The genera Muellerina Bassiouni, 1965 and Thaerocythere Hazel, 1967 from the Neogene of Northwest Europe

Twenty one species of the genera Muellerina and Thaerocythere are considered. Of these, 11 species, including the type species of both genera, have been described previously. Two species are left in open nomenclature due to inadequate numbers but eight are described as new. These species are: Muellerina latimarginata (Speyer, 1863), M. abyssicola (Sars, 1866), M. lacunosa (Jones, 1857), M. praeabyssicola Uffenorde, 1981, M. parvipunctata Uffenorde, 1981, Thaerocythere crenulata (Sars, 1866), T. hoptonensis (Brady, Crosskey & Robertson, 1874), T. mayburyae Cronin, 1991, T. oedichilus (Brady, 1878), T. trachypora (Jones, 1857), T. trapezia (Brady, 1878), and the new species M. dolabrata, M. metabyssicola, M. orygyma, M. pliocenica, T. biformis, T. praecrenulata, T. vermiculata, T. woutersi and T. wittei. The phyletic relationships of these northwestern European species are considered.


INTRODUCTION
The type material of Jones (1857Jones ( , 1870, Jones & Sherborn (1887, Brady, Crosskey & Robertson (1874) and Horne & Kerr (1989), which are presently deposited at the British (Natural History) and Hancock museums, have recently been re-examinedl. In the light of a recent study by Liebau (1991) and the subsequent re-examination of northwestern European Muellerina and Thaerocythere species, the present authors have attempted to unravel both the taxonomic and evolutionary problems that presently surround the Plio-Pleistocene species of these genera. The biostratigraphical and ecostratigraphical implication!; arising from this will be published elsewhere (Wood & Whatley, in press; Wilkinson et al., in press).

PREVIOUS WORK
The most significant contributions, in the context of the present study, are undoubtedly those of Wouters (1979), Uffenorde (198@-1990), Liebau (1991) and Cronin (1991). The investigation of adductor muscle scar patterns and carapace characteristics of Muellerr'na by Wouters 1c1979) suggested a tendency towards subdivision within the former and changes in the outline and ornamentation of the valves. He suggested that a gradual change in the composition (grouping sensu Wouters, 1979, p. 225) of the muscle scars could be seen through time. However, the evolution of the muellerinids within the southern North Sea basin was justly considered by Uffenorde (1986) to be much more complicated and not just a simple case of muscle scar subdivision or the abatement of reticulation (Uffenorde, 1980(Uffenorde, , 1986. 1nve:stigations into the phylogentical behaviour of sculptural components (macrorecticulation) in Tertiary to Recent trachylebrids and hemicytherids (Liebau, 1977(Liebau, , 1978 has led to the taxonomical revision of 'Quadracythere-like hemicytherids' in which Liebau (1991) erected a number of new Thaerocythere subgenera and species. Similarly, Cronin (1 99 1) has described a number of new Muellerina and Thaerocythere species from the Pliocene/Pleistocene of Tjornes, Iceland.

LITHOSTRATIGRAPHY, BIOSTRATIGRAPHY AND BIO-CHRONOLOGY OF NORTHWESTERN EUROPEAN NEO-GENE DEPOSITS
The lithostratigraphical, biostratigraphical and biochronolgical framework for the British and Dutch Neogene successions have recently been reviewed by . In the same paper, a number of new ostracod-based ecozones were erected on the basis of palaeoclimatic-related changes in the generic composition of Upper Miocene and Pliocene ostracod assemblages. The extent of this new ecozonation and previous foraminifera1 and palynological biostratigraphical schemes are presented in Figs 1 and 2.    Puri, 1953Subfamily Hemicytherinae Puri, 1953 Genus Muellerina Bassiouni, 1965 Type species. Cythere latimarginata Speyer, 1863. Emend. Diagnosis. Carapace subovate to elongate/subquadrate, with thick, generally smooth, marginal rims imparting a truncated aspect to the end margins in dorsal view. Anterior and posterior margins may be denticulated. Dorsal and ventral margins converging posteriorly. Upper part of posterior margin straight or weakly concave, lower part convex, together forming weak posteroverntral caudal process. Dimorphic, males proportionally more elongate, less inflated in dorsal view. Ornament pitted/reticulate with a single concentric row of fossae inside the marginal rim and subcentral tubercle. Maximum length below mid height, greatest height at or just anterior of anterior cardinal angle, maximum width at sub-ce:ntral tubercle.
