Ozarkodina remscheidensis plexus conodonts from the upper Ludlow (Silurian) of the Welsh Borderland and Wales

Three subspecies of the conodont Ozarkodina remscheidensis Ziegler, 1960, are described from the late Ludlow (Silurian) of the Welsh Borderland and Wales, including a new subspecies O. r. baccata. Four discrete element types which occur in association are also described and interpreted as possible members of the Ozarkodina remscheidensis apparatus.

S assov (1971) dshkova (1972) Jeppsson (1 975) Ebner (1 976) Viira (1 983) Drygant (1984)  (Aldridgc & Schonlaub, 1989)) or geographically separated (e.g. Apsidognathus tuberculatus tuberculatus Walliser, 1964 andA . t. arcticus Armstrong, 1990). Subspecies have also been applied to morphological variants that overlap in space and time (e.g. Higgins (1975) for the Carboniferous species Gnathodus girtyi). Some authors have preferred to designate morphotypes for the latter situation (e.g. Klapper & Murphy (1974) for Ozarkodina conjuens), giving each morphotype a different Greek Ictter. The subspecies designated here, 0. r. baccata, forms a distinct and potentially biostratigraphically useful population in late Ludlow samples from the Welsh Borderland, although specimens of the other two subspecies do occur in association. As subspecific categories already exist for 0. remscheidensis, it is preferential to differentiate this population as a third subspecies, rather than add morphotype letters to one o f the existing subspecies. Posterior process with four to six denticles; two proximal denticles of similar shape and size to cusp; one specimen with tiny accessory denticle fused to proximal denticle. Remaining denticles increasingly smaller distally as process diminishes posteriorly to three quarters medial height; process terminates with small step-like extension beyond last denticle. Posterior aboral surface slightly concave. Anterior process same length as posterior; denticles number four in juvenile specimens and up to six in   (Walliser, 1964 (Ziegler, 1960). Pa element, PM X 1277, sample 31b/3, Whitcliffe Formation, foreshore of Severn Estuary, Tite's Point, Gloucestershire (loc. 31b), x45: Fig. 21. Lateral; Fig. 24. Oral.

