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Orientation and Anatomical Notation in Conodonts

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Orientation and Anatomical Notation in Conodonts J. Paleont., 74(1), 2000, pp. 113±122 Copyright q 2000, The Paleontological Society 0022-3360/00/0074-0122$03.00 ORIENTATION AND ANATOMICAL NOTATION IN CONODONTS MARK A. PURNELL, PHILIP C. J. DONOGHUE,* AND RICHARD J. ALDRIDGE Department of Geology, University of Leicester, Leicester LE1 7RH, U.K. ,[email protected]., ,[email protected]. ABSTRACTÐAll aspects of conodont paleontology rely on the identi®cation and description of homologous anatomical units or elements. But the current schemes of anatomical notation and terms for orientation were formulated at a time when little was known of conodont anatomy or skeletal architecture, resulting in some confusion and dif®culties in their application. With improving knowledge of cono- donts, these problems are becoming increasingly acute. In an attempt to address current problems, we introduce new terms for orientation in conodonts and their elements, and a modi®ed scheme of anatomical notation. The principal axes of the conodont body are identi®ed as rostrocaudal, dorsoventral, and mediolateral, with opposite lateral sides designated dextral and sinistral. Anatomical notation is de®ned according to topological relationships between elements with reference to the principal axes of the body and takes the form of letters with numeric subscripts (e.g., P1,P2,S0-S4). The ozarkodinid apparatus serves as a standard, but the Pn-Sn scheme can be applied rigorously to all taxa that are known from natural assemblages or where an hypothesis of topological homology can be inferred from secondary morphological criteria. INTRODUCTION homology as an hypothesis of similarity that is based on topo- LL ASPECTS of conodont paleontology rely ultimately on the logical relations and which contains potential phylogenetic in- A description of elements, and this requires a means of iden- formation (see Rieppel, 1994 for discussion). Topology refers to tifying direction and communicating the relative disposition of the numbers of, and the relative spatial relationships between, morphological features. Equally important is a scheme for iden- recognizable anatomical units. Similarity, in this context, does tifying the various elements of the conodont apparatus. Such not mean morphological similarity of the units under consider- schemes allow the formulation and testing of hypotheses of ho- ation. We should also point out that homology is hierarchical, mology, the development of multielement taxonomy, and the operating at different levels including genes, development, and analysis of phylogenies and evolutionary patterns. But the cur- structure. It is also contextual as illustrated by the classic ex- rent systems of terminology for orientation and anatomical no- ample of batwings and birdwings; these structures are homolo- gous as tetrapod forelimbs but not as wings. In this paper we tation were formulated when little was known about conodont deal only with homology of whole elements within the conodont anatomy and biology. With the discovery and interpretation of apparatus (not homology of the apparatus, nor homology of parts conodonts with preserved soft-tissues (e.g., Aldridge et al., of elements). 1993; Gabbott et al., 1995), we now have direct evidence of the orientation of the feeding apparatus within animals of different CURRENT PROBLEMS taxa. From detailed analysis of natural assemblages, the archi- Dif®culties with conventions of orientation.ÐThe basic terms tecture of the apparatus in ozarkodinids, prioniodontids, and of orientation currently applied to conodont elements include prioniodinids is now known in some detail (Aldridge et al., anterior, posterior, oral, aboral, upper, lower, inner, outer, left, 1987, 1995; Purnell and Donoghue, 1997, 1998; Repetski et al., right, sinistral, and dextral. These terms have universal biolog- 1998; Purnell and von Bitter, 1996). These new data have shown ical meanings, but as applied to conodont elements they have that several historical assumptions were in error, and continuing distinct and different meanings. The orientation of conodont el- traditional usage of terms is misleading for comparisons within ements is currently determined according to arbitrary morpho- the conodonts, and between conodonts and other animals. In this logical criteria, based on a datum de®ned as ``an imaginary sur- contribution we propose a new terminology for element orien- face that includes the apexes of the cusp and basal cavity'' tation and anatomical notation based on the biological infor- (Sweet, 1981a, p. W6). In most cases the concave side of the mation now known. cusp within this plane is ``posterior,'' the tip of the cavity is Although this paper is aimed speci®cally at current problems ``up,'' and the upper margin of the element's base or posterior in conodonts, we cannot avoid a brief discussion of general ter- process indicates ``horizontal'' (for more precise de®nitions see minology. Conodonts are not unique in having evolved a com- Sweet, 1981a, p. W6-W9). This convention has a number of plex and