Row of four adductor muscle scars, middle two may be subdivided; two frontal scars, ventral scar may be bilobate. Hinge holamphidont with short thick teeth. Narrow anterior and posterior vestibula may be present; marginal pore canals sinuous/straight, simple, 2&30 anteriorly; 1&14 posteriorly. Remarks. The re-examination of adductor muscle scar patterns in Muellerina, using a quite separate database of Upper Miocene to Recent species from northwestern Europe, clearly supports the original conclusions of Wouters (1979). Using composite counts for muscle scar groups 1/2 and 3/4 our results confirm the progressive division of median scars from the Miocene to the Recent. The adductor muscle scar pattern of 200 adult specimens, belonging to six species, were analysed from Neogene sites in France, England and The Netherlands. The relative percentage of the three adductor muscle scar groupings, through time, are represented in Fig. 7.  1965 Muellerina abyssicola (Sars); Bassiouni: 510, pl. 1, figs 3-6. 1969 Murrayiira latimarginata (Speyer); Yassini: 84, pl. 24, fig. 5;pl. 39, figs 15 & 16 (non Speyer, 1863). Diagnosis (adapted after Athersuch et al., 1989) (Lord, 1980), Bay of Biscay (Yassini, 1969;Peypouquet, 1971) and the Hoxnian of Denmark and northern Germany (Bassiouni, 1965). Its present distribution on the northeast Atlantic shelf, as recorded by , is from the Bay of Biscay north to the Barents Sea. Muellerina lacunosa (Jones, 1857) 1857  (Doppert, 1980) of the Oosterhout Formation, Holland; Coralline Crag Formation, Waltonian, Butleyan and Newbournian stages of the Red Crag Formation, East Anglia, UK. Remarks. Muellerina lacunosa (Jones) bears a strong resemblance to its nominated ancestor M . parvipunctata Uffenorde (1981) and also to Muellerina orygyma sp. nov. However, M . lacunosa can be easily distinguished from M . orygyma sp. nov. as the ornament of the former is constructed from a dense network of large fossae and also comprises two posterior tubercles. M . parvipunctata although similar in outline has an ornament dominated by a strong, posteriorly bifurcate, ventromarginal ridge and rather small cellate fossae. Uffenorde,198 1 Distribution. Upper Miocene; Reinbekium and Gramium of northwest Germany. Remarks. This species is similar in outline to M . lacunosa (see remarks) but appears to be restricted both stratigraphically and geographically to the Upper Miocene of northwest Germany where it can occur with both M . praeabyssicola Uffenorde, 1981 and Muellerina latimarginata morphotype 1 Uffenorde (198 I ,p.165). Derivation of name. Latin, dolabratus=shaped like an axe. With reference to the 'axe head' shape of the carapace in lateral view. Diagnosis. A species characterized by its axe head shaped carapace in lateral view. Almost symmetrical about the median height line. Anterior marginal reticulate with fine radial ridges. Reticulation composed of 15-1 8 large, circular/subovate, occasionally linked, cellate fossae with normal pores proximal to muri. Additional, scattered normal pores, common. Posterodorsal and posteroventral tubercles present, the latter more strongly so.  Wouters, 1979)  Description. Carapace subquadrate, dorsal margin convex, ventral sinuous. Anterior margin broadly round, posterior somewhat truncated. In dorsal view the outline is sinuous but truncated in front of the posterior marginal rim. Maximum length well below mid height. Eye tubercles conical, more weakly developed :in males. Anterior rim possesses four to seven fine, transverse, arcuate/sinuous riblets. Ornament consists of 15-1 7 subcircular/ovate cellate fossae, occasionally linked below a sturdy subcentral tubercle. Single, delicate, rib angled from the posteromedian region to the subcentral tubercle. Sensillum pores simple, small, intermural or peripheral on the solum. Avestibulate, inner lamalla broad, marginal pore canals simple straight, 24-26 anteriorly, nine posteriorly, 17 orally. Holotype ARV-F MPK9772 OSCl OR7a 0.77 0.41 Paratype LV A-1 MPK9773 OSCl OR15 0.68 0.37 Distribution. Lower Pliocene; FB zone, Oosterhout Formation, the Netherlands; mid-Pliocene; Coralline Crag Formation, Orford and Sutton Knoll, East Anglia, UK. Description. Carapace elongate subquadrate/ovate to trapeziform. Dorsal margin of LV concave, raised anterior cardinal angle, RV sinuous. Ventral margin, in both LV and RV, convex. Anterior margin rounded. In dorsal view, posterior listrically truncated. Maximum length just below the mid height. Margin rims broad, rounded in dorsal view. Conspicuous eye tubercle situated just beneath anterior cardinal angle. Coarsely reticulate with rounded/subovate fossae, quadrate submarginally. Fossae and muri suppressed with respect to marginal reticulum. Plicate ventromarginal ridge terminates in the posteroventral region in the form of an alate swelling. Posterodorsal tubercle more weakly developed. Arcuate, ridge, forming an inter-connection between tubercles situated behind mid-point on the dorsal margin and below the anterior cardinal angle. Avestibulate, moderately broad inner lamalla, marginal pore canals, straight, numerous; 24 anteriorly, 11 orally and nine posteriorly. Remarks. Although much larger, both the outline of the carapace and the disposition of reticulation of this species are similar to those of M . praeabyssicola and M . abyssicola. However, the posterior tubercles of M . praeabyssicola are greatly inflated and dorsomedian swellings weaker. Although retaining the 'S-shaped dorsomedian structure in the form of an insubstantial, raked, murus; the ornament of M . abyssicola is conservative: by comparison.

