaclimacograptus bohemicus, Neolagarograptus rickardsi, Normalograptus? aff. scalaris, cf. Paradiversograptuscapillaris,Petalolithusminor,P.praecursor,P.ovatoelongatus?, Pristiograptus regularis sl, Pseudorthograptus insectiformis, Pseudoretiolites perlatus? and Torquigraptus? decipiens, which identify the mid Aeronian convolutus Biozone, and Monograptus bjerreskovae, Monograptus ex gr. marri, Paradiversograptus runcinatus?, Petalolithus cf. altissimus, Pristiograptus renaudi, Stimulograptus becki and S. halli?, which indicate the early Telychian guerichi Biozone. The graptolite faunas are generally of low diversity, with most levels yielding between one and three species, occasionally rising to as many as seven species. The palaeogeography of the depositional basin, coupled with the low diversity and the abundance of Pristiograptus and Normalograptus species, suggests a marine-shelf setting. This is also indicated by the common occurrence of benthic shelly fossils, including articulated bivalves and gastropods. In general, the graptolite faunas are of much lower diversity than contemporaneous marine assemblages from the British Isles or the Czech Republic, and probably represent a graptolite biotope of “cratonic invaders.” Graptolites are very well preserved, displaying detail of fusellar banding (growth increments) and perhaps ultrastructure.

Graptolites provide one of the most enduring and high-resolution biostratigraphies for Ordovician and Silurian rocks and are instrumental in correlating rocks of that age worldwide. In Saudi Arabia, the Qusaiba Member of the Qalibah Formation has yielded rich graptolite faunas of Llandovery age (Rickards and Koren’, 1974; El-Khayal, 1985, 1986, 1987a, b, c; McClure, 1988; Legrand inMahmoud et al., 1992 and references therein). Most of the faunas described from the Qusaiba Member indicate graptolite biozones of the Rhuddanian or Aeronian stages of the Llandovery (Figure 1). Graptolites referable to the late Llandovery Telychian Stage have also been noted (Legrand inVaslet et al., 1987; Legrand inVaslet, 1989) though the assemblages have not been figured or documented in detail.

In this paper the graptolites from three shallow core holes drilled by Saudi Aramco in central Saudi Arabia are documented (Figures 2 to 6 and Plates). The graptolites identify the convolutus Biozone of the mid Aeronian, widely recognised on the Arabian Plate and identified as maximum flooding surface S10 by Sharland et al. (2001); and the younger guerichi Biozone of the early Telychian (Figure 1). The fauna enables the first detailed evaluation of early Telychian graptolites from the Arabian Peninsula.

A generalised stratigraphy for the Saudi Arabian Silurian is shown on Figure 3a and the Baq’a area Qalibah Formation outcrop succession is illustrated on Figure 3b. Natural exposures of Silurian shale such as those in the Baq’a and Qasim areas can be deeply weathered and/or covered. The core holes were drilled by Saudi Aramco during 2002 to obtain unweathered samples for palynology and also a continuous stratigraphic record of the Qalibah Formation for sedimentological study. The graptolites are preserved in grey hemipelagic to thinly graded mudstones recovered from cores cut in three shallow core holes drilled through the Qusaiba Member of the Qalibah Formation. Graptolitebearing samples were identified by examining bedding plane surfaces of the two-inch cores. One hundred and twenty eight graptolite-bearing core samples (such as that illustrated in Figure 7) were recovered, though further splitting of selected samples identified graptolites on numerous additional bedding surfaces. Selected and figured graptolites are stored in the Palaeontology Department collections of the Natural History Museum, Cromwell Road, South Kensington, London, United Kingdom. The remaining graptolite samples and cores reside with Saudi Aramco in Dhahran, Saudi Arabia.

For comparative purposes, specimens of Neolagarograptus tenuis from the British Geological Survey collection, Keyworth, Nottinghamshire, were also examined. These are indicated by the prefix BGS whereas the Natural History Museum specimens are denoted with the prefix NHM.

In this section the graptolite assemblages from the three core holes are described. The stratigraphy is summarised in Figures 3 to 6. The graptolite biozonation employed here is that developed for the British Isles (e.g. Loydell, 1992, 1998; Figure 1). Reference is also made to the closely comparable graptolite biozonation used in the Czech Republic and Spain, particularly for the early Telychian (Štorch, 1994; Gutiérrez-Marco and Štorch, 1998). The term turriculatus Biozone sl (Figure 1) refers to the earliest graptolite biozone of the Telychian used in its historical sense. This interval is now divided into the guerichi and turriculatus ss biozones (Figure 1). Depth measurements are given in feet, from the original core hole depths.

