The taxonomic status of the minute foraminifera Discorbina minutissima Chaster (1892), D. chasteri Heron-Allen & Earland (1913) and related species

This study presents a revision, using type material, of the tiny foraminifera Discorbina minutissima Chaster, D. chasteri Heron-Allen & Earland (erected as a nomen novum for D. minutissima Chaster, non Seguenza) and D. chasteri var. bispinosa Heron-Allen & Earland. All are referred to the genus Rotaliella Grell. Heron-Allen & Earland’s new name chasteri is shown to be only applicable as a replacement name for Chaster’s species; their own material from Clare Island is a new species, named here R. heronalleni sp.nov. In addition, two other species — Rotaliella ? simplex (Sidebottom) and R. antarctica sp.nov. — have been recognised amongst the specimens labelled as D. chasteri in the Heron-Allen & Earland Collection in the Natural History Museum (BMNH). Lectotypes are designated for Rotaliella chasteri and R. bispinosa.


INTRODUCTION
While working on the minute benthic foraminifera of the Gulf of Elat and the NE Atlantic, we found a few specimens which resemble the species Discorbina minutissima and D. chasteri auctt. The original descriptions and illustrations of these tiny species, whose test diameters average only 100 microns, were made by Chaster (1892) and , respectively. Their drawings, however, are quite schematic and difficult to compare with the modern SEM images of our specimens. We therefore examined the relevant type material, deposited in the Natural History Museum, London (BMNH) as well as other specimens labelled as minutissirna/ckasteri in the Heron-Allen & Earland Collection of the BMNH and in the Cushman Collection of the National Museum of Natural History, Smithsonian Institution, Washington, D.C. (USNM). The results of the study, presented here, reveal five different species are involved. No doubt many other species have erroneously been referred to minutissima/chasteri in the past and await further study.
Because of its small size, D. minutissima/chasteri has only rarely been cited in the recent literature of benthic foraminifera (Boltovskoy & Lena, 1970;Boltovskoy et al., 1980), reflecting a generallackof interest insuch smallsized forms inforaminiferal research. For years the limited possibilities of optical equipment made the identifiction of such 'microforaminifera' difficult. The advent of scanning electron microscopy, however, now makes it perfectly feasible to study in detail species smaller than 100ym. This paper is a contribution to the better understanding of this group, whose possible greater diversity is still to be discovered.

HISTORICAL REVIEW
The history of Discorbina minutisima/chasteri is a good example of how confusing the determination of these tiny foraminifera can be. D. minutissima was described by Chaster in 1892, as a small foraminifer having a ...' test circular in outline, depressed, consisting of about two convolutions; segments few, about three in the last convolution, inflated and smooth superiorly; inferior surface deeply umbilicated in the centre, and marked with radiating lines; periphery rounded'. The species was isolated from shore mud off Southport, NW England, at a depth of 20-50m. Later it was reported from the Recent and Quaternary of Ireland by Wright (1903) and Gough (1907), the latter illustrating it with a copy of Chaster's original drawings (Fig. 1A).
In 1913, Heron-Allen & Earland in their classic work on the foraminifera of Clare Island, W Ireland, described a species which they considered conspecificwith D. minutissimachaster. They pointed out, however, that Chaster's name could not be used because of primary homynymy ...' the specific name minutissima having been used in 1880 by Seguenza for a Discorbina of the orbicularis type ..., Chaster's name must lapse'.
They, therefore, gave it the new name of Discorbina chasteri.  illustrated D. ckasteri with three views (reproduced here as Fig. lB), one of them representing a form with an oval outline. According to these authors, this oval form, in which '... the chambers are longer and set eccentrically so as to give an oval contour of the test', was identical with Pulvinulina simplex described by Sidebottom (1909) fromDelosIslandin theMediterranean.They suggested that chasteri may have a circular or oval form of the test. Both occurred in the same samples, but the circular form was more abundant. D. chasteri was reported elsewhere by Heron-Allen & Earland, from the NorthSea (l913,1916a, 1916b)   In 1913 Heron-Allen & Earland also described a variety of this species, D. chasteri var. bispinosa, in which ...' the surface of each of the last two or three chambers is furnished with a pair of short but stout spines, projecting forwards. The spines average .Olmm in length'. They observed that in this varie ty...' the surface of the test is also rough compared with the highly vitreous surface of the type' (see Fig. 1B). It seems that it was relatively rare compared to the nonspinose form and was reported from the shallow waters off the coasts of southern Comwall, SW England (Heron-Allen & Earland, 1916a) and western Scotland (Heron-Allen & Earland, 1916b).
Subsequently, Cushman (1931) transferred Discorbinn chasteri and D. chasteri var. bispinosa to the genus Discorbis. In this, his monograph of the Foraminifera of the Atlantic Ocean, he described chasteri and illustrated it with a copy of the original drawinds by Heron-Allen & Earland (1913) and pictures of two specimens collected off the Faroe Islands, at 128 metres, sent to him by Earland. Cushman had some doubts concerning the homogeneity of the species, observing that its wide distribution, from the North Sea to the Antarctic, was quite unusual, if 'these are really all the same'. He particularly noticed that the specimen from the east coast of Australia, identified by Sidebottom (1918) as D. chnsteri var.
bispinosa, was quite different, being keeled rather than having a broadly rounded periphery. Cushman (1948) also included D. chnsteri in his monograph of the Arctic Foraminifera. He gave the same description as in 1931, but the illustrated specimen, this time, was much larger, with numerous chambers and several rows of pustules on the umbilical side. He observed that this specimen resembles 'Discorbinn obtiisa d'orbigny' illustrated by Parker & Jones (1865) and suggested that both species were identical.