Muellerina parvipunctata
A fourth species, described by C. A. Maybury @en. comm.) from the Re:donian (Upper Pliocene) of northwest France, may also be a member of the 'abyssicokz' lineage.  (Wouters, 1979), Belgium. Pliocene, Coralline Crag Formation at Sutton Knoll and Orford, Suffolk, UK. Description. Carapace, robust, elongate/subrectangular. Dorsal and ventral margins sinuous. Anterior broadly rounded; posterior somewhat truncated. In dorsal view the outline is inflated with the maximum width behind mid-length. Moderately well developed eye tubercle present just below the anterior cardinal angle. Ornament coarsely pitted/reticulate, fossae inside marginal rim rectangular. Rounded/subovate fossae surround the subcentral tubercle while a discrete cluster occur anterodorsally of this tubercle. Secondary, fine, puncta especially visible in juveniles. Weak, longitudinal, plicate muri, arising from the subcentral tubercle terminating posterodorsally. Very narrow anterior vestibulum, posterior avestibulate, ventral frontal muscle scar bilobate, inner lamella moderately broad. Marginal pore canals simple, straight/sinuous, 27 anteriorly, 14 posteriorly and at least four orally. Remarks. Muellerina orygyma sp. nov. occurs most commonly in association with M . lacunosa (Jones) in the early Pliocene successions of the Netherlands. Muellerina orygyma sp. nov can be distinguished from the latter species by both its swollen mediodorsal region and reduced posterior tubercles, however, it is apparent that intermediate forms also exist between the two end member species; this being especially prevalent within ecozone B, mid Pliocene .  fig. 19. listric truncated in front of posterior margin. Maximum height at or just in front of anterior cardinal angle. Triangular eye tubercle situated in front of the anterior cardinal angle. Anterior margin consists of an outer denticulated lip, broad marginal rim with elongate: pits, parallel with margin, occasionally connected to form a sinuous depression. This area is separated from a single concentric row of amorphous fossae by a delicate excavate rib. Poijterior margin broad, with five subcircular depressions, lipped, denti'culated posteroventrally. Ornament, tuberculate, prominent dlorsomedian, posterodorsal and posteroventral swellings. The latter structure tapers anteriorly to form a plicate ridge terminating in the ventromedian field. Dorsal tubercles inter-connectNed via an excavated rib. Scattered, subcircular, cellate fossae clustered posterodorsally. Sensillum pores common, intramural and simple. Avestibulate, inner lamella moderately broad, marginal pore canals simple, straight; 24 anteriorly, 201 orally and 13 posteriorly. Family Thaerocytheridae Hazel, 1967 Subfamily Thaerocytherinae Hazel, 1967 Genus Thaeroc-ythere Hazel, 1967  (Notopleura) and a questioned third have been erected by Liebau (1991) using sculpture mapping of the mesh (or cell) patterns, pore cones and muscle scars. It is clear from Liebau's work that a number of the Tertiary thaerocytherid species, especially those outside of the European realm, have no or only questionable affiliations with these new subgenera; most notably are T. sugittata Liebau, 1991;T. mayburyae Cronin, 1991; T. woutersi sp. nov.; T. ruespelensis (Uffenorde, 1981); T. schmidtae (Malkin, 1953) and T. carolinensis Hazel, 1977. Without creating a number of new monospecific subgenera, which the present authors feel would only cause additional complications, difficulties in assigning species to the subgenera level cannot be overcome. We have, therefore, refrained from using the new subgenera with respect to this genus.
