Globorotalia margaritae evoluta Cita 1973 from: Bolli, H.M.Saunders, J.B. (1985): Oligocene to Holocene low latitude planktic foraminifers. In: Plankton Stratigraphy Eds: Bolli, H.M.Saunders, J.B. p. 155-262 | . |
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Subspecies Globorotalia margaritae evoluta Cita 1973 |
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| Discussion / Comments: |
Bolli & Saunders (1985):
G. margaritae s.l. is a widely recognized index species for the Pliocene. On palcomagnetic evidence quoted by Cita (1975) its absolute range is +/- 5.25 to 3.32 m.y. Compared with other Pliocene planktic index species, G. margaritae is remarkably cosmopolitan with a worldwide distribution through the tropical, subtropical and temperate Atlantic, Pacific and Indian Ocean provinces, and the Mediterranean area. However, both in deep water and in shallow water, more marginal assemblages it may be very infrequent. Though morphological differences between the earliest and the latest G. margaritae are considerable, it is often difficult to place intermediate specimens with certainty into one of the three subspecies primitiva, margaritae and evoluta proposed by Cita (1973). The reason for this is the continuous variability of specimens from the small non keeled and almost non lobate primitiva to the large, keeled and strongly lobate end form evoluta. While the distinction between G. margaritae primitiva and G. margaritae margaritae (acquisition of a peripheral keel) is usually not difficult, that between G. margaritae margaritae and G. margaritae evoluta on the other hand is more difficult as keeled forms vary in amount of keel and lobateness of test; while the absolute size of the test can be affected by environmental factors. Cita offers the following characters of distinction between the subspecies: G. margaritae primitiva differs from G. margaritae margaritae in its smaller size, less elongate equatorial outline, slower increase in size of chambers, more symmetrical axial profile, and complete lack of a distinct keel. Globorotalia margaritae evoluta differs from G. margaritae margaritae by its greater size, less elongated equatorial outline, slower increase in height of the chambers, and more symmetrical axial profile. Figs. 30.17-19, 12-14, 6-8, 2-4 show a sequence of G. margaritae subspecies arranged in stratigraphic order, taken from Krasheninnikov & Pflaurnann (1977). Catalano & Sprovieri (1969) described a G. praemargaritae (Fig. 31.7) from the Late Miocene (N 17, Globorotalia tumida plesiotumida Zone) of Central Sicily. With its distinctly lobate, nearly circular equatorial periphery and fairly inflated chambers we consider the species to fall within the G. scitula plexus. Compared with G. margaritae, in particular with the early subspecies primitiva, chambers in the last whorl of G. praemargaritae increase less rapidly in size, resulting in a more circular peripheral outline of the test. |
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Bolli & Saunders (1985):
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| References: |
Cita,M.B. (1973):
Pliocene biostratigraphy and chronostratigraphy . Rep. Deep Sea drill. Proj. Vol. 13 p. 1343-79
Bolli,H.M. and Saunders,J.B. (1985):
Oligocene to Holocene low latitude planktic foraminifers.
In: Plankton Stratigraphy Eds: Bolli, H.M.Saunders, J.B. p. 155-262
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