New British Pliocene species of the Australian Recent genus Austroparadoxostoma Hartmann, 1979 (Ostracoda)

Two new Pliocene species of the genus Austroparadoxostoma Hartmann, 1979; Austroparadoxostoma pliocenica and Austroparadoxostoma sp. are described from the Upper Pliocene deposits of St. Erth, Cornwall. This genus has hereto only been recorded from the Recent of the west coast of Australia.


INTRODUCTION
'The Pliocene specimens described and illustrated herein were initially difficult t o assign to any previously described paradoxostomatid genus. They appeared most closely to resemble the genusAustropar~zdoxostomu Hartmann, 1979 a Recent form from Goode Beach, (Frenchman Bay), near Albany on the west coast of Auslralia. Unfortunately, however, Hartmann did not figure an external view of the type species., A . ventro-mar6:inalis and his description of the species lacked refer,ence t o the hinge and inner lamella. These features could not be appreciated from his il1ustrai:ions which were, in addition, based upon a single male specimen and one empty carapace. Electron micrographs of the Pliocene specimens were sent to Hartmann ftor comparison with the type material. Hartmann (lit. comm., 1981) confirmed that the Pliocene material could be firmly assigned to Austroparadoxostoma, but thought that the hingle structure ofA. ventromarginalis differed from that of the Pliocene specimens. His photographs, he maintained, were too small to obtain detail of the hinge elemLents and suggested that the hinge structure may indeed not be uniform within the genus, as is the case of the closely related Paradoxostoma Fischer, 185.5.
'The occurrence, therefore, of further specimens of Hartmann's genus in the St. Erth Beds has permitted us t o situdy the hinge and inner lamella and offer an emended carapace diagnosis for the genus.
'The type material is housed in the Department o f Palaeontology, British Museum (Natural History).

SYSTEMATIC DESCRIPTlONS
Class  (1973) and Maybury & Whatley (1980). This paper represents the third in a series in which this diverse fauna is discussed.
(PI. l., figs. 1-7; Fig. 1). Derivation of name. Latin, with reference to the occurrence of the species in the Pliocene deposits of Cornwall. Diagnosis. A medium sized paradoxostomatid with 2 well developed ridges; one traversing the entire length of the ventral margin, the other commencing at the ventral extremity of the caudal process; in the RV this ridge is obliquely disposed; but in the LV it parallels the keel-like, ventrolateral ridge. In both valves the dorsalmost ridge terminates near mid-length. Surface smooth. Normal pore canals simple, open, round, scattered and few in number. Inner lamella, hinge and muscle scars as for the genus. The inner lamella bears a weakly developed selvage and flange.
Holotype. RV, no. 0s 12 130. Paratypes: LV, 0s 12 13 1 ; RV, 0s 12132 and LV, 0s 12133. Description. Carapace almost assumes a parallelogram form in lateral view. Anterior margins oblique. Posterior margin in RV gently curved inward from a subdorsal, bluntly truncated caudal process to a well defined, ventrolateral ridge but in LV it is oblique with a rounded caudal process. In both valves the posterior and ventral margins meet obtusely, whereas the anteroventral angle is acute. Dorsal margin of RV convex anteriorly and slightly concave posteriorly, but gently arcuate in LV. Ventral margin of both valves convex to slightly sinuous with a prominent ventrolateral ridge produced dorsal to the ventral margin and projecting to the vertex of the valves which is at mid-length. The ventrolateral ridge does not obscure the ventral margin, but overhangs the anteroventral angle of both valves. Maximum length measured from the anterior extremity of the ventrolateral ridge to the posterodorsal extremity of the caudal process; maximum width at mid-length and coincident with the vertex of both valves. Each valve is approximately 3 times as long as it is high. In dorsal aspect, each valve forms a shallow arc within the greater arc of the ventrolateral ridge. The commissure is straight, but slightly concave in the anterior Weakly calcified, transparent to opaque.

Material
Surface smooth with, in addition to the ventrolateral ridge, a second ridge commencing at the ventral extremity of the caudal process. In the RV this ridge is obliquely disposed in the posterior l/4 and then becomes somewhat meandrine and less elevated. It does not traverse the anterior l/4 of the valve. In the LV, however, it is parallel to the ventrolateral ridge and tapers slightly posteriorly, terminating a little before mid-length. Normal pores simple, open and circular in cross section. They are few in number and irregularly situated.
Inner lamella very weakly calcified. There is a subtriangular vestibule anteriorly and a very narrow, crescentic vestibule posteriorly; otherwise the line of concresence is coincident with the inner margin. Anteriorly and posteriorly the inner lamella is relatively wide with its inner margin asymmetrically curved. Selvage and flange very poorly developed. There are 4 straight marginal pore canals anteriorly and one bifurcate and one straight canal posteriorly and 7 to 9 straight canals ventrally.
The hinge structure does not readily approximate to that of any previously described ostracod. In the RV there is no anterior terminal element, but the valve edge is thickened somewhat peripherally. The median element is composed of a broad, deep groove, which is locellate dorsally and closed ventrally by a broad ridge. The median groove is of constant width, except in its anterior l/4 where it tapers slightly distally. The ventral ridge is widest medianly and tapers anteriorly.  The adductor muscles are situated medianly with the dorsal-most scar just below mid-height. They comprise an oblique row of 4 contiguous or closely adjacent, elongate scars. Anterodorsal to these is a fulcra1 point situated very close to an irregularly shaped frontal scar.
Instar stages from Adult to ?A-3 have been recognised and it is notable that the juvenile valves are so weakly calcified that there is a tendency toward splitting along the juncture between the inner and outer lamellae; hence only the outer lamella is preserved. Erth. It has been recovered from both the brown calcareous and blue marine clay levels and from a mixed sample in which clay dominated. The occurrence of the new species in this mixed sample has rendered it impossible for the authors to give anything but a general description of the type level. It has not been found in deposits of equivalent age from East Anglia, N.W. France or the Vienna Basin studied by the authors.
A . pliocenica differs from the type species, A ventromarginalis in being slightly less elongate, in the details of its muscle scar arrangement and possibly in its hinge structure. It is notable that both species occur as rare members of their respective faunas and that the genus itself should show such a widely disjunct geographical distribution over the time interval extending from the Upper Pliocene to the present day.
(Pl. 1, fig. 8) Diagnosis. A ustroparadoxostoma with smooth surface and few open normal pore canals; characterised by well developed ventrolateral ridge and short ridge extending obliquely anteroventrally from just above the apex of the bluntly truncated caudal process below which are a series of short radiating, subvertical grooves. Internal details as for genus. Material. 1 adult RV. Locality and horizon. As for A . pliocenica sp. nov. Dimensions (mm) Length Height RV, 0s 12134 0.52 0.25 Remarks. This species more closely resembles A . pliocenica sp. nov. than the type species. It differs from the RV of the former in its possession of fine, subvertical, radiating grooves and in being proportionally and absolutely higher and less elongate. In addition, the ridge extending from the caudal process is not as well developed as in A . pliocenica. The species is left in open nomenclature because of the extreme paucity of material.