The taxonomy and apparatus structure of the Silurian distomodontid conodont Coryssognathus Link & Druce, 1972

Collections of discrete conodont elements from the Upper Whitcliffe Formation of the Welsh Borderland indicate a septimembrate plan for the feeding apparatus of Coryssognathus, comprising Pa, Pb, Pc, M, Sa/Sb, Sb and Sc elements. Each element is paired, and relative frequencies suggest that there was a total of 16 elements in the apparatus, including two indistinguishable pairs of Sc elements. Associated small coniform elements appear to represent discrete denticles of crown tissue that were sequentially incorporated into multidenticulate elements during ontogeny.


INTRODUCTION
Conodonts are almost entirely known from the scattered elements of their apatitic feeding a p p a r a t u s e s . The multielement composition of these apparatuses can be partially or completely deduced through analyses of collections of discrete elements, but the only direct evidence of apparatus architecture comes from the rare occurrences of bedding-plane assemblages. These are preserved when individual conodonts died a n d decayed without a n y disturbance from currents, scavengers or infauna. Each bedding-plane assemblage comprises t h e complete apparatus of a single conodont, collapsed onto the bedding plane from its original three-dimensional geometry. Most bedding-plane assemblages are of Carboniferous age and represent just one conodont order, the ozarkodinids. These include the apparatuses preserved in the heads of the conodont animal specimens from the Granton Shrimp Bed, Edinburgh (Briggs et al., 1983;Aldridge et al., 1986. By studying the arrangement of assemblages preserved in different orientation? it has been possible to reconstuct the three-dimensional architecture of ozarkodinid conodonts, with the ramiform S and M elements forming a basket in front of one pair each of Pb and Pa elements, both oriented perpendicular to the trunk of the animal (Aldridge et al., 1987). The apparatuses of representatives of all other conodont orders are much less well known, with the numbers and types of elements present often a matter of debate.
Conodonts were most diverse a n d variable in t h e Ordovician, but there is little direct evidence of the apparatus structure of most Ordovician taxa. The prioniodontid conodonts were one of the most characteristic groups, diminishing somewhat in the Silurian and becoming extinct by the latest Devonian or earliest Carboniferous (Sweet, 1988;Dzik, 1991;. Reconstructions based on collections of discrete elements have indicated that the prioniodontid feeding apparatus normally comprised six or seven different element types with the P positions occupied by pastinate coniform or pectiniform elements or their platformed equivalents (Sweet, 1988). The only prioniodontid bedding-plane assemblages known are of the giant U p p e r 0 r d o v i ci a n species P r o m is s II m p zr l ch r 11 m Kovacs-Endrody, a n d these display octomembrate apparatuses that differ considerably in architecture from the ozarkodinids (Theron et al., 1990). In particular, Proiiiissuin has three pairs of P elements in contrast to the two of ozarkodinids, a n d t h e M element is associated geometrically w i t h the P elements rather t h a n the S elements. In ozarkodinids, the P elements are situated posterior to the set of ramiform M and S elements, whereas in Proinissirni they appear to have been positioned above or below (Theron et al., 1990;Aldridge & Theron, 1993).
Not all prioniodontid taxa may conform to the Pron7issic~ir plan, but the bedding-plane assemblages provide an initial template for reconstructing the apparatuses of other genera. For example, the suggestion that the Silurian genera Pterospathodzis and Pranognathus had paired Pa, Pb and Pc elements (Mannik & Aldridge, 1989) is consistent with the pattern shown by Promissirin, although Pterospnthodirs appears to have had a reduced series of S elements and lacks a truly symmetrical Sa element. Apart from the Pterospathodontidae, at least three other prioniodontid families a r e represented in the Silurian: the Distomodontidae, the Icriodellidae and the Icriodontidae, with rare Canadian specimens referred to "Apparatus A" by Uyeno (1981) perhaps representing the Rhipidognathidae .
