Architecture and functional morphology of the skeletal apparatus of ozarkodinid conodonts

Transcription

1 Architecture and functional morphology of the keletal apparatu of ozarkodinid conodont MARK A. PURNELL and PHILIP C. J. DONOGHUE Department of Geology, Univerity of Leiceter, Leiceter LE1 7RH, UK CONTENTS page 1. Introduction Architectural recontruction of the conodont apparatu Problem, material and method The architecture of Idiognathodu Simulation of Idiognathodu collape pattern The function of the apparatu 1558 Appendix 1. Decription of the apparatu of Idiognathodu 1562 Reference 1563 SUMMARY Ozarkodinid conodont were one of the mot ucceful group of agnathan vertebrate. Only the oropharyngeal feeding apparatu of conodont wa mineralized, and the keletal element were generally diarticulated on the death and decay of the body. Occaionally, however, they were preerved in aociation a natural aemblage, foilized in itu after pot-mortem collape of the apparatu. From analyi of element arrangement in natural aemblage of Idiognathodu from the Pennylvanian of Illinoi we have produced a precie cale model of the feeding apparatu of ozarkodinid conodont. At the front lay an axial Sa element, flanked by two group of four cloe-et elongate Sb and Sc element which were inclined obliquely inward and forward; above thee element lay a pair of arched and inward pointing M element. Behind the S M array lay tranverely oriented and bilaterally oppoed Pb and Pa element. Our model hed new light on food acquiition in conodont. We propoe that the anterior S and M element of ozarkodinid conodont were attached to cartilaginou plate. In order for the animal to feed, thee plate were firt everted, and then drawn back and upward over the anterior edge of an underlying cartilage. Thee movement produced a highly effective graping action, the cup and denticle of the element converging to grab and impale any food item that lay anterior to the open array. According to thi hypothei, the anterior part of the conodont apparatu i comparable to, and poibly homologou with, the lingual apparatu of extant agnathan; the element themelve, however, have no direct homologue. 1. INTRODUCTION For more than a century, quetion of conodont palaeobiology were conidered intereting but eoteric. The lat few year, however, have een a revolution in our undertanding of conodont anatomy, affinitie and functional morphology, and thi ha led to a dramatic hift in focu. Conodont are now widely thought to be vertebrate or craniate, and have an important role to play in undertanding the origin and early diverification of the clade (e.g. Sanom et al. 1992; Aldridge et al. 1993; Purnell et al. 1995; Janvier 1996). Not only are they among the firt craniate to appear in the foil record, they are Preent addre: School of Earth Science, Univerity of Birmingham, Edgbaton, Birmingham B15 2TT, UK (p.c.j. donoghue@bham.ac.uk). alo far more divere than any other group of jawle fih. With thi new focu, analyi of conodont functional morphology take on a new ignificance. Recent work ha etablihed that many conodont, including ome primitive taxa with coniform element, were macrophagou, probably predatory organim, and that thoe conodont with more complex apparatue ued their phophatic element to grap (Brigg et al. 1983), lice and cruh their food (Purnell & von Bitter 1992; Purnell 1995). The tronget evidence for thee concluion come from microwear analye of urface feature on conodont element (Purnell 1995), but thi tudy, and all rigorou analyi of conodont functional morphology, relie to ome extent on a ound undertanding of the arrangement of the element in the conodont apparatu. 352, c 1997 The Royal Society Printed in Great Britain 1545 TEX Paper

2 1546 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function Pa Schmidt 1934 Pb Dzik 1991 S M Pa Pb S M (a) Rhode 1952 (b) (d) Aldridge et al Nicoll 1985, 1987, 1995 (c) (e) Figure 1. Hypothee of element arrangement in ozarkodinid conodont. Front, ide and top view of the apparatu are projected onto the ide of each box; element morphology i diagrammatic, but baed on Idiognathodu; (a) alo how P, M, S element notation ued in the text. (a) Linear arrangement of Schmidt (1934), Pa element anterior. (b) Linear arrangement of Rhode (1952), neither anterior poterior nor doro-ventral axe were indicated by Rhode. (c) Linear arrangement of Nicoll (1985, 1987, 1995; Nicoll & Rexroad 1987), M element anterior, S element denticle directed ventrally, Sb 1 element (hi Sd) et back from other S element. Nicoll did not recontruct Idiognathodu, and it i not clear how he would orientate M element of Idiognathodu morphology. (d) Vertical arrangement of Dzik (1991) (modified from Dzik 1976, 1986); M element anterior, dorally directed end of element are poterior according to conventional deignation. (e) Arrangement of Aldridge et al. (1987); S and M element anterior. The development of idea about conodont keletal architecture (ee figure 1) ha cloely paralleled hypothee of biological affinity and functional morphology (ee Aldridge (1987) for a review). Undertanding of architecture underpin analyi of function, but many tudie (e.g. Schmidt 1934; Lindtröm 1964, 1973, 1974; Nicoll 1995) have confued the two by uing cenario of function to contruct and contrain model of element arrangement. Thi lack of methodological rigour ha contributed to the

