New data on the ostracod genus Aratrocypris Whatley et al. 1985, with descriptions of species from the Upper Cretaceous of Europe and the Cainozoic of the North Atlantic

Paradoxostoma ? cretacea Bonnema (1941), is shown to belong to the deep sea cyprid genus Aratrocypris Whatley et al (1985). The implications of the discovery of this genus, hitherto known only from the Palaeocene to Recent, in the Upper Cretaceous of the Netherlands are discussed with respect to the palaeodepth of the Dutch Chalk Sea and to the origin and palaeozoogeographical history of Aratrocypris. The new species Aratrocypris gigantea, from the Recent and Aratrocypris maddocksae from the Cainozoic, are described from the North Atlantic and other, nomina nuda species are described or discussed from the same area.


IN T K 0 D LJ C T I 0 N
In June 1987, while perusing the Ellis and Messina ostracod catalogue for early records of Purudo~wsromu, the seni0.r author encountered the subsurface Cretaceous record from i.he northeast Netherlands of Punidosostornu ? C I Y~I U ( ' C~U Bonnema (1941). Although the original illustrations are small, they and Bonnema's description are suggestive of At.utt.oc:\'pri.s Whatley et ul. ( 1985). The subsequent request for assistance in tracking down the original material was met by an immediate reply from LW who provided details of the original borehole, some of the cores of which are housed in the NI-therlmds Geological Survey, and who also \ent specimens. which clearly belong to At.utt-oc~?j/7t~is. In subsequent correspond'ence and at a meeting in October lYX7. the two authors decided to collaborate and subsequently GC was also invited to contribute. In 1969. Maddocks described a strange cyprid ostracod with ;in anterior flange-like projection from the sub-Recent of the eastern Pacific. She ascribed it to the genus Airsrtulowiu. Subsequently, Whatley of u/. ( 1 98.5). having discovered a number of species with the same curious 'plough'-like structure anteriorly, in DSDP cores of lipper Palaeocene to Pleistocenes age in the S.W. Pacific, erected the genus At-utr.oc~vpt~i.s to accommodate them. These authors also rioted the record o f the genus from Recent sediments in tlhe North West Atlantic by Cronin (1983) as Airstt.trlot~cLu. sp. G. and Whatley and Coles ( 19x7) subsequently recorded it from the early Pliocene of the central North Atlantic.
The late Palaeocene and early Eocene representatives of Arutroc.ypr-i.s in the S.W. Pacific/Australasian region seem to have lived in shelf environments in that they occur in association with the thermophylic genus Cythrrrlloidw Alexander and also with ornate species with well developed eye tubercles. However, from the late Palaeogene onwards, all records of Arutrmypris are from deep water bathyal and abyssal (greater than I000m) environments, where they occur in association with typically deep water ostracod taxa.
Whatley er ul., (1985) also suggest that not only did At.utr.oc~vpr.is originate in the S.W. Pacific/Australasian area, but that this was the locus of its involuntary induction into the deep sea by means of tectonic processes as outlined in Whatley rt ul. ( 1983), Whatley ( I 983). They also suggect that from this initial locus, the genus migrated by bathyal and abyssal pathways to other parts ofthe worlds oceans, arriving late in the North Atlantic.
Although Bonnema's original illustrations of Purudoxosromu ? cwtuccu are very small, they are unmistakably of At-utt-ocyprYs. Together with the anteroventral 'plough'. other features typical of post-Cretaceous species of the genus are already well developed in A.cwruc.ru. Most of these were noted by Bonnema ( I94 I ) in his original description. The most important, as discussed in Whatley rt ul. (1985) are: i ) The hingeline is short and is situated very posteriorly and inclined at a steep angle towards the posterior. It is also more or less parallel with the blade of the 'plough'.
i i ) The smaller left valve is very strongly overlapped antero-dorsally by the right .
i i i ) The anterior and ventral surfaces of the 'plough' are strongly reinforced and dentate.
iv) The 'plough' has a deep concavity in its ventral surface.
v ) The adductor scars seem to be situated well back in the carapace, thus allowing the valves to gape anteriorly.
Whatley P I ul. speculate on the possible function of the 'plough' and conclude that it must be associated with feeding. Whatever the particular feeding adaptation was, Arat t n r : v l , t i s ('t'rtu('ru was already employing it in the Upper Cretaceous.