In this contribution, we provide a reconstruction of the apparatus of the late Silurian species Coryssognatlizrs dirhrirs (Rhodes), which serves as a guide to the apparatus structure of distomodontid conodonts. In Britain, this species first appears in the upper beds of the Upper Bringewood Formation of latest Gorstian age and spans the Ludfordian, with the highest record in the lowest beds of the Downton Castle Sandstone Formation (Aldridge, 1985;recorded as Distomodus? dubius and Pelekysgnathus dubius). The species is most abundant in the Upper Whitcliffe Formation (upper Ludfordian), and our investigation is primarily based on collections from this formation and its correlatives. The most complete reconstruction of the C o r y s s o g n a t h u s apparatus published to date is that of van den Boogaard (1990), who recognised Pa, Pb, M, Sa, Sb, Sc and coniform elements. Here, we distinguish Pa, Pb, Pc, M, Sa/Sb, Sb, Sc and coniform elements, deduce the numbers of each in the apparatus and suggest a geometric configuration.  Thomas, 1949, by Jeppsson (1972, a designation supported by Nicoll (1982) on the basis of associated coniform elements similar to those of other Pelekysgnathus species. However, it is clear that Coryssopathus Pa elements grew by successively incorporating neighbouring denticles (Jeppsson, 1972;van den Boogaard, 1990), in contrast with the Pa element of Pelekysgnathus, which has a posterior cusp and basal cavity tips only beneath the cusp and the immediately proximal denticle (van den Boogaard, 1990). A further distinction between the two genera lies in the rest of the apparatus, which in Pelekysgnathus is composed of coniform elements (Klapper & Philip, 1972). The Sc element of Coryssogiiathus dubius was originally figured by Rhodes (1953) as Cordylodus? dubius, but the apparatus of the Lower Ordovician genus Cordylodus is not comparable with that of Coryssognathus (Sweet, 1988). The genera Distomodus Branson & Branson, 1947, Rotuiidacodiria Carls & Gandl, 1969, and Deiitacodiiia Wang, 1980, all include elements similar to those in the apparatus of C. dubius. Distoniodus has a distinctive, multiprocessed Pa element, which, at least beyond the earliest juvenile stages, cannot be seen to have grown by successive incorporation of coniform elements. Elements referred by Carls & Gandl (1969) to Roturidacodina appear to belong to species of the Devonian genus Icriodus, which has a distinctive Pa element (Serpagli, 1983). Deiitacodina is a younger name than Coryssogizathus and the nature of any Pa element associated with the specimens figured by Wang (1980) is unclear; a multidenticulate element identified as Pelekysgnathus cf. index Klapper & Murphy by Wang (1980, p. 374, pl. 2, figs 33-35) does occur in the same collections. The senior subjective synonym Astacoderma spinosum Harley has been suppressed by the International Commission on Zoological Nomenclature following a submission by Jeppsson & Aldridge (1988).
Coryssognathus dubius (Rhodes, 1953) 1972 Distomodus dubius (Rhodes); Jeppsson: 5658, pl. 1, figs 1 (M), 2 4 (Sc), 5 (Sa/Sb), 6 (Pb?), 7-9 (Pc), 10-13 (condorm  (1972, 1975, 1979b, 1984) from the Ludlow of Sk5ne a n d Gotland, by Link & Druce (1972) from the Ludlow of the Yass Basin, Australia, by Helfrich (1974) from the Wills Creek Formation of Virginia, U S A , by Harley (1861), Rhodes (1953) and Aldridge (1975Aldridge ( , 1985 from the U p p e r Bringewood Formation to D o w n t o n Castle Sandstone Formation of the Welsh Borderland, by Wang (1980) from the u p p e r Silurian of the Qujing District, Junnan, China, by Viira (1982) from the upper part of the Paadla Stage of Estonia, and by van den Boogaard (1990) from the Ludlow of Irian Jaya, Indonesia. Samples examined in this s t u d y a r e from t h e U p p e r Whitcliffe Formation and its lateral equivalents, Ludlow Series of Wales and the Welsh Borderland, unless stated.  (Rhodes, 1953), Pa elements from the Upper Whitcliffe Formation. Description. Pa element ( Fig. 2 and PI. 1, figs 1-14): scaphate, with anterior, erect cusp, which is not markedly larger than other denticles. Posterior process with one to four compressed, triangular denticles, inclined to posterior; distal denticle often largest. Denticles may be partially fused, and proximal denticle may be fused to cusp (PI. 1, fig. 9). All denticles lenticular in section, those closest to cusp smooth, posteriormost denticles in mature specimens often with small longitudinal ridge on inner and outer lateral faces. Anterior process adenticulate, weakly developed as extension to ridge on anterior side of cusp.
Lateral process of juvenile specimens present only as fold on inner lateral surface; mature specimens have welldeveloped posteriorly curved process arising immediately posterior of cusp and bearing one to three denticles (PI. 1,figs 3,4,(9)(10)(11)(12)(13)(14); denticles similar to those on posterior process. Basal cavity deepest at junction of posterior and lateral processes; thick layer of spongelike basal body up to 80ym thick present in all specimens. Cavity extends under entire element. White matter clearly defines denticles and characteristically extends downwards almost to termination of anterior process (Fig. 2).