Qusaiba-1 Core Hole

The Qusaiba-1 core hole was spudded on a calcareous duricrust in the Qasim area north-northwest of the deserted Qusaiba village (Figure 2b). The stratigraphic terminology applied to the Silurian section here was summarised by Manivit et al. (1986) from local surface sections. At that time the Silurian was termed the Tayyarat Formation but this was changed to Qalibah Formation because of nomenclatural priority (Mahmoud et al., 1992). The upper 97 feet of the core consists of red, brown to tan, bioturbated sandstone and subordinate mudstones. No palynological age could be determined for this unit because of weathering. Fifty-four core samples of grey silty mudstones from between depths 133.5 ft and 253.3 ft in the Qusaiba-1 core hole yielded the following graptolites (Figure 4): Campograptus undulatus (Elles and Wood 1913)?, Lituigraptus convolutus (Hisinger 1837), Metaclimacograptus bohemicus (Perner 1897), Metaclimacograptus sp., Neolagarograptus rickardsi (El-Khayal 1987c), Normalograptus? aff. scalaris (Hisinger 1837), cf. Paradiversograptus capillaris (Carruthers 1867), Petalolithus minor (Elles 1897), Petalolithus ovatoelongatus (Kurck 1882)?, Petalolithus sp., Pristiograptus regularis (Törnquist 1899) sl, Pristiograptus sp., Pseudorthograptus insectiformis (Nicholson 1868), Pseudoretiolites perlatus (Nicholson 1868)?, Retiolites sp. and Torquigraptus? decipiens (Törnquist 1899). Other fauna includes bivalves (some articulated), gastropods, orthoconic nautiloids and burrows, some of which are pyrite-lined.

This fauna includes many of the elements listed and described by El-Khayal (1985, 1987a, 1987b) from the convolutus Biozone in the Qusayba Shales Member [sic] at Qusaiba in the Al-Qasim Province of Saudi Arabia. Although attempts have been made, in southern Britain, to subdivide the interval of the convolutus Biozone into lower and upper divisions (Zalasiewicz, 1996), the graptolites from this core hole are insufficiently diverse to enable this distinction in Saudi Arabia.

Baq’a-3 Core Hole

The Baq’a-3 core hole was spudded on the lower seven feet of the Tawil Formation northeast of Baq’a (Figure 2a). It penetrated 152 ft of the Sharawra Member of the Qalibah Formation and reached total depth in the Qusaiba Member (Figure 5). Twenty core samples yielded graptolites from between depths of 303 ft and 517.8 ft. These identify the early Telychian, at a level probably equivalent to the turriculatus Biozone sl of Britain between 303 ft and 336 ft (Figure 1). Through this interval only Stimulograptus becki (Barrande 1850) was encountered, together with occasional fragments of organic films that resemble possible plant fragments. Although graptolites referred to ‘Monograptus’ becki have previously been noted from Aeronian horizons in the Qusaiba Member (Legrand inVaslet, 1989), the species is normally associated with Telychian strata where, for example, it is recorded from the linnaei Biozone of the Czech Republic (Štorch, 1994) and Spain (Gutiérrez-Marco and Štorch, 1998), and the proteus Subzone of the turriculatus Biozone sl of Wales (i.e. Loydell, 1993). Since in this core hole early Telychian faunas of guerichi Biozone age are undoubtedly present at greater depth (Figure 5), it seems likely that the occurrence of S. becki indicates a level midway through the turriculatus Biozone sl of southern Britain (Figure 1). As such, this interval represents the youngest graptolite-bearing strata in the three core holes studied (Figure 5).