3 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue 1547 diverity of alternative model of keletal arrangement that have been propoed, ome of which are little more than pure peculation (ee 2 below). Part of the blame, however, alo lie in a paucity of good foil material and a conequent lack of morphological contraint. Until the foilized remain of the conodont body were found it wa not poible to determine anterior poterior and doro ventral axe with certainty, but the foil that provide the mean to unravelling the primary, in vivo patial arrangement of conodont element were firt decribed more than 60 year ago (Schmidt 1934; Scott 1934). Thee foil preerve together number of different conodont element, either a aociation on bedding plane or a cluter of element fued together by diagenetic mineral. Some of thee element aociation are faecal or diarticulated accumulation that preerve little or nothing of primary keletal architecture, but the remaining natural aemblage repreent collape of the three-dimenional oropharyngeal apparatu onto a two dimenional bedding plane. Different pattern of element arrangement in natural aemblage therefore repreent different orientation of apparatu collape, the limited number of recurring pattern reflecting the attitude of the dead conodont on the ea floor (cf. Dzik 1986). A conodont carca lying on it belly produced one characteritic pattern (figure 2 and 3), lying on it ide another (figure 4 and 5) and lying head down (or up) in the ediment produced yet another (figure 6 and 7). All thee orientation of collape are poible, a are all the intermediate orientation, but they are not all equally likely, and the majority of natural aemblage reflect collape in which the conodont carca lay in an orientation omewhere between thoe illutrated (ee Purnell & Donoghue (1998) for a more detailed dicuion of natural aemblage collape orientation). Compared to normal collection of dijunct conodont element, natural aemblage are extremely rare, but depite thi they are of paramount importance in conodont palaeontology. They erve a reference in the development of conodont taxonomy and anatomical notation, and provide template for recontructing the apparatue of the vat majority of taxa that are known only from diociated remain. They are alo fundamental in the recognition of homologie between taxa and in the interpretation of evolutionary pathway (Purnell & Donoghue 1998). Conodont have no cloe living relative, and without homologou tructure in extant organim to aid interpretation, analyi of natural aemblage i the only rigorou method of recontructing the original patial arrangement of conodont element in the feeding apparatu. 2. ARCHITECTURAL RECONSTRUCTIONS OF THE CONODONT APPARATUS Recent claification of conodont recognize up to even order (Sweet 1988; Dzik 1991; Aldridge & Smith 1993). Four have apparatue compoed of morphologically imple element, and the architecture of ome of thee, including taxa aigned by Sweet (1988) to the order Bellodellida and Panderodontida, ha recently been reviewed by Sanom et al. (1994). Of the three order characterized by more complex element morphology, the architecture of prioniodontid wa addreed by Aldridge et al. (1995) and an analyi of prioniodinid apparatue i in preparation (Purnell & von Bitter 1996). The third order, the Ozarkodinida (enu Sweet 1988), i the focu of thi paper. Repreentative of thi group dominate conodont fauna through mot of the Palaeozoic, in term of both abundance and diverity, and mot bedding plane aemblage and cluter are ozarkodinid. Almot all attempt at recontructing the conodont apparatu have therefore dealt primarily with ozarkodinid taxa. With few exception (e.g. Lindtröm 1964, 1973, 1974; Nicoll 1995), mot analye of conodont apparatu arrangement have been baed on bedding plane aemblage and cluter which are thought to retain omething of the original patial relationhip of the element. Thee analye have conformed to two ditinct methodologie. Both recognize that the extremely rare natural aemblage with bilaterally ymmetrical arrangement of element (e.g. figure 2 and 3) record primary architectural information, but they differ in the way they treat aymmetric aemblage (e.g. figure 4 9). Mot analye (e.g. figure 1a c) have aumed that deviation from ymmetry reflect pot-mortem movement of the element with recurrent aymmetric pattern produced by rotation and tranlation of element by compreion and decompoition or by ytematic mucle relaxation contraction effect (ee Aldridge (1987) and Purnell & Donoghue (1998) for a review). Thi need to invoke ad hoc pot-mortem effect repreent a ignificant weakne in thi approach but everal author realized that different apparatu pattern reflected different orientation of collape of the original three-dimenional tructure. For example, baed on an interpretation that their collection contained only a few more laterally than doroventrally collaped apparatue, Schmidt & Müller (1964) concluded that the conodont animal wa neither doro-ventrally nor laterally flattened. Avcin (1974) recognized that different attitude of repoe of the conodont carca would produce different aemblage configuration, but he ruled out doro-ventral collape a impoible, given the extreme lateral flattening of what he mitakenly took to be the conodont animal (i.e. Typhloeu). Obervation uch a thee paved the way for a more rigorou approach to recontructing apparatu architecture. Thi methodology differ from that outlined above in that it aim i to produce a ingle model of apparatu architecture that can account for a variety of natural aemblage pattern without recoure to ad hoc hypothee of pot-mortem mucle relaxation and contraction. Norby (1976, 1979), for example, realized the difficultie of producing aymmetric aemblage pattern from a linear model of element arrangement, and uggeted that the element

4 1548 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function Figure 2. Natural aemblage of Idiognathodu from the Pennylvanian Modeto Formation, Bailey Fall, Illinoi, USA; Univerity of Illinoi pecimen UI X-1480, originally figured by Du Boi (1943). All four P element, the remain of at leat ix S element, and one M element are preerved in the part; counterpart not illutrated. See figure 3a for cale. in the apparatu may have been oriented ide by ide, with their long axe vertical. Dzik (1976) noted that the natural aemblage illutrated by Rhode (1952, plate 126, figure 11; figure 2 and 3 herein) and Mahkova (1972, plate 1) were doro-ventrally and laterally flattened, repectively, and propoed a imilar arrangement of element with their long axe vertical and cup oppoed acro the midline of the apparatu a the only one that could account for both aemblage pattern. Dzik later hypothei of keletal architecture (1986, 1991, alo dicued in Dzik 1994) modified hi earlier arrangement a little in order to account better for oberved natural aemblage pattern; hi 1991 model i illutrated in figure 1d and i dicued in more detail below ( 3). Thi approach wa further developed by Aldridge et al. (1987) through incorporation of a phyical modelling technique derived from that of Brigg & William (1981). Aldridge et al. (1987) took the apparatu of the firt-dicovered conodont animal pecimen (IGSE 13822) a the primary data for a phyical model of element arrangement (figure 1e) which they then teted by attempting to imulate photographically a variety of recurrent pattern of apparatu collape, both ymmetrical and aymmetrical. The architectural model they propoed wa followed in everal ubequent paper (e.g. Purnell & von Bitter 1992; Aldridge et al. 1993, 1994, 1995; Purnell 1993, 1994). Thi phyical modelling method ha ince been uccefully ued to recontruct the apparatu of the giant conodont Promium pulchrum (Aldridge et al. 1995) and our new model of ozarkodinid architecture i baed on imilar technique. 3. PROBLEMS, MATERIALS AND METHODS Architectural hypothee that can account for all recurrent natural aemblage pattern are uperior to thoe that require ad hoc pot-mortem movement of element. Thi make teting of recontruction imple: if they cannot account for the detail of element arrangement oberved in natural aemblage, they mut be rejected or modified. All linear model (e.g. Schmidt 1934; Rhode 1952; Jeppon 1971; Nicoll 1977, 1985, 1987, 1995; Wallier 1994; figure 1a c herein) fail thi tet becaue they cannot account for the aymmetrical pattern oberved in the majority of natural aemblage. The model propoed by Aldridge et al. (1987) and Dzik (1991) (figure 1d and e) are in much cloer accord with oberved pattern, but there are till a number of ignificant dicrepancie. Aldridge et al. (1987), and later Dzik (1991), identified many of the important general feature of ozarkodinid architecture, uch a the orientation of the P element, and the anterior poterior patial differentiation within the apparatu. The overall pattern of element arrangement and orientation in the model of Aldridge et al. (1987) correpond well with natural aemblage pattern, but thee author were alo aware of a number of limitation. They noted that, in the model, the element were more widely paced than in nature, and that detail of the model, epecially the relative poition of the ramiform element, remain to be refined. In particular, the M element [in natural aemblage] commonly diplay an independence from the S element, uggeting that