Pb a n d Pc elements (Fig. 3 and P1. 2, figs 1-15): the morphological terminology of Jeppsson (1972, p. 57) is used in this description. Each element pastinate with weakly developed processes. Cusp tall, prominent, displaying sharp postero-lateral and inner antero-lateral edges; inner face between lateral edges proximally falcoid to weakly biconcave, outer face strongly convex. Cusp twisted, becoming lenticular distally, so that posterior lateral edge migrates into lateral position. Short processes arise from sharp edges; those of Pc element more strongly developed than Pb element. Base hyaline, with distinct transition to white matter of cusp; denticles also albid when present.
Pb element (Pl. 2, figs 1-6): with three weakly developed, simple processes. Inner antero-lateral process with single triangular denticle showing distinct suture at base and occasionally barely attached to element (PI. 2, fig. 3 ) . Postero-lateral process rarely develops small stubby denticle fused to inner base of cusp. Outer antero-lateral process adenticulate and only visible proximally, blending into strongly convex outer face distally.
Pc element (Pl. 2, figs 7-15): with inner antero-lateral process better developed than outer antero-lateral and postero-lateral processes. Inner antero-lateral process adenticulate or with up to three denticles of lenticular cross section and circular bases; distal denticle normally best developed. Stubby triangular denticle on posterior lateral Explanation for Plate 2 Figs 1-18. Coryssognathus dubius (Rhodes, 1953) Fig. 15. Pc element, lateral, PM X 1202, sample and locality as for fig. 3, X80. Fig. 16. M element, posterior, I' M X 1242, sample and locality as for fig. 1, X62. Fig. 17. M element, posterior, PM X 1250, sample and locality as for fig. 1, X75. Fig. 18. M element, posterior, PM X 1208, sample and locality as for fig. 3, X100. process blends into inner side of the cusp. Outer anterior process minor a n d adenticulate. Basal cavity triangular, m a r g i n s w h e n complete a r e convex between outer anterior and posterior lateral processes and concave either side of outer anterior lateral process (PI. 2, fig. 11). Within basal cavity, each denticle has own basal pit with later lamellae enclosing each denticle (Pl. 2, figs 9, 11); cavity commonly contains basal body.
M element (Fig. 3 and PI. 2, figs 16-18): makellate form. C u s p c u r v e d posteriorly a n d inwards, compressed, lenticular in cross section with sharp lateral edges. Proximally, posterior face of cusp more strongly convex than anterior which is almost flat i n s o m e specimens. O u t e r lateral p a r t of posterior face striated. Outer lateral process very short, but more strongly developed than inner; bears small compressed denticle separate from margin. Inner lateral process rarely denticulate with denticle confluent with inner lateral margin (Pl. 2, fig. 18). Shallow lenticular basal cavity w i t h posterior margin slightly pinched and anterior margin weakly convex (Fig. 3). Thickened area above cavity hyaline with a distinct transition to white matter of cusp and denticles.

S d S b C
anterior faces flattened; single costa on posterior face. Lateral costae confluent w i t h short, downwardly a n d outwardly directed processes, each bearing a single appressed denticle with its base at cavity margin; processes may be accentuated by breakage of part of cavity lips (PI. 3, fig. 2). Posterior process at base of cusp, short with peg-like denticle e x t e n d i n g axially from costa o n the posterior face of the cusp. Base hyaline; cusp, denticles, costae and posterior process composed of distinct white matter.
Well developed costa on posterior face of cusp e x t e n d s from base t o tip a n d m a y be twisted sinistrally (PI. 3, fig. 4) or dextrally (PI. 3, fig. 5 ) .
Cusp triangular at base with convex lateral faces, becoming lenticular towards tip (Fig. 4). Small posterior process with peg-like denticle developed at base of posterior costa. Striae at junction between base and cusp, diverging either side of posterior process. One or two lateral denticles on either side of cavity; lenticular to rounded in cross section. Deep triangular cavity ranging from equilateral to inequilateral. Hyaline base extends into lower part of cusp at posterior but less extensive anteriorly. Lateral denticles, posterior process and cusp albid.