sometimes contradictory terminology. Aspects of ana- potential problems (see, e.g., MuÈller, 1956 for discussion), but tomical nomenclature in birds, for example, have been some- until now the scheme has persisted without causing serious dif- what confused by different authors applying different names to ®culty. This is primarily because terms can generally be applied the same structure, or the same name to different structures. consistently; because comparisons have been made only be- Rowe (1986) discussed this problem in detail. His review of the tween different conodont elements, not between conodonts and literature on avian anatomical nomenclature concluded that an- other organisms; and because there has been little need to de- atomical terminology should be based on homology and that scribe the orientation of elements in life. However, conodonts prevailing terms founded on inaccurate anatomy or incorrect hy- no longer exist as a discrete group outside comparative biology, potheses of homology should be replaced (Rowe, 1986, p. 343, and with increasing knowledge of conodont biology the prob- and references therein). But what is homology? The meaning of lems associated with conventional arbitrary terminology have homology is the subject of continuing debate (see, e.g., Hall, become more acute. Purnell and Donoghue (1998, p. 89) re- 1994; Tautz, 1998), and a precise de®nition remains elusive. For cently noted that ``In no apparatuses for which the architecture the purposes of this paper we adopt an operational concept of is known do . conventional designations coincide fully or con- sistently with true biological orientations.'' The evidence that the P elements of the ozarkodinid apparatus, for example, were * Present address: School of Earth Sciences, University of Birmingham, oriented with their ``posterior'' process directed dorsally is now Birmingham B15 2TT, U.K. ,[email protected]. overwhelming. Dzik (1994) has also expressed similar concerns, 113 114 JOURNAL OF PALEONTOLOGY, V. 74, NO. 1, 2000 but his proposal for a biological system of orientation was de- may be necessary to invent intermediate categories such as Sa- rived from an hypothesis of apparatus architecture (Dzik, 1991) b or Sb-c'' (Sweet, 1988, p. 25), but S notation is applied in- that is contradicted by natural assemblages preserving intact ap- consistently and can be confusing, with the same term having paratuses (Purnell and Donoghue, 1998). Consequently some of different meanings in different contexts. A number of authors Dzik's terminology for element orientation is incorrect. Jeppsson (e.g., Aldridge et al., 1987; Nicoll, 1985, 1987) have applied the (1997) has also proposed new biological terms for orientation, notation ``Sd'' to the element that is intermediate in location (but but his mesial-distal terminology for panderodontids is also not symmetry) between the Sa and the Sb positions. According based on an hypothesis of element orientation that is contra- to Sweet (1981b, 1988), however, ``Sd'' refers to an axial po- dicted by natural assemblages; he identi®es the furrow on pan- sition occupied by a quadriramate element and should not be derodontid elements as distal, but in reconstructions of the Pan- applied to ozarkodinids (Sweet, 1988; Over, 1992). Purnell and derodus apparatus (see Smith et al., 1987; Sansom et al., 1994) Donoghue (1998) suggested that the four lateral S positions in it faces forwards, not distally. Nevertheless, we agree with Dzik ozarkodinids be identi®ed as Sb1,Sb2,Sc1, and Sc2 (see also and Jeppsson that our knowledge of conodont anatomy and ap- Aldridge et al., 1995, ®g. 1), but this is not without dif®culties. paratus architecture has reached the point where the use of an The architectures of the S arrays in ozarkodinids and in the arbitrary scheme can no longer be justi®ed, and in certain cir- prioniodontid Promissum pulchrum are known in detail from cumstances the use of biological terms cannot be avoided (see natural assemblages, and both contain the same number of ele- ments in the same relative positions (Aldridge et al., 1995; Pur- e.g., Donoghue and Purnell, 1999a, 1999b). nell and Donoghue, 1998). But despite the clear homologies (see Dif®culties with anatomical notation.ÐAnatomical notation below) Aldridge et al. (1995) labeled those of Promissum Sb1, in conodonts takes on particular signi®cance because of its im- Sd, Sb , Sc rather than Sb ,Sb,Sc,Sc. This was explicitly to portance in disentangling biologically-valid ``multielement tax- 2 1 2 1 2 avoid applying the term ``Sb2'' to quadriramate elements, which onomy'' from the taxonomically illegal practice of erecting dis- on morphological grounds are widely designated as Sd elements crete component elements as taxa without regard for the fact in the literature. Thus the Sd element of Promissum is homol- that several different ``taxa'' came from a single individual. It ogous with the Sb in ozarkodinids, and the Sb of Promissum is no coincidence that the advent and proliferation of multiele- 2 2 is homologous with the ozarkodinid Sc1. This is not a satisfac- ment taxonomy in the 1960s and 1970s (e.g., BergstroÈm and tory situation.
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