The discovery ofA~.utrocypri.s in the Dutch Chalk throws new light on the original and migrational history of the genus. While it does not necessarily negate the suggestion that the genus first entered the deep sea in the antipodes and spread into other oceans from there, it does demonstrate that its origins were not necessarily in that region and that they may best be sought in the Upper Cretaceous of Europe. Only the discovery of further fossil material of the genus will resolve this paradox. Unfortunately, the small size and fragile nature of Aratrocypris renders it likely to be overlooked or not preserved.
Of the four wells in the eastern Netherlands which Bonnema (1941) examined for Ostracoda, only one yielded Paradoxostorna:.' cretacea. This well, De Krim NNIV (Noord Netherland IV), is located near Coevorden, a township that was later to become well known for discoveries of natural gas.
Unfortunately, Bonnema placed all specimens of one species, irrespective of their stratigraphical provenance, in a single slide. The species was reported to be abundant in the 'Mergel', and no more detailed age or level was indicated, neither in the text nor on the slide with the original material.
In slide no. 0.2243 of Bonnema's ostracod collection, which is now housed in the Geological Survey of the Netherlands in Haarlem, some tens of specimens of the species are present. Three specimens are stored in a separate slide, no. 0.2360, and were apparently used for the original illustrations.
In order to obtain more detail on the distribution of the species through the Chalk, fifteen additional samples were taken from the original core material. The depth range of the samples was 325m (top of the Chalk is at 3 Ism) to 582m. Aratrocypris cretaceu was found in only four of the samples, the lowest occurrence being the sample at 493-499m and the highest the top sample at 325m.
Well preserved planktonic and benthonic foraminifera occur abundantly in these samples which, together with other elements of the rich microfaunaenable the section of the core from which Arutrocypris has been recovered to be dated as Coniacian to Santonian.
The associated ostracod fauna is rich and diverse and among the more abundant taxa are Purvucythereis suhparva, Phacorhahdotus serniplicatus, Xestoleheris ovata, Irnhotepia rnarssoni, Cardohairdia rninutu, Cytherelloidea circurnvallata, A versovalva vscriptu and Krithe honnernai, together with species of Cytherellu, Neonesideu and Bythoceratina. This ostracod association indicates a shallow water environment. Many of the species have eyes, indicating deposition on the shelf. Some of the genera present in this Upper Cretaceous assemblage have subsequently migrated downslope and many or all of their species are deep water indicators in the Cainozoic (e.g. Krithe, Phacorhuhdotus, Aversovulva, Curdohairdia, Bythoceratina). The principle of shallow water ostracods subsequently invading deeper water is well documented (Whatley, 1983). Arutrocypris which, in the Cainozoic and Recent, has only once (Cronin 1983), 454-783m) been encountered in palaeodepths less than 1 000m (and usually much deeper) is a striking example.   Arutim.ypri.s gigunteu sp.nov. tate 'plough' Description. Small, thin-shelled, subovate to sub-elliptical in lateral view. In dorsal view, widest in the posterior third and tapering regularly towards the laterally compressed anterior. Anterior margin with straight to slightly convex antero-dorsal slope extending from the anterior cardinal angle (at orjust behind mid-length) to the tipof the 'plough'. 'Plough' like process well developed, dentate and with ventral incurvature. Posterior margin rounded or subrounded. Dorsal margin short, straight and steeply inclined towards the posterior. Ventral margin posterior of the 'plough' straight. Greatest length below mid-height; greatest height at the anterior cardinal angle; greatest width in the posterior third. Right valve larger than and strongly overlapping the left valve, especially antero-dorsally and ventrally. Surface smooth, internal features as for genus. Dimensions. ( (Data from the present study and from Whatley et ul. 1985. Note that the species with the longest stratigraphical ranges also have the greatest range in size. In almost all cases, the largest individuals are also the youngest).

