[Palaeontology, Vol. 52, Part 2, 2009, pp. 337–342] ORDOVICIAN FISH FROM THE ARABIAN PENINSULA by IVAN J. SANSOM*, C. GILES MILLER , ALAN HEWARDà,–, NEIL S. DAVIES*,**, GRAHAM A. BOOTHà, RICHARD A. FORTEY and FLORENTIN PARIS§ *Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK; e-mail: [email protected] Department of Palaeontology, The Natural History Museum, London SW7 5BD, UK; e-mails: [email protected] and [email protected] àPetroleum Development Oman, Muscat, Oman; e-mail: [email protected] §Ge´osciences, Universite´ de Rennes, 35042 Rennes, France; e-mail: fl[email protected] –Present address: Petrogas E&P, Muscat, Oman; e-mail: [email protected] **Present address: Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada; e-mail: [email protected] Typescript received 25 February 2008; accepted in revised form 19 May 2008 Abstract: Over the past three decades Ordovician pteras- morphs from the Arabian margin of Gondwana. These are pidomorphs (armoured jawless fish) have been recorded among the oldest arandaspids known, and greatly extend from the fringes of the Gondwana palaeocontinent, in par- the palaeogeographical distribution of the clade around the ticular Australia and South America. These occurrences are periGondwanan margin. Their occurrence within a very dominated by arandaspid agnathans, the oldest known narrow, nearshore ecological niche suggests that similar group of vertebrates with extensive biomineralisation of Middle Ordovician palaeoenvironmental settings should be the dermoskeleton. Here we describe specimens of arandas- targeted for further sampling. pid agnathans, referable to the genus Sacabambaspis Gagnier, Blieck and Rodrigo, from the Ordovician of Key words: Ordovician, pteraspidomorphs, Gondwana pal- Oman, which represent the earliest record of pteraspido- aeocontinent, Sacabambaspis, Oman. Ordovician pteraspidomorphs from Gondwana are referable to Sacabambaspis from the Amdeh Formation rare. They have a sporadic appearance, have hitherto of Oman, greatly extending the palaeogeographical dis- been described exclusively from Australia and South tribution of the genus around the margins of Gond- America, principally from four basins with a strati- wana and indicating a potential for future discoveries graphical range from the Floian through to the Sand- in intervening sedimentary sequences of similar age and bian, and are almost exclusively from the family environmental setting. Arandaspididae. Ritchie and Gilbert-Tomlinson (1977) were the first to describe arandaspids based upon col- lections from the Amadeus Basin in central Australia, STRATIGRAPHY OF THE AMDEH with subsequent work by Ritchie (1985) and Young FORMATION (1997) extending the taxonomic and stratigraphical range of pteraspidomorphs within the Larapinta Group. The arandaspid material described here comes from a The description of allied taxa in Bolivia (Gagnier et al. similar stratigraphical level at two different locations 1986), the Precordillera of Argentina (Albanesi et al. (Text-fig. 1) within the Amdeh Formation, S of Muscat, 1995) and the Cordillera Oriental also of Argentina (Al- Oman. The oldest material comes from Wadi Daiqa from banesi and Astini 2002) greatly expanded their known a coarse-grained, angular sandstone within a sequence of geographical range. The South American discoveries sandstones and shales probably of the Am4 Member. opened up a wealth of information on the genus Sac- Additional specimens have also been recovered from fine- abambaspis, which has led to a number of papers grained crinoidal calcarenites from the Am4–5 members at focussing specifically on the palaeobiology of this taxon Wadi Qahza. (for example see Gagnier 1993a, b; Sansom et al. 2005; The stratigraphy of the Amdeh Formation is, as yet, Pradel et al. 2007). It has now become the text-book poorly constrained with reliable dates only from the example of a primitive armoured agnathan and has an upper two members of the 3400 m thick sequence (Text- extremely important role in our understanding of early fig. 2). The lithostratigraphy of the Amdeh Formation vertebrate evolution. We herein report new material was initially established by Lovelock et al. (1981) and has ª The Palaeontological Association doi: 10.1111/j.1475-4983.2009.00846.x 337 338 PALAEONTOLOGY, VOLUME 52 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). subsequently been modified during the regional geological Wadi al Janhi and Wadi al Mayh, near the base of the mapping of Le Me´tour et al. (1986). Although these are Upper Siltstone Member (Am5) are suggestive of a Dapin- in substantial agreement, there are some subtle, but gian age (F. Paris, unpublished data), whilst the sparse important, differences between the two with respect to acritarch assemblage with Arkonia reported by Lovelock the boundary between the Upper Quartzite Member and et al. (1981) suggests, on reinterpretation, an age not the Upper Siltstone Member (sensu Lovelock et al. 1981) older than Darriwilian but the preservation of both and Am4 and Am5 (sensu Le Me´tour et al. 1986). How- chitinozoans and acritarchs from this horizon prevents a ever, the pteraspidomorph material described here can be firm biostratigraphical assignment to this part of the closely tied into dates based upon palynological and trilo- sequence. Le Me´tour et al. (1986) and Villey et al. (1986) bite studies. place this sequence at Wadi Qahza higher in Am5 than An uppermost Dapingian? to early Darriwilian age for we consider here. the probable Am4 Wadi Daiqa (Dayqah) pteraspido- Identification of the trilobite Neseuretus tristani from morph specimens is derived from the co-occurrence of Am5 in the Wadi Sarin area suggests close correlation the chitinozoans Lagenochitina obeligis, Laufeldochitina ba- with at least some part of the Hanadir Shale of Saudi culiformis and Belonechitina gr. micracantha (F. Paris, Arabia, which has been tied into the Llanvirnian sections unpublished data), the close similarity of the recovered in Iberia and elsewhere in southern Europe (Fortey and acritarch assemblage to the assemblage VK2 (hirundo Morris 1982; El-Khayal and Romano 1985) and which graptolite zone) reported by Quintavalle et al. (2000), and also supports an assignment to the Darriwilian (Lovelock the absence of taxa characteristic of the middle and upper et al. 1981; R. A. Fortey, unpublished data). These out- Darriwilian (G. A. Booth pers. obs.). Le Me´tour et al. crops were considered Middle Shale Member by Lovelock (1986) map the rocks of this inlier as Am5, yet the sedi- et al. (1981) but were mapped as Am5 by Le Me´tour mentary facies and lithologies are more typical of the et al. (1986) and have the sedimentary facies and litholo- sandier Am4. gies characteristic of the latter unit elsewhere. The chitinozoa Belonechitina gr. micracantha and Euc- Am4 is the probable equivalent of the Ghudun Forma- onochitina cf. vulgaris from Wadi Qahza, a tributary of tion in the subsurface of interior Oman and, in part, the SANSOM ET AL.: ORDOVICIAN FISH FROM THE ARABIAN PENINSULA 339 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 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 dissolu- tion 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). 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 Porophora- spis (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 associa- tion with the trilobites Neseuretus and Ogyginus, the bival- ved mollusc Redonia, crinoid remains and orthoconic nautiloids, are indicative of nearshore conditions (Lovelock et al. 1981; Fortey and Morris 1982). This inter- TEXT-FIG. 2. A stratigraphical log of the Amdeh Formation