Catalog - Globorotalia (Globorotalia) truncatulinoides pachytheca

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Globorotalia (Globorotalia) truncatulinoides pachytheca

Citation: Globorotalia (Globorotalia) truncatulinoides pachytheca Blow 1969

Type sample (& lithostrat): Manchioneal Formation (sensu lato).

Type age (chronostrat): Pleistocene, Zone N.22 Globorotalia (G.) truncatulinoides truncatulinoides Partial-range zone)

Type locality: Holotype (figs. 13-15) from a San San Bay road-cutting, Navy Island Member, Jamaica, West Indies; and figured paratype (figs. 1-5) from the Sarmi Forniation (sample Ba. 1 6), Otim River Section, Island of New Guinea, West Irian.

Test coiled in a low trochospire with about 11-12 chambers comprising the spire and with five chambers in the last whorl. Dorsal side of the test is flat and the ventral side of the test is vaulted and obtusely conical. In equatorial profile, the test is not lobulate but almost smoothly circular. In dorsal aspect the chambers are slightly inflated, closely appressed and slightly embracing. The dorsal intercameral sutures are subradial to slightly recurved with distinct retortion at their distal ends; dorsal sutures are limbate but this is only clearly visible when the test is wet due to the overlaying sheath-like structure. The peripheral margin is carinate but much of the carina is buried beneath the sheath-like covering and is only clearly seen on the last chamber of the final convolution. Ventral sutures not sharply incised but broadly depressed and marked only by broad furrows between the moderately inflated ventral chamber surfaces; ventral sutures are largely obscured by the exterior sheath-like structure. Umbilicus, narrow but deep, not sharply delimited by a sudden 'in-turning' of the umbilical shoulders of the enclosing chambers. Aperture, interiomarginal umbilical-extraumbilical, a low slit-like opening bordered only by a weakly developed lip. Wall, calcareous, radial hyaline in the inner parts of the wall;
this radial hyaline wall with normal wall-pores being overlain by a sheath-like structure. In optical microscopy, in reflected light, this sheath-like structure has a 'sugary' texture but from a comparison with stereoscan investigations of a paratype, the sheath-like structure is believed to consist of a felted mass of interlocking granules or lath-like sclerites with anastomosing fine caniculae. The sheath-like structure of the holotype covers most of the test in a continuous manner but appears to be thinner over the area of the final chamber and may be absent over the apertural face of this chamber. The sheath buries most of the external expression of the carina and the external expression of the dorsal intercameral sutures although these characters can be seen when the specimen is wet.

Maximum diameter of holotype 0.58 mm.

