Catalog - Globorotalia (Morozovella) aequa tholiformis

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Globorotalia (Morozovella) aequa tholiformis

Citation: Globorotalia (Morozovella) aequa subsp. tholiformis Blow 1979

Type specimens: Holotype:PM PF 64128; Paratype:PM PF 64252

Type age (chronostrat): Upper Paleocene-Lower Eocene, Landenian Cuisian [upper Thanetian lower Ypresian], Zone P. 7 (Globorotalia (Acarinina) wilcoxensis berggreni Zone of Blow, 1979, op. cit., p. 271); ranges from upper Paleocene, Zone P.5, to Lower Eocene, Zone P.8b.

Type locality: DSDP Station 47 2, Shatsky Rise, lat. 32 0 26.9' N., long. 157 0 42.7' E., northwestern Pacific Ocean: holotype and two paratypes (pl. 1 19, figs. 1-3) from sample 8 3, 82-85 cm; four paratypes (pl. 125, figs. l, 2; pl. 127, fig. 9; pl. 129, fig. 6) from sample 8 2, 71-73 cm; one paratype (pl. 133, fig. 9) from sample 8/ 1, 77-79 cm, DSDP Station 20-C, Rio Grande Rise, lat. 28 0 31.47' S., long. 260 50.73' W., South Atlantic Ocean; two paratypes (pl. 102, figs. 7, 8) from sample 6/3, 76-78 cm; one paratype (pl. 137, fig. l) from sample 5, 72-74 cm.

The test is coiled in a low fairly tightly expresssed trochospire with about 9-10 chambers comprising the spire and with 4 1/2 chambers in the last convolution of the test. The dorsal side of the test is sensibly flat and the dorsal sides of the chambers are not inflated. The ventral side of the test is strongly vaulted so that in axial-apertural profile the test is acutely plano-conic. The equatorial profile is moderately lobate and the chambers increase rapidly in size, as added in the progression of the trochospire, so that the last formed chamber always appears to comprise about one-half of the test in dorsal aspect. A peripheral-test murococarina is developed around the equatorial peripheral margin of the test and this margin of the test is acute, especially for the later chambers of the last whorl. In ventral aspect, the umbilicus is small but open and deep. The aperture is a low-arched opening situated interiomarginally at the base of the apertural face of the last chamber and extends from the umbilicus to about one-half of the way from the umbilicus towards the peripheral margin. A small dorsal sutural opening is present in the spiral suture near the junction of this suture with the dorsal intercameral suture between the last and penultimate chambers; this sutural opening is not considered as a true functional dorsal supplementary aperture (Blow, 1979, op. cit., p. 492).

Maximum diameter of holotype 0.34 mm.

Globorotalia (Morozovella) aequa tholiformis Blow, 1979 resembles G. (M.) aequa aequa Cushman and Renz in dorsal aspect and both forms display a moderately lobate equatorial periphery and a similar rate of chamber increase. However, tholiformis differs from aequa aequa in the much stronger vaultation of the ventral side which is combined with an acute expression of the umbilical extremities of the later chambers so that in axial profile, the test of tholiformis is sharply conic ventrally. In general, the umbilicus of tholiformis is smaller than in aequa aequa but it is always open and deep. The paratypes figured on pl. 102, figs. 7 and 8, show the rapid rate of increase in size of the chambers as added in the progression of the trochospire whilst the small but open and deep umbilicus is clearly shown in the paratype illustrated on pl. 102, fig. 7. The holotype figured on pl. 119, fig. 2, is flanked by two paratypes figured in dorsal aspect (figs. I and 3) which show the strongly developed peripheral test muricocarina which is usually much better organised and developed than seen in specimens referable to G. (M.) aequa aequa. The two paratypes figured on pl. 125, figs. I and 2, show the strongly vaulted ventral side as well as the rapid increase in size of the chambers during ontogeny and, in this respect, the chambers of tholiformis are, like those of aequa aequa, always considerably longer tangentially than radially broad. In the paratype figured in ventral aspect on pl. 125, fig. 2, the umbilicus is smaller than usual and in this feature alone, this specimen of tholiformis shows a transitional feature of the umbilical condition seen in G. (M.) aequa dolabrata Jenkins.
Plate 127, fig. 9 and pl. 129, fig. 6, illustrates two further paratypes in dorsal aspect showing the typical rapid increase of chamber size and the well developed murwocarina. However, these two paratypes also show quite clearly the small dorsal intercameral openings which appear as 'supplementary apertures' (but see Blow, 1979 op. cit., p. 498). The paratypes illustrated on pl. 133, fig. 9 and pl. 137, fig. l , document the persistence of typical G. (M.) aequa thol(formis into horizons as young as those included in Subzone P.8b.
Globorotalia (M.) aequa tholiformis differs from G. (M.) aequa dolabrata Jenkins, 1965 (see Blow, 1979, op. cit., p. 981) in retaining the rapid increase of chamber size typical for aequa (sensu stricto), a more acute peripheral margin and the consistently (albeit small) umbilicus. Furthermore, the umbilical extremities of the last convolution of chambers in dolabrata become rounded and the test, although generally plano-conic, has a rounded umbilical portion so that the conical ventral side appears truncated in axial profile.
'Globorotalia (M.) aequa tholiformis can be considered as the late direct descendant of G. (M.) aequa aequa in which the biocharacters of the latter taxon become progressively accentuated as to form a more specialised and extreme morphotype. Although the characters of coiling, rate of chamber increase, the recurved nature of the dorsal intercameral sutures and the tangentially long chambers persist in only a very little changed form within the morphogenesis from aequa aequa (sensu stricto) to aequa tholiformis, the morphogenetic changes in the characters of the ventral side are strongly pronounced. Thus, in tholiformis the strongly developed vaultation of the ventral side combined with the acute prolongations of the umbilical shoulders of the last convolution of chambers make the taxon easily distinguishable from Cushman and Renz's taxon. Furthermore, the small, but deeply open, umbilicus and much better organised and developed, peripheral test muricocarina provide additional points of difference between the two forms.""

References:

Blow, W. H. (1979). The Cainozoic Globigerinida: A study of the morphology, taxonomy, evolutionary relationships and stratigraphical distribution of some Globigerinida (mainly Globigerinacea). E. J. Brill, Leiden. 2: 1-1413. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia (Morozovella) aequa subsp. tholiformis Blow 1979

Globorotalia (Morozovella) aequa tholiformis

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