Catalog - Lunatriella spinifera

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Lunatriella spinifera

Citation: Lunatriella spinifera Eicher and Worstell 1970

Type sample (& lithostrat): Bridge Creek Member of the Greenhorn Limestone

Type age (chronostrat): Lower Turonian

Type locality: On Bridge Creek in Sections 14, 22, and 23, T 23 S, R 42 W, Hamilton County, Kansas.

Test elongate, compressed; initial biserial portion consisting of up to four pairs of subglobular chambers ; later chambers becoming increasingly elongate vertically, with curved axes convex outward ; final one to three elongate chambers in fully developed specimens irregularly uniserial, the chambers arcing alternately right and left; commonly the last chamber and rarely the last two chambers of the uniserial portion having a large aboral spine like elongation protruding at an angle that varies greatly among specimens; sutures distinct, depressed, strongly curved in later chambers of biserial portion, slanting alternately right and left in uniserial portion; aperture in early chambers a high arch bounded by lateral lips, which become increasingly elongate as chambers increase in height. In elongate but still biserially arranged chambers the lateral lips join below the aperture to form a trough like flange (here termed the "apertural trough"), which extends downward to the top of the chamber below. In uniserially arranged chambers the apertural trough commonly forms a delicate flying buttress separated from the main body of the test by a window; wall of radially built calcite, finely perforate.

Length of holotype 0.36 mm. Length of paratypes ranges from 0.21 mm. to 0.36 mm.

Preservation: Most of the specimens in hand are not very well-preserved. and surface textures have been considerably roughened by solution; many, however, show structural details of the apertural troughs. Unfortunately, the actual apertural openings are indistinct in all specimens studied.

Derivation: Lunatriella spinifera is inferred to have descended from a small species of Heterohelix that is associated with it in almost every occurrence. This species, here referred to Heterohelix pulchra (Brotzen), is characterized by a gently tapering test composed of chambers that change from subglobular in the early portion to elongate and slightly reniform in the later portion. And by prominent laterral apertural lips H. pulchra was initially described from the Senonian of Sweden in 1936. Montanaro-Gallitelli (1957, p. 1 37) considered H. pseudotessera (Cushman. 1938) to be a junior synonym. Specimens of H. pulchra from the western interior, illustrated in figures 1-4 of plate 1 are 0.22, 0.22, 0.30, and 0.25 mm. in length.

The initial chambers of L. spinifera are identical with those of H. pulchra. H. pulchra occurs at most localities stratigraphically slightly below L. spinifera, but where the two species coexist they intergrade morphologically. Forms transitional between H. pulchra and L. spinifera are shown in figures 4 and 5 of plate 1. These facts suggest that H. pulchra gave rise to L. spinifera by progressive elongation of the last few chambers. production of spinelike chamber projections. and downward elongation of the lateral apertural lips.

Although the specimens at hand appear morphologically identical with Brotzen's species, they may represent an early Turonian homeomorph, and not the actual beginning of true H. pulchra. The species here referred to H. pulchra has a short range within the Turonian in the western interior. Although its disappearance from the western interior in the middle Turonian may not represent its upper range limit as yet it has apparently not been reported from upper