SYSTEMATIC DESCRl PTlONS
Five species are distinguished herein from material in the BMNH and USNM, labelled as minutissimnlchnsferi, including the type specimens. All are transferred to Rotalielln Grell (1954), a genus characterized by a minute, trochospiral test composed of few inflated chambers. The aperture is umbilical and covered by a more or less developed umbilical flap. The thin translucid wall is finely perforate on the spiral side and covered with radial grooves on the umbilical side. Tiny denticles occur around the umbilicus. A distinctive character of Rotnlielln, furthermore, is a small, hemispherical or elongated space, situated between the proloculus and the subsequent two chambers, which is visible as a prolocular boss on the spiral side; this structure was called the 'Zwischenkammer' by Grell(1954), who suggested that this is an additional chamber formed during the embryonic stage. It seems, however, that this'prolocular boss' is a pseudochamber, rather than a real chamber, as it never contains the cytoplasm (this is based on the original illustration of Rotalielln heterocnryoticn Grell(l954) and our own observations of cultured species of Rotalielln).
Our SEM study of the syntypes of Discorbirin rniriirtissirnn and Heron-Allen & Earland's Clare Island material of D. clznstcri clearly show there are two species involved. We have designated a lectotype fromchaster's material. This is referred to Rotnlielln clrnsteri (Heron-Allen & Earland), as their i i o m w nouiiiii was erected as a replacement name for riiirrirtissirrrrr (noir Seguenza, 1880). The other species, as illustrated by Heron- Earland (1913, P1. 13, Figs 1-3), is named Rotaliella heronalleni sp.nov. We propose to raise Discorbina chusteri var. bispinosa Heron-Allen & Earland to specific status, as Rotaliella bispinosa. The differences between the oval form of 'D. chasteri' (see above) and Rotaliella ? simplex (Sidebottom) are also presented. A new species, Rotaliella antarctica, is described from Heron-Allen & Earland material collected by the 'Terra Nova' and 'Discovery' expeditions to the Antarctic.
We have examined with the scanning electron microscope the specimens identified, as D. chasteri, by Sidebottom (1918) from the east coast of Australia and these can be referred to Heronallenita dorsocostata Seiglie & Bermddez (1965). Cushman's (1948)  Description (lectotype). Test minute (approximately 115pm maximum diameter), trochospiral, circular in outline; proloculus oval followed by 5 inflated, crescentic chambers, 3 in the last whorl; periphery broadly rounded; sutures slightly depressed, curved on the spiral side, nearly radial on the umbilical side; umbilicus deep and open, aperture not directly observed, covered by an umbilical flap; wall translucent, smooth, finely perforate on the spiral side, umbilical side imperforate, with numerous denticles at the umbilical margin and with fine grooves radiating from umbilicus towards periphery, the longest ones just visible on the spiral side. Remarks. The remaining syntypes in Chaster's original slides from off Southport (1915.10.25.438 and1915.4.1.620), and a slide in the Heron-Allen Students Collection, collected by Wright from Strangford Lough, Northern Ireland, were examined. Of the syntypes, 2 have a circular, slightly lobate test with numerous radial grooves on the umbilical side, as illustratedbychaster (1892);oneof themischosenaslectotype. Two others have an oval test, smaller and less arcuate chambers, and a smaller umbilicus, and are conspecific with the specimens illustrated fromClare Island by Heron-Allen & Earland (1913). They were either unaware that two species were involved or, more likely, as followers of the 'English School', they had a wide concept of the species in general. To them, they would have no hesitation in synonymising the two forms, in spite of the fact that the original illustrations are quite different (see Fig. l).Aswe havenowshownthat theyaredistinct, thenomen novum chasteri must apply only to Chaster's species, as lectotypified by us. The other is named Rotaliella heronalleni sp.nov. (see below).
Amongst theother species belonging toRotaliella,R. chastteri most resembles the type species, R. heterocaryotica Grell. The latter, however, is much smaller, with a more lobate periphery, more elevated spiral face and 2, instead of 3-4 radial grooves on the umbilical side. Because of the fact that the number of the umbilical grooves is constant in cultured species of Rotaliella (from observations of slides kindly sent to us by Professor K. Grell and from our own cultured material), this character seems to be a valuable taxonomic criterion.  . They differ from R. chasteri (Heron-Allen & Earland), as lectotypified by us, in having an oval, rather than a circular form of test, usually 4 instead of 3 chambers in the last whorl, more highly arched and less embracing chambers, a smaller umbilicus, a smoother wall, and less numerous grooves and denticles. In some paratypes the last chambers are quite long, giving an elongated test (Pl. 1, Figs 4a, b). The difference between this form and the species described asPuluinulina simplex by Sidebottom (1909) are given below.  ', on 20.8.1911, in 11 fathoms (20m); Heron-Allen & Earland Collection. Figured in P1.l, Figs 5a, b. Description (lectotype). Test minute (100pm maximum diameter), trochospiral, ovate-rectangular in outline with broadly rounded periphery, proloculus oval, followed by hemispherical deuteroloculus and 4 flattened inflated chambers, embryonic pseudochamber visible only in transmitted light; spiral sutures large, curved, umbilical sutures deep, radial; umbilicus open, with short, large denticles at umbilical margin; aperture umbilical, covered with denticulate umbilical flap; wall calcareous, translucent, relatively thick and rugose, the spiral side, except proloculus, perforate, with a few short spines on periphery; umbilical side imperforate, marked with long radial grooves, which reach the periphery and are just visible on the spiral side in the form of 'drip-point' terminations, the long grooves are separated by short, secondary grooves. Remarks. R. bispinosa was first described by Heron-Allen & Earland (1913) as a variety of D. chasteri. It is raised here to species level because of the relatively thick, rugose wall, deep, umbilical grooves whose terminations are just visible on the spiral side, and the short, peripherally situated spines. The spines, however, are not always present. Two such specimens, but with the thickly calcified wall, distinctive spiral side, and the 'drip-point" terminations of the umbilical grooves, so characteristic of bispinosa, were observed in the Clare Island material.

