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December, 1946

No. 283. A New Species of Amphistegina from the Eocene of Ecuador


The species here described is unique in its general shape and the form of the supplementary chambers. Sections are given to show the internal structure.

AMPHISTEGINA ELLIOTTI Cushman and Stainforth, n. sp. (Pl. 20. figs. 1‑6)

Test strongly conical in shape, the dorsal side nearly flat or only slightly convex, the ventral side nearly as deep as wide, periphery sub-acute; chambers of the dorsal side involute, strongly curved, the ventral side with supplementary chambers distinctly curved and in some specimens apparently tending to become slightly labyrinthic; sutures rather indistinct except in abraded specimens, but very strongly curved; wall especially on the ventral side marked by prominent, raised papillae, the dorsal side also with distinct papillae in well preserved specimens; aperture on the ventral side, elongate, at the inner margin of the last‑formed chamber. Height 0.75‑1.00 mm.; diameter 0.75‑1.50 mm.

Holotype (Cushman Coll. No. 47222) from the middle Eocene, Estero Pollo, a small tributary on the left bank of Rio Verde, about 22‑1/2 km. in a straight line from the river mouth, northwest Ecuador. The samples were collected by D. H. Elliott and are numbers 13144, 13451‑B in the sample lists of International Petroleum Co. in Guayaquil. The species is known only from this one locality.

The species differs from Amphistegina lopeztrigoi Palmer from the Eocene of Cuba in the more conical form and the smaller papillae.

Loose specimens of A. elliotti are abundant in the washed residue of shaley pockets within a limestone exposed in strike‑section in the stream. The species can also be recognized in thin‑sections of the limestone. The accompanying fauna is a Discocyclina‑Archaeolithamnion assemblage such as typifies the “Guayaquil limestone” of Sheppard.[1] Although there are outcrops of typical “Guayaquil limestone” in the Rio Verde area, the Estero Pollo exposure is atypical in containing megascopic clastic material. On faunal grounds it must be regarded as a slight facies‑variant of the “Guayaquil limestone.”

In thin‑sections of the Estero Pollo limestone and of a few free specimens of larger foraminifera picked from the shaley material, the following species were noted:

Discocyclina (Discocyclina) anconensis Barker ?

Discocyclina (Discocyclina) meroensis (Berry)

Discocyclina (Discocyclina) sheppardi Barker ?

Discocyclina (Asterocyclina) sp.

Pseudophragmina (Proporocyclina) peruviana (Cushman) ?

The specific determinations are partly tentative because of the nature of the material, but the subgeneric determination of Asterocyclina sp. is positive, based on free specimens from the shaley material. It is a quad­rate species closely related to A. rutteni Vaughan and undoubtedly related to ‘Asterodiscocyclina’ stewarti Berry from the middle Eocene of Peru. The assemblage as a whole shows a distinct affinity to that recently described from the Upper Scotland formation of Barbados, of which Vaughan[2] states: “The foraminiferal fauna of the Upper Scotland formation is obviously Middle Eocene, and it may be considered the type Middle Eocene of America.”

The limestone on Estero Pollo is overlain by beds containing a suite of small foraminifera and radiolaria which indicate high middle to upper Eocene age.

On the foregoing evidence, Amphistegina elliotti must be considered a middle Eocene species, probably from the lower part of that interval.

Explanation of Plate 20

FIGS. 1-6. Amphistegina elliotti Cushman and Stainforth, n. sp. Eocene, Ecuador. x 35. 1, 2, Transverse sections of the dorsal portion. 3, Transverse section of labyrinthic central portion. 4, Peripheral view showing conical ventral side. 5, Dorsal view of slightly eroded specimen showing arrangement of chambers on dorsal side. 6, Ventral view showing labyrinthic chambers. 1-3 4, 6, Paratypes. 5, Holotype.

7-9. Cribropyrgo robusta Cushman and Bermudez, n. gen., n. sp. Recent, off Cuba. x 20. 7, Apertural view. 8, Side view. 9, Transverse section. 8, Holotype. 7, 9, Paratypes.

[1] Sheppard, G. “The geology of southwestern Ecuador.” London, 1937

[2] Vaughan, T. W. “American Paleocene and Eocene Larger Foraminifera.” Mem. 9, Geol. Soc. Amer., pt. I, 1945.