There is an interval in the succession without graptolites and the next graptolite-bearing horizon is at 367.1 ft. Between 367.1 ft and 406.05 ft the faunas are poor, but the interval appears to lie within the early Telychian based on superposition. At 367.1 ft the long-ranging Telychian Monograptus ex gr. marriPerner 1897 occurs, together with shell fragments. The graptolite identification is consistent with the occurrence of S. becki in strata above, and Monograptus bjerreskovaeLoydell 1993 and Pristiograptus renaudi (Philippot 1950) in strata below (from depth 454.4 ft). In Britain, M. ex gr. marri appears in the renaudi Subzone of the guerichi Biozone (early turriculatus Biozone sl; Figure 1), at a level above the first occurrence of M. bjerreskovae and P. renaudi, but below the first occurrence of S. becki. There is a gap in the sequence and the next graptolite-bearing horizon is at 401.25 ft. From this depth to 406.05 ft two samples yield the graptolites Metaclimacograptus sp. and Monograptus sp., a sponge spicule and pyritized burrows. The graptolites are not age-diagnostic but appear to be within the early Telychian, based on stratigraphic position (Figure 5).

There is an unfossiliferous interval in the succession with the next graptolite-bearing sample at 454.4 ft. From 454.4 ft to 517.8 ft the graptolites indicate an early Telychian age at the level of the guerichi Biozone, and the same stratigraphic level as that recorded between 37.1 ft and 135.55 ft in the Baq’a-4 core hole. Here the graptolites comprise Monograptus bjerreskovae, Petalolithus? sp., Pristiograptus renaudi, Pristiograptus spp. and Retiolites sp. Other fauna include orthoconic nautiloids and chitinozoan chains. This graptolite assemblage indicates the guerichi Biozone (Figure 1). Both P. renaudi and M. bjerreskovae first appear in the runcinatus Subzone, the lowermost subzone of the guerichi Biozone (see Loydell, 1992, 1993). P. renaudi reaches its acme in the renaudi Subzone of that biozone (Figure 1). The order of appearance of the Telychian graptolites in this core hole, with P. renaudi and M. bjerreskovae at the base, followed by the appearance of M. ex gr. marri and S. becki, is replicated in the British succession (Loydell, 1992, 1993), and in part in the Czech Republic and Spanish successions (Štorch, 1994; Gutiérrez-Marco and Štorch, 1998).

Baq’a-4 Core Hole

The Baq’a-4 core hole was spudded on the erosional surface of the Qusaiba Member just north of the Hawban Playa (Qa’ Hawban, Figure 2a). The section is severely weathered to a depth of about 30 ft and unsuitable for palaeontological study. The upper part of the section penetrated in the Baq’a-4 overlaps with that of the lower part of the Baq’a-3. A depth of 118 ft in the Baq’a-4 approximately correlates with a depth of 508 ft in the Baq’a-3, based on graptolites and wire line character. The combined thickness of the Qusaiba Member in these core holes is about 500 ft. Fifty-four core samples from between depths of 37.1 ft and 275.3 ft in the Baq’a-4 core hole yielded graptolites (Figure 6). The graptolites identify the Telychian guerichi Biozone between 37.1 ft and 135.55 ft (Figure 6). The fauna comprises Monograptus bjerreskovae, Paradiversograptus runcinatus (Lapworth 1876)?, Petalolithus cf. altissimus (Elles and Wood 1908), Pristiograptus renaudi, Pristiograptus spp. and Stimulograptus halli (Barrande 1850)? Other fauna includes orthoconic nautiloids, scolecodonts, gastropods, ostracods and unidentifiable organic fragments. Although low-diversity, these graptolites indicate the guerichi Biozone of the earliest Telychian Stage (Figures 1, 6 and Baq’a-3 discussion). Some specimens in this interval resemble the early Telychian P. runcinatus, though it is possible that these are narrow specimens of M. bjerreskovae.

There is an interval without graptolites in the succession represented in this core hole (Figure 6) and the next graptolite-bearing horizon is at 218.55 ft. The Aeronian convolutus Biozone is indicated at this depth by possible Campograptus lobiferus (M’Coy 1850) and possible Metaclimacograptus bohemicus, together with Pristiograptus regularis sl, and Retiolites sp., but there is no firm evidence for the presence of graptolites indicative of the intervening sedgwickii Biozone sl (Figure 1). Between 268.9 ft and 275.3 ft, the convolutus Biozone is indicated by Lituigraptus convolutus, Normalograptus? aff. scalaris, Petalolithus praecursorBoucek and Přibyl 1941?; bivalves and gastropods also occur. Except for the absence of Neolagarograptus rickardsi this fauna is similar to that encountered in the Qusaiba-1 core hole.

