Igorina


Classification: pf_cenozoic -> Truncorotaloididae -> Igorina
Sister taxa: Acarinina, Praemurica, Igorina, Planorotalites, Astrorotalia, Morozovella, Morozovelloides,
Daughter taxa: (blue => in age window 0-300Ma)
Like I. broedermanni but with more plano-convex test; more chambers (8-9) in final whorl; wider and deeper umbilicus; and tendency towards an intraumbilically restricted aperture.
Biconvex to planoconvex test with 6-7 chambers in fine whorl. Like I. lodoensis but with less lobulate periphery and flatter spiral side.
Test small, lobulate, equally biconvex, densely muricate; narrow umbilicu; essentially straight radial sutures on the umbilical side.
Test small, biconvex, moderately lobulate, densely and finely praemuricate test; axial periphery subrounded to subacute, noncarinate; spiral sutures depressed, strongly recurved, tangential to inner whorl, often obscured by dense murical network; umbilicus small, shallow as a result of tight coiling mode.
Test moderately to strongly biconvex; sutures on spiral side strongly recurved yielding trapezoidal-shaped chambers; peripheral margin distinctly carinate, particularly on last chambers.
Test small, essentially circular, biconvex, cancellate, pustulose test with 5-6 chambers in last whorl; sutures on umbilical side radial, depressed, on spiral side moderately to strongly curved, depressed/weakly incised; axial periphery subacute and non-carinate; umbilicus narrow, shallow, aperture an interiomarginal, umbilical-extraumbilical arch extending towards, but not reaching, the periphery.
Specimens which cannot be assigned to established species

Taxonomy

Citation: Igorina Davidzon, 1976
Rank: Genus
Type species: Acarinina tadjikistanensis Bykova, 1953
Taxonomic discussion: As Pearson (1993:212) observed, the group of "biconvex, muricate morozovellids" (see Boersma and Premoli Silva, 1983; Premoli Silva and Boersma, 1989)— including the pusilla -laevigata, convexa, and broedermanni plexus of forms—is probably distinct from Acarinina and mainstream morozovellids. He considered these forms inappropriately placed in Igorina Davidzon (Loeblich and Tappan, 1988), inasmuch as the type species of the latter, Globorotalia tadjikistanensis Bykova, was considered an enigmatic, biconvex morozovellid from the upper part of Zone P3a, and probably is related to conicotruncata. Recent examination and SEM reillustration of the holotypes of Globorotalia tadjikistanensis Bykova (pl. 11: figs. 4-6) and Globorotalia convexa Subbotina (pl. 11: figs. 1-3), however, have shown these forms to be synonymous, with tadjikistanensis having priority.
Igorinids are characterized by their small, coarsely cancellate tests, evolute coiling, and distinctive blunt, praemuricate surface texture. [Olsson et al. 1999]

Catalog entries: Igorina;

Type images:

Distinguishing features: Test small, biconvex, evolute, sometimes with a keel; chambers, ovoid or low conical, 5-6 in final whorl
Wall, coarsely cancellate, praemuricate, often pustulose; .

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.

Description


Diagnostic characters: Test small, biconvex, may have a peripheral keel; chambers, ovoid or low conical in shape, 5-6 in final whorl; wall, coarsely cancellate, praemuricate, often with a thick, encrusted pustulose layer covering the praemuricate wall; aperture, interiomarginal, umbilical-extraumbilical, a low arch bordered by a thin lip. [Olsson et al. 1999]

Biogeography and Palaeobiology


Phylogenetic relations: We believe that the Igorinids evolved from Praemurica about the time of appearance of the first morozovellids (M. praeangulata, M. angulata). Like many globigerininid groups, the igorinids show a trend toward chamber compression and development of a peripheral keel, but this trend is reversed in the Eocene with the evolution of the I. broedermanni group. [Olsson et al. 1999]

Most likely ancestor: Praemurica - at confidence level 0 (out of 5). Data source: Olsson et al. 1999.

Biostratigraphic distribution

Geological Range:
Last occurrence (top): in upper part of Lutetian Stage (69% up, 43.2Ma, in Lutetian stage). Data source: Total of range of species in this database
First occurrence (base): near top of Danian Stage (85% up, 62.3Ma, in Danian stage). Data source: Total of range of species in this database

Plot of occurrence data:

Primary source for this page: Olsson et al. 1999 - Atlas of Paleocene Planktonic Foraminifera, p. 68

References:

Davidzon, R.M., (1976). Novyy paleogenovyy rod planktonnykh foraminifer. Trudy Vsesoyuznogo Nauchno-issledovatel’skogo Geologorazvedochnogo Neftyanogo Instituta (VNIGNI), Tadzhikskoe Otdelenie,, 183: 197-199.

Davidzon, R.M., (1976). [A new Paleogene genus of planktonic foraminifera.] [Russian.]. Dushanbe: Vses. Nauchno-Issled. Geologorazved. Neft. Inst. (VNIGNI), Tadzhik. Otdel., Trudy,, 183: ++.

Olsson, R.K.; Hemleben, C.; Berggren, W.A. & Huber, B.T., (1999). Atlas of Paleocene Planktonic Foraminifera. Smithsonian Contributions to Paleobiology, 85. Smithsonian Institution Press, Washington, DC, 1-252 pp.


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Igorina compiled by the pforams@mikrotax project team viewed: 15-12-2018

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