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Keywords:

  • Ordovician;
  • pteraspidomorphs;
  • Gondwana palaeocontinent;
  • Sacabambaspis;
  • Oman

Abstract

  1. Top of page
  2. Abstract
  3. Stratigraphy of the Amdeh Formation
  4. The fish from the Amdeh Formation
  5. Palaeogeographical conclusions
  6. Acknowledgments
  7. References

Abstract:  Over the past three decades Ordovician pteraspidomorphs (armoured jawless fish) have been recorded from the fringes of the Gondwana palaeocontinent, in particular Australia and South America. These occurrences are dominated by arandaspid agnathans, the oldest known group of vertebrates with extensive biomineralisation of the dermoskeleton. Here we describe specimens of arandaspid agnathans, referable to the genus Sacabambaspis Gagnier, Blieck and Rodrigo, from the Ordovician of Oman, which represent the earliest record of pteraspidomorphs from the Arabian margin of Gondwana. These are among the oldest arandaspids known, and greatly extend the palaeogeographical distribution of the clade around the periGondwanan margin. Their occurrence within a very narrow, nearshore ecological niche suggests that similar Middle Ordovician palaeoenvironmental settings should be targeted for further sampling.

Ordovician pteraspidomorphs from Gondwana are rare. They have a sporadic appearance, have hitherto been described exclusively from Australia and South America, principally from four basins with a stratigraphical range from the Floian through to the Sandbian, and are almost exclusively from the family Arandaspididae. Ritchie and Gilbert-Tomlinson (1977) were the first to describe arandaspids based upon collections from the Amadeus Basin in central Australia, with subsequent work by Ritchie (1985) and Young (1997) extending the taxonomic and stratigraphical range of pteraspidomorphs within the Larapinta Group. The description of allied taxa in Bolivia (Gagnier et al. 1986), the Precordillera of Argentina (Albanesi et al. 1995) and the Cordillera Oriental also of Argentina (Albanesi and Astini 2002) greatly expanded their known geographical range. The South American discoveries opened up a wealth of information on the genus Sacabambaspis, which has led to a number of papers focussing specifically on the palaeobiology of this taxon (for example see Gagnier 1993a, b; Sansom et al. 2005; Pradel et al. 2007). It has now become the text-book example of a primitive armoured agnathan and has an extremely important role in our understanding of early vertebrate evolution. We herein report new material referable to Sacabambaspis from the Amdeh Formation of Oman, greatly extending the palaeogeographical distribution of the genus around the margins of Gondwana and indicating a potential for future discoveries in intervening sedimentary sequences of similar age and environmental setting.

Stratigraphy of the Amdeh Formation

  1. Top of page
  2. Abstract
  3. Stratigraphy of the Amdeh Formation
  4. The fish from the Amdeh Formation
  5. Palaeogeographical conclusions
  6. Acknowledgments
  7. References

The arandaspid material described here comes from a similar stratigraphical level at two different locations (Text-fig. 1) within the Amdeh Formation, S of Muscat, Oman. The oldest material comes from Wadi Daiqa from a coarse-grained, angular sandstone within a sequence of sandstones and shales probably of the Am4 Member. Additional specimens have also been recovered from fine-grained crinoidal calcarenites from the Am4–5 members at Wadi Qahza.

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Figure TEXT-FIG. 1..  Outcrop map of the Amdeh Formation in the Saih Hatat region of northern Oman, showing the position of the fish bearing localities at Wadi Qahza and Wadi Daiqa (modified after Lovelock et al. 1981).

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The stratigraphy of the Amdeh Formation is, as yet, poorly constrained with reliable dates only from the upper two members of the 3400 m thick sequence (Text-fig. 2). The lithostratigraphy of the Amdeh Formation was initially established by Lovelock et al. (1981) and has subsequently been modified during the regional geological mapping of Le Métour et al. (1986). Although these are in substantial agreement, there are some subtle, but important, differences between the two with respect to the boundary between the Upper Quartzite Member and the Upper Siltstone Member (sensu Lovelock et al. 1981) and Am4 and Am5 (sensu Le Métour et al. 1986). However, the pteraspidomorph material described here can be closely tied into dates based upon palynological and trilobite studies.

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Figure TEXT-FIG. 2..  A stratigraphical log of the Amdeh Formation showing the lithostratigraphical schemes of Lovelock et al. (1981) and Le Métour et al. (1986), the positions of the pteraspidomorph finds and the biostratigraphical control using palynomorphs and trilobite faunas.

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An uppermost Dapingian? to early Darriwilian age for the probable Am4 Wadi Daiqa (Dayqah) pteraspidomorph specimens is derived from the co-occurrence of the chitinozoans Lagenochitina obeligis, Laufeldochitina baculiformis and Belonechitina gr. micracantha (F. Paris, unpublished data), the close similarity of the recovered acritarch assemblage to the assemblage VK2 (hirundo graptolite zone) reported by Quintavalle et al. (2000), and the absence of taxa characteristic of the middle and upper Darriwilian (G. A. Booth pers. obs.). Le Métour et al. (1986) map the rocks of this inlier as Am5, yet the sedimentary facies and lithologies are more typical of the sandier Am4.

