Catalog - Subbotina triangularis cancellata

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Subbotina triangularis cancellata

Citation: Subbotina triangularis cancellata Blow 1979

Type specimens: Plate 80, figs. 2-9 (7 = holotype); Plate 238, fig. 6; Text-figure L, fig. nos. (iii)-(vii). BP Cat. No. 41/31

Type sample (& lithostrat): DSDP Leg 3, Station 20C, Core 6, Section 4, sample at 72-74 cms.

Type locality: DSDP Hole 20C/6/4, 72-74cm, South Atlantic Ocean

The test is coiled in a low trochospire with about 10-12 chambers comprising the spire and with four chambers visible in the last convolution of the test. The chambers of the last whorl are inflated and subglobular but are moderately appressed and embracing. The dorsal intercameral sutures are depressed but not incised and are radially or subradially disposed. The ventral intercameral sutures are also depressed, slightly incised and are radially disposed. The equatorial profile is lobulate and oval in outline whilst the axial profile is smoothly rounded. The umbilicus is small, but open and deep, and the umbilical depression is fairly sharply delimited by the umbilical shoulders of the last whorl of chambers. The aperture is virtually limited in lateral extent to the limits of the umbilical depression, although it extends a little further towards the anterior side of the last chamber than it does towards the posterior side of this chamber. Tnus, the aperture is a low arched opening slightly asymmetrically placed with respect to the centre of the umbilicus. The aperture is bordered by a strongly developed porticus which can be seen clearly cutting across the surface structures of the primary wall of the last chamber; from this, the porticus can be seen to be a structure additional to the primary wall of the last chamber and is not a simple, direct, reflexed continuation of the chamber wall. The porticus is not significantly thickened. The primary walls of the test bear large mural-pores which open into very large, sharply defined pore-pits with massively developed inter-pore ridges.

Maximum diameter of holotype 0.29 mm

Remarks: From the paratypes figured on Plate 8o, figs. 2-6 and 8-9, it will be seen that the morphotypes are all characterised by a constantly expressed, extremely coarsely developed, wall texture. This wall texture consists of large mural-pores which open into very large pore- pits and with massive inter-pore ridges. Furthermore, this wall texture is also remarkably uniform throughout the ontogeny of each of the specimens although, perhaps, the last chamber of each specimen has slightly smaller pore-pits and mural-pores than over the earlier chambers of each specimen. Elsewhere in this work (see p. 377), the writer has pointed out that, very frequently, the absolute size of the mural-pores and, especially, the pore-pits is strictly related to, and dependent on, the stage of calcification developed for the various chambers of the tests in many taxa of the Globigerinacea. Very often this variation in size of the pore-pits and mural-pores is also related to ontogeny so that larger pore-pits are seen for chambers formed earlier in ontogeny as compared to the size of the pore-pits seen in the ontogenetically later chambers of single specimens (compare, for instance and example, Plate 69, figs. 1, 3, 4, 5, 6 and 8). However, in Subbotina triangularis cancellata n.subsp. there is no distinct ontogenetically controlled variation of pore- pit and mural-pore size but rather there is a constant development of the mural-pores and pore-pits both within a single specimen and throughout the members of a population as a whole. Accordingly, the writer feels justified in recognising these morphotypes as worthy of some formal nomenclatorial and taxonomic distinction. The basic morphology of the morphotypes included in the new taxon differs only a very little from the typical morphology of specimens referred to Subbotina triangularis triangularis (White) sense Bolli, 1957 (see p. 1282, and Plate 91, fig. 7, for example). However, in S. triangularis cancellata n.subsp. the chambers are a little more closely appressed and embracing with less distinctly incised intercameral sutures as compared to these features in forms referable to S. triangularis triangularis. It would seem that S. triangularis cancellata n.subsp. has a somewhat more massively thickened wall than as compared to the walls of specimens included in S. triangularis triangularis. It would seem that the massive inter-pore ridges of cancellata is the one particular feature which provides the most unambiguous means of differentiating the two taxa. Whether these massive inter-pore ridges are a resultant of greater calcification over the walls of the fundamental triangularis-morphotypes due to some ecological condition is not yet known. The specimens of the primary type-series of cancellate n.subsp. coexist in the same sample with normal triangularis triangularis morphotypes and, thus, do not have an overt climatic ecological dependance which, excludes S. triangularis triangularis. However, there may be some bathymetric preference for the two morphotypes.

Because of the very distinctive primary wall texture for the last chamber, the holotypi, and paratypic specimens of S. triangularis cancellata n.subsp., illustrated here, enable the nature of the porticus to be resolved without ambiguity. Thus, the paratypic specimen: illustrated on Plate 80, fig. 4, is examined at higher magnification of Plate 238, fig. 6 from these photographs it can be seen that the porticus cuts across the large mural-pore (and their associated pore-pits) of the last chamber and is a structure totally additional to the primary wall of the last chamber. The portical structure can actually be seen infilling one of the pore-pits of the primary wall in the central part of the photograph given on Plate 238, fig. 6. In the writer's opinion, these specimens of S. triangularis cancellata n.subsp. render it imperative to recognise the morphological peculiarities of the porticu: and to distinguish it from a simple apertural-lip (as seen in Globigerina bulloides, type-species of the genus Globigerina, for example) which is a direct but reflexed continuation of the primary cI amber wall.

Reference should be made to the discussion given in Chapter 6, The Morphological Basis for the Supraspecific Classification of the Globigerinacea, for some general comments concerning the variation in size of the mural-pores and pore-pits seen within specimens referable to many taxa of the Globigerinacea. Whilst the writer doubts the validity of these characters for general taxonomic differentiation of the Globigerinacea, he finds the constancy of expression of large pore-pits in S. triangularis cancellata n.subsp. is worthy of some taxonomic recognition even if only at the subspecific level. Accordingly, cancellata is fol((tally recognised on the presence of large pore-pits but also, more especially, on the nature of the massive inter-pore ridges and the distinctive pattern of pore-pits. The conditions which give rise to these features in cancellata are not yet understood or even evaluated either in terms of the processes of biomineralisation or to any possible ecological factor which may potentiate the development of the peculiar wall structure. The total range of S. triangularis cancellata n.subsp. is probably considerably less than seen for S. triangularis triangularis (White) sense Bolli, 1957 (see below).

Stratigraphic Range, from Zone P.2 to within Zone P.4 (range not yet fully evaluated), ?uppermost Danian to the later part of the Thanetian, Palaeocene Epoch.

References:

Blow, W. H. (1979). The Cainozoic Globigerinida: A study of the morphology, taxonomy, evolutionary relationships and stratigraphical distribution of some Globigerinida (mainly Globigerinacea). E. J. Brill, Leiden. 2: 1-1413. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Subbotina triangularis cancellata Blow 1979

Subbotina triangularis cancellata

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