The taxonomic position of Orthoplecta Brady, 1884 (Foraminiferida)

The type species of Orthoplecta, contrary to previous descriptions, possesses an Elongobula-like furrowed apertural face, with a distinct toothplate. Orthoplecta is reclassified in the Buliminacea in the family Orthoplectidae, herein raised to family status. The Orthoplectidae is redefined to include the genera Orthoplecta, Elongobula and Floresina. The occurrence in Orthoplecta of morphological features characteristic of both Floresina and Buliminoides raises questions concerning the evolution of these foraminifera.


INTRODUCTION
Among the many taxadescribed by Brady ( 1 884) from material collected by the 'Challenger' expedition, Cassidulinu (Orthoplecta) clavata is one of the more exotic species. It's remarkable cylindrical form, together with it's puzzling suture pattern set it quite apart from the other cassidulinids (Fig. I). The aperture, according to Brady ( 1884, p.432) is characterised as 'an oblique loop-likeopening on the faceofthe final segment close to the distal end'.
Loeblich & Tappan (1955) studied type material as well as additional specimens in their redescription of Orthoplectu clavata, which drew attention to the irregular chamber arrangement and to the subterminal, ovate aperture, thus endorsing Brady 's original description (Fig. 2). Orthoplectu, along with Islundiella, Cussidulinoides and Stichocussidulinu, was classified by Loeblich & Tappan (1964) in the new family Islandiellidae, because it possessed a radial wall structure and a coiling mode neither cassiduline nor biserial in the earlier stages. Fig. 2. A reproduction of the drawings of Orthoplecra clavata by Loeblich & Tappan, 1955. Note that the aperture is drawn as in the original figure by Brady. The dissected specimen on the right-hand side shows the peculiar coiling mode.

Fig. 1.
A reproduction of Brady's (1884) drawing of Cassidulina (Orthoplecta) clavara. Note especially the supposed aperture close to the apex of the test. Haynes (1981) placed the Islandiellidae in the Cassidulinacea, with the same member taxa as proposed by Loeblich & Tappan, including Orthoplecta. In 1984, Loeblich & Tappan created for Orthoplecta the new subfamily Orthoplectinae, in the Islandiellidae, Eouvigerinacea, differentiating it from the other islandiellids on account of its elongate, uncoiled test and lack of distinct chambers and septa. The subfamily definition also included the presence of a spiralling internal column, equated with a modification of the toothplate, and an elongate, subterminal aperture just above the base of the final chamber. In their latest classification, the Orthoplectinae remained monotypic, but now as a subfamily of the Cassidulinidae, Cassidulinacea, the Islandiellidae having been synonymised into the Cassidulinidae (Loeblich & Tappan Fig5 Close-up of the apertural face. The aperture is hidden by a low, broad flap, with grooves radiating away from it (scale bar -10pm). Fig.6. Close-up of the test wall. Note the smooth nature of the wall and the minute pores (scale bar -5pm). Figs 3,4. Stereopair of the specimen dissected by H. Tappan (scale bar -100pm). Figs 7-8. Close-ups of same dissected specimen; Fig.7, The toothplate (t) is clearly visible as the thin plate running from foramen to foramen (scale bar -50pm). Fig.8. Just below the toothplate (t), the foramen (f) can be seen, showing the resorption of the apertural flap. Note also the overall organisation, including the partial compression of the individual chambers (scale bar -20pm). Micrographs shown in Figs 3,4,7,8, taken using the 'Environmental Chamber' technique, without coating.

DISCUSSION
When Brady described Orthoplecta clavata, he apparently had only one specimen to hand. Unfortunately, that specimen has a somewhat obscure aperture and, being arcuate, was mounted so that the apical aperture cannot be seen. The very irregular sutural pattern is rather confusing, so that it seems that Brady mistook an inclusion in one of the last chamners for the aperture.
Loeblich & Tappan (1955) studied the holotype and a supplementary dissected specimen in London, the latter originating from the Heron-Allen & Earland Collection (ex Millett), from 'Challenger' station 185, off Raine Island. Tappan's accurate drawings (see Text- fig.2) clearly show the peculiar coiling mode and explain the seemingly erratic suture pattern (Loeblich & Tappan, 1955). However, the true nature of the coiling was not elucidated and apparently the specimen was not turned over so that no critical observation of the aperture was undertaken. Brady 's description therefore remained standing. These drawings were later reproduced in the successive classifications of Loeblich & Tappan (1964;1988).
The illustrations presented here (PI. 1) of 0. c'lavata show, for the first time, the true apertural features and thecharacteristic furrowed apertural face. The aperture is hidden by a low, broad flap, arising from the lowermost peripheral side, while the rest of the apertural face is furrowed by seven grooves which continue for some length over the test. However, grooves belonging to earlier chambers do not show on the surface ofthe test, probably due to its relatively thick secondary lamination.
The apertural face of Orthoplecta resembles that of Floresina Revets, 1990b, except that in the former the aperture is always hidden by an apertural flap. The toothplate is also quite similar, except that in Orthoplecta one border remains attached. The central part of the toothplate is also straight rather than curved as a trough throughout. Interestingly, the apertural flap seems to be resorbed when the aperture is converted into a foramen, an observation earlier made on Elongobula milletti (Revets. manuscript). Thus, while the obstruction of the apertural flap is removed, it is replaced by a toothplate. On the other hand, the shape of the individual chambers and the coiling mode is more reminiscent of Bidiminoides, now excluded from the buliminids (Revets, 1990a), or some of the more elongate Elongohula species. However, the pores in Orthoplecta are extremely minute, contrary to species of both Elongohula and Buliminoides. The set of characteristics exhibited by Orthoplecta, taken together however, is more than sufficient toreclassify it close to Flor-esina and Elon~qohulu. The subfamily Orthoplectinae is here raised to full family status in order to receive these three genera (Orthoplecta. Floresina and Elongohula) since they have more characters in common than with other genera in the Buliminacea.
The joint-occurrence of characters in Orthoplecta, considered to be diagnostic for several different genera, notably Elongohula, Floresina and Bidiminoides, poses some interesting problems from the evolutionary point of view. Convergence is an attractive proposition, but the above mentioned I70 genera seem to be too closely related to account for this similarity solely by convergence. On the other hand, true recombination of the genetic elements responsible for these morphological features is too strong a hypothesis since different genera are being considered. It is possible that Orthoplectu amongst others, is the result of a hitherto unsuspected evolutionary mechanism, and therefore deserves closer scrutiny. It seems likely that Orthoplectu is a fairly recent offshoot of the Elf)n~obulu-Flore.~inu line, not only on morphological grounds, but also based on the biogeographic and stratigraphic infomation available. Orthoplectu is only known as a Recent taxon, and its occurrence seems to be restricted to the tropical Pacific Province (see Text- fig.3), contrary to Floresinu, which is also encountered in the tropical parts of the New World.

Manuscript received January 1990
Revised Manuscript accepted June 1990