Paragloborotalia siakensis

Classification: pf_cenozoic -> Globigerinidae -> Paragloborotalia -> Paragloborotalia siakensis
Sister taxa: P. acrostoma, P. incognita, P. pseudocontinuosa, P. semivera, P. kugleri, P. pseudokugleri, P. mayeri, P. siakensis, P. birnageae, P. continuosa, P. opima, P. nana, P. griffinoides, P. sp.,


Citation: Paragloborotalia siakensis (LeRoy, 1939)
Rank: species
Basionym: Globigerina siakensisLeRoy, 1939
Taxonomic discussion:

Bolli and Saunders (1982a) argued that siakensis is synonymous with P. mayeri, but many workers differentiate the two forms based on the radial spiral sutures and lower arched aperture in siakensis, and the slightly curved spiral sutures and higher-arched aperture in mayeri (e.g., Kennett and Srinivasan, 1983; Spezzaferri and Premoli Silva, 1991; Spezzaferri, 1994; Pearson and Wade, 2009). Other than these subtle differences, the two taxa are very similar and share a highest occurrence in the early late Miocene (top of Zone M11/N14; Wade and others, 2011). Paragloborotalia siakensis is the ‘primitive’ form, initially very small with 5 chambers in the final whorl and radial spiral sutures in the early to mid-Oligocene, increasing in size and giving rise to P. mayeri s.s. with 5 chambers in the final whorl and slightly curved spiral sutures in the latest Oligocene; both taxa then increase to 6-7 chambers in the early to middle Miocene. According to Spezzaferri and Premoli Silva (1991), small specimens of P. siakensis (150 μm) appear in Subzone P21a of the Gulf of Mexico, while even larger specimens (>250 μm) appear within Zone P22 in the Indian Ocean. [Leckie et al. 2018]

In the near topotype material discussed and illustrated by Zachariasse and Sudijono (2012), specimens of P. siakensis generally have 5 chambers in the final whorl with a range of 4½-6½ chambers. They note that kummerform chambers are common, including one of the two specimens illustrated by LeRoy (1944). The aperture is variable from low to moderately high arch, with well-developed lip; some specimens lack a well-developed lip and instead have an imperforate rim around the aperture (e.g., Pearson and Wade, 2009). [Leckie et al. 2018]

Catalog entries: Globigerina siakensis

Type images:

Distinguishing features: Like P. nana but with more chambers (5-7) in the final whorl, less embracing chambers, more rapid rate of chamber expansion, and a higher arched aperture.

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:

Paragloborotalia siakensis is distinguished from opima in always having at least 5 chambers in the final whorl, a more ovate, and lobulate equatorial outline due to greater chamber expansion, less embracing chambers, and more open umbilicus. The holotype of P. siakensis is slightly larger than the holotype of P. mayeri; the arched aperture of siakensis is not as high as the mayeri holotype; the holotype of siakensis has a prominent lip bordering the aperture, while the holotype of mayeri has an imperforate band and lacks a lip. Paragloborotalia siakensis is further distinguished from mayeri in having radial rather than slightly curved spiral-side sutures. This radial suture pattern is the more primitive condition, giving rise to the derived curved sutures in the latest Oligocene, a pattern that repeats itself in other early Miocene ‘globorotaliid’ lineages. Both taxa have similarly variable apertural characteristics, stratigraphic trend of increasing chamber number, paleobiogeographic distribution, and highest occurrence.

Paragloborotalia siakensis is distinguished from pseudokugleri by having a more rapid chamber expansion rate and more ovate equatorial outline, and in having a higher arched aperture. In addition, pseudokugleri commonly has slightly curved spiral sutures. Distinguished from P. birnageae by having a more rapid chamber expansion rate and more ovate equatorial outline, by having depressed sutures and more inflated, less embracing chambers, and a distinctly higher arched aperture. Distinguished from Fohsella? peripheroronda in having a broadly rounded periphery due to more spherical chambers, radial rather than curved spiral-side sutures, and a higher arched aperture. [Leckie et al. 2018]

Wall type: Normal perforate, coarsely cancellate, sparsely spinose in life (Hilgen and others, 2000; Zachariasse and Sudijono, 2012; Sanchez and others, 2014), heavy gametogenetic calcification is often present.