Ludlow Series
Upper Whitcliffe Formation (and lateral equivalents) more mature specimens, of uneven size, crowded close to cusp, becoming slightly more isolated distally. Terminal denticle slightly lower than adjacent denticle, with anterior margin sloping very slightly to anterior and rounded antero-aborally. Anterior process with straight aboral margin, decreasing much more gradually in height than posterior process. Shallow cavity extends entire length of element, flared at mid-length with rounded lips; flaring asymmetrical, much stronger on one side of element. Oral surface of flare pinched. White matter present in cusp and all denticles, mostly as single solid block; base of white matter parallel to aboral margin of element or rising anteriorly from beneath cusp (Fig. 2 (Ziegler, 1956). Mashkova (1 972), however, used a bedding plane assemblage to reconstruct the apparatus of 0. steinhornensis and showed that it possessed an Sa element with a denticulate posterior process. On Sa elements of the reconstructed apparatuses of the taxa discussed here a posterior process is not developed (see Mashkova (1972), pl. 2), and Klapper 8z Murphy (1974, pp. 39, 40) argued that the appropriate species name should be remscheidensis rather than steinhornensis. 0. r. baccata is distinct from other late Silurian subspecies, but is closer to 0. r. eosteinhornensis than to 0. r. remscheidensis. There is a clear angle between the basal margins of the anterior and posterior process of the Pa element of 0. r. remscheidensis, which also displays well developed denticles towards the anterior. The Pa element of 0. r. baccata is distinct from that of 0. r. eosteinhornensis as it has a central, asymmetrically flared cavity pinched on one side and broader denticles which are not  (Walliser, 1964 fused above the cavity. Philip (1966, fig. 8A) figured a similar Pa element from the Lower Devonian of Australia as Spathognathodus steinhornensis buchanensis Philip, but the denticles vary from relatively large, crowded and almost discrete to small, crowded and fused. The anteriormost denticles of S. s. buchanensis are sometimes enlarged (Philip 1966, fig. 8B), whereas the denticles on 0. r. baccata do not show a marked size increase anteriorly and the anteriormost denticle is always smaller and lower than its neighbour. The majority of S. s. buchanensis specitnens figured by Philip (1966, pl. 2) are markedly bowed in oral view whereas all the specimens of 0. r. baccata from the Welsh Borderland have straight to very slightly bowed blades (Pl. 1). Schonlaub (1980, pl. 6, fig. 11 as 0. r. rernscheidensis) and Drygant (1984, pl. 13, fig. 11 as Spathognathodus costeinhornensis) also figured specimens with similar dentition to 0. r. baccata but the nature of the cavity is unclear on both illustrations.
The apparatus of 0. r. baccata is unknown. It is probable that the Pb element is indistinguishable from that of 0. r. eosteinhornensis, which is relatively over-represented in our collections (Fig. 3). Aldridge (1985, p. 90) suggested that the Pa element 'is associated with ramiform elements bearing crowded denticles of alternating size'. An Sb element with appropriate morphology (Fig. 5f) occurs in the Whitcliffe Formation and is possibly from the apparatus of 0. r . baccata, although it has not yet been found in association with the Pa element. No other unassigned elements with alternating denticulation have been recovered from the late Silurian of the Welsh Borderland. Specimens identified (Figs 3, 4) as 0. r. cf. baccata or 0. r. cf. eosteinhornensis are incomplete or abraded. Many of the faunas have been significantly affected by hydrodynamic sorting and Pa element over-representation is common in other species from the same samples. This could explain the poor representation of the other elements of the apparatus. Jeppsson (1975) noted that conodonts of the 0. steinhornensis (remscheidensis) group tend to form local populations. 0. r. baccata probably represents a localized subspecies of 0. remscheidensis that may have been confined to the Welsh Borderland in the late Silurian.

Ozarkodina remscheidensis remeheidensis
Holotype. Geologisches Landesamptes Nordrhein -Westfalen, Krefeld, FRG., No. G 88 b. Figured by Ziegler, 1960, pl. 13, fig.  4, from Pack 6, sample E, Remscheid Anticline, Germany (see Ziegler 1960, fig. 2). Material. Two Pa elements. Localities and horizons. (Figs 3,4) Whitcliffe Formation: sample 31b/3, Tite's Point, Gloucestershire (loc. 31b). Upper Perton Beds: sample 161/1, Perton, Hereford & Worcester (loc. 23b). A single specimen (identified by RJA) was also reported from 60 cm below the Temeside bone bed in the Ledbury Formation (Piidoli) of the Teme River Section, Ludlow (GR SO 522 742), by Antia (1979, p. 117). Description. Pa element carminate with cusp slightly to anterior of cavity. Aboral margins of processes inclined upwards away from cavity to form angle of 150". Denticulation uneven. Posterior process decreases in height distally, although termination broken on one specimen; proximal denticle large and similar to cusp with following three denticles decreasing rapidly in size. Posterior and anterior processes thickened parallel to oral margin at level half way between aboral margin and base of denticles. Anterior process longer than posterior, of constant height, with proximal two denticles small, followed by a tiny fused denticle. Fourth, fifth and sixth denticles larger, triangular, more isolated and of roughly equal size. Terminal denticle small, extending to three quarters height of rest of blade. Cavity lips asymmetrical, more laterally flared on one side and slightly obliquely pinched either side of blade; in lateral view, cavity lips inclined parallel to posterior aboral margin. Cavity tapering, extending under entire length of anterior process and unbroken part of posterior process. White matter fills cusp and all denticles with base subparallel to aboral margin. Accessory and first. denticle, and between first and second denticles (Fig. 2).