5 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue 1549 Pa Pb Sb d Sc Sc Sa 1 M Sb 1 Pa d Pb d Sc d 1 mm (a) Figure 3. (a) Compoite camera lucida drawing of pecimen UI X-1480, counterpart and part (counterpart on bottom). (b) Photograph of model taken from above, behind and lightly to the left to imulate collape pattern of UI X-1480; mall cube indicate orientation of principal axe of apparatu relative to ea floor at time of collape, x = 59, y =30,z=8. Du Boi (1943, plate 25, fig. 4) figured another Idiognathodu aemblage exhibiting a imilar pattern of element arrangement, but reflecting a lightly more poterior angle of collape (x =71,y=17,z=9 ). they may have been operated by different mucle (p. 74). Dzik (1991, p. 274) alo pointed out that the orientation of the S element in thi model, with their cup directed anteriorly, wa a poor fit with natural aemblage ; in particular, it i difficult to account for the conitent inward inclination of S element denticle in aemblage approaching doro-ventral collape orientation (e.g. figure 2 and 3a). Dzik own model (figure 1d), however, i not without it problem: the vertical orientation of the S element i not matched by lateral or oblique lateral collape pattern (e.g. figure 4, 5a, 8 and 9a), and hi hypoth-

6 1550 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function Figure 4. Natural aemblage of Idiognathodu from the Pennylvanian Modeto Formation, Bailey Fall, Illinoi, USA; Natural Hitory Mueum pecimen PM X (a) Part; (b) counterpart; ee figure 5a for cale.

7 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue mm Pa Pa d Pb Pb d Sc 2 d M d M Sc 2 Sc 1 Sb 1 Sb 1 d Sc 1 d (a) Sb 2 d Sb 2 Sa Figure 5. (a) Compoite camera lucida drawing of pecimen PM X 2217, counterpart and part (counterpart on bottom). (b) Photograph of model taken from the right ide and lightly below to imulate collape pattern of PM X 2217; mall cube indicate orientation of principal axe of apparatu relative to ea floor at time of collape, x =0,y=8,z=82. Idiognathodu aemblage with a imilar pattern of element arrangement, reflecting imilar collape orientation, have been figured by Du Boi (1943, plate 25, fig. 17; plate 25, fig. 3 and 11, a lightly more poterior collape, x =29, y=3,z=61 ) and Avcin (1974, plate 2, fig. 12).

8 1552 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function Figure 6. Natural aemblage of Idiognathodu from the Pennylvanian Modeto Formation, Bailey Fall, Illinoi, USA; Natural Hitory Mueum pecimen PM X P element and four S element are preerved on the part; no counterpart. See figure 7a for cale. ei that the element of the ymmetry tranition erie were arranged with their cup in direct oppoition acro the axi, in a tructure the hape of an anteriorly open V with a vertical cloure, alo place element in poition that are not oberved in natural aemblage. It i thee difficultie, together with the acquiition of new material and re-examination of exiting collection, that prompted u to produce our new model of ozarkodinid architecture. Natural aemblage of Idiognathodu (enu Baeeman 1973; Grayon et al. 1991) outnumber thoe of all other taxa, and the morphology of all the element of it apparatu i well known (ee, for example, Grayon et al. 1991). Our architectural recontruction i, therefore, baed on Idiognathodu. We have re-examined all ignificant collection of Idiognathodu bedding plane aemblage, including the material of Du Boi (1943), Rhode (1952), Avcin (1974), Aldridge et al. (1987) and Purnell (1993), and new or unpublihed material from Bailey Fall and Wolf Covered Bridge in Illinoi, USA (ee Purnell (1994) for tratigraphic and locality detail). In order to produce the mot accurate recontruction poible, we ued regreion derived from meaurement of Idiognathodu bedding plane aemblage (Purnell 1993, 1994) to calculate the ize of element in an apparatu with Pa element 2 mm long, and produced 1:50 cale model of all of the element. The configuration of the element in the model wa determined uing a combination of phyical modelling and photographic technique (Brigg & William 1981; Aldridge et al. 1987, 1995), with final teting achieved by uing photograph of the model to imulate pattern of element collape in natural aemblage. The reult of thi teting are reproduced here a figure 3, 5, 7 and 9. Due to the limitation of page pace, the aemblage and imulated collape pattern illutrated are jut example which demontrate the range of pattern oberved in Idiognathodu. We alo include a tereo pair (figure 10) and an anterior view (figure 11) of our model to illutrate detail of the apparatu not evident in other photograph. 4. THE ARCHITECTURE OF IDIOGNATHODUS The baic contraint on apparatu orientation are derived from the Scottih Carboniferou animal foil. Thee indicate unequivocally that the S and M element were at the front of the apparatu and that the poterior P element were oriented with their long axe normal to the long axi of the conodont body (Aldridge et al. 1987). Doral and ventral have been difficult to determine with certainty (Aldridge et al. 1987), but recognition of cartilaginou eye capule, poible otic tructure, and an equivocal doral nerve cord (Aldridge et al. 1993) all

