Globorotaloides hexagonus

Classification: pf_cenozoic -> Globigerinidae -> Globorotaloides -> Globorotaloides hexagonus
Sister taxa: G. atlanticus, G. stainforthi, G. eovariabilis, G. hexagonus, G. quadrocameratus, G. suteri, G. testarugosus, G. variabilis, G. sp.,


Citation: Globorotaloides hexagonus (Natland, 1938)
Rank: species
Basionym: Globigerina hexagona Natland, 1938
Taxonomic discussion:

We take Blow’s (1979:176) view that Globorotaloides hexagonus descended from G. variabilis in the mid- to late Oligocene (Fig. 4.1). We note, however, that this is only a tentative model because G. variabilis and G. hexagonus morphotypes are rare at that time and the number of specimens available for comparison is very limited. Moreover, there is a large degree of morphological similarity between Globorotaloides hexagonus and G. eovariabilis such that G. eovariabilis could be the true ancestor of G. hexagonus and G. variabilis a phylogenetic side branch. As discussed above, it is possible that G. hexagonus might be the senior synonym of G. eovariabilis, although, as illustrated on Plate 4.6, late Oligocene G. hexagonus morphotypes are considerably larger than Eocene to lower Oligocene G. eovariabilis. Living G. hexagonus has occasionally been observed with a bulla.

Among the living Globorotaloides there appear to be two species, G. hexagonus and a less well known morphotype that can be assigned to ‘Globorotalia (Clavatorella) oveyi’ Buckley, 1973 (M. Kučera, personal communication). We follow Kennett and Srinivasan (1983) in considering the latter morphotype as belonging to Globorotaloides. Globorotaloides oveyi differs from G. hexagonus in having distinctly curved sutures, more numerous chambers in the final whorl and pronounced apertural lips (reminiscent of Clavatorella bermudezi Lipps, 1964). We find no evidence of Globorotaloides oveyi in the Oligocene. In both modern species of Globorotaloides, tooth-like corners of relict apertural lips project into the umbilicus. Based on the presence of these ‘tooth-like projections’, Parker (1962) placed hexagonus in genus Globoquadrina. As discussed by Lipps (1964), however, the similarities with Globoquadrina end there since G. hexagonus always possesses a flattened spiral and typically also has a more highly cancellate wall. Hemleben and others (1989) describe G. hexagonus as an ‘Indo-Pacific’ species, although this restriction is uncertain. Several studies have suggested that the taxon disappeared from the Atlantic Ocean in the Pleistocene, approximately 60,000 years ago B.P. (Pflaumann, 1986; Kučera and others, 2005), whereas other studies have found G. hexagonus in core-top samples from the Caribbean (Saunders and others, 1973), and equatorial Atlantic (Weaver and Raymo, 1989), suggesting it does occur in the Holocene. Apparently this species has narrow environmental preferences. Its occurrence is likely linked to temperature and/or nutrient content of sub-thermocline water masses, which it evidently prefers (see Spezzaferri and Premoli Silva, 1991; Ortiz and others, 1996), potentially even at an entire ocean-scale.

[Coxall & Spezzaferri 2018]

Catalog entries: Globigerina hexagona, Globorotalia (Clavatorella) oveyi

Type images:

Distinguishing features: Very low trochospiral, spiral side almost flat, equatorial periphery lobulate, chambers spherical

NB These concise distinguishing features statements are used in the tables of daughter-taxa to act as quick summaries of the differences between e.g. species of one genus.
They are being edited as the site is developed and comments on them are especially welcome.


Diagnostic characters:

Differs from Globorotaloides eovariabilis by the larger size, slightly more inflated chambers and nonspinose wall. It differs from G. variabilis, from which it was probably descended, by the more rounded chambers, wider umbilicus and typically the lack of a bulla-like final chamber. Note that the flattened area in the umbilical region of the holotype of G. hexagonus (Pl. 4.6, fig. 1) we believe is an adhering coating or glue and not a small umbilical bulla as could be perceived. [Coxall & Spezzaferri 2018]

Wall type: Nonspinose. Normal perforate, coarsely cancellate, sacculifer-type wall texture, with a distinctly honeycomb appearance. [Coxall & Spezzaferri 2018]

Test morphology: Test outline lobate to strongly lobate, axial periphery rounded, biconvex, oval to egg-shaped in edge view; 3-3½ whorls of inflated chambers arranged in a flattened trochospire; 11-14 chambers in adult tests, 4½-6½ in the final whorl increasing gradually in size; umbilicus shallow to moderately deep and narrow; umbilical sutures depressed, radial and slightly curved; spiral initially indistinct, later weakly depressed, radial sutures; aperture a low umbilical-extraumbilical arch surrounded by a broad lip that extends into the umbilical area. Relict apertural lips often preserved as teeth within umbilical region. [Coxall & Spezzaferri 2018]