remscheidensis ssp.
M element morphotype A (Fig. 2, Fig. 5c) ?1975 Hindeodella steinhornensis ssp. 1. Jeppsson: 4 3 4 ,pl. 11,fig. 50. Material. Three specimens. Localities and horizons. (Figs 3, 4) Whitcliffe Formation: sample 8/l, Diddlebury, Corve Dale, Shropshire (loc. 8). Upper Perton Beds: sample 160/1, Perton, Hereford & Worcester (loc. 23a). Description. Bipennate; cusp stout with convex inner face, flatter outer face, sharp anterior and posterior edges. Anterior process as downward extension of cusp with three strongly fused denticles. Posterior process increases in height distally, most complete specimen (Fig. 5c) broken after sixth denticle; proximal denticles slender, small, sharp and crowded, becoming wider and more isolated distally. Cavity flared beneath cusp, tapering gradually to termination of anterior process and present as thin groove under entire preserved part of posterior process. Basal body may be retained within flared cavity, extending below cavity lips. White matter extends under entire element apart from basal cavity lips (Fig. 2). Remarks. A specimen of this morphology was figured as a subspecies of 0. steinhornensis by Jeppsson (1975) although alternating denticulation typical of the remscheidensis (steinhornensis) plexus is absent. The specimen figured by Nicoll & Rexroad (1987, pl. 3.1, fig. 4) as 0. r . eosteinhornensis is very similar, but the anterior processes on the Welsh Borderland specimens are more pointed and the specimens have a fused denticle next to the cusp on the posterior process (Fig. 5c). This is a candidate M element for the apparatus of 0. r. baccata, but the available material is too limited to test this possibility.
The reconstruction of 0. r. rernscheidensis by Mawson (1986) includes a broadly similar M element, but the Welsh specimens do not possess alternating denticulation. Lane & Ormiston (1979) also figured a similar M element as 0. r . remscheidensis but with less fused denticles. The specimen figured by Mastandrea (1985a, pl . 1, fig. 4) has a straight, not curved, posterior process, similar to that identified by Bultynck (1 971,pl. 4, fig. 7) as Neoprioniodus bicurvatus (Branson & Mehl). A specimen figured by Borremans & Bultynck (1986, pl . 1, fig. 20) as 0. r. remscheidensis has the same distinctive curved posterior process but the specimen is poorly preserved and the denticulation is not evident on the illustration. Sb element. (Fig. 2, Fig. 5 0 Material. Two specimens. Localities and horizons. (Figs 3, 4) Whitcliffe Formation: sample 39/1, Aston Munslow, Corve Dale, Shropshire (loc. 7a); sample 8/1, Diddlebury, Corve Dale, Shropshire (loc. 8). Description. Bipennate. Cusp lenticular in section, posteriorly inclined; posterior edge rounded, anterior edge sharp. Anterior process curved slightly inwards with alternating denticulation; repeated sequence of two broad denticles followed by narrow fused denticle, denticles increasing in size until break in specimen after fourth large denticle. Posterior process proximally straight but curved downwards distally at about 45' , increasing in height towards abrupt tip; proximal denticle small and fused to base of cusp, followed by two larger denticles. Alternating denticulation on straight part of process, with increasing size until largest denticle in third group of denticles is almost cusp sized; downcurved distal part of process with two large medial denticles flanked by smaller neighbours. Cavity small, lips flared to greater extent on inner margin. White matter reflects the inclination of the denticles on the anterior process and alternating denticulation of the posterior process (Fig. 2). Remarks. The element possesses alternating denticulation which is a characteristic of some members of the remscheidensis plexus. The element possibly belongs to 0. remscheidensis eosteinhornensis, but its posterior process is curved downwards through 45" and is unlike the Sb element figured from the apparatus of 0. r. eosteinhornensis by Mashkova (1972, pl. 2, fig. 28). It is a possible candidate Sb element for 0. r . baccata.

Manuscript received July 1994
Manuscript accepted February 1995