While the assemblages as a whole are similar enough to previously described material to draw robust biostratigraphic conclusions, some of the taxa show significant differences from previously described material, and these differences are described below.

The neolagarograptid, N. rickardsi, present in the convolutus Biozone is morphologically intermediate between N. tenuis (Portlock 1843) and N. impolitus (Štorch 1998) and is redescribed herein.

Normalograptus? aff. scalaris is morphologically close to a Pseudoglyptograptus such as P. barriei (Zalasiewicz and Tunnicliff 1994), a magnus Biozone species (Figure 1), having small but distinct lappets on the thecae, especially proximally. It is an interesting intermediate link between the two taxa, with the real scalaris appearing not to have discernable lappets (Zalasiewicz, 1996, figures 2g-j). This suggests that Normalograptus is polyphyletic, some late forms of this genus having evolved from Pseudoglyptograptus, which in turn evolved from early Normalograptus, and in turn implies the acquisition and then subsequent loss of a morphological character – the proximal lappets – during evolution. There is not yet enough known about this lineage, however, to draw any biostratigraphically reliable conclusions.

Genus NeolagarograptusŠtorch 1998 

Type species:Lagarograptus helenaeŠtorch 1988 

Neolagarograptus rickardsi (El-Khayal 1987c)

Plate 1a1c; Plate 2d; Plate 4a4g 

Holotype: KSUSTQ 229, lodged at the Geology Department, King Saud University, Riyadh. Figured by El-Khayal (1987c) plate 1, figures 1, 8; from the convolutus Zone of the Qusaiba Shale, Tabuk Formation, Al-Qusaiba, Al-Qasim Province, Saudi Arabia.

Material described herein: Specimens: NHM QQ 200(1) – Plate 2d and Plate 4a, 4cf; NHM QQ 206(1) – Plate 1c and Plate 4g; NHM QQ 207(1) figured in Plate 1a, 1b and Plate 4b; additional material is lodged with Saudi Aramco in Dhahran.

Revised diagnosis: A neolagarograptid with an open dorsal curvature proximally, straightening distally. Rhabdosome widens from c. 0.4 mm proximally to c. 0.8 mm distally, and 2TRDs (two theca repeat distance sensuHowe, 1983) range between 2.5 and 3.0 mm. Sicula between 3 and 4 mm long, reaching to, at or just above the theca 2 aperture. Triangular, ventral apertural processes present on all thecae.

Zone: convolutus Biozone

Description: The longest specimen seen in this material comprises 18 thecae. The rhabdosome has an open dorsal curve proximally, and straightens distally. The sicula is approximately 3.0 mm long, reaching to, at or about the theca 2 aperture. It is slender (0.15–0.2 mm across at the base), gently curved, and possesses a curved virgella 0.2–0.4 mm long; theca 1 arises 0.6–0.8 mm above the sicula aperture. The proximal width is c. 0.4 mm, the maximum observed width of 0.5 mm being attained at theca 5 and maintained thereafter. 2TRDs vary between 2.5 and 3.0 mm, and show no obvious pattern within this range along the rhabdosome. The most complete specimen (Plate 4b) shows a slight increase in thecal spacing distally, while another specimen (Plate 4c) shows a slight decrease. The ventral thecal walls locally show evidence of a slight geniculum just distal to the aperture of the preceding theca, and then expand very slightly distally; apertural margins throughout bear distinctive processes that appear triangular in outline.

Remarks: This material can be confidently referred to N. rickardsi, as originally described from surface exposures of the Qusaiba Shale near Qusaiba by El-Khayal (1987c); this followed earlier mentions of “Lagarograptus cf. tenuis” (Portlock 1843) from the Qusaiba Shale by Rickards (1976) and Rickards et al. (1977, p. 104). Our material shows some minor differences from the type material of El-Khayal: the sicula is shorter (c. 3 mm, compared to c. 4 mm in El-Khayal’s material) and reaches to perhaps a slightly lower level on the rhabdosome; the thecal spacing is more variable; and our specimens do not reach such great widths distally (to 0.8 mm) as that described by El-Khayal (1987c), presumably because they largely comprise relatively early astogenetic stages (stages in development of colonies).

