A new record of Aratrocypris Whatley

During analyses of ditch cuttings samples from wells 34/10-53A and 35/9-F-1H drilled by Statoil ASA in the North Viking Graben, North Sea, 16 specimens pertaining to Aratrocypris were found. One specimen was found at 1260 m (depth below KB) in 35/9- F-1H within an interval of Lower Paleocene claystones, and based on its white chalky preservation characteristics is considered reworked from Maastrichtian chalk. The remaining 15 specimens were found among ostracod-rich microfossil assemblages of Early Cretaceous age in a sample at 4190 m of 34/10-53A, and at 2166 m of 35/9-F-1H. Although these samples contain numerous ostracods, the occurrence at such a low stratigraphical level was unexpected since Aratrocypris , hitherto, has not been found older than Coniacian/Santonian. Their preservation condition as orange-stained opaque carapaces is the same as that of the remainder of the assemblage found at that level in the Lower Cretaceous. For this reason the possibility of it being caved from higher levels of the well is ruled out. The find, therefore, represents the earliest record of the genus, and is important also when considering its early zoogeographical distribution.

Ostracods occur in well 35/9-F-1H at two levels: Lower Paleocene and Lower Barremian–Lower Hauterivian. Each assemblage has a very different style of preservation which allows them to be easily separated. Ostracod assemblages of the younger level are white, translucent carapaces, a preservational style consistent with the abundant foraminifera …

inhabited 'outer sublittoral-upper bathyal' palaeodepths mainly in the Central North Sea and Viking Graben (King et al., 1989).Upon examining the same biofacies and stratigraphical level of well 34/10-53A, additional specimens have been found which confirm the provenance and antiquity of the new Aratrocypris data.

the genus Aratrocypris
Aratrocypris is a very distinctive cyprid genus by virtue of its plough-like structure situated antero-ventrally.Seven species have been described from the Upper Cretaceous-Recent (Whatley et al., 1985;1989).The Cretaceous-Paleocene species lived in shelf environments, whereas younger species inhabit deep-water bathyal and abyssal environments (Whatley et al., 1989).
Unfortunately, the 'plough' structure characteristic of Aratrocypris, is not well preserved in the new material.However, other features discussed by Whatley et al. (1989) as typical of the genus are easily discernable (Text-fig.1).Identifiable features unique to Aratrocypris observable in the new Lower Cretaceous specimens are: (1) short hinge line situated very posteriorly and inclined at a steep angle, and parallel to anterior margin; (2) the smaller left valve is very strongly overlapped antero-dorsally by the right; (3) internal opening to the 'plough' at anterior extremity; and (4) antero-ventral incurvature and edge of the 'plough' ventrally is preserved.While the possibility that all the specimens are juvenile cannot be ruled out, the small size (length range 0.23-0.30mm) is in keeping with the size trend: mean length increases from 0.39 mm to 0.72 mm over its Upper Cretaceous-Recent range (Whatley et al., 1989).
Although very small and incompletely preserved, the overall carapace shape shows little variation and it is possible make a judgement regarding the affinity of the Lower Cretaceous material with other species of Aratrocypris.Its outline with rounded posterior and postero-ventral angle is most similar to A. cretacea (Bonnema, 1941) described from the Coniacian-Santonian of the Upper Cretaceous of the Netherlands (Whatley et al., 1985).On the basis of outline and overall shape an assignment to A. cretacea seems correct.If so then A. cretacea has changed very little in morphology over its considerable age range.Further comparisons must await the finding of additional Cretaceous material.
Although Aratrocypris is known world-wide, mainly from the bathyal environment, the previous record of A. cretacea in the Dutch Chalk indicates that an origin for the genus in the shelf seas of NW Europe is most likely.The new material reported here shows that Aratrocypris existed as early as Hauterivian/ Barremian times in Europe, and occurred in shelf seas for a long time before its eventual migration into the deep sea during the early Paleogene.

Aratrocypris Whatley et al., 1985, ostracoda, from the Lower cretaceous of the north Sea: a range extension for the genus MICHAEL
AyRESS * & ALEVTINA DORN Ichron Limited, Century House, Gadbrook Business Centre, Northwich, Cheshire CW9 7TL, UK.The new find indicates that the genus existed in shelf seas of NW Europe as early as the Lower Cretaceous.using the scanning electron microscope at the University of Liverpool.Graham Coles read an early version of the manuscript and made useful positive comments.Helpful comments by the reviewer are also acknowledged.Witte, L. & Coles, G. 1989.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.Journal of Micropalaeontology, 8: 207-214.
*Corresponding author (e-mail: mike.ayress@ichron.com)concLuSIonS Several specimens from the Lower Barremian-Lower Hauterivian of wells 34/10-53A and 35/9-F-1H in the North Viking Graben, North Sea are shown to belong to the cyprid genus Aratrocypris Whatley et al., 1985.Preservational characteristics indicate that specimens are in situ and not caved from younger strata.Hitherto, the earliest record of the genus was A. cretacea (Bonnema, 1941) from the Upper Cretaceous of The Netherlands.