Globorotalia opima opima Bolli 1957 from: Bolli, H.M. (1957): Planktonic foraminifera from the Oligocene-Miocene Cipero and Lengua formations of Trinidad, B.W.I.. In: Studies in Foraminifery, Bulletin of the U.S. National Museum Vol. 215 Eds: Loeblich, A.R.Tappan, H.Beckmann, J.P.Bolli, H.M.Gallitelli, E.M.Troelsen, J.C. p. 97-123 | . |
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Subspecies Globorotalia opima opima Bolli 1957 |
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| Diagnosis / Definition: |
Bolli (1957):
Shape of test very low trochospiral; equatorial periphery slightly lobate; axial periphery rounded; due to the rapid increase in size of the chambers in the last whorl, the spiral side often appears slightly concave. Wall calcareous, perforate, surface finely pitted. Chambers spherical, 10-12, arranged in about 2 1/2 whorls; the 4-5 chambers of the last whorl increase rapidly in size. Sutures on spiral side radial, depressed; umbilical side radial, depressed. Umbilicus narrow, deep. Aperture a fairly low arch, a slight rim or lip is only occasionally observed, interiomarginal, umbilical extraumbilical. Coiling random. Largest diameter of holotype 0.55 min. |
| Discussion / Comments: |
Bolli & Saunders (1985):
The larger subspecies opima and the smaller sized nana are distinguished by size only. Number and arrangement of chambers in the last whorl, robust wall structure with coarse pores and low aperture are common to both. With its more restricted range, G. opima opima has turned out to be a valuable and widely used index fossil within the Oligocene, and a zone based on its total range has been established. As only the large sized opima subspecies has a restricted, stratigraphically valuable range, its clear separation from nana is essential. Originally, only 'small' as against 'large' size was cited as a distinction for the two subspecies with measurements only given for the holotypes. Investigators therefore had to decide arbitrarily whether to place intermediate specimens into one or the other subspecies, with the result that broadening of the concept of G. opima opima sometimes resulted in an over extension of the length of the zone. To determine whether the two subspecies can be divided more accurately by size we have measured the greatest diameter of a sequence of different sized specimens from the type locality of the two subspecies, with the result that the following four size intervals can be quite clearly distinguished: 0.25-0.32, 0.32-0.38, 0.39-0.43 and 0.45-0.5 mm or slightly larger. The smallest group belongs unquestionably to the subspecies nana, the largest to opima. Of the two intermediate size groups the larger, 0.39-0.43 mm can also be assigned to opima as it represents specimens equivalent to the largest group but apparently not fully grown and therefore lacking the final chamber. The three groups 0.25-0.32,0.39-0.43 and 0.45-0.5 mm consist of specimens having the last whorl formed by 4 chambers. Only rarely does one find here specimens with 5 chambers. Such an unusual specimen was unfortunately chosen as the holotype for G. opima opima, while the paratype figured along with it is the far more common 4 chambered form. Specimens of the size range 0.32-0.38 mm often possess 5 chambers in the last whorl instead of 4, unlike the smaller and larger groups. This is also the size interval that divides the smaller, typical subspecies nana from the larger opima. Because of its intermediate size, this group with its 5 chambered specimens is difficult to assign to either of the two subspecies. Population studies in more samples from different levels within the zone and different areas should eventually aid in deciding whether these intermediate sized 5 chambered forms belong to the opima complex at all or whether they might possibly be ancestral to Globorotalia mayeri. Based on the measurements made on the populations at the type locality, the following subdivision into the subspecies nana and opima is indicated:
0.25-0.32 mm G. opima nana
0.32-0.38 ram: intermediate sized specimens often with
5 chambers, not typical for nana or opima, probably
restricted to the Globorotalia opima opima Zone
0.39-0.5 ram or slightly larger: G. opima opima
Typical G. opima opima differ from G. mayeri in having the last whorl usually formed by 4 instead of 5 or 6 chambers. In side view opima tests are higher than those of mayeri. The equatorial peripheral outline appears more circular in opima compared with an often slightly elongate one in mayeri. While the last chamber in G. mayeri specimens may be sack like and drawn back ('sacculiferid'), resulting in a curved suture between the last two chambers, this feature is unknown in G. opima opima. The aperture in G. opima opima is a low slit compared with a distinctly higher arched one in G. mayeri. The differences between G. opima nana and G. continuosa are virtually the same as those between G. opima opima and G. mayeri. It is possible that the 5 chambered, medium to small sized atypical G. opima may represent the ancestral stock for G. mayeri, which develops more typically above this stratigraphic level. Likewise it is difficult to draw a distinct limit between the 4 chambered G. opima nana and G. continuosa. Specimens that cannot clearly be assigned to one or the other taxon are present from the Globigerina ciperoensis ciperoensis to the Catapsydrax stainforthi Zone (Bolli & Saunders, 1982a, pl. 4). The two subspecies opima and nana have been recorded from low and mid latitudes including the Mediterranean area. To what extent ecological factors controlled the presence of the two subspecies is not yet fully understood. There are indications for a temperature control as pointed out by Stainforth et al. (1975) for Ecuador, where the subspecies ruma is said to be abundant in a cool water fauna but opima is absent. Bolli (1957), Blow (1969) and Stainforth et al. (1975) quote G. opima nana as occurring from the Middle Eocene Truncorotaloides rohri Zone to the top of the Middle Oligocene Globigerina ciperoensis ciperoensis Zone. Re examination of samples through the Middle and Late Eocene and Early Oligocene confirm the presence as early as the Globorotalia lehneri Zone of small, 4 chambered specimens morphologically very close to the type nana. Some of the specimens in the type locality sample of the Truncorotaloides rohri Zone in Trinidad closely resemble G. cf. bolivariana, a near homeomorph of G. opima opima, though they are smaller in size. It would appear that at least some of the Eocene and Early Oligocene small 4 chambered specimens that are morphologically close to G. opima nana may in fact he homeomorphs and not be related genetically.
Bolli (1957):
Globorotalia opima opima, new species, new subspecies, is distinguished from G. mayeri Cushman and Ellisor by the greater thickness of the test and in having 4-5 chambers in the last whorl, instead of 5-6. The apertural lip, which is usually present in G. mayeri, is only occasionally seen in the new subspecies. G. opima opima has a very restricted range whereas G. mayeri can be followed through most of the Cipero formation into the lower Lengua. G. opima opima differs from G. opima nana, new species, new subspecies, by its larger size. It has a more restricted stratigraphic range. |
| Synonym list: |
Bolli & Saunders (1985):
Bolli (1957):
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| References: |
Bolli,H.M. (1957):
Planktonic foraminifera from the Oligocene-Miocene Cipero and Lengua formations of Trinidad, B.W.I..
In: Studies in Foraminifery, Bulletin of the U.S. National Museum Vol. 215 Eds: Loeblich, A.R.Tappan, H.Beckmann, J.P.Bolli, H.M.Gallitelli, E.M.Troelsen, J.C. p. 97-123
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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