The chitinozoa Belonechitina gr. micracantha and Euconochitina cf. vulgaris from Wadi Qahza, a tributary of Wadi al Janhi and Wadi al Mayh, near the base of the Upper Siltstone Member (Am5) are suggestive of a Dapingian age (F. Paris, unpublished data), whilst the sparse acritarch assemblage with Arkonia reported by Lovelock et al. (1981) suggests, on reinterpretation, an age not older than Darriwilian but the preservation of both chitinozoans and acritarchs from this horizon prevents a firm biostratigraphical assignment to this part of the sequence. Le Métour et al. (1986) and Villey et al. (1986) place this sequence at Wadi Qahza higher in Am5 than we consider here.

Identification of the trilobite Neseuretus tristani from Am5 in the Wadi Sarin area suggests close correlation with at least some part of the Hanadir Shale of Saudi Arabia, which has been tied into the Llanvirnian sections in Iberia and elsewhere in southern Europe (Fortey and Morris 1982; El-Khayal and Romano 1985) and which also supports an assignment to the Darriwilian (Lovelock et al. 1981; R. A. Fortey, unpublished data). These outcrops were considered Middle Shale Member by Lovelock et al. (1981) but were mapped as Am5 by Le Métour et al. (1986) and have the sedimentary facies and lithologies characteristic of the latter unit elsewhere.

Am4 is the probable equivalent of the Ghudun Formation in the subsurface of interior Oman and, in part, the Am5 of the Saih Nihayda Formation. This is a revision to the correlation of Droste (1997), as the trilobite-bearing shell beds of the Wadi Sarin area being considered here to be Am5 rather than Am3.

The fish from the Amdeh Formation

  1. Top of page
  2. Abstract
  3. Stratigraphy of the Amdeh Formation
  4. The fish from the Amdeh Formation
  5. Palaeogeographical conclusions
  6. Acknowledgments
  7. References

At both Wadi Qahza and Wadi Daiqa, fragments of arandaspid headshield and broken scales have been recovered. Bulk sampling has yielded microremains from the fine-grained crinoidal calcarenites within the Am4–5 members at Wadi Qahza via acetic acid dissolution and a combination of hydrogen peroxide and freeze/thaw disaggregation. Larger specimens, readily identifiable in hand specimen, come from the Wadi Daiqa locality.

The recovered specimens show the typical ‘oak-leaf’ shaped ornament characteristic of the dermal armour of arandaspid fish (Text-fig. 3) and are considered to be fragments of Sacabambaspis. The specimens collected to date are indistinguishable in morphology from those illustrated by Gagnier (1993a), particularly in the flank scales illustrated by Gagnier (1993a, pl. 8C) and here in Text-figure 3B. Histologically, the dermal armour of Sacabambaspis is composed of a thin glassy layer of enameloid covering a dentine ridge, which surmounts an acellular bony base (Sansom et al. 2005).

image

Figure TEXT-FIG. 3..  A–G, specimens of Sacabambaspis sp. from the Amdeh Formation, Ordovician, Saih Hatat region, Oman. A, B, D, fragments of headshield (A, D) and flank scale (B) from probable Am4 Member, Dapingian? – early Darriwilian, Middle Ordovician, Wadi Daiqa, magnification ×10, ×14 and ×10 respectively. C, E–G, from Am4–5, Dapingian?–Darriwilian, Middle Ordovician, Wadi Qahza; magnification ×90, ×82, ×84 and ×165 respectively. Specimens are housed at the Natural History Museum, Muscat, Oman, accession numbers: A, ONHM3654; B, ONHM3655; C, ONHM3656; D, ONHM3657; E, ONHM3658; F, ONHM3659; G, ONHM3660.

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Although the nature of the material from which the dates have been derived lacks high resolution, the earliest Amdeh specimens are at least contemporaneous with the oldest occurrence of Sacabambaspis (known from the Darriwilian and Darriwilian/Sandbian of Australia and South America respectively). The Omani material is only predated by the poorly understood arandaspid Porophoraspis (basal Floian) in the Gondwanan pteraspidomorph record (Young 1997).

