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Palaeontology, 2010, Pp PALA 1019 Dispatch: 15.10.10 Journal: PALA CE: Archana Journal Name Manuscript No. B Author Received: No. of pages: 17 PE: Raymond [Palaeontology, 2010, pp. 1–17] 1 2 3 TAPHONOMY AND AFFINITY OF AN ENIGMATIC 4 5 SILURIAN VERTEBRATE, JAMOYTIUS KERWOODI 6 7 WHITE 8 9 by ROBERT S. SANSOM*, KIM FREEDMAN* , SARAH E. GABBOTT*, 10 RICHARD J. ALDRIDGE* and MARK A. PURNELL* 11 *Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK; e-mails [email protected], [email protected], [email protected], 12 [email protected] 13 6 Wescott Road, Wokingham, Berkshire RG40 2ES, UK; e-mail [email protected] 14 Typescript received 14 July 2009; accepted in revised form 23 April 2010 15 16 17 Abstract: The anatomy and affinities of Jamoytius kerwoodi pairs of branchial openings, optic capsules, a circular, subter- 18 White have long been controversial, because its complex minal mouth and a single terminal nasal opening. Interpreta- 19 taphonomy makes unequivocal interpretation impossible tions of paired ‘appendages’ remain equivocal. Phylogenetic 20 with the methodology used in previous studies. Topological analysis places Jamoytius and Euphanerops together (Jamoytii- 21 analysis, model reconstruction and elemental analysis, fol- formes), as stem-gnathostomes rather than lamprey related 22 lowed by anatomical interpretation, allow features to be or sister taxon to Anaspida. 23 identified more rigorously and support the hypothesis that 24 Jamoytius is a jawless vertebrate. The preserved features of Key words: Jamoytius, Euphanerops, phylogeny, taphonomy, 25 Jamoytius include W-shaped phosphatic scales, 10 or more Vertebrata, Gnathostomata, Silurian. 26 27 28 29 W hite (1946) first described Jamoytius kerwoodi on the of the in-group taxa included in the phylogenetic investi- 30 basis of two specimens from a Lower Silurian (Llando- gation has affected the placement of Jamoytius (Donoghue 31 very) horizon of the Lesmahagow inlier of Lanarkshire, et al. 2000; Donoghue and Smith 2001; Gess et al. 2006). 32 Scotland, and considered it to be the most primitive The position of Jamoytius on cladograms has conse- 33 known vertebrate. Numerous subsequent authors have quently not stabilized, though Jamoytius usually appears 34 disputed this conclusion or have disagreed with aspects of as a sister taxon to the lampreys, the anaspids, or Eupha- 35 White’s (1946) interpretation (Data S1). Evidence from nerops longaevus Woodward, 1900. 36 additional specimens collected at Lesmahagow localities Janvier and Lund (1983, p. 412) wrote, ‘These various 2 37 (see Ritchie 1968, 1985 for synopses of locality and stra- interpretations cause one to wonder about the degree of 38 tigraphy details) prompted Ritchie (1960, 1963, 1968, imagination involved in the study of Jamoytius and other 39 1984) to redescribe Jamoytius as an ‘unspecialized anas- fossils preserved as tarry impressions.’ So why has it 40 pid’, possibly related to the extant jawless vertebrates. As proved so difficult to produce a definitive interpretation 41 shown by Plate 1, most of Ritchie’s (1960, 1968) interpre- of Jamoytius and to determine its affinities? The principal 42 tations of the anatomical features of Jamoytius differ from problem is one that affects interpretation of many prob- 43 those of White (1946). lematic fossils – disentangling the different aspects of the 44 Despite Ritchie’s treatment, Jamoytius continued to process of anatomical reconstruction. The selection of an 45 generate debate. Various authors challenged Ritchie’s appropriate anatomical comparator upon which to base 46 work (e.g. Janvier 1981; Forey and Gardiner 1981) or sug- hypotheses of homology (comparisons being drawn either 47 gested affinities with a range of subsequently discovered directly to a specific extant organism or clade, or indi- 48 fossils (e.g. Janvier and Busch 1984; Briggs and Clarkson rectly through a fossil intermediate) can be especially 49 1987). Cladistic analyses of the jawless vertebrates (e.g. problematic (Donoghue and Purnell 2009); the choice of 50 Janvier 1981, 1996a, b; Forey 1984, 1995; Forey and Jan- interpretative model needs careful justification on the 51 vier 1993) have also failed to clarify the affinities of basis of characters present, preferably unequivocally, in 52 Jamoytius; the lack of agreement regarding its anatomical the fossils. In the case of Jamoytius, almost all workers 53 homologies has meant that different analyses have used have considered it as a jawless vertebrate without explicit 54 different character codings. In addition to coding, choice justification, and consequently, anatomical interpretations, ª The Palaeontological Association doi: 10.1111/j.1475-4983.2010.01019.x 1 2 PALAEONTOLOGY 1 homology statements and phylogenetic analyses are all at phylogenetic analyses were employed to investigate the 2 risk of circularity. Jamoytius is generally preserved in two placement of the organism in an evolutionary context. 