Originally assigned by El-Khayal (1987c) to Lagarograptus, this species better fits within NeolagarograptusŠtorch (1998), a genus erected following the redescription of the type species of Lagarograptus, L. inexpeditus Obut and Sobolevskaya, 1968 (in Obut et al., 1968), by Koren’ and Bjerreskov (1997). These authors showed that Lagarograptus is an earlier and morphologically distinct entity, possessing such features as genicular flanges and hoods, absent from the taxa that Štorch (1998) later included within Neolagarograptus.

Neolagarograptus rickardsi shows close similarities to both N. impolitus (from the leptotheca and convolutus biozones of Bohemia) and N. tenuis [a common species in the lower part of the sedgwickii Biozone of Britain, redescribed by Hutt (1968), and recorded also from Alaska (Churkin and Carter, 1970) and doubtfully from Russia (Obut and Sennikov, 1980)]. N. impolitus is closely similar in general outline and overall dimensions, but possesses ventral apertural processes only on the most proximal thecae rather than throughout. N. tenuis is similar in possessing ventral apertural processes throughout, but differs in possessing a more slender proximal end (0.20-0.25 mm at theca 1 cf. 0.4 mm: Plate 4i-4k, herein; it also shows a distinctive, somewhat enigmatic (perhaps taphonomically enhanced?) kink in the sicula at around the origin of theca 1 (Plate 4i–k, herein); this is not seen in our material of N. rickardsi, but there is a hint of such a feature in some of the type material figured by El-Khayal (1987c, plate 1, figures 1, 2, 8). N. rickardsi resembles the rare incomplete fragments from the convolutus Biozone of central Wales figured as Lagarograptus sp. by Zalasiewicz (1996, text-figures 3l, m), but there is insufficient material of the latter to establish their identity with certainty.

The morphology and biostratigraphic level of the Saudi Arabian material is consistent with it being an evolutionary transition, or ‘missing link’, between impolitus and tenuis. Simply following this logic would suggest either that the level in the Qusaiba Member is high in the convolutus Biozone, and/or that the Qusaiba neolagarograptid is an intermediate form transitional to evolving into tenuis before the latter migrated into other parts of the Silurian world ocean.

There are suggestions, though, of a more complex scenario. N. rickardsi is present through a considerable stratigraphic interval (103 ft; see Figure 4) of the Qusaiba Member, associated with a fauna which otherwise suggests the convolutus Biozone. Thus, it seems that N. rickardsi co-existed with N. impolitus as distinct, geographically separated entities through a substantial part of the convolutus Biozone, before the oceanographic changes associated with the transition to the sedgwickii Biozone (Loydell, 1998; and below) when N. tenuis become widespread. Further data are needed to constrain the manner in which this replacement took place.

Graptolites are water-mass specific, and like modern plankton were probably influenced by nutrient supply and water temperature (Finney and Berry, 1997; Zalasiewicz, 2001; Williams et al., 2003). Graptolite diversity may have peaked at marine-shelf margins, where upwelling deepwater currents supplied nutrient-rich waters (Finney and Berry, 1997) and where graptolites may have lived in low-oxygen conditions, perhaps to avoid predators or competition (Berry et al., 1989). Oceanwards and landwards, graptolite diversity may have been much reduced. In general, the graptolite faunas from the Qusaiba Member are of low diversity, and the abundance of Normalograptus and Pristiograptus suggest that the assemblages represent “cratonic invaders” on a deep-marine shelf setting. This is also suggested by the common preservation of the graptolites with orthoconic nautiloids, bivalves, gastropods and burrows, broadly comparable biofacies having been recorded, for instance, from the Wenlock successions of the Builth District in east-central Wales (Zalasiewicz and Williams, 1999; Williams and Zalasiewicz, 2004).

Stratigraphic Interval of the convolutus Biozone

Although it is difficult to assess the true diversity of the graptolite faunas at any one horizon, given the small amount of surface area exposed on the core slabs, the overall fauna from the convolutus Biozone is poorly diverse, with about 12 species. This contrasts with the more richly diverse graptolite faunas of Britain (e.g. Hutt, 1974, 1975; Zalasiewicz, 1996) and Bohemia (e.g. Štorch, 1994, 1998) for the same interval. Indeed, in many areas the convolutus Biozone represents the acme of diversity for Aeronian graptolites, correlating with a global sea level high (Loydell, 1998). In the Qusaiba-1 and Baq’a-4 core holes most horizons from the convolutus Biozone yield from one to three species, occasionally rising to seven species. This low diversity and the preservation of the graptolites with shelly fossils, including bivalves and gastropods, suggests a marine shelf setting. The graptolites may represent “cratonic invaders” of a shelf-marine graptolite biotope (Finney and Berry, 1997).