9 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue 1553 Pa Pa d Pb d Sb 1 d Pb (a) Sc 2 Sc 1 Sb 1 1 mm Figure 7. (a) Camera lucida drawing of pecimen PM X (b) Photograph of model taken from behind, left and lightly below to imulate collape pattern of PM X 2218; mall cube indicate orientation of principal axe of apparatu relative to ea floor at time of collape, x =67,y=14,z=18. Similar Idiognathodu aemblage have been figured by Du Boi (1943, plate 25, fig. 5, x =67,y=10,z=21 ; fig. 13, x =64,y=5,z=26 ) and Avcin (1974, plate 2, fig. 19, x =71,y=9,z=17, re-illutrated by Aldridge et al. (1987), fig. 4.4).

10 1554 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function Figure 8. Natural aemblage of Idiognathodu from the Pennylvanian Modeto Formation, Bailey Fall, Illinoi, USA; Natural Hitory Mueum pecimen PM X (a) Part; (b) counterpart; ee figure 9a for cale.

11 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue mm Sb 2 Pa Pb M Sc 2 Pa d Sc 1 Pb d Sb 1 Sc 2 d Sc 1 d M d Sb 1 d Sa Sb 2 d (a) Figure 9. (a) Compoite camera lucida drawing of pecimen PM X 2219, part and counterpart (part on bottom). (b) Photograph of model taken from above, right, and lightly behind to imulate collape pattern of PM X 2219; mall cube indicate orientation of principal axe of apparatu relative to ea floor at time of collape, x =12,y=43, z=44. A imilar orientation of collape i recorded by the pecimen figured by Avcin (1974, plate 1, fig. 8, plate 2, fig. 1, x =1,y=40,z=50 ; refigured by Aldridge et al. (1987), fig. 4.8A).

12 1556 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function Figure 10. Stereo pair of the Idiognathodu apparatu model, viewed from above front. indicate that the apparatu wa oriented uch that the poterior of the P element (according to conventional deignation) wa directed dorally. A full decription of the apparatu architecture of Idiognathodu appear in Appendix 1. It i important to note here, however, that our recontruction (figure 3, 5, 7 and 9 11) differ from thoe propoed previouly in two important repect. Firt, the Pa element in our model are arranged with the left element behind the right. Thi may appear a rather ubtle difference, but it ha far reaching implication for functional tudie and the detection of conodont microwear pattern (Purnell 1995). Secondly, and more obviouly, our model differ from that propoed by Aldridge et al. (1987; figure 1e) in the arrangement of the S and M element at the anterior of the apparatu. Their recontruction placed the S element in parallel, with approximately equal forward inclination, with no vertical diplacement from one element to the next, and with no inward inclination. The M element flanked the S array, and had a imilar general orientation, the long axi parallel to thoe of the S element. In our recontruction, the long axe of the S element diverge anteriorly; the axi of the Sa element i horizontal, and the angle of forward inclination of the other S element decreae away from the agittal plane; the inward inclination of the S element increae away from the vertically oriented Sa. The M element are located above, and oriented obliquely to the S element. Thee difference are not trivial; they repreent a ignificant improvement in our undertanding of the keletal anatomy of conodont, and are fundamental to analyi of functional morphology and the problem of food acquiition in conodont (ee 6). The orientation of the S and M element in our recontruction alo differ from Dzik hypothei of architecture (1991; figure 1d). He conidered the S element to be vertical, their long axe parallel, and their cup directed inward at 90 to the agittal plane; he alo placed the M element at the front of the apparatu. 5. SIMULATIONS OF IDIOGNATHODUS COLLAPSE PATTERNS The model of apparatu architecture decribed above tand or fall according to how cloely it can imulate the pattern of element ditribution in natural aemblage of Idiognathodu. The pecimen in figure 2 and 3a i the mot widely illutrated natural aemblage (originally figured by Du Boi (1943), plate 25, figure 14), primarily becaue of it clear bilateral ymmetry. Previou attempt to imulate the collape of thi aemblage (e.g. Aldridge et al. 1987, figure 4.12; Purnell et al. 1995, figure 6) have, however, incorrectly identified the left and right ide of the apparatu, and have therefore produced incorrect imulation. A preerved on the pecimen part (figure 2) the apparatu ha collaped obliquely, from below and in front toward top and behind. Thi orientation cannot be imulated photographically (it would require the bae-board of the model to be completely tranparent), o our imulation i of the whole apparatu a drawn in the camera lucida (figure 3a) with the counterpart on the bottom. Detail uch a the overlap between the Pa and Pb

