The Rockalliidae, a new family of Cainozoic Cytheracean Ostracoda

The Rockalliadae, a new family of Cytheracean Ostracoda, is erected based on Rockallia Whatley, Frame & Whittaker, 1978 and an undescribed genus from the Tertiary of Argentinian Patagonia. Rockallia, hitherto monotypic, is augmented by the description herein of five new species: R. eocenica, R. vscripta and R. inceptiocelata from the Eocene, Neogene and Quaternary of the S.W. Pacific respectively and R. woutersi and R. sp. from the Oligocene and Miocene of N.W. Europe. The older species seem to have lived in relatively shallow water while the younger species are exclusively bathyal and abyssal. The genus is rediagnosed and its origins, evolutionary development and dispersal are discussed.


INTRODUCTION
As the trivial name of the type species (enigmatica) suggests, the genus Rockallia presents a number of problems with respect to its origins and affinities. When originally described by Whatley, Frame & Whittaker (1978), only a single species was known. These authors faced something of a dilemma in assigning the original material to a supra-generic category and summed up the situation in the following way: "This new genus is difficult to assign with certainty to any group of Ostracoda. Although some 200 specimens have been encountered to date none have well preserved appendages although some have 'mummified' soft parts. The possession of four adductor scars in a vertical line would seem to indicate cytheracean affinities but the situation of these scars in a dorso-median position, the narrow and primitive inner lamella and the shape and outline are suggestive of the Platycopina. The genus is, on these grounds and in the absence of soft parts, tentatively referred to the latter suborder despite the fact that the overlap relationship of the valves militates against this." The present authors are of the firm opinion that this tentative assignment to the Platycopina is quite incorrect and that the genus is a member of the superfamily Cytheracea of the Podocopina. The new material described in this paper and further study of the type species, allow us to be certain of this, the principal evidence being the possession of four adductor scars and a single frontal scar.
One of us (R. C. W.) has received written communications from a number of colleagues concerning the taxonomic status of the genus. However, in the light of the new species described herein, the authors are confident of the correctness of erecting a new cytherid family to accommodate Rockallia and an, as yet, undescribed genus from the Tertiary of Argentina. One of us (R. C . W.) has seen the latter in the collection of Lic.H.V. Valicenti. It is of Oligocene age and, in its left valve outline, is very similar to Rockallia although the right valve is somewhat dissimilar. In size, musculature and the nature of the hinge and inner lamella, the two genera are very similar although the undescribed genus (PI. 2, figs. 17-19) has an eye, a structure which is lacking in all species of Rockallia. The ornament is also different. A description of this genus will be published elsewhere in due course.
The importance of the Argentinian material, however, is not merely in establishing the new family but, more importantly, when it is published it will suggest affinities to, and possible origins of, the Rockalliadae in the Cytherideidae, notably the Neocytherideinae. These affinities and possible origins are attested to also in Rockallia by its elongate shape and, among other features, the strongly developed fulcra1 point.
As Fig. 1 shows, Rockallia is now known to range from the Middle Eocene to the Recent. Although there are / \ / many stratigraphical and geographical gaps in the fossil record of the genus, many of which will probably be filled in due course, it is, nonetheless, possible to suggest an outline evolutionary history of the genus.
