Morozovella protocarina: a new species of Palaeocene planktonic Foraminiferida

Detailed study of the Palaeocene Morozovella lineage of Deep Sea Drilling Project Sites 577 (Shatsky Rise, North Pacific) and 527 (Walvis Ridge, South Atlantic) have revealed a distinct morphological stage transitional between M. angulata (White) and M. velascoensis (Cushman & Ponten). This stage is characterised by the development of the first peripheral pseudo-keel of the Cenozoic Globigerinidae and is named Morozovella protocarina. The biostratigraphic application of this species clarifies problems of recognising the Late Palaeocene planktonic foraminiferal zones.


INTRODUCTION
Members of the Palaeocene Morozovella lineage are widely used zonal indicators in the tropical latitudes (Berggren et al., 1985;Berggren & Miller, 1988;Toumarkine & Luterbacher, 1985). The lineage originates from a subbotinid ancestor in the Danian and evolves via several intermediate morphologies to the species M . velascoensis in the late Palaeocene. Corfield & Granlund (1988) have examined the phylogenetic development of this lineage using morphometric techniques and have identified an intermediate morphology between M . angulata (the marker species for Zone P3a) and M . velascoensis (the marker species for zone PS) which is also easily recognisable using the ordinary light microscope. Separate specieslevel status is here proposed for this morphotype.

STRATIGRAPHY AND MICROPALAEONTOLOGY
DSDP Site 577 was drilled in 2685m water depth on the Shatsky Rise, North West Pacific (Figure 1). The position of this site close to the western margin of the sub-tropical gyre and above the Calcite Compensation Depth (CCD) combine to provide a wide species diversity of tropical planktonic foraminifera with good preservation. In total 118.8m of unlithified nannofossil oozes were recovered at Site 577. The section is interrupted by an unconformity at 60m below sea floor, where sediments of middle Miocene age lie atop Middle Eocene strata, and also at 91mbsf, in the Late Palaeocene.
Morozovella protocarina first appears at 102.5 mbsf and last occurs at 86.8mbsf. Using a timescale developed from the available magnetostratigraphic, nannofossil and carbon isotopic data as described by Corfield (1987) this gives an age estimate for the first  appearance datum of this species of 63.1Ma (Chron 27N) and 59.0Ma (Chron 25N) for the last occurrence datum. Figure 2 illustrates the range of this species in relation to the other Palaeocene morozovellids at this site together with the marine magnetic anomaly chronology of Berggren er al. (1985). The holotype and paratypes of Morozovella protocarina are deposited in the Sedgwick Museum, University of Cambridge.

SYSTEMATIC PALAEONTOLOGY
Family  Boersma (1984) and Corfield (1987). The agreement is close except for the LOD of M . angulata and the FAD of M . velascoensis.
400pm from the tip of the final chamber to the tip of the antepenultimate chamber in umbilical view. Detailed size data are given in Plate 1. The term pseudo-keel is used to distinguish this structure from the true keel of Neogene Globorotalia which is formed as part of the primary chamber wall. The pseudo-keel is formed by the packing together of muricae which are part of the secondary calcification. Remarks. This species forms part of the structural transition from Subbotina pseudobulloides of the Early Palaeocene to Morozovella velascoensis of the Late Palaeocene ( Fig. 1 in Corfield & Granlund, 1988). In terms of phylogenetic development it lies between the species Morozovella angulata and Morozovella velascoensis. M . angulata sensu strict0 does not possess the pseudo-keel of muricae or the mounds of muricae on the umbilical shoulders, whereas M . velascoensis possesses a pseudo-keel that runs completely around the margin of the organism as well as well-developed umbilical muricae that form characteristic horns.
Plate 1 in Corfield & Granlund (1988)  This new species corresponds to those morphotypes thought by Blow (1979), to lie at the most ornate end of the spectrum of variation associated with M . angufata. I believe that the features associated with this form are sufficiently well developed to warrant individual specific status.
A comparison was made between the biostratigraphy of Site 527 as performed by the shipboard biostratigrapher (Boersma, 1984) and Corfield (1987). For all planktonic foraminifera1 datums except the LOD of M. angulata and the FAD of M. velascoensis the agreement of depth and age estimates between the two authors was very close (Figure 3 ) . In contrast the identification of the LOD of M. angulata and the FAD of M. velascoensis was poor. This suggests that the two authors were employing different species concepts in the identification of these two important datums. This discrepancy may be attributed to the large variation which is associated with the transition from M . angulata to M. velascoensis. This suggests that the new taxon described here is of value in describing and circumscribing this morphologically transitional form. Fig. 4 illustrates the ranges of the Palaeocene and Early Eocene morozovellids in the Atlantic DSDP Site 527 using the same timescale as that used for Figure 2. It is clear that the FAD of M . profocarina is isochronous between the two sites. The range of this species however is much shorter in this temperate site, with an LOD of 61.9Ma in 527 compared with 59.OMa in 577.
This new species is the first of the Tertiary planktonic foraminifera to acquire a keel composed of muricae. This accounts for the choice of the name protocarinu (proto = first, carina = keel).  Fig. 2. Holotype. side view. Fig. 3. Paratype, spiral view. Fig.  4. Paratype, umbilical view. Fig. 5. Paratype, umbilical view. Fig. 6. Juvenile, umbilical view. Fig. 7. Juvenile, side view. Fig.  8. Juvenile, spiral view. Fig. 9. High power of final chamber of holotype showing pseudokeel and partially developed unbilical muricae, umbilical view. Fig. 10. High power of umbiiical muricae of holotype, oblique view. Fig. 11. High power of pseudokeel on early chambers of final whorl of holotype. oblique view. Fig. 12. High power of pseudokeel on final chamber of holotype, oblique view. ary point of view. It is ancestral to the Late Palaeocene morozovellids M . velascoensis, M . finchi and M . occlusa (Corfield & Granlund, 1988), and, in addition, is a contributor to the peak in taxonomic turnover at 63.5Ma referred to as Turnover A by Corfield & Shackleton (1988). This peak represents the initial burst of speciation within the planktonic foraminifera as they recolonised the early Tertiary oceans following the extinctions at the Cretaceous/Tertiary boundary.