Stratigraphic Interval of the sedgwickii Biozone sl

The presence of guerichi Biozone assemblages overlying convolutus Biozone assemblages, separated by a non-graptolitic interval some 70 ft (c. 20 m) thick, is noteworthy. In typical deepwater Llandovery successions, this gap is occupied by the sedgwickii and halli biozones, commonly lumped together in biostratigraphic zonations. The Qusaiba Member, deposited essentially on a marine-shelf (see above), may have been prone to reverting to oxygenated, and therefore graptolite-free, conditions during regressions. It may be significant that graptolites of the sedgwickii Biozone sl have not been identified in the Llandovery successions of Bornholm, Spain, and the Girvan area, which were also essentially of a cratonic nature, while good convolutus Biozone and turriculatus Biozone sl assemblages have been described in these localities (Bjerreskov, 1975; Gutierrez-Marco and Štorch, 1998; Floyd and Williams, 2003 for 2002). Loydell (1998) interprets the sedgwickii Biozone sl as generally representing a time of regression, which would explain a reduced, or absent, fauna during this interval in cratonic environments. The sequence stratigraphy of the Qusaiba Member is certainly consistent with there being a notable regression in the Arabian Peninsula during the sedgwickii Biozone sl (Miller and Melvin, 2005; Page et al., in press).

Stratigraphic Interval of the guerichi Biozone

Graptolite faunas from the early Telychian are of very low diversity. Those characterised by S. becki from the upper part of Baq’a-3 are associated with fragments of possible carbonised plant material and may represent a marine-shelf graptolite biotope from which more open-marine, oceanic-influenced species were excluded. The assemblage characterised by P. renaudi and M. bjerreskovae, present in both Baq’a-3 and -4 core holes, typically yields one to three species at any one horizon. Retiolites sp., orthoconic nautiloids and benthic shelly faunas are often associated. These low-diversity Pristiograptus-dominated faunas also suggest a marine biotope of “cratonic invaders” and are of much lower diversity than faunas of similar age from the Czech Republic or Britain (cf. Štorch, 1994; Loydell, 1992, 1993).

The graptolites from the Qusaiba Member are very well preserved, often showing detail of fusellar banding (Plate 2a–2c; Plate 6a, 6c, 6f and 6g) and perhaps original ultrastructure. Such preservation is rare (Underwood, 1992) and is more dependent on location and level, rather than particular species or facies (Crowther, 1981), suggesting that it is governed by local taphonomic pathways. The graptolite rhabdosome is composed of structural protein, widely held to be collagen on the basis of studies of preserved ultrastructure (Towe and Urbanek, 1971; Crowther and Rickards, 1977). Although collagen is an insoluble protein, relatively inaccessible to microbial decomposition, it is biodegradable and its preservation requires favourable burial conditions excluding microorganisms and limiting the extent of peptide bond hydrolysis (Collins and Gernaey, 2001).

There is good evidence for fluctuating levels of oxygenation at the sediment-water interface, with graptolite preservation limited to the most oxygen-restricted levels. Early diagenetic pyrite framboids coat the inner surfaces of many of the graptolites (e.g. Plate 2a; Plate 6g6i) and some are preserved as steinkerns (Plate 6d, 6g, 6i, 6n and 6o). The graptolite rhabdosome may have provided a sulphate-reducing microenvironment in dysoxic ambient conditions (Underwood and Bottrell, 1994; Wignall, 1994) or a site concentrating reactive iron minerals in euxinic conditions (Berner, 1984), although the lack of disseminated pyrite suggests the latter is unlikely (Hudson, 1982). Likewise, the lack of bioturbation seen on the graptolite-bearing levels is, by definition, indicative of oxygen-restriction.

The presence of trace fossils, such as ?Chondrites, and extensive bioturbation elsewhere, points to periodically increased sediment ventilation. Burrowing can speed both aerobic and anaerobic decomposition during these intervals (Aller and Yingst, 1978; Hines and Jones, 1985), consistent with the decreased proportion of macroscopic organic matter and lighter-coloured lithofacies that characterise these intervals.