Rockallia probably originated in cither the late Cretaceous o r early Tertiary, most likely in the S. W. Pacific/ Australian region. It may have descended direct from some neocytherideinid ancestor w h k h it shares in common with the Argentinian Tertiary new genus mentioned above, or it may have descended from this postulated, ancestor via thc latter genus. The latter genus, on balance, retains more neocytherideinid characters than does Rockallia so that the second possibility seems the more likely one.
To date, thc earliest known species of Rockullia is R . eocenicu sp. nov. from the Middle Eocene of the site 207, S. W. Pacific. This site is on the southern end of the Lord Howe Rise (lat. 36" 5 7 . 7 5 ' s ; long. 165" 26.06'E) at 1389 m. water depth. The Middle Eocene palaeodepth was probably somewhat less than that of the present day because, although R . eocenica has been obtained from a pelagic ooze, the physiography of the Lord Howe Rise, surrounded as it is (and was) by deeper water, would preclude the introduction of clastic sediment and thus allow the calcareous ooze to be deposited at shelf depth 2 without dilution. The shallowness of the type locality is further emphasized by the presence o f a considerable number of sighted species among the ornate genera o f the ostracod fauna and by the occurrence of Cytherelloidea Alexander (1929), as a dominant genus.
In stratigraphical terms, the next species of Rockallia known is R . woutersi sp. nov. from the Middle Oligocene-Upper Miocene o f continental N.W. Europe. In the Upper Miocenc of Germany, this species occurs together with Rockulliu sp. Both are probably outer shelf species which seem to have invaded possibly shallower water envionments in continental Europe during transgressive phases; this would have introduced oceanic Atlantic water masses i n t o the North Sea basin.
The first unequivocal record ofRockalliu from bathyal and abvssal depths is in the Middle Miocene of the S. W. Pacific and this species, R . vscripta sp. nov. ranges in this area into the Pleistocene. Interestingly, it has not been found further north in thc Pacific nor at the three Neogenc sites we have investigated in the Indian Ocean. Rockallia vscripta seems to have been largely replaced at the end of the Pliocene in this area by R . inceptiocelata sp. nov. although, as yet, we have only found this in the Quaternary at site 209 on the Queensland Plateau (lat. 15" 56.19' S; long. 152" 11.27' E ) at a present day  Rockullia enigmaticu bears a closer relationship t o R. woutersi sp. nov. and to Rockullia sp. than it does t o any of the Pacific species, although this relationship is by n o means close. It does suggest, however, that since R. enigmutica appears to be virtually confined in bathyal and abyssal depths to a fairly restricted area of the N.E. Atlantic, that this species entered the deep sea in the European region and, thcrefore, that both the Pacific arid European Palaeogene and early Neogene "shallow" water stocks gave rise, independently to species which entered the deep sea. This suggestion could, however, be nullified by the discovery of new material of the genus in pre-Quaternary deep water deposits in the Atlantic. T h e species of Rockallia increase fairly regularly in size with time from the smallest, R . eocenicu sp. nov. to R . enigmutica as shown in table 1. 5 , fig. 9. R . enigmufic differs from the other species of the genus described below, in that the vertical component o f the ribs, which form its reticulation, dominate and radiate rather regularly from the mid-dorsal region. The faint secondary papillate ridges on the ribs anteriorly and posteriorly are very well developed on this species. Also, the smooth nodcs which externally denote the position o f the adductor and frontal scars are particularly strongly developed in the female right valve. R. enigmuticu is the largest of the species known to date.