3 dimensions, and investigations have often concentrated 4 on relating the shape of its features to the three-dimen- Elemental analysis. Topological data were complemented 5 sional anatomical parts of presumably related organisms. by determination of the chemistry and preservation of 6 Different workers have alternatively described the same particular features to provide evidence of original histol- 7 circular features in Jamoytius as mouth, eyes and nasal ogy and ⁄ or composition (e.g. Butterfield 2002; Gabbott 8 structures (Pl. 1 and Data S1). These interpretations are et al. 2004). Elemental mapping of some features of Ja- 9 thus inherently equivocal. Anatomical and phylogenetic moytius was performed on a LEO 435 VP Scanning Elec- 10 claims, and counterclaims, will continue to be insecure tron Microscope with an Oxford Instruments ISIS 300 11 without further information about the topology, compo- EDX spectrometer operating in variable pressure mode at 12 sition and taphonomic history of anatomical features, 10 Pascals with an accelerating voltage of 10 kV and a 13 combined with explicit articulation of the methodology beam current of 500 picoamps for 1000 frames (approxi- 14 used to reach anatomical interpretations. mately 14 h of run time). The specimens analysed in this 15 study have not been subjected to hydrofluoric acid treat- 16 ment (Ritchie 1963, 1968); smaller specimens were 17 MATERIALS AND METHODS selected because of SEM chamber size limits. 18 19 Topological reconstruction and comparative anatomy. In Phylogenetic analysis. Data matrices were constructed in 20 order to address the problems of circularity and equivo- MacClade 4.06 (Maddison and Maddison 2003). Heuristic 21 cal interpretations, a stepwise methodology separating searches were performed using PAUP 4.0 (Swofford 2002) 22 topological considerations from anatomical interpreta- with 1000 random sequence addition replicates and TBR 23 tion was applied as advocated by Donoghue and Purnell (Tree bisection and reconnection) branch swapping. 24 (2009). First, the features of the fossils were identified Where appropriate, characters were reweighted according 25 and described (i.e. number and shape of distinct body to their rescaled consistency indices. To investigate the 26 parts, and the topological relationship between those alternative topologies that satisfy phylogenetic relation- 27 body parts). By comparing specimens preserved in dif- ships proposed on the basis of molecular data, heuristic 28 ferent orientations, three-dimensional reconstruction was searches were conducted using backbone constraint trees 29 possible (cf. Briggs and Williams 1981; Purnell and constructed in MacClade 4.06 (see below). 30 Donoghue 1999). Throughout this process of interpreta- 31 tion and reconstruction, no assumptions were made Institutional abbreviations and publicly held material. NHM, Nat- 32 about the affinities of the organism or the homology of ural History Museum, London, P11284 (holotype), P11285, 33 its features. P47784-7; AMS, Australian Museum, Sydney, F64401, F102841- 34 Following topological description, an explicitly justified 6; NMS National Museum of Scotland, Edinburgh, 1959.1, 35 interpretative model was selected upon which to base 1966.3.1-3; BGS, British Geological Survey, Keyworth, 11882-3; Hunterian Museum, Glasgow, V.7792, V.8036V.8141, V.8148, 36 anatomical hypotheses. Putative homologies were identi- GLHAM101382; UOE, University of Edinburgh, FR1628, 37 fied through the consideration of topological relationships FR1476, 20129-32, 20145, 20159-62. 38 between body parts (Rieppel and Kearney 2002) and were 39 informed by evolutionary and potential taphonomic 40 transformational sequences (i.e. assessing whether the BODY PARTS, TOPOLOGICAL 41 appearance of a feature, or its absence, represents the ori- ANALYSIS AND RECONSTRUCTION 42 ginal anatomical condition, or the results of post-mortem 43 processes of decay and preservation). The intrinsic Body shape 44 properties and composition of the body parts provided 45 additional constraints on interpretations. In general aspect, Jamoytius has an elongate lozenge- 46 Following translation of topological structures into shaped body exhibiting a size range of 140–180 by 30– 47 anatomical interpretations, taxonomic assessments and 48 49 50 51 EXPLANATION OF PLATE 1 52 Jamoytius kerwoodi White holotype (NHM P11284a) immersed in 90 per cent ethanol with incident polarized light and filter, 53 illustrating the conflicting interpretations of White (1946), in bold, and Ritchie (1960, 1963, 1968, 1984) in plain text. Scale bar
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