( Fig. 4 a n d PI. 3, figs 6 -8 ) : tertiopedate, similar to morphotype B but with cusp inclined laterally, a n d denticles arranged asymmetrically either side of cavity. Costa extends entire length of posterior face of cusp to posterior process. Cusp proximally inequilateral triangular to circular with costae occasionally developed; cross section lenticular towards tip, often with one lateral face less convex than the other (Fig. 4). Posterior process with single peg-like denticle inclined slightly denticles on outer margin much more closely spaced than those on inner margin. Denticles lenticular and bicostate in cross section, more strongly developed towards cavity margin.  Fig. 3. Detail of denticles on lower basal margin, X250. Fig. 4. Sc element, lateral, I'M X 1249, sample and locality as for fig. 1, X60. Fig. 5. Sc element, lateral, PM X 1240, sample and locality as for fig. 1, X65. Fig. 6.  Sb elemeiit (Fig. 4 and P1. 3, figs 9-12): tertiopedate, distinctly asymmetrical, with inner costa extending into a downwardly directed process. Cusp twisted slightly inwards, almost ovoid at base, becoming lenticular towards apex. Inner margin flattened or weakly convex (in contrast to Sa/Sb element) and lateral margins weakly to strongly convex (Fig. 4). Uni-, bi-and tricostate forms occur, all with costa on posterior face. Posterior process with peg-like denticle curved slightly inwards. Inner lateral process with single denticle, fused to costa. Outer lateral process directed downwards and outwards with two or three sharp, discrete denticles. Cavity deep and ovoid in cross section with inner margin weakly convex, flattened or sometimes slightly concave. White matter distribution similar to Sa/Sb elements. Sc elerrlent (Fig. 3, P1. 4, figs 1-5): modified dolabrate, with cusp strongly curved to posterior. Cusp bicostate, costae running entire length of anterior and posterior margins; lenticular in transverse section towards tip, proximally flattened on one lateral face and becoming weakly concave near anterior margin. Upper margin of base supports up to four peg-like denticles, with denticle closest to cavity usually most strongly developed, although all denticles m a y be of equal proportion. Denticles bi-costate and Cavity deep, lenticular, with margin flattened on one side (Fig. 3). Area below denticles dark brown, grading into distinctive triangular amber, hyaline area at the base of the cusp (Fig. 3). White matter extends almost entire length of anterior margin but terminates at junction of cusp with base on the posterior margin producing a distinctive oblique line across element (Fig. 3). Remarks. Jeppsson (1972) and Clark et al. (1981) considered Pelekysgnathus dubius and Coryssognathus dentatus to be conspecific, while Klapper and Murphy (1974) regarded 6). them as congeneric. Van den Boogaard (1990) assigned both to Coryssognathus, but distinguished C. dentatus by the presence of a denticulate lateral process on mature Pa elements. Specimens from the Upper Whitcliffe Formation show wide ontogenetic variation (Fig. 2), mature specimens having a lateral process with up to three denticles. One specimen (Pl. 1, figs 11, 12) from Aston Munslow (sample 3911) is almost identical to the holotype of C . dentatus figured by Link & Druce (1972, pl. 2, figs 15, 18, 19), but is part of a population that contains typical C. dubius forms. The size and shape of the denticles on the posterior blade, the position of the cusp and the posteriorly facing inner lateral process suggest C. dentatus is conspecific with C. dubius. One specimen figured by Link & Druce (1972, pl. 2, figs 13, 14) possesses a denticle to the anterior of the cusp, a feature not seen on any specimens from the Welsh ~~ ~.
The suite of elements assigned by Jeppsson (1972Jeppsson ( , 1975 to Distomodus dubius does l o t inc-lude-a Pa element. The element formerly identified as Coryssognathus dentatus and Pelykysgnathus dubius has been suggested by Cooper (1974), Aldridge (1975) and van den Boogaard (1990) to be the possible Pa element for this type of apparatus. Collections from the Upper Whitcliffe Formation (Fig. 1) support this contention as the Pa element commonly occurs associated with the D. dubius suite but never without. All the evidence, including that from the Welsh Borderland, indicates that the names D. dubius and C. dubius are based on material from a single apparatus, and are thus synonymous.
It is common in faunas of scattered conodont elements for the Pa element to be over-represented. However, the Pa of C. dubius is always under-represented in Upper Whitcliffe faunas (Figs 1, 5) and Jeppsson (1979b) noted that in SkAne, the Pa is restricted to a much shorter interval than the ramiforms; when present, is under-represented and of a much more juvenile growth stage. Merrill & Powell (1980) described Pennsylvanian conodont collections that showed similar under-representation and relatively small size of Pa elements, and concluded that the Pa elements were less well developed than their associated ramiforms during early stages of apparatus growth. Van den Boogaard (1990) suggested that the low proportions of Fa elements in collections of Coryssognathus could similarly be explained if juvenile apparatuses had coniform elements that had not yet fused into multi-denticulate elements or if the fragile juvenile Pa elements suffered post-mortem breakage. From our experience of handling juvenile C. dubius Pa elements, they are easily broken. Samples from the Welsh Borderland with most coniform elements also contain relatively few Pa elements (Fig. 1). These large collections of coniforms include some elements morphologically similar to denticles of juvenile Pa elements of C. dubius (cf. Fig. 2). Juvenile Pa elements may, therefore, have been broken during deposition or laboratory processing and their dissociated denticles counted among the coniform elements.