Remarks
Although several hundred specimens now exist in U . C. W. collections of this, the only extant. species, none have been recovered with well preserved soft parts, although some specimens have "mummified" appendages. This is unfortunate, since a knowledge of thc morphology of the appendages would probably be of considerable value in better understanding the taxonomic relationships of the genus.
(PI. 1, figs. 2, 3, 5-10) Derivation of name. Greek, with reference to the fact that, at the time of writing, this is the only known species of thc genus from the Eocene. Diagnosis. A very small to small species of Kockalliu with pronounced sexual dimorphism and characterized by a deeply incised arcuate sulcus posteriorly which separates the elevated posterolateral surface from the less elevated posterior rim.

5
New Cainozoic Ostr-&coda and which radiate from the dorsal margin, tend to dominate. The ribs are rather narrow and rounded in profile.
The fossae tend to increase in size distally from the centre and also to become less angular in outline. The adductor muscle scar node is situated dorsomedianly and is well marked, it is enclosed by a sub-vertically aligned "V"shaped rib rather like that in R. vscripta sp. nov. but much less well defined. The frontal muscle scar node is present but indistinct. Normal pores rather few, situated on the ribs and apparently simple. Their distribution is as in other species of the genus. The anterior and posterior pore conuli characteristic of the genus are present but while strongly developed posteriorly they are somewhat subdued anteriorly. Internal features as for genus. Muscle scars situated dorsomedianly, largely obscured. Sexual dimorphism pronounced ; males more elongate than females. Diagnosis. Rockallia characterized by the development of "keyhole" celation within the fossae of the mid ventrolateral region and by the irregular nature of the fossae in this region and, more particularly, in the mid dorsolateral area. In this latter area the fossae are strongly drawn out into a vertical alignment. The most ventral of the adductor scars bears a strong sub-conical boss.
Holotype. Female LV, 0s 12112. Ornament strongly and irregularly reticulate and somewhat variable in expression between individuals although the basic distribution of the fossae is constant throughout the species. The fossae are parallel, and aligned with, the free margins. Also, around the periphery, the interfossal ribs parallel the margins. In the mid dorso-lateral region the fossal pattern is very confused and irregular and three sub-vertical rows of fossae, which extend from the dorsal margin just behind centre, are strongly drawn out along vertical axes. This vertical alignment persists to the area immediately posterior to the adductor node. In general the fossae increase in size towards the periphcry. In the mid posterolateral area they are more rounded in profile than elsewhere. T h e costae vary somewhat in thickness between individuals. Adductor and frontal scar nodes well marked. Mid ventrolaterally, the fossae exhibit "keyhole" celation. Normal pore canals few, wellspaced; pores simple and situated on the costae. Two pore conuli occur adjacent t o the anterior margin and two similar conuli subadjacent to the posterior margin. Inner lamella rather wide for the genus. Radial pore canals few, simple and straight. Hinge lophodont with locellate gutter-like accommodation groove above median elcment of left valve, which latter is somewhat expanded distally. Adductor muscle scars small and dorsomedian in position; they comprise a vertical row of 4 scars, the median two of which interlock and the ventral of which bears a strong conical boss. Dorsal and slightly anterior to the dorsal adductor scar is a strong sub-oval fulcra1 point. The front scar is typical of thc genus. Males are more elongate and less tumid posteriorly than females.

Dimensions. (mm)
Length  Description. Small to medium size. Subrectangular in lateral view. Anterior margin broadly rounded in left valve, more asymmetrical in right valve with ventral ape;u. Posterior margin broadly and regularly rounded in left valve, bluntly pointed with apex above mid-height in right. Dorsal margin of both valves straight or very slightly concave between pronounced cardinal angles. \~eritral margins straight to gently convex in left valve; that of right valve with distinct keel-like projection just behind centre and with slight oral concavity. Length approximately twice the height. Maximum length at or just below mid-height. Maximum height in anterior quarter of left valve; through ventral "keel" in right. Ornament strongly reticulate. Around. and subparallel to, the free margins extend a pair of strong ribs with deep ovai to subrectangular fossae. These ribs are particularly strongly developed posteriorly and ventrally. Over the remainder of the lateral surface, the fossae generally increase in size away from the centre and are smallest mid-dorsally. Over much of the surface the horizontal component of the ribs forming the recticulum are as strong as those of the vertical components. The ribs are smooth except anteriorly and dorsally where they may bear faint papillate ridges parallel to their long axes. However, a strong sub-vertically aligned "V" shaped rib, open dorsally, encloses the adductor scars, the external reflection of which is marked by a smooth node situated somewhat above mid-point. The frontal scar is marked by the presence of a much less well developed node. Normal pore canals rather few and well-spaced; they are apparently simple and are situated o n the top, or the flanks of the ribs. A prominent pore conulus occurs near mid-point. Two prominent pore conuli occur close to the anterior margin and two similar pore conuli occur posteriorly but at a little distance from the posterior margin. Inner lamella of medium width; widest at the end margins where narrow crescentic vestibules occur. Selvage well developed, broad and subperipheral. Radial pore canals simple, straight; 6-7 anteriorly, 5-6 posteriorly. Hinge lophodont. In the right valve the terminal elements are very weak, smooth knob-like teeth and the median element is a shallow smooth groove open to the interior. Complementary structures occur in the left valve hinge, above the median element of which is a narrow "gutter"-like accommodation groove. Adductor muscle scars small and situated above mid-height. They comprise a verticle row of 4 scars, the median two of which interlock. The frontal scar is relatively large and "heart"-shaped. Dorsal and slightly anterior to the adductors is a large sub-ovate fulcra1 point. Sexual dimorphism not particularly marked; males somewhat more elongate than females.