Test morphology: Test large in size; low trochospiral, strongly lobulate in equatorial outline, chambers globular, inflated, slightly embracing; some specimens may develop a kummerform final chamber; 4½-5½ chambers in the ultimate whorl of mid- to late Oligocene forms, up to 6-7 chambers in latest Oligocene to middle Miocene forms, increasing rapidly in size in initial whorl and slowly in the final whorl; in spiral view chambers subspherical, arranged in 2½-3 whorls, sutures depressed, radial; in umbilical view chambers spherical to subspherical, sutures depressed, radial, umbilicus narrow moderately deep; aperture umbilical-extraumbilical, low to moderately high arch, bordered by a prominent lip; in edge view chambers spherical, spiral side flat to slightly convex, umbilical side slightly convex, periphery broadly rounded. [Leckie et al. 2018]

Size: Maximum diameter of holotype 0.40 mm, maximum thickness 0.28 mm (original measurements); maximum diameter 0.41 mm, thickness 0.25 mm (remeasured this study). [Leckie et al. 2018]

Character matrix

test outline:Lobatechamber arrangement:Trochospiraledge view:Equally biconvexaperture:Umbilical-extraumbilical
sp chamber shape:Globularcoiling axis:Lowperiphery:N/Aaperture border:Thick lip
umb chbr shape:Globularumbilicus:Narrowperiph margin shape:Broadly roundedaccessory apertures:None
spiral sutures:Weakly depressedumb depth:Shallowwall texture:Cancellateshell porosity:Macroperforate: >2.5µm
umbilical or test sutures:Moderately depressedfinal-whorl chambers:4.5-7.0 N.B. These characters are used for advanced search. N/A - not applicable

Biogeography and Palaeobiology

Geographic distribution: According to Kennett and Srinivasan (1983), siakensis is more common in equatorial and warm subtropical locations, while mayeri is more common in temperate locations. Spezzaferri (1994) reports a cosmopolitan distribution for siakensis. [Leckie et al. 2018]

Isotope paleobiology: Late Oligocene P. siakensis from Trinidad suggest an upper thermocline depth of calcification with no photosymbionts (Pearson and Wade, 2009). Matsui and others (2016) interpreted a lower mixed-layer habitat for the late Oligocene siakensis group (including mayeri and semivera) from the equatorial Pacific Ocean. Gasperi and Kennett (1993) reported consistently low δ18O values through the lower and middle Miocene of DSDP Site 289 in the western equatorial Pacific suggesting a mixed-layer habitat. [Leckie et al. 2018]

Phylogenetic relations: Paragloborotalia siakensis was probably derived from nana by the addition of another chamber in the final whorl, less embracing chambers, more rapid rate of chamber expansion, and a higher arched aperture. Alternatively, siakensis may have been derived from opima based on many of the same morphological changes, but the earliest forms of siakensis were small suggesting that nana was the more likely ancestor. Paragloborotalia mayeri is most likely descended from siakensis in the latest Oligocene (Kennett and Srinivasan, 1983; Premoli Silva and Spezzaferri, 1991; Spezzaferri, 1994). [Leckie et al. 2018]

Most likely ancestor: Paragloborotalia nana - at confidence level 1 (out of 5). Data source: Leckie et al. 2018.
Likely descendants: Paragloborotalia mayeri;

Biostratigraphic distribution

Geological Range:
Notes: Lower Oligocene Zone O3 to upper Miocene Zone M11 (Spezzaferri and Premoli Silva, 1991; Spezzaferri, 1994; Wade and others, 2011). [Leckie et al. 2018]
Last occurrence (top): within M11 zone (10.46-11.63Ma, top in Tortonian stage). Data source: Leckie et al. 2018
First occurrence (base): within O3 zone (29.18-30.28Ma, base in Rupelian stage). Data source: Leckie et al. 2018

Plot of occurrence data:

Primary source for this page: Leckie et al. 2018 - Olig Atlas chap.5 p.166


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Paragloborotalia siakensis compiled by the pforams@mikrotax project team viewed: 3-7-2020

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