Biol. Lett. (2007) 3, 72–75 doi:10.1098/rsbl.2006.0557 Published online 14 November 2006 Palaeontology The tail of the Ordovician fish Sacabambaspis Alan Pradel 1 , Ivan. J. Sansom 2 , Pierre-Yves Gagnier 3 , Ricardo Cespedes 4 and Philippe Janvier 1,5, * 1 UMR 5143 du CNRS, Pale ´obiodiversite ´ et Pale ´oenvironnements, De ´partement Histoire de la Terre, CP38, Muse ´um National d’Histoire Naturelle, 8 rue Buffon, 75005 Paris, France 2 Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK 3 De ´partement des Galeries, Muse ´um National d’Histoire Naturelle, 57, rue Cuvier, 75230 Paris, Cedex 05, France 4 Museo de Historia Natural Alcide d’Orbigny, Av. Potosi 1458, Cochabamba, Bolivia 5 The Natural History Museum, London SW7 5BD, UK *Author for correspondence (janvier@mnhn.fr). The tail of the earliest known articulated fully skeletonized vertebrate, the arandaspid Saca- bambaspis from the Ordovician of Bolivia, is redescribed on the basis of further preparation of the only specimen in which it is most extensively preserved. The first, but soon discarded, recon- struction, which assumed the presence of a long horizontal notochordal lobe separating equal sized dorsal and ventral fin webs, appears to have considerable merit. Although the ventral web is significantly smaller than the dorsal one, the presence of a very long notochordal lobe bearing a small terminal web is confirmed. The discrepancy in the size of the ventral and dorsal webs rather suggests that the tail was hypocercal, a condition that would better accord with the caudal morphology of the living agnathans and the other jawless stem gnathostomes. Keywords: Vertebrata; Arandaspida; Ordovician; caudal fin 1. INTRODUCTION The anatomy of the earliest known articulated vertebrate possessing an extensive dermal skeleton, Sacabambaspis janvieri (Gagnier, Blieck & Rodrigo, 1986), from the Ordovician (Llanvirn and Caradoc) of South America, has been described in detail by Gagnier (1993a), on the basis of several specimens from the locality of Sacabambilla, Cochabamba area, Bolivia. Although the head armour, body scales and histology of this ‘ostracoderm’ (armoured jawless vertebrate) are now relatively well known (Gagnier 1993a,b; Sansom et al. 2005), the morphology of its caudal fin remains a puzzle and has been interpreted in a number of different ways. Further preparation of the only specimen that displays the caudal fin web now allows its reconstruc- tion, which lends support to Gagnier’s (1989) long debated reconstruction although with some modifi- cation, and provides clear evidence for the structure of the oldest recorded ostracoderm tail fin. 2. MATERIAL AND METHODS The material considered comes from the Ordovician (Caradoc) Anzaldo Formation of Bolivia. The articulated Sacabambaspis material from Sacabambilla consists of a number of three-dimensional specimens preserved in a very large concretion and, at least, six dorsoventrally flattened specimens preserved in a large sandstone slab. The specimens are housed in the Museo de Historia Natural Alcide d’Orbigny (MHNC), Cochabamba and (as a temporary deposit) in the Museum National d’Histoire Naturelle, Paris. The specimen MHNC 1182 (figure 1a), which displays the caudal fin, comes from the sandstone slab and has been further prepared by removing a small part of the overlying head shield of another, neighbouring, articulated specimen (MHNC 1180). The dermoskele- ton of the caudal region has been removed with dilute hydrochloric acid, an elastomer cast of the resulting external mould made, whitened with magnesium and photographed. 3. PREVIOUS INTERPRETATIONS The overall morphology of Sacabambaspis has pre- viously been reconstructed on the basis of a dozen of more or less complete articulated specimens. These show elongate, dorsally flattened and ventrally inflated head shields, and a trunk covered with elongated flank scales arranged in chevrons. Based upon a single specimen (MHNC 1182, which forms the basis for the current study), Gagnier recon- structed the tail region as having a symmetrical caudal fin web with an elongate cylindrical process emerging from the rear, originally interpreted as a horizontal notochordal lobe, analogous to that of the living coelacanth. This early reconstruction of the tail by Gagnier (1989, fig. 2) (Blieck et al. 1991, fig. 10a; Gagnier & Blieck 1992, fig. 3) still appears in some popular illustrations. Gagnier (1993a) mentioned a second specimen (MHNC 1186) that may display part of the tail, but the latter only shows a poorly informative patch of fin web. Shortly after Gagnier’s first descriptions, this interpretation of the tail was questioned (Soehn & Wilson 1990; Sansom et al. 2001), because no other fossil or living jawless vertebrate possesses a caudal fin with a long, axial, notochordal lobe, and owing to issues over the preservation of the tail region in this single specimen. The posterior extremity of the presumed notochordal lobe of the specimen MHNC 1182 was partly covered by the head shield of another Sacabambaspis specimen (MHNC 1180; figure 1a). Therefore, it was assumed that the minute square-shaped scales of the presumed notochordal lobe were in fact not part of the tail, but merely the impression of either an isolated epibranchial plate (figure 1g) or the shield margin of another, under- lying, specimen. Subsequent reconstructions of Sacabambaspis thus show (as dashed lines) a leaf shaped, isocercal caudal fin, ending with an incom- plete axial lobe (Gagnier 1992, fig. 4, 1993a, fig. 4; Janvier 1996, figs 1.1, 4.2b(i)). 4. DESCRIPTION Considering the importance of this unique source of information about the structure of the tail in Ordovi- cian vertebrates, since no other caudal fin is known to date in Ordovician fully skeletonized vertebrates, we decided to further prepare this specimen at the cost of the destruction of a small part of the overlying head shield MHNC 1180 that hid it. As described by Gagnier (1993a,b) and reiterated here, the body scales of MHNC 1182 (figure 1a) are exposed in ventral view and pass progressively to large patches of minute, elongated scales arranged in rows, which clearly indicate the presence of caudal fin webs Received 1 August 2006 Accepted 2 October 2006 72 This journal is q 2006 The Royal Society