13 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue 1557 Figure 11. Anterior view of the Idiognathodu apparatu model. element, the orientation of the S element denticle inward and toward the anterior, the location of the Sc element cup, and the poition of the preerved initral M element are all accurately matched in the imulation (figure 3b). The main viual difference between the imulation and the pecimen arie from the forehortening of element caued by the oblique angle of photography; in reality, the long axe of element all came to lie in the ame plane during collape, but thi cannot be imulated photographically. Figure 4 and 5a illutrate a lateral collape. In thi orientation, the P element could probably have fallen either way, but they have come to ret with the more anterior dextral element in front of the initral element. The imulation of thi aemblage (figure 5b) accurately reproduce the relative juxtapoition and orientation of the S and M element, a hown clearly by the Sa, Sb and M element. The lightly teeper forward inclination of the S element in the aemblage probably reflect the reorientation of element long axe a they came to lie on the bedding plane. Although the lack of a counterpart and probable burial of ome element beneath other mean that only eight element of the apparatu are evident in the aemblage hown in figure 6 and 7a, the pattern of element arrangement exhibited by the pecimen i accurately imulated by photographing the model from behind, to the left and lightly below (figure 7b). The pattern of element poition and orientation preerved in the pecimen hown in figure 8 and 9a i matched almot exactly by imulating oblique collape from above, right, and lightly behind (figure 9b). The unuual arrangement of the M element, at firt ight anomalou in having the initral M parallel to the S element, but the dextral M lying acro them, i faithfully reproduced in the imulation. The location of the S element, thoe on the initral ide lying above and behind their dextral counterpart, i reproduced accurately, with the initral Sb 2 element, for example, located in the pace between the initral Sc 1 and the dextral Sc 2 in both the foil and the imulation. The P element are lying with the initral member of each pair offet above and behind the other. Thee illutration erve only a example, but they clearly demontrate that our apparatu model pae the tet of being able to imulate the range of different element arrangement in natural aemblage of Idiognathodu (ee Purnell & Donoghue (1998) for more example). The fidelity with which thee photograph reproduce foil pattern reflecting different orientation of collape provide compelling evidence that our model i accurate, and given the number of variable involved and the complexity of the apparatu, it i inconceivable that a ignificantly different apparatu architecture could produce equally accurate imulation. Our model mut therefore be cloe to the in vivo keletal architecture of Idiognathodu. Analyi of collape pattern in natural aemblage of other taxa (Purnell & Donoghue 1998) indicate that the model alo reflect the architecture of ozarkodinid conodont a a whole.

14 1558 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function 6. THE FUNCTION OF THE APPARATUS Prior to the dicovery of the firt conodont animal (Brigg et al. 1983), a general lack of biological contraint rendered rigorou functional analyi of conodont effectively impoible (Bengton 1980). Since then, however, a number of tudie have conidered the function of element a component of an integrated feeding tructure in the head of an eellike marine animal. Some have uggeted that the apparatu wa a tiue covered filter-feeding device (Nicoll 1985, 1987, 1995; Nicoll & Rexroad 1987), but thi hypothei i refuted by analyi of apparatu growth rate (Purnell 1993, 1994) and by the demontration of hearing microwear on the urface of ome conodont element (Purnell 1995). The available evidence indicate that the conodont apparatu had a toothlike function, and that in ozarkodinid the poterior P element proceed food by cruhing and/or licing (Brigg et al. 1983; Aldridge & Brigg 1986; Aldridge et al. 1987; Purnell & von Bitter 1992; Purnell 1993, 1994, 1995). The quetion of food acquiition and the function of the anterior S and M element i more problematic. Apparatu location, architecture, ontogeny, element morphology and wear pattern on P element accord with a general hypothei that the S and M element performed a graping function, but the mean by which thi wa achieved i unknown. Mot detailed analye of function have focued on the Pa element, and although Brigg et al. (1983) and ubequent author have peculated that S and M element may have operated in a manner broadly analogou to the lingual apparatu of hagfih, tatement concerning S and M function are generally vague. In part, thi i becaue it i difficult to enviage how element arranged in the parallel array propoed by Aldridge et al. (1987) actually graped. If the anterior S and M array wa a graping device it could not have been tatic, and movement of element during function ha been potulated everal time (e.g. Jeppon 1971). Aldridge et al. (1987) uggeted that a 90 rotation of each ide of the ramiform array wa required in order to bring the cup of the S and M element into oppoition. Our revied model of apparatu architecture hed new light on thi problem. Our model provide both firm phyical contraint and a foundation upon which to contruct hypothee of the mechanic of element motion. Further, biological contraint are impoed by the phylogenetic poition of conodont. The evidence that conodont were jawle vertebrate i now compelling (ee Aldridge & Purnell (1996) for a recent review), and thi ha provided a group of living relative with which conodont can be compared. Comparion mut, however, be made with care. Conodont may hare a number of important character with extant agnathan, but they are clearly a ditinct and pecialized clade. In particular, functional analogie between the conodont apparatu and the lingual apparatu of lamprey and hagfih mut be drawn with caution becaue the ynapomorphy that unite the Conodonta i the phophatic feeding apparatu. It i alo poible that new evidence may weaken or refute the hypothei that conodont were jawle vertebrate. Thi would not alter our architectural recontruction, however, and our hypothee of function would therefore remain, albeit without the upport of analogie with agnathan. The fact that our model can imulate the range of natural aemblage collape pattern indicate that the element generally lay in the ame table configuration in all dead but undecompoed conodont. The Sb Sc element were arranged a two obliquely oppoed et of cloely paced ubparallel element, their functional urface were directed obliquely doral, and the aboral urface of the element in each et lay in approximately the ame plane. Irrepective of phylogenetic relationhip, thi arrangement i comparable with the everible lingual apparatu of extant agnathan; in both lamprey and hagfih, the keratin biting element of the lingual apparatu are arranged a oblique et attached to the doral ide of a cartilage plate or plate. We interpret the juxtapoition and orientation of the Sb Sc element alo to reflect their location on a pair of underlying cartilaginou dental plate. The hypothei that conodont element at on upporting tructure i not new (e.g. Kirk 1929; Smith et al. 1987, and reference therein), but it ha not been previouly uggeted that oppoable dental plate united the Sb-Sc element a integrated functional unit. Without uch aboral cartilage upport, the control of movement and orientation of each element would have required a eparate et of complex muculature. Thi hypothei i difficult to reconcile with the lack of pace between the Sb-Sc element; alo, it i not upported by analogie with any other agnathan. The orientation and the poition of the Sa element ugget that it did not it on the ame cartilage plate a the Sb Sc element, but on a eparate medial ridge or plate of cartilage. For thi reaon we propoe eparate plate for the initral and dextral ide of the apparatu, imilar to the ituation in lamprey, rather than the ingle flexible dental plate of hagfih (Yalden 1985). The poition and orientation of the M element, and the inclination of their denticle, are markedly different to S element and ugget that their motion wa alo omewhat different. They may have at on lateral projection of the Sb Sc plate, capable of a degree of independent articulation, or on eparate baal plate. Note that we do not equate cartilage upport tructure with the conodont baal body. The proce of graping clearly required both opening and cloing of the apparatu, and the firt tep in undertanding the operation of the apparatu i to ae the point of the cycle repreented by the configuration of element in the architectural model. The location of the apparatue in the Granton conodont animal pecimen indicate that unle the mouth wa in an unuually poterior poition, the S and M element mut have moved forward in order to have graped food. Thi ugget that our model repreent an apparatu near to cloure. The orientation of the Sb Sc element cup and denticle, in oblique oppoition, and the poition and orientation of the