Diagnosis. Elongate, subovate in lateral view; dorsal margin obliquely convex, ventral sinuous. Ornament consists of subovate fossae, marginally subquadrate; radial muri raised with respect to concentric component; pore-conuli common, situated on micropappilate solum where they are proximal to the muri, also present in subconcentric row mid-way between centre and margin (most common above mid-height) were they may be depressed. Well developed anterodorsal and short posterodorsal ridges, latter angled between posterior cardinal angle and subcentral tubercle. Thaerocythere oedichilus (Brady, 1878) 1878 Cythere oedichilus nov. sp. Brady: 388, pl. 64, figs la-ld. 1967 Thaerocythere? oedichilus (Brady) Remarks. On the basis of macroreticulation Liebau (1991) placed this species and both T. crenulata and T. sagittata Liebau, 1991 within Thaerocythere (Thaerocythere) Liebau, 1991. The outline of this species resembles T. crenulata in many respects although its ornament is markedly different.

Site Length Height
Thaerocythere trachypora (Jones, 1857) (PI. 3, fig. 1 Remarks. It appears likely that the rather worn specimens of T . whatleyi Cronin, 1991 from the Pliocene of Iceland are synonymous with this species. T. trachypora has retained a number of traits common to T. vermiculata sp. nov., notably two equally robust, anteromedian, longitudinal ridges and the general structure of both raised and depressed muri in the posterodorsal region. Thaerocythere trapezia (Brady, 1878) 1878 Cythere trapezia nov. sp. pl Wouters, pers. comm.). Description. Heavily calcified, sub-trapezoidal in lateral view. Dorsal margin of RV straight with posterior irregularities, LV arched, ventral margin convex with convexity behind anterior margin. Anterior margin finely denticulated, obliquely rounded with broad angular margin. Posterior denticulated, subtruncated, dorsal region straight or concave, extremity below mid height. Eye tubercle heavily swollen, broadly conical and mounted on submarginal murus. Ornament composed large, irregular, subtriangular/subovate. Radial and concentric murus equally well developed.
Pore-conuli commonly developed, with apophysis, peripheral on micropapillate solum. Robust, submarginal, ridge connecting anterior cardinal angle with posteroventral region, posterior termination inflated, angular and bifurcate. Dorsal margin reinforced with irregular, lateral, ridge forming an inclined 'U' at the posterior cardinal angle. Two 'pinched' nodes developed below dorsal margin in the dorsomedian field.
Inner lamella broad, radial pore canals numerous and straight; 45 anteriorly, 6 orally, 17 posteriorly; selvage broad, well developed. Remarks. Two ecophenotypes of T. biformis sp. nov. occur, both forms exhibit an exceptionally high level of reciprocity in the disposition of macroreticulation. The first, and oldest form, is predominantly found in sediments of upper Miocene age (Diest Formation, Belgium, K. Wouters pers comm., and the FC1 Zone, Oosterhout Formation) and is considered by  to have existed in a subtropical environment. This thermophilic form is characterized by finely chiselled muri. The second, cryophilic, form is restricted to ecozone A (first phase Lower Pliocene cooling) of the Netherlands and is characterized by a plicate swelling of muri and ridges. The earlier, thermophilic, form of T. biformis sp. nov. is similar, in many respects, to T. gramanni Liebau, 1991  Derivation of name. Latin; yrae = before. Pertaining to its possible ancestral links with T. crenulata (Sars, 1865). Diagnosis. Carapace subquadrate in lateral view; anterior obliquely rounded, finely denticulated. Ornament composed of a first order polygonal, sub-radial, reticulation with low muri; more elongate parallel to anterior margin. Second order fabric of small sub-triangular/polygonal muri, surrounding commonly marginal pore-conuli. First and second order reticulation, sub-radial disposed, about finely punctate subcentral tubercle. Polygonal, first order fossae with low relief muri, more elongate parallelled to anterior margin. Second order composed of a fine tesselation of subtriangular/polygonal micro-muri surrounding, commonly peripherally sited, pore-conuli. Muri suppressed or absent at dorsal margin.
Inner lamella moderately wide, avestibulate, marginal radial pore canais, numerous; 12 posteriorly, nine orally and 27 anteriorly, selvage moderately developed. Remarks. Proposed ancestor of I: crenulata (Sars, 1866). Size and distribution of primary macroirecticulation in both species is clearly analogous, however, the secondary tessellation of the solum in the Pleistocene/Recent species is much simplified, being finely punctate or papillate. Both species are cryophilic, T.
Selvage broad, well developed. Remarks. Its outline in lateral view is similar to that of another Upper Miocene species T. nodoreticulata (Bassiouni, 1962). Both species are characterized by a serrated dorsal margin, however, 7 ' . woutersi sp. nov. is both longer and possess a rather distinctive ornament, whereas in T. nodoreticufata the celation of the fossae is complete. Distribution. Pliocene; Coralline Crag Formation, Orford, Suffolk, UK; ?Mactra Zone, Tjornes Iceland (Cronin, 1991). Remarks. In lateral view T. sp. B is clearly allied with T. hopfonensis [(Brady et al., 1874)