From the stereoscan picture of a paratype (figs. 1-5) it will be seen that the sheath-like structure over the area of the ventral chamber surfaces does not show a mass of pustules just closely packed together but rather a sheet of interlocking granules or lath-like sclerites without any obviously euhedral crystalites. The sheath-like structure is ramified by a mesh-work of very fine caniculae and the normal wall-pores do not pass through this layer. The degree of organisation of the constituents of the sheath is considerable and the presence of the fine caniculae is most important and points to a genetically controlled mode of formation. As discussed for Globorotalia (T.) crassaformis ronda and G. (T.) tosaensis tosaensis it may be that the continuous sheath-like structure results from a proliferation and fusing together of the pustules. However, no traces of the original pustule boundaries remain and the fine caniculae seem to ramify the whole structure without interruption. In the writer's opinion, this sheath-like structure does not result from a purely random ecophenotypic response to change of habitat (migration to deep-water). It would be expected that if this was the case there should be extensive modification to the surface of the final chamber since this chamber would have been formed in the new environment. Further it must be remembered that the tests of specimens referable to the Globigerinacea is entirely internal to much of the cytoplasm and is, therefore, not in direct contact with the exterior environment. Further, as in any living organism, any internal skeletal structure must be in equilibrium with the organism's own physiology and thus must be an endoadaptive feature and not an exoadaptive feature. The postulation that this endoadaptive function may be modified as a result of change of habitat is strongly discounted in the occurrence of 'thin '-walled and 'thick '-walled forms together in sediment of shallow-water origin such as the samples of provenance of the holotype and paratype of the taxon. Dr. R. Cifelli has recorded (pers. comm.) the presence of the 'thick'-walled form (now referred to pachytheca) in the shallow-water sediments near the crests of St. Paul's Rocks in the Central-Southern Atlantic. These forms could not have been derived upwards from the deep-water environments adjacent to St. Paul's Rocks. The writer has noted the concurrence of truncatulinoides (s.s.) and truncatulinoides pachytheca in samples from a variety of environments and the two forms occur together in shallow-water (less than 250 metres) deposits in Jamaica (holotype sample), West Irian (paratype sample), Java, Venezuela and elsewhere. The two forms also occur together m samples from deep-sea cores and from depths greater than 2,500 metres. In the writer's opinion the complex structure of the sheath in pachvtheca rules out any phenotypic secondary origin
In addition to the presence of the sheath-like structure G. (G.) truncatulinoides pachytheca differs from G. (G.) truncatulinoides truncatulinoides in a number of other features; these include, the more inflated chambers of pachytheca as compared to the flattened, laterally compressed chambers of truncatulinoides (sensu stricto), the tighter coiling and more embracing chambers, the small umbilicus and the generally more rounded test of pachytheca as compared to the corresponding characters in truncatulinoides truncatulinoides. The generally more rounded test of pachytheca cannot be entirely explained by postulating the burial of the more sharply angulate structures of truncatulinoides truncatulinoides, for in some cases, such as the pointed ventral terminations of the chambers, the deposition of additional material should tend to accentuate the angularity of the structures. It would seem likely that there are two genetic patterns one of which characterizes G. (T.) crassaformis oceanica and the other form G. (T) crassaformis ronda; the descendants of these two forms seem to have passed through similar morphological stages of development to give grossly homeomorphic forms but which are distinguishable in terms of the presence or absence of the complex sheath-like structure. In a similar manner to the evolution of truncatillinoides (sensu stricto) from G. (T ) tosaensis tenuitheca, the evolution of G. (G.) truncatulinoides pachytheca from G. (T) tosaensis tosaensis is distinguished by the first evolutionary appearance of a true carina. The two evolutionary sequences do not appear to be quite synchronous and the various stages appear at slightly different times.
The writer is aware that many workers will probably criticise him for considering a separate ancestry for G. (T.) tosaensis tosaensis and G (T.) tosaensis tenuitheca as well as for G. (G.) truncatulinoides truncatulinoides and G. (G.) truncatulinoides pachytheca and yet retaining a subspecific terminology for the forms. This seems, at first sight, to be bad taxonomic and nomenclatorial practice and runs contrary to the usual biological concepts. However, the writer believes that G. (T) crassaformis (s. l.) should be considered as a remarkably plastic stock genetically which is, in effect, a plexus of forms with a basic genetic pattern in common but which has an associated group of 'recessive' genes which only become operative under certain conditions of adaptive response. Similarly, G. (T.) tosaensis (s.l.) may be considered to be the further differentiated descendants of the fundamental
G. (T.) crassaformis (s.l.) stock but the potential of the 'recessive' genes is also retained. Finally, the same concept applies to a basic G. (G.) truncatulinoides stock which has inherited the modified basic genetic patterns present in crassaformis (s.l.) and tosaensis (s.l.) but which has also inherited the potential of the 'recessive' genes to operate under certain adaptive conditions. Thus, the writer envisages a basic genetic pattern in crassaformis (s.l.) which is modified successively in tosaensis (s.l.) and truncatulinoides (s.l.) by normal mutational changes to give the successive basic genotypes. The other group of genes, which is additional to the basic genetic pattern, does not undergo mutational change but the group persists unchanged and js only operative under certam circumstances. The writer does not consider, for example, that a specimen of pachytheca ('thick '-wall) is a single individual ecophenotype of truncatulinoides (sensu stricto), for if this were so then there would be no constant expression of the uniform development of the sheath-like structure; this is contrary to observation. Thus, the writer cannot accept that the development of the 'pachytheca' morphology is a late ontogenetic stage of development repeated in specimens of truncatulinoides (s.s.) for reasons of change of habitat. The more reasonable explanation is to consider dual stable states of the basic genetic pattern and that one such state allows the 'recessive' group of genes to operate in response to a combination of both exo- and endoadaptive factors. Thus, the development of the two morpholocial forms is probably governed by the overall genetic complex (both basic genetic pattern and the group of 'recessive' genes) resulting from the fusion of the gametes. In one case, the genetic complex resulting from the fusion of the gametes is in the one stable state in which the 'recessive' group can operate whilst in other case, the 'recessive' group is unable to do so. Hence, it would seem likely that pachytheca could result from the sexual reproduction of truncatulinoides (s.s.) as well as from the asexual reproduction of pachytheca itself. Similarly, the reverse could probably occur and the truncatulinoides (s.s.) morphotype might well result from the sexual reproduction of the pachytheca morphotype depending on which stable state in the genetic pattern was formed at the time of fusion of the gametes. The conditions governing which stable state resulted from the fusion of gametes might very well be ecologically controlled in the same way as 'direction of coiling' is controlled by water-mass characteristics. Indeed, the genetic mechanism for both processes might be the same and the skeletal response, to which ever stable genetic state was initially formed, cross linked to an endoadaptive physiological process in the cytoplasm. Hence, the writer does not consider each and every specimen of pachytheca (for example) to be an individual ecophenotype of truncatulinoides (s.s.) but rather the population of specimens referable to pachytheca to be the morphological (skeletal) response to a particular genetic stable state which in turn is dependant on the balance of adaptive conditions at the time of reproduction of the progenitors of the new generation.
In terms of morphogenesis, the evolution of pachytheca appears to be from tosaensis (s.s.) and this, in turn, from crassaformis ronda. Likewise, the morphogenetic series from G (T.) crassaformis crassaformis G. (T) tosaensis tenuitheca G. (G ) truncatulinoides truncatulinoides appears to outline another evolutionary line. However, in terms of probable genetic relationships, the evolution of the whole group probably proceeded as mutational changes for each of the stages represented by crassaformis (s.l.), tosaensis (s.l.) and truncatulinoides (s.l.) with the differentiation into 'thick' - and 'thin' -walled forms appearing in each of the major stages; this, however, is not to say that the two morphotypes are simple ecophenotypes.

References:

Blow, W. H. (1969). Late middle Eocene to Recent planktonic foraminiferal biostratigraphy. In, Bronnimann, P. & Renz, H. H. (eds) Proceedings of the First International Conference on Planktonic Microfossils, Geneva, 1967. E J Brill, Leiden 380-381. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia (Globorotalia) truncatulinoides pachytheca Blow 1969

Globorotalia (Globorotalia) truncatulinoides pachytheca

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