T H E APPARATUS STRUCTURE OF CORYSSOGNATHUS
To assess the apparatus composition of C. dubius, nine samples have been selected that each yielded more than 100 well-preserved elements with a wide size distribution. These samples are the least likely to have been affected by sorting (McGoff 1991), although all samples will have been transported to some degree in the subtidal storm depositional environment of the Upper Whitcliffe Formation. The ratio of Pa elements to other elements remains relatively constant (Fig. 5). Totals from all samples given in figure 1 also show similar relative proportions Figure 5 shows a high relative frequency of P b / P c elements and sc elements in each sample. This suggests that the apparatus contained two pairs of Pbl Pc elements, and that the Pb and Pc positions were both occupied in Coryssognathus, as in Pterospathodus, Pratzogllathus and Promissum (see Mannik & Aldridge, 1989;Theron et al., 1990). However, Pb and Pc elements have been counted together for the present study ( Fig. 1) as abraded PbiPc elements are very difficult to separate. The low numbers of A, B and C morphotypes of the Sa/Sb element ( Fig. 1) suggests that they represent one morphologically plastic Sa/Sb element varying from nearly symmetrical to twisted and somewhat asymmetrical. A similar element was identified in the Pterospathodus apparatus by Mannik & Aldridge (1989), whose Sa 1 Sb designation is followed here pending further confirmation that the axial Sa position is not occupied in apparatuses of this type. The consistently high relative abundance of Sc elements (Figs 1, 5 ) indicates that the apparatus contains two pairs of indistinguishable Sc elements. M elements are more abundant than Pa, Sa/Sb, and Sb elements in most samples, but they are not common enough to suggest the presence of two pairs of M elements in the apparatus. We therefore propose a ratio of 1:2:1:1:1:2 between Pa, PbIPc, M, Sa/Sb, Sb and Sc elements and reconstruct the apparatus of C. dubizrs as septimembrate with a total of 16 elements (Fig. 6). There is no direct evidence from bedding-plane assemblages or clusters of the relative positions of the elements in Coryssogimthzrs, but we have followed the Promissum template in allying the M elements geometrically with the P elements. The relative positions of the P+M group of elements and the S group are schematic, although the proximal ends of the latter do lie to the anterior in most specimens of Proinisszrin (Theron et nl., 1990;Aldridge & Theron, 1993).
The relative numbers of coniform elements vary from sample to sample (Fig. 1). It was suggested by Jeppsson (1972) and later shown by van den Boogaard (1990) that elements of the apparatus incorporated these cones during ontogeny to become increasingly denticulate. Denticles on Pc elements each have a basal pit, incorporated by later lamellae (Pl. 2, figs 9, 11) and some Sc elements show a suture between the cone-like denticle and the base of the cusp (Pl. 4, fig. 2) suggesting incorporation at a later growth stage. The cones are the right size to have been part of a multidenticulate element of the apparatus. The discrete cones, therefore, represent either denticles broken from processes of multidenticulate elements or cones not b_e.ween ~k~~~!~~ incorporated as denticles. It is probable that the discrete cones were attached to the parent element during life by basal tissue, and that fusion by overgrowing crown tissue occurred sequentially for more distal denticles during ontogeny. Some other Silurian and early Devonian prioniodontid taxa appear to have possessed apparatuses similar to that of C. d u b i u s . Early Silurian specimens described a s Rotundacodina aff. R. dubia by Mabillard & Aldridge (1983, pl. 4, figs 1-6) compare closely with Pb, Pc and S elements of C. d u b i u s , a l t h o u g h n o Pa element h a s a s yet been recognised. The early Silurian genus Distoiizodus has been reconstructed with a six-element apparatus (Cooper, 1975;Barrick & Klapper, 1976), but a seventh, erect (Pc) element is evident in material from South China (Aldridge & Wang, u n p u b l i s h e d collections). Similarly, Serpagli (1983) reconstructed the lcriodus woschmidti Ziegler apparatus as seximembrate, although a seventh element type is probably represented by erect specimens he illustrated as " possible e morphotypes" (Serpagli, 1983, fig. 4 G-I).