Rotaliella heronalleni
The species Glabrafella arcfica Scott & Vilks (1991) which also often has (but not always) spines around the periphery, differs from R. bispinosa in having more arcuate chambers and more numerous umbilical grooves. Another spinose form R. keigwini Pawlowski (1991), differs from bispinosa in having doral, rather than peripheral spines, angular, rather than globular chambers, a semicircular protuberance on the first chamber, and in being smaller; neither, moreover, appear to have the test rugosity of bispinosa. (Sidebottom, 1909) (Pl. 2, Figs 4a, b) 1909 Puluinulina simplex Sidebottom: 9,P1. 4,Figs 4,5. Remarks. One specimen of this type was found on a Clare Island Survey slide from stataion 13, Inishgowla Harbour, Clew Bay, Co. Mayo; dredged by the SS 'Helga' in 4 fathoms (7m). It was in the Heron-Allen & Earland Collection and had been identified by them as the 'oval form' of chasteri, for which they considered P. simplex Sidebottom to be one and the same. However, a SEM study of the syntypes of Sidebottom's species, from Delos (in the BMNH), shows that there are differences between the two in the form of the chambers and position of the umbilicus. R.? simplex has rapidly enlarged chambers, with the last chamber particularly large and strongly inflated on the umbilical side, making the position of the umbilicus eccentric.

Rotaliella simplex
The generic position of this species is unclear. It resembles other Rotaliella in having umbilical grooves and denticles. The prolocular pseudochamber, which is characteristic of this genus, was not observed in this specimen. sp.nov. (Pl. 2,Figs la,b;2a,b;3,a b) 1922 Discorbinba clzasferi Heron-Allen & Earland; Heron-Allen & Earland: 202 (non) Heron-Allen  broadly rounded, sutures slightly depressed, curved on the spiral side, and radial on the umbilical side; umbilicus large, partly obscured by numerous denticles around the margin, aperture umbilical, covered with an umbilical flap; wall translucent, smooth, with a few fine perforations on the spiral side, umbilical side imperforate, with numerous fine grooves radiating from umbilicus towards periphery, without reaching peripheral border. Measurements. 15 specimens from British Antarctic ('Terra Nova') Expedition station 220 (38) anfarcticu is the largest known species in the Rotaliellinae. It differs from the other species of Rotaliella in having an abundantly ornamented umbilical side (often in excess of 15 long grooves) and large embracing chambers. Some specimens have a large proloculus followed by the deuteroloculus and only one true chamber (PI. 2, Figs 3a, b), but their size is only slightly smaller than that of the larger multilocular specimens. It is possible that these specimens are the gamonts of the species, which reach maturity at the one-chamber stage, as this phenomenon has already been observed in another, unpublished species of Rotaliella (Pawlowski & Lee, in press).

Rotaliella antarctica
About 40 specimens have been studied in detail from the Heron-Allen & Earland Collection in the BMNH. Recorded as Discorbina (or Discorbis) chusteri (see synonymy for change in generic combination), it is found in the South Atlantic and Antarctica, in the following localities: the Ross Sea (Heron-Allen & Earland, 1922), the Falkland Islands (Heron-Allen & Earland, 1932;Earland, 1934) and South Georgia (Earland, 1933). It was most abundant at 'Terra Nova' station 220, off Cape Adare, depth 80-90m, in sediments composed mostly of organic debris, hydrozoan fragments, diatoms, sponge spicules and mineral particles.