Size: Holotype maximum diameter 0.39 mm, breadth 0.33 mm, thickness, 0.18 mm. [Coxall & Spezzaferri 2018]

Character matrix

test outline:Lobatechamber arrangement:Pseudoplanispiraledge view:Equally biconvexaperture:Umbilical-extraumbilical
sp chamber shape:Globularcoiling axis:Very lowperiphery:N/Aaperture border:Thick lip
umb chbr shape:Globularumbilicus:Wideperiph margin shape:Broadly roundedaccessory apertures:N/A
spiral sutures:Moderately depressedumb depth:Shallowwall texture:Cancellateshell porosity:Macroperforate: >2.5µm
umbilical or test sutures:Moderately depressedfinal-whorl chambers:4.5-6.5 N.B. These characters are used for advanced search. N/A - not applicable

Biogeography and Palaeobiology

Geographic distribution: Today as in the past it occurs in subtropical and equatorial environments. It is typically rare. Higher abundance levels may be associated with high nutrient systems, including the Arabian Sea (Kucera, unpublished) and California Current (Ortiz and others, 1996). Based on distribution patterns Globorotaloides hexagonus has been identified as an indicator of Oligocene to Miocene upwelling (Spezzaferri, 1995). [Coxall & Spezzaferri 2018]

Isotope paleobiology: Stable isotopes of Globorotaloides hexagonus from plankton tows and a Holocene core-top sample register high δ18O and low δ13C compared to other species, indicating a deep sub-thermocline habitat (Ortiz and others, 1996; Birch and others, 2013). This is consistent with observations from depth stratified plankton nets that show this species and G. oveyi consistently living below the thermocline and down to at least 800 m water depth (Ortiz and others, 1996; M. Kučera, oral communication). Strong negative δ13C disequilibrium in this species may be controlled by physiological processes related to slow growth at low temperatures (Ortiz and others, 1996) or enhanced metabolic 12C incorporation due to small test sizes (Birch and others, 2013). [Coxall & Spezzaferri 2018]

Phylogenetic relations: We suggest that Globorotaloides hexagonus evolved from Globorotaloides variabilis in the uppermost lower Oligocene (Zone O4), rather than Clavatorella bermudezi as previously considered (Jenkins and Orr, 1972; Srinivasan and Kennett, 1975; Kennett and Srinivasan, 1983), since its first appearance predates that of C. bermudezi by more than 10 million years. [Coxall & Spezzaferri 2018]


Similar species:

Most likely ancestor: Globorotaloides variabilis - at confidence level 2 (out of 5). Data source: Kennett & Srinivasan 1983.
Likely descendants: Clavatorella bermudezi;

Biostratigraphic distribution

Geological Range:
Notes: Upper Oligocene Zone O4 (rare) (Quilty, 1976; Spezzaferri and Premoli Silva, 1991; Spezzaferri, 1994, this study: Pl. 4.6, Figs. 13-15) to Recent (Hemleben and others, 1989; Ortiz and others, 1996; Kučera and others, 2005). The holotype is from a seafloor sample (“dark green clay with abundant foraminifera”) collected off Long Beach, California at a water depth of 884 m. The lowest occurrence of G. hexagonus is poorly constrained and is difficult to determine due to a general scarcity of the taxon at the beginning of its range as well as similarities with G. eovariabilis. Spezzaferri (1994) illustrated a specimen recorded as “Globorotaloides aff. G. hexagonus” from Zone P22 (O6/O7) of ODP Site 667 (Spezzaferri, 1994, pl. 36, figs. 1a-c) that we suggest is attributable to G. hexagonus. Table 7 of Spezzaferri’s article shows Globorotaloides aff. G. hexagonus ranging from Subzone P21a (O4) to lower Miocene Zone N5 (M2) at this Atlantic Ocean Site. At DSDP Site 354, also in the low latitude Atlantic Ocean, this taxon is recorded as first appearing in Zone P22. Although some of Spezzaferri’s Globorotaloides aff. G. hexagonus can now be placed in G. atlanticus, new observations confirm that Subzone P21a (Zone O3/O4) marks the lowest occurrence.
Today, G. hexagonus is described as an “Indo-Pacific species”, having reportedly become extinct in the Atlantic approximately 60,000 years ago B.P. (Pflaumann, 1986; Hemleben and others, 1989; Kučera and others, 2005). [Coxall & Spezzaferri 2018]
Last occurrence (top): Extant Data source: present in the plankton (SCOR WG138)
First occurrence (base): within O4 zone (28.09-29.18Ma, base in Rupelian stage). Data source: Coxall & Spezzaferri 2018

Plot of occurrence data:

Primary source for this page: Coxall & Spezzaferri 2018 - Olig Atlas chap.4 p.98; Kennett & Srinivasan 1983, p.216


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Globorotaloides hexagonus compiled by the pforams@mikrotax project team viewed: 19-9-2020

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