Catalog - Globigerina incompta

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Globigerina incompta

Citation: Globigerina incompta Cifelli 1961

Diagnosis: Test small, coiled low trochospirally, slightly convex on the dorsal side; periphery lobulate, subquadrilateral in side view, rounded in edge view; chambers inflated to slightly appressed, numbering about 9 to 14 and arranged in 2 to slightly less than 3 whorls, with marked increase in size of chambers between whorls; usually 4, but sometimes 4 and a portion of a fifth, chambers visible from the ventral side; final chamber overlapping on ventral side and thus oblique to axis of coiling; sutures radial, deeply depressed; umbilicus deep, but not particularly broad; aperture an elongate rounded opening at the base of the final chamber extending into and beyond the umbilicus, close to the periphery; only the aperture of the final chamber visible from the surface; a thin porous lip projecting at the top of the aperture; wall thin, porous; surface spinose.

Chambers: The initial whorl consists of a minute proloculus and 4 or 5 additional small chambers. An abrupt increase in size generally occurs at about the sixth and again at about the eighth chamber. There are 4 or 4½ chambers in the second whorl and from 1 to 3 in the final, incomplete whorl. The cameral and spiral sutures are deeply depressed and sharply defined throughout so that the details of the chambers are clearly visible on the test. The last 4 chambers are relatively large and the increase in size of these is uniform. The final chamber is usually highly inflated and is mostly, but not always, the largest. Because of the uniformly large size of the last four chambers the periphery, in ventral view, is subquadrilateral. In most specimens there are 4 chambers visible from the ventral side but in some there are 4 and a portion of a fifth; the peripheries of these are less quadrilateral. The final chamber overlaps considerably more on the ventral side than do the previous ones, but the amount of overlap is highly variable.

Aperture: The aperture is at the base of the final chamber and opens into a deep but fairly narrow umbilicus. It extends beyond the umbilicus in most specimens, occasionally almost reaching the periphery. The aperture varies from an elongate narrow opening to one that is rather broad and semi-circular. The aperture of the penultimate chamber can sometimes be seen through this opening, but it connects with the final chamber and does not appear to open directly into the umbilicus. The lip of the aperture of the final chamber is thin, delicate and porous; it was not observed on all of the specimens.

Surface: Live and wet preserved specimens are covered with fine, elongate spines. These are easily destroyed, however, and are practically never seen on dry specimens. The wall is thin and highly porous and translucent; through it the protoplasm can be seen. It is a dark olive color. The surface of dry specimens is spinose.

Coiling: No actual count of coiling ratios wa made, but in the material examined the majority of specimens are dextral. The coiling direction of this species, however, may be temperature controlled, as has been reported to be the case in other pelagic Foraminifera.

Holotype: 14 chambers max diamter .36mm thickness 0.21mm. Paratypes: 11-13 chambers diameter 0.28-0.51mm thickness 0.16-0.34mm

Comparisons and remarks. This species is common in parts of the North Atlantic and will probably be familiar to persons acquainted with Recent pelagic Foraminifera. However, because of its small size and rather generalized characters it probably has, in past, been regarded as a juvenile or variant form of some other species with which it is associated. My first impression was that this species representedan immature stage of Globigerina eggeri Rhumbler, which it closely resembles, in mode of coiling and in inflation of chambers. It was not possible to establish a transition, however, and after study of many specimens I concluded that this small form was a separate species. It is distinguishable from G. eggeri in the following ways:

The test of Globigerina incompta is coiled low trochospirally while that of G. eggeri is high, even In small specimens.
The umbilicus is a larger, more open area in G. eggeri. The apertures of the chambers on the ventral side can often be seen in the umbilicus in G. eggeri, while only the aperture of the final chamber is visible in the umbilical area of G. incompta.
The aperture of G. eggeri opens to, and does not extend beyond, the umbilicus. In G. incompta the aperture also opens to the umbilicus, but is variable in fength, extending outward beyond the umbilicus and almost to the periphery.

Of particular importance also is the fact that G. incompta was found to be very common in the plankton tows studied from the North Atlantic while G. eggen: was rare. In fact, G. incompta appears to be one of the most characteristic species of the North Atlantic Slope waters, along with G. inflata and G. bulloides. In one 200 meter plankton sample collected in August, 1960, at latitude 39 longitude 710 06' W, G. incompta was the dominant species and made up 40 percent of the foraminiferal population. In eleven 200 meter plankton samples collected August, 1960, from the North Atlantic Slope waters, the Gulf Stream and the Sargasso Sea, the ratio of G. eggeri to G. incompta varied from 0 57% (Table 1). If G. incompta were an early stage of G. eggeri it would be difficult to understand why so small a percentage of individuals succeed in achieving an adult stage.