The sediments of the Amdeh Formation represent shallow water deposition, within a mixed Skolithos-Cruziana ichnofacies containing trace fossils of both suspension- and deposit-feeding tracemakers (Cruziana furcifera, C. rugosa, Daedalus, Phycodes, Planolites, Rusophycus, Skolithos linearis, Teichichnus) and, in association with the trilobites Neseuretus and Ogyginus, the bivalved mollusc Redonia, crinoid remains and orthoconic nautiloids, are indicative of nearshore conditions (Lovelock et al. 1981; Fortey and Morris 1982). This interpretation accords with the environment of deposition of the other Ordovician Gondwanan pteraspidomorph occurrences including the Anzaldo Formation (Bolivia, yields Sacabambaspis janvieri; Davies et al. 2007), the Trapiche Group (Argentina, yields Sacabambaspis sp.; Albanesi et al. 1995; Albanesi and Astini 2002), and the Stairway Sandstone Formation (Australia, yields Arandaspis prionotolepis together with other arandaspids; Wells et al. 1970). It appears that the arandaspids occupied an almost exclusive nearshore niche throughout their spatial and temporal range.

Palaeogeographical conclusions

  1. Top of page
  2. Abstract
  3. Stratigraphy of the Amdeh Formation
  4. The fish from the Amdeh Formation
  5. Palaeogeographical conclusions
  6. Acknowledgments
  7. References

The distribution of pteraspidomorphs during the Ordovician has led to two models for the palaeogeographical distribution of Lower Palaeozoic vertebrates. The first of these can be summarised as an ‘out-of-Gondwana’ model (Elliott et al. 1991), wherein pteraspidomorphs are postulated to have an origin in Australia, migrated through South America and then crossed an oceanic barrier to reach the Laurentian palaeocontinent, diversifying into the astraspids and eriptychiids of the Harding Sandstone Formation of Colorado, USA along the way. This model has been criticised as a somewhat literal reading of the stratigraphical distribution of Ordovician pteraspidomorphs, although more recent work has also seen a strong stratigraphical, rather than phylogenetic, bias to a review of Ordovician vertebrates (Turner et al. 2004). Gondwanan-centred models have been questioned by other discoveries, most critically the extension of the vertebrate record into the Cambrian to include conodonts (Aldridge et al. 1993; Donoghue et al. 2000), Anatolepis (Smith et al. 1996) and the naked agnathans from the Chengjiang Lagerstätte (Shu et al. 1999).

These new data from Oman indicate that arandaspids are a truly peri-Gondwanan group (Text-fig. 4), and lends considerable support to an alternative hypothesis that dispersal and vicariance both had roles to play in the distribution of Lower Palaeozoic vertebrates, largely influenced by their bodyplans (Smith et al. 2002). The heavily armoured arandaspids and astraspids, endemic to Gondwana and Laurentia respectively, would have had a low dispersal potential, especially across oceanic barriers, as they lacked a significantly mineralised backbone, whereas the microsquamous forms, such as thelodonts and chondrichthyans, rapidly became cosmopolitan after their first appearance in the fossil record (Smith et al. 2002). Although taphonomic (Young et al. 2007), sequence stratigraphical (Smith et al. 2001; Kidwell and Holland 2002) and sampling biases all have a profound influence on the Ordovician pteraspidomorph record, given their presence in siliciclastic nearshore sequences perched on cratonic interiors, the presence of arandaspids in Oman indicates considerable potential for further discoveries from strata representing shallow water deposition, particularly with the ichnofacies and faunal associations represented in the Amdeh, Stairway and Anzaldo Formations, in periGondwanan Ordovician strata and especially in the hitherto unexplored shallow marine siliciclastics that dominate this stratigraphical interval in the rest of the Arabian Peninsula.

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Figure TEXT-FIG. 4..  Palaeogeographical distribution of arandaspids around the margins of the Gondwana supercontinent. Locality 1, Anzaldo Formation, Bolivia (Gagnier et al. 1986); 2, Sepulturas Formation, Argentina (Albanesi and Astini 2002); 3, Trapiche Group, Argentina (Albanesi et al. 1995); 4, Larapinta Group, Australia (Ritchie and Gilbert-Tomlinson 1977; Young 1997); 5, Amdeh Formation, Oman. Palaeogeographical reconstruction based upon Veevers (2004).

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Acknowledgments

  1. Top of page
  2. Abstract
  3. Stratigraphy of the Amdeh Formation
  4. The fish from the Amdeh Formation
  5. Palaeogeographical conclusions
  6. Acknowledgments
  7. References

Acknowledgments.  IJS and NSD were supported by Natural Environment Research Council Grant Ref NE/B503576/1. Philippe Janvier, Pierre- Yves Gagnier, Guillermo Albanesi and Alex Ritchie are thanked for access to arandaspid specimens in their collections and ongoing discussions over the palaeobiology of these primitive fish. Two anonymous reviewers are thanked for their comments and advice on further developing this manuscript. Special acknowledgement is made to The Ministry of Natural Heritage and Culture, The Ministry of Oil, Gas and Petroleum Development Oman for their permission to publish this paper.

References

  1. Top of page
  2. Abstract
  3. Stratigraphy of the Amdeh Formation
  4. The fish from the Amdeh Formation
  5. Palaeogeographical conclusions
  6. Acknowledgments
  7. References
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