We acknowledge the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company (Saudi Aramco) for granting permission to publish this paper. We thank Wyn Hughes (Saudi Aramco Geological Technical Services Division) for a thoughtful critical review. Muhittin Senalp (Saudi Aramco Upstream Ventures Department) kindly provided the geological inset maps used in Figure 2. We are grateful to Claire Mellish (Natural History Museum) and Paul Shepherd (British Geological Survey) for assistance with curation of the Saudi specimens and access to comparative material. We thank Grenville Turner and Tony Milodowski (BGS) for scanning electron microscopy, Tim Cullen (BGS) for preparing the digital images and also Vincente Tegelan and Sana Al-Badah (Saudi Aramco Graphic Design Unit, Cartographic Imaging Division) for drafting our figures. The final design and drafting by Nestor Niño Buhay is appreciated. Finally we thank the two anonymous GeoArabia reviewers for their constructive comments and suggestions. Cambridge Earth Sciences Contribution Number ES8859.

Jan Zalasiewicz has been a Lecturer in Geology at the University of Leicester, UK, since 1994. He formerly worked for the British Geological Survey (1979-1994) as a Field Geologist and Biostratigrapher. Jan received his BSc in Geology from the University of Sheffield in 1975, and a PhD from Cambridge in 1981 (Stratigraphy and Palaeontology of the Arenig area, north Wales). Currently he is Chair of the Stratigraphy Commission of the Geological Society of London, member of the Editorial Board of the Geological Quarterly (Warsaw), and member of the International Subcommission for Stratigraphic Classification. His current research interests include graptolite biostratigraphy and taxonomy; and the deposition, diagenesis and low-grade metamorphism of mudrocks.

Mark Williams is a Lecturer in Palaeobiology at the University of Leicester, UK. He was previously a Senior Lecturer in Geosciences at the University of Portsmouth, and worked for the British Antarctic Survey and British Geological Survey as a Stratigrapher, Palaeontologist and Palaeoclimatologist between 1995 and 2005. Mark was educated at the universities of Hull and Leicester in the UK, and a post-doctoral Alexander von Humboldt Fellow at the universities of Frankfurt in Germany and Lyon in France (1991-1993), and post-doctoral fellow at the University of Leicester (1993-1995). Amongst others, his main research interests are: warm climates in Earth history as a proxy for future global climate change, high resolution biostratigraphy (particularly using graptolites, ostracods and integrated multi-fossil studies), the Cambrian radiation and arthropod evolution, Carboniferous stratigraphy and the colonization of non-marine aquatic environments, and microfossil signatures of tsunami events.

Merrell Miller is a Palynologist and the Team Leader for the Clastic Systems and Palynology Group with Saudi Aramco’s Geological Technical Services Division in Dhahran. He received a Masters degree from Ohio State University in 1976 and since then has gained experience in industrial palynology working with Texaco and Amoco. From 1978 to 1991, he worked extensively on Early Palaeozoic palynology for Amoco Research including North African and Arabian Plate biostratigraphy and source rocks. He was in Amoco’s international technical service organisation from 1992 until 1999. Merrell is a member of the AASP, The Micropalaeontological Society, CIMP, Dhahran Geoscience Society, the Paleontological Society and SEPM. His publications include palaeoenvironmental, taxonomic and biostratigraphic topics.

Alex Page is a Research Associate at the Department of Earth Sciences, University of Cambridge where he received his BA and MSc in Natural Sciences (Geological Sciences). From 2003 to 2007 Alex was a PhD student at the Department of Geology, University of Leicester. He is a member of the Palaeontological Association and Paleontological Society. His research interests include the palaeoecology and taphonomy of graptolitic mudrocks, ocean anoxia and the carbon cycle during the Early Palaeozoic.

Edward Blackett is a Director of EB Geoscience Ltd., specialising in undertaking site investigations and geotechnical design. He received his BSc in Geology from the University of Derby in 2001, and is a Fellow of the Geological Society of London. Professional work includes pile design, rock mechanics, site investigation, project engineering and reporting within rail, domestic and highways environments, working for both contractors and consultancies. Academic interests include Early Palaeozoic stratigraphy and palaeontology; undertaking research in graptolite taxonomy and biostratigraphy (based at the University of Leicester, with support from the British Geological Survey).