Catalog - Globorotalia crassacarina

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Globorotalia crassacarina

Citation: Globorotalia crassacarina Scott et al. 1990

Type specimens: Holotype and 26 paratypes registered in as TF1634

Type sample (& lithostrat): DSDP Site 284A-2-5-20

Type age (chronostrat): Nukumaruan

Type locality: DSDP Site 284A 2-5-20; Tasman Sea;

Type repository: Lower Hutt; N.Z Geological Survey foraminiferal collections

Spiral orientation
Shell outline: In crust-free specimens the lobulate outline is nearly quadrate (Fig. 72A) with a broad re-entrant (sometimes shallow) above the aperture and weaker re-entrants at other chamber junctions. The latter are usually masked in encrusted individuals and lobulation is suppressed, although quadrate shape is preserved. Individuals with a kummerform final chamber tend to have elliptical outlines.
Chamber packing and coiling: There are 3 whorls with 4 chambers in the outer and about 5 in the middle. Coiling is predominantly sinistral (c.215 specimens; 15 dextral).
Chamber shape: Late-formed chambers are narrow and elongated. Leading and trailing sections of the outer margins of the last 2-3 chambers are short but may be well defined. They are symmetrically curved on the last chamber of some individuals but commonly the leading section is longer (Fig. 72A) and may show greater curvature.
The central section usually forms a low arch. Sections of the inner margin are weakly to moderately concave. Outer margins of chambers in the middle whorl are relatively less elongate, more strongly arched and lack distinct leading and trailing sections. A keel is commonly present on the later chambers of this whorl and on all chambers of the outer whorl, although it may not always run the full length of the outer margin (as in Fig. 72G). Chamber surfaces are evenly juxtaposed and sutures are not depressed.

Axial orientation
Shell outline: The spiral face is almost linear. In some specimens it may be slightly concave due to the ori- entation of the surfaces of chambers in the outer whorl. The umbilical face is steeply elevated and cone-like, particularly in crust-free individuals. When the last 2 chambers are comparable in elevation only a weak apex is formed (as in Fig. 72H).
Outline of early whorls: Typically, there is a very low dome (Fig. 72B). Sometimes it is inconspicuous and may also be obscured by the greater elevation of chambers in the outer whorl.
Outline of last chamber: The spiral section is linear and may be orientated upward towards the periphery (creating a partially concave outline for the spiral face of the shell, as in Fig. 72H). The junction of spiral and umbilical faces is angular and is the site of a narrow keel. However, a ridge is seldom identifiable in axial orientation (as in Fig. 72E), although it may be prominent on the spiral surface (as in Fig. 72D). The periphery may be located at the junction of spiral and umbilical faces but a more common position is on the upper section of the latter, in a broad zone of gentle convexity (Fig. 71). The outline of the umbilical face is weakly convex and inclined at c.30o to the coiling axis; curvature tends to increase towards the apex in some encrusted specimens. The apex sometimes curves inward towards the umbilical area.
Outline of earlier chambers: The apex of the (n-l)th chamber is seen behind the umbilical area and, in encrusted specimens, is commonly about as elevated as that of the last chamber (as in Fig. 72H). The remainder of the outline of the shell is usually formed by the (n-2)th chamber (occasionally the (n-1)th). Their contours are similar to that of the last chamber.
Aperture: A low, slit-like opening with a narrow lip (Fig. 72B). There is a weakly defined apertural face (reflecting the subangular form of the leading section of the outer margin in spiral orientation). It is best developed in crust-free specimens.

Umbilical orientation and wall topography
Sutures are convexly curved in the direction of coiling and depressed, forming narrow channels. They are partially masked in encrusted specimens. The umbilicus is open and is usually quite narrow. But there are some wider examples (Fig. 72C). Chamber surfaces are weakly convex in the direction of coiling but are sharply elevated towards the coiling axis. Narrow pores are densely and uniformly distributed over the flat wall surface. Small pustules may be thinly scattered on spiral surfaces but are stronger and less sparse on umbilical surfaces (Fig. 72C). The keel is much more conspicuous on the spiral than on the umbilical surface. Encrusted individuals are common and the additional deposits obscure primary topographic features on all but the last chamber.

1982 Globorotalia crassaformis Hornibrook, fig. 6d-f.

VARIATION: At DSDP Sites 284 and 284A most samples conform closely with the pattern seen in the described sample. Some specimens from DSDP Site 284A cores 2-4-100 and 2-1-20 have a wide umbilicus. Typical specimens are present in DSDP Site 284 core 3 but only encrusted specimens (random coiling) occur in the base of DSDP Site 284A core 1. Because of very small sample sizes, variation in collections from onshore sections is poorly known. Specimens are crust-free and do not reach the size of the largest specimens from the oceanic site at DSDP Site 284. Ventroconical form is weaker.

DISCRIMINATION Globorotalia crassacarina closely resembles G. crassaconica. Encrusted shells of these taxa may be easily confused and they are best separated using crust-free specimens. Globorotalia crassacarina is often more regularly quadrate (spiral profile) because of the lesser extension of the length of the last chamber. Keel formation is usually stronger in G. crassacarina and tends to be initiated earlier in ontogeny. The low, slit-like aperture is a more constant feature in G. crassacarina than in G. crassaconica. Low arched, rather than slit-like, apertures seen in some oceanic specimens of G. crassaconica are not seen in G. crassacarina. This feature is particularly useful in separating encrusted individuals. The apertural face is not as well defined as in G. crassaconica. Globorotalia crassacarina is readily separated from G. crassaformis by the development of keels and its stronger ventroconical form. Discrimination from Globorotalia crassula is discussed under that species.

RELATIONSHIPS Wall topography and aperture shape link this species with G. crassaformis and, to a lesser extent, the association is supported by chamber and shell shape. However, while aperture shape (commonly a conservative feature) is often similar in these taxa, it may be incorrect to infer that the species branched directly from the latter. Phyletically, it is possibly significant that G. crassacarina appears subsequent to G. crassaconica, and has stronger keels which often form earlier in ontogeny. Cifelli and Scott (1986) noted that, within globorotalid lineages, keels appear initially as weak folds on the late chambers and subsequently strengthen and appear earlier in ontogeny. Globorotalia crassaconica and G. crassacarina fit this pattern. The substantial interval (c.1 m.y.) between the taxa might account for the greater evolutionary advance of the latter species. We suggest that G. crassaconica and G. crassacarina maybe components of a lineage within the G. crassaformis plexus the history of which is at present only partially known in the New Zealand region.

DISTRIBUTION Nukumaruan to Quaternary. The records at DSDP Sites 284 and 284A are the most extensive available. There it appears at the base of core 5 (Nukumaruan) and extends into core 1 (Quaternary). In the latter core it is very rare, suggesting that only expatriates reached the region at that period. Onshore records include V20/f88 and W19/f8492 (northern Hawkes Bay; Nukumaruan) and S27/f8751 (Makara Stream section, southern Wairarapa; Nukumaruan).

References:

Scott, G. H., Bishop, S. & Burt, B. J. (1990). Guide to some Neogene Globorotalids (Foraminiferida) from New Zealand. New Zealand Geological Survey, Paleontological Bulletin. 61: 1-135. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia crassacarina Scott et al. 1990

Globorotalia crassacarina

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