Comparing favorably and apparently identical with Globigerina incompta is G. dutertrei d'Orbigny as originally described and figured by d'Orbigny (1839, p. 84, pI. 4, figs . 19-21) . However, the lectotype of G. dutertrei selected by Banner and Blow (1960, p. 11, pI. 2, fig. 1) is not identical. The lectotype as figured is in my opinion, inseparable from G. eggeri. I have observed both forms in a bottom sample from off the coast of Cuba, one of the areas from which d'Orbigny ecorded G. dutertrei. The form comparable to d'Orbigny's figure also compares favorably and is identical with G. incompta, although the Cuban specimens have somewhat fewer chambers and slightly less lobulate ripheries. The other form, referable to the lectotype, is clearly of the G. eggeri type; besides being larger and having more chambers, it has a higher, less flat coil and has a larger umbilicus and a very broad bilical aperture. D'Orbigny did not distinguish the two forms and apparently included both in his concept of G. dutertrei. It is unfortunate that the lectotype which was selected is not the form figured by d'Orbigny, since that specimen now becomes the name bearer of the species; d'Orbigny's figure no longer has menclatural status, even though it appears to be an accurate illustration. This further complicates a species that has already caused much difficulty in interpretation. G. dutertrei now becomes a senior subjective synonym of G. eggeri.

A species very similar to Globigerina incompta is G. megastoma Earland from the Antarctic region. The latter species, however, has a higher spire and a broader aperture. It is also a larger form and has a more elongate final chamber.

The relationships of this species to the form occurring in the Antarctic region and identified as GlobigerinaGlobigerina dutertrei by authors is problematiG. Brady (1884, p. 601) regarded G. dutertrei as a typical Antarctic species and believed that it was the southern counterpart of G. pachyderma which, according to him, does not occur in the Antarctic region. HeronAllen and Earland (1922, pp. 189-192) recognized G. dutertrei as a dominant form in the Antarctic but maintained that it is replaced by and intergrades with G. pachyderma in the lowermost latitudes. They felt that G. pachyderma is actually a variety of G. dutertrei, "due to the suppression of the comparatively large single aperture of the typical G. dutertrei combined with a reduction of the general dimensions and particularly with a massive thickening of the shell wall" (Heron-Allen and Earland, 1922, p. 192). Phleger, Parker and Pierson (1953, p. 13) and Uchio (1960, p. 5) , on the other hand, consider it unlikely that the thick-walled Antarctic form is conspecific with the species described by d'Orbigny from the coast of Cuba. Uchio (1960, p. 5) found that G. dutertrei (of authors, not d'Orbigny) is dominant in the Antarctic sediments, while G. pachyderma, a small form, is of secondary importance. G. cf. G. bulloides, the only foraminifer occurring in the Antarctic plankton tows, is rare in the bottom sediments. G. pachyderma intergrades with G. dutertrei and Uchio believes that the Antarctic G. dutertrei is the adult stage of G. pachyderma (Uchio, 1960, p. 5). The original G. dutertrei of d'Orbigny, he feels, is the early stage of G. eggeri.

In the Arctic region G. pachyderma is dominant in the bottom sediments while a form resembling a juvenile G. eggeri or G. bulloides is the only foraminifer occurring in the plankton tows of the upper 200 meters of water. The line figure of the plankton form given by Be (1960, fig. lc) is suggestive of a type comparable to G. incompta although the Arctic form appears to be smaller. In addition, the coil of the Arctic form is sinistral while that of the present species is dextral. Like Uchio, Be considers this form to be a life stage of G. pachyderma. Unlike Uchio, however, Be regards G. pachyderma to be the adult stage. According to Be (1960, p. 65) .the juvenile achieves the adult characters by descending to deep waters, below 200 meters. The morphologic changes involved are "in the addition of a reduced final chamber and in the crystalline thickening of the test."

Part of the difficulty in comparing the Arctic and Antarctic forms is that despite the fair abundance of records and discussions of these forms there have been very few descriptions and, more important, illustrations. Without adequate illustrations and descriptions it is impossible to make meaningful comparisons. The figure of G. dutertrei given by Brady (1884, pI. 81, fig. 1) shows a form that appears to be comparable to G. pachyderma Earland. The Antarctic form figured by Ovey (in Wiseman and Ovey, 1950, p. 65, pI. 2, fig. la-c) compares well with G. incompta, but only a single specimen is figured, and nothing can be determined about the population structure. Conceivably the species described by d'Orbigny and revived here might be a life stage of G. pachyderma; the interpretations of Uchio and Be seem reasonable if only the areas that they have studied are considered. However, it hardly seems justifiable to regard G. incompta a life ft m of G. pachyderma in the lower latitudes where G. incompta is common and C: pachyderma is rare or absent. Moreover, it is difficult to see how G. pachyderma could be an adult stage in the Arctic and a young stage in the AntarctiG. Another possibility is that G. incompta is a geographic subspecies of G. pachyderma. However, this cannot be determined at the present since th'e descriptions and illustrations are inadequate and the records of occurrences of the high latitude forms cannot be evaluated. For now it seems best to retain for the lower latitudes, at least, a species comparable to the one that d'Orbigny originally described as G. dutertrei, even though a name change is involved.

References:

Cifelli, R. (1961). Globigerina incompta, a new species of pelagic foraminifera from the North Atlantic. Contributions from the Cushman Foundation for Foraminiferal Research. 12: 83-86. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globigerina incompta Cifelli 1961

Globigerina incompta

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