Dimensions (mm)
Length Height Remarks. This species differs from other members of the genus in the virtual absence of vertical alignment of the ribs, in its posession of the "V"-shaped rib enclosing the adductor scar node and in its possession of two strong ribs subparallel to the posterior margin. It does not seem to be as closely related to R. enigmatica or R. woutersi as these two species are to each other. On the basis of similarities in ornament, it is probable that R. vscripta was derived from R. eocenica and also that it is ancestral to R. inceptiocelata. The single specimen from the Pleistocene of site 281 (PI. 2, fig. 6) which is at present tentatively assigned to R. vscripta may, with the acquisition of more material, prove to belong to a distinct species. Anterior margin asymmetrically rounded in both valves with, in the left valve and female right valve, a slightly convex anterodorsal slope and in the male right valve a more convex anterodorsal slope ; anteroventral margin well rounded; extremity below mid-height. Posterior margin broadly and regularly rounded in female, bluntly acuminate in male; extremity at mid-height. Dorsal margin straight in both valves with well marked cardinal angles, particularly the posterior of the male. Ventral margin gently convex in left valve; biconvex in right with shallow concavity and distinct keel-like process just behind mid-ventral. Maximum length just below midheight; maximum height just behind centre in both valves; maximum width in posterior third. Ornament strongly reticulate with wide, smooth (except anteriorly and posteriorly where delicate papillate ridges occur) interpunctate elevated areas and rather small (for the genus), deeply excavated circular to ovate punctae, the solae of which are both smooth and secondarily reticulate.  The elevated areas of the reticulum are relatively wide and this feature imparts a celated appearance to the species. The ornamental features on most of the valves are vertically aligned and radiate from the mid-dorsal region; around the free margins it parallels the margin, particularly ventrally where the fossae become elongate parallel to the long axis of the valve. In the right valve a distinct ventrolateral sub-alar ridge is developed posteriorly. Normal pore canals rather few and wellspaced; pores apparently simple and opening on the elevations where they are either flush with the surface or surrounded by elevated lips. Two large and complex pore conuli occur at a little distance from the posterior margin, each pore of which contains a tube-like cylindrical structure. Two similar but less prominent and less complex pore conuli occur anteriorly, adjacent to the margin. Inner lamella is of medium width; widest at the end margins where narrow crescentic vestibules occur. Selvage strong, broad and subperipheral. Radial pore canals not seen. Hinge lophodont with, in the right valve, two smooth knob-like terminal teeth connected by a broad smooth groove above which is a narrow "gutter"like accommodation groove. Adductor muscle scars situated medianly and comprising a subvertical row of 4 closely adjacent and interlocking scars of which the second scar from dorsal forms an isosceles triangle. Immediately anterodorsal to the adductors is a deep subcircular fulcra1 point. The frontal scar is large and reniform. Sexually dimorphic; males more elongate than females.

Dimensions. (mm)
Length Height Paratype 0 C, OS 12125 0.42 0.22 The LV paratype is from locality 1 of Wouters and the carapace is from his locality 4. See "Remarks" below. Remarks. This species differs from other members of the genus in the shape of its anterior margin and the wide smooth nature of its costae. It is the only European species to possess a very pronounced posterior cardinal angle in the right valve. R . woutersi is thought to be ancestral to R. enigmatica and these two species contain morphological similarities, particularly with respect to the overall expression of their ornament. R . enigmatica is considerably larger, however, and has narrow and more radiate ribs. Neither species seems closely related to the Pacific species. Rockallia sp. from the Miocene of Germany is very similar and possibly derived from R . woutersi. Both these species seem to be restricted to Continental Europe and to have inhabited deeper shelf, rather than bathyal and abyssal environments. Distribution. The species ranges from the Rupelian (M. Oligocene) to the Upper Miocene and has been found in northern Germany, Denmark and the Netherlands. It has been found by Wouters at the following localities: Material. 2 adult left valves. Diagnosis. A very small and imperfectly known species of Rockallia characterized by rather rounded end margins and by a reticulum with small circular to oval fossae and wide interpunctate areas, the product of fairly advanced celation. Internal details as for genus.