15 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue 1559 (a) (b) Figure 12. Operation of the anterior array of the ozarkodinid apparatu illutrated by Idiognathodu. Element drawn with dotted line indicate their poition when the apparatu i everted and open; element drawn with olid line are in the retracted, cloed poition. (a) Lateral view of the dextral ide of the apparatu; arrow indicate net movement of Sa, Sb 1,Sb 2,Sc 2 and M element during retraction and cloure of the anterior array. (b) Anterior view of the whole apparatu; arrow indicate net movement of S and M element. M element, curving round above and in front of the S element, upport thi interpretation, a doe the fact that the poition of the P element would have prevented ignificant poterior movement of the S element. It i pertinent to note that the lingual apparatu of extant agnathan come to ret in a cloed poition in dead animal. The Granton foil alo preerve evidence of paired eye and otic capule (Aldridge et al. 1993), and conodont mut have had a true head and a differentiated brain. Thi, and the oblique doral orientation of the S element indicate that a ignificant component of ventral movement wa required in order to open the conodont apparatu. Baic biological contraint demand that movement of the cartilage plate bearing the conodont element required a ytem of antagonitic mucle. One end of each of thee mucle inerted, probably via a tendon, onto one of the dental plate, and the other end mut have been attached to another keletal cartilage. Retractor mucle could have inerted onto cartilage aociated with branchial tructure, the braincae, or other hypothetical poterior keletal tructure, but protractor mucle which brought about the neceary anterior and ventral motion of the dental plate mut have inerted onto cartilage that were ventral to the element and their dental plate. The number, ize and hape of thee keletal cartilage in conodont i a matter of peculation, but it i likely, baed again on biomechanical contraint and alo by analogy with living agnathan, that their hape exerted a fundamental control on the movement of the dental plate. In both lamprey and hagfih, cloure of the lingual apparatu (i.e. biting ) i brought about by retraction of the dental plate into a cartilage decribed a pulley or U-haped (Yalden 1985), and we propoe a imilar mechanim wa reponible for cloure of the conodont apparatu (i.e.

16 1560 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function (a) (b) (c) Figure 13. Relative motion of S and M element during cloure of the anterior array. Element drawn with dotted line indicate their poition when the apparatu i everted and open; element drawn with olid line are in the retracted poition. (a) Lateral view of dextral S element and Sa element; arrow indicate motion of Sa, Sb 1,Sb 2, and Sc 2 element relative to a fixed point at the dital end of the poterior proce of the Sb 1 element. (b) Anterior view of dextral M element; arrow indicate motion relative to cup of Sa element. (c) Anterior view dextral S element and Sa element; arrow indicate motion of Sa, Sb 1,Sb 2, and Sc 2 element relative to a fixed point at the dital end of the anterior proce of the Sb 1 element. graping). The anterior and ventral motion involved in opening of the conodont apparatu, therefore, reulted from pulling of the dental plate forward out of the laterally confining U-hape, and pivoting them over the anterior edge of the ventral cartilage into a ubvertical poition in which the S element denticle were directed anteriorly. The protractor mucle reponible for uch motion mut have wrapped round the anterior end of the ventral cartilage and inerted omewhere on it ventral ide. Thi hypothei of element motion i illutrated in figure 12. Becaue the apparatu came to ret near to cloure, the firt tage in the graping cycle mut have been opening of the apparatu, but it i cloure that i important for undertanding how graping wa achieved, and our illutration and dicuion therefore concentrate on element movement during retraction of the apparatu. The apparatu wa, however, opened by reveral of the element movement illutrated. Cloure of the apparatu wa brought about by the action of retractor mucle inerted onto the dental plate bearing the Sb Sc and poibly the M element. Thi reulted in a net poterior and inward rotation of the element a indicated by the arrow on figure 12. Thi motion need not have been a teady, mooth action; a the dental plate pivoted over the anterior edge of the underlying cartilage the apparatu may have napped back into the cloed poition, in a manner imilar to cloure of the hagfih lingual apparatu (Dawon 1963; Kreja 1990a). The retracted, reting poition of the Sa element wa anterior and more ventral of the adjacent element; thi, and the hape of the medial cartilage on which we ugget it lay, reulted in a different trajectory a the apparatu cloed. Relative to the other S element it moved up and back, paing through the axial pace between the Sb 1 element. The M element, during cloure, moved backward and wung inward. Although the net effect of retraction of the apparatu wa to move the S and M element backward, upward and inward (figure 12), the mechanim by which food wa graped by the element i more clearly illutrated by conidering their relative rather than their abolute motion (figure 13). During cloure, the Sb Sc element wung upward through an arc approximately parallel to the curvature of the cup and denticle (figure 13a), a motion comparable to the cloure of the lower jaw in mammal (e.g. Crompton & Hiiemae 1970). At the ame time they rotated inward, again along trajectorie approximately parallel to the orientation of the cup and denticle (figure 13c). Through the ame phae of cloure the M element rotated inward, downward, and lightly forward (figure 13b). The combined effect of thee movement would have produced a highly effective graping action, the cup and denticle of the element converging to grab and impale any food item that lay anterior of the open array. The poterior component of apparatu retraction would have imultaneouly drawn food back into the mouth. It i poible the Sb Sc element were retracted a little further than the poition illutrated (i.e. the configuration of element in the model) o that the cup