SYSTEMATIC DESCRIPTIONS
Apart from being the smallest known species of genus, A . c'retacea differs from the type species, A. rectoporrectt, in the shape of the 'plough' and in lacking the strongly truncated posterior margin and the postero-ventral spine on its left valve. A. vuccamuris differs in size and shape and also bears a weak ornament of ribs anteriorly. A . praealtu is proportionally much higher and its left valve has a blunt posteroventral spine which is either absent or only inconspicuously developed in A . cwtuceu. The most similar of previously illustrated species is Aratrocypris sp. of Whatley et ul. ( l Y85) which is herein subsumed within A . maddocksue sp. nov. This latter species, although considerably larger, is similar in overall shape, particularly in its subrounded posterior margin. The morphology of the 'plough' in the two species is also similar and it is probably that they are closely related.  Remarks. Al-utr-oi~ypri.sgigunteu, in shape and outline, most closely resembles the Upper Cretaceous A . i'rotuwu but it can be easily distinguished from this, and from all other species of the genus by its much greater size. It is almost twice as large a A. i'retuceu. Distribution. The species was found in two surface sediment samples, taken with an oversize box-corer in deep water off the coast of Sierra Leone. The samples are from depths of 3436m and 3648m and were collected during 'Geotropex 83'.  Whatley ct ul. ( 1985) as Arutrocypris sp.). Whatley et ul., principally in its rounded rather than spinose posteroventral corner and in being much less umbonate dorsally. While similar in outline, the present species lacks the anterior ornament of A. vaccamaris Whatley et al. It shares its rounded posteroventral margin with A . cwtuceu (Bonnema), but the latter species is smaller and also differs in the outline of its dorsal margin. In including Arutr.ocypris sp. of Whatley et ul. 1985 in the synonomy of this species, we have reservations only in respect of the single specimen from the Pleistocene (0s 1224 I , PI. 2, Fig.   16). The Pacific Palaeogene specimens (PI. 2,figs 14,IS,17) are clearly conspecific with our Palaeogene material from the North Atlantic. We also note that one of the specimens of A rec'toporrectu illustrated by Whatley et ul. 1985 (PI. I , Fig. 1 1 ) is more similar to A. muddocksue than other members of the former species but the similarity is largely in the nature of the posterior margin and not, for example, in the dorsal margin. Distribution. Whatley etul. ( 1 985

OTHER ARATROCYPRZS SPECIES IN THE NORTH ATLANTIC
At.utrocypris sp. Cronin and Compton-Gooding, 1987 This species is represented by only 3 specimens from the Middle Eocene of DSDP Site 612 (PDWD 1404m) off New Jersey, USA. This is clearly distinct from all other known species of the genus. The single illustrated RV is small (length 0.43mm), elongates in outline with a very bluntly truncated posterior margin and a small, slightly upturned antero-ventral plough-like process. Unusually for the genus, the slope of the hinge margin is not parallel with the anteroventral margin of the 'plough'.
Austruloeciu sp. G. Cronin 1983. This represents another distinct species of Arutrocypris which was recorded from the Recent slope off Florida between 454-783m. Small (length 0.44mm), subovate in outline, very broadly rounded posteriorly, regularly convex dorsally. Antero-ventral 'plough' small, finely denticulate. Inner lamella wide and notched anteriorly. It is most similar in shape to A . prurulutu Whatley et ul. (1985), but lacks the postero-ventral spine and differs in the shape of the anteroventral 'plough'.
This species, which wasoriginally recorded only from the Upper Palaeocene to Miocene of DSDP sites 56,207 and 277 in the SW Pacific, has subsequently been recorded at Site 558 in the North Atlantic. The details of its occurrence are given below:

PHYLOGENETIC CONSIDERATIONS
With respect to the phylogenetic relationships between the various species ofAt.utt.oc,.vpt.i.s, we believe, largely based on the rounded nature of the posterior margin and particularly the postero-ventral comer, that there is probably a relationship between A. mudk)c~ksuc~ and both A. e~tx~tucru and A . ,qi,qtit~t~u. The exact nature of this relationship is difficult to elucidate. However, we suggest that A . mtrridotksue may have evolved from A . u.otuc'eu or to have shared with it a