17 Conodont apparatu architecture and function M. A. Purnell and P. C. J. Donoghue 1561 were brought into more direct oppoition. However, we enviage a graping, rather than a biting function for the S and M element, and thi doe not require complete cloure of the array. It doe eem likely, however, that the Sa element continued it backward arc, the poterior proce moving through the horizontal to a poition of poterior inclination. Through thi cycle of retraction, the cup and lateral procee of the Sa element would have moved from a poition cloe to the cup of the Sb 1 element when fully everted, pat the cup of each of the other Sb Sc element in turn. Thu, at the earliet tage of cloure the Sa element would have performed a graping function, but a retraction continued, food impaled on the Sb Sc element would have been lifted off and moved backward toward the P element. It ha been uggeted that with the anterior S and M array in retracted poition, conodont element may have been withdrawn into encloing pocket of epithelium (Aldridge et al. 1987; cf. Bengton 1976). However, the evidence that S element were cloely juxtapoed, our hypothei that they lay on cartilage dental plate and were till functional when retracted (cf. Bengton 1983), and recent reinterpretation of element hitology (Sanom 1996; Donoghue 1998) together indicate that hi hypothei i no longer tenable. In ozarkodinid conodont, food captured by the S and M element wa liced and cruhed by the Pb and Pa element (e.g. Brigg et al. 1983; Purnell & von Bitter 1992; Purnell 1995). Morphology, occluion and wear pattern indicate that P element operated by being rotated againt each other acro the axi (Nicoll 1987; Weddige 1990; Purnell & von Bitter 1992); harp blade-like element, for example, functioned like a pair of errated cior (Purnell & von Bitter 1992; Purnell 1995). The relative movement of thee element i undertood, but how motion wa produced i not. It i poible that rotation wa produced by mechanim of retraction imilar to thoe propoed for the S and M element, but the preciion with which element were brought into repeated contact argue againt thi (Purnell 1995). Rather, it eem more likely that the P element were located at the entrance to the pharynx (Aldridge et al. 1995), and movement wa controlled by antagonitic mucle located above and below the element in the doral and ventral wall of the pharynx. Janvier peculative uggetion that the P element were attached to a tranverely moving tructure derived from a velum of larval lamprey type (Janvier 1996, p. 277) i conitent with their pharyngeal location, but i difficult to tet. However, the velum in larval lamprey puhe water into the pharynx, and it primary motion i anterior poterior (Mallatt 1996). The muculature of thi tructure would, therefore, require ignificant remodelling if it were to bring about the axially directed rotational action of ozarkodinid P element. Furthermore, faint tranvere trace preerved immediately behind the eye of one conodont pecimen (IGSE 13821; Brigg et al. 1983, figure 2C and 3A) have been interpreted a poible branchial tructure (Aldridge et al. 1993). If correct, thi would ugget that the velum in conodont (if they poeed one) wa located anterior of the P element. The architecture of the conodont apparatu i conitent with a mode of operation analogou to that of extant agnathan. The poibility that they were imilar in detail, a we ugget above, lend trong upport to the hypothei that the anterior portion of the conodont feeding apparatu a a whole i homologou with the lingual apparatu of extant agnathan (cf. Aldridge et al. 1986; contra Janvier 1996, p. 267). The conodont element themelve, however, are certainly not homologou with the keratin teeth of agnathan (contra Kreja 1990b). The poible homology of the bilaterally operating feeding apparatu upport the hypothei that it i a ynapomorphy of craniate (e.g. Janvier 1981, 1996; Purnell 1993), and i not, a ha been uggeted recently (Mallatt 1996), a derived feature of extant agnathan. Thi hypothei of homology, and the corollary that the S and M element lay in the conodont mouth, alo caue ome difficultie for Mallatt functional cenario for the origin of jaw (1996). Regarding conodont affinity, homology of the feeding apparatu doe not help to reolve thi contentiou iue; a a pleiomorphic character hared by all craniate it provide no indication of the cloene of relationhip between one agnathan group and another (contra Yalden 1985). Along with the evidence for the patial arrangement of the element, however, apparatu homologie do contradict recent uggetion that the conodont apparatu wa comparable to the oral plate of otracoderm (Janvier 1996) or wa jaw-like in it arrangement (e.g. Gee 1996, p. 67). Similarly, the uggetion that the S and M element were the pharyngeal denticle of a uction-feeding animal (Janvier 1995, 1996) can be reconciled neither with our hypothee of architecture, function and homology, nor with evidence that agnathan are unable to generate trong uction (Mallatt 1996). Our hypothei of retraction and graping in ozarkodinid conodont, although contructed within the framework provided by our model of apparatu architecture and contrained by analogie with living agnathan, i upported only by indirect evidence. It i, however, both plauible and tetable. Location of point of maximum food contact and tre can be predicted, and thee hould correpond to maximum wear and denticle breakage. It i alo poible that microwear analyi will reveal cratche on cup that will confirm or refute our hypothei of the relative motion of the element. Undoubtedly, the conodont head and it variou organ were upported by keletal cartilage which, except for the clerotic cartilage, have not been preerved in the conodont animal foil found o far. The poibility of future dicoverie of foil reflecting a different taphonomic hitory from the Granton pecimen, and which preerve keletal cartilage, provide a potential tet of our hypothei of the cartilage involved in the function of the conodont apparatu.

18 1562 M. A. Purnell and P. C. J. Donoghue Conodont apparatu architecture and function For loan and acce to material we thank Profeor Richard Aldridge, Univerity of Leiceter, Dr Rod Norby, Illinoi Geological Survey, and Profeor Peter von Bitter, Royal Ontario Mueum. Dr Rod Norby, Profeor Peter von Bitter and Dr C. Piu Wiebel aited in collecting material from Bailey Fall. We thank Profeor Richard Aldridge for dicuion, and Profeor Derek Brigg and an anonymou reviewer for critical comment on thi and earlier verion of the manucript. Photographic aitance wa provided by Mr Ian Pateron and Mr Colin Brook. Mr David York aited in rod bending and drilling. Thi work wa funded by NERC Fellowhip GT5/F/GS/95/6 (M.A.P.), and a Univerity of Leiceter potgraduate tudenthip (P.C.J.D.). APPENDIX 1. DESCRIPTION OF THE APPARATUS OF IDIOGNATHODUS In thi decription, a plane parallel to the long axi of the animal and orthogonal to the agittal plane i taken a horizontal. The whole model (1:50 cale), meaured from the tip of the cup of the Sa element to the blade of the Pa element, i 25 cm long, and an animal with 2 mm long Pa element would, therefore, have had an apparatu 5 mm long. The element of the apparatu of Idiognathodu grew iometrically (Purnell 1993, 1994), and auming the ame to be true of the whole apparatu, dimenion expreed a proportion are applicable to Idiognathodu apparatue of any ize. We therefore give dimenion a proportion of total apparatu length. At it widet (between the dital tip of the M element), the apparatu i 60% of length, and it full doro-ventral depth (between ventral end of anterior procee of Sb 1 element and dital tip of Sc 2 element) i 50% of length. Note that our uage of element notation and the problem of element orientation are dicued elewhere (Purnell & Donoghue 1998). Detailed decription of the element of Idiognathodu i beyond the cope of thi paper, but brief clarification of the morphology of the element occupying S poition i neceary. Sa element: hortet of S element, poterior proce approximately half length of that of Sc. Sb 1 element: bipennate, lack pronounced cup, have fairly long anterior proce that curve harply inward through approximately 90 ; poterior proce about three-quarter the length of that of the Sb 2 and Sc element. Sb 2 element: bipennate, lack pronounced cup, have fairly long anterior proce that curve gently inward and downward. Sc element: bipennate, with pronounced cup and hort incurved and downcurved anterior proce bearing recurved elongate denticle; anterior proce of Sc 1 more tightly incurved than Sc 2, in ome pecimen aboral edge of anterior proce of Sc 2 i recurved and more hook-like than Sc 1. Poterior inclination of denticle on the poterior procee of the element decreae from an angle of approximately 50 (with repect to the poterior proce) in Sb 1 element to approximately in Sc element. The denticle of Sb 1 element are alo more trongly incurved. The Sa i the mot anterior of the S element; it lie on the agittal plane with it poterior proce approximately horizontal and it cup vertical (figure 5, 10 and 11). It i flanked by four pair of ymmetrically arranged S element: Sb 1,Sb 2,Sc 1 and Sc 2 (in equence, away from the axi). The Sb 1 cup are et back approximately 10% of apparatu length from the Sa cup and lie approximately 4% of apparatu length from the agittal plane. The Sb 2 cup are approximately 5% behind the Sa, and approximately 5.5% from the agittal plane. The two Sc element on each ide are tightly grouped, their cup lightly behind that of the Sa, and the Sc 2 cup approximately 9% of apparatu length from the agittal plane. Poteriorly, the poterior procee of the Sb and Sc element terminate approximately 4 5% of apparatu length from the agittal plane; the Sb 1 poterior procee are parallel to the agittal plane, but thoe of the other element diverge anteriorly, the Sb 2 element at approximately 5, the Sc element at approximately 15 with repect to the agittal plane. The vertical dipoition of the element i relative to a horizontal datum along the bae of the poterior proce of the Sa. The Sb 1 element are the mot ventral in the apparatu (figure 5, 10 and 11), and the other element have progreively more doral location away from the axi. Relative to the datum, the baal cavity beneath the cup of each Sb 1 element i approximately 6% of apparatu length below, that of each Sb 2 element i very lightly above, that of each Sc 1 element approximately 4% above, and that of each Sc 2 element 10% above. The poterior tip of the Sc 2 element terminate approximately 35% of apparatu length above the datum, and apart from the horizontal Sa element, all the S element are oriented with their poterior proce tilted forward. Relative to the horizontal, thi angle decreae from approximately 45 in the Sb 1 through approximately 35 in the Sb 2 to approximately 30 in the Sc element. The Sb Sc element are alo inclined inward; the plane in which the denticle of the poterior proce lie i inclined at approximately 45 to the agittal plane in Sb 1 element and increae through to approximately 60 in Sc element (figure 7, 10 and 11). The M element are located above the S element, the baal cavity of each i approximately 20% of apparatu length above the datum, and approximately 14% from the agittal plane; the cup tip approximately 9% from the agittal plane. The orientation of the M element i very different to that of the S element. Each lie with it lateral procee in a plane that i approximately vertical in anterior apect, but which curve gently inward when viewed from above. At the poterior of the M element, thi plane lie at an average angle of approximately 30 to the agittal plane, increaing to 50 at the cup. Viewed from the ide (figure 5), the M element are pitched gently forward, but the cup themelve are directed downward at approximately 20 from the horizontal. The Pb element lie 72% of apparatu length behind the Sa cup, and the Pa element at the back of the apparatu, approximately 28% behind the Pb element. The element on the left ide of each pair i the more poterior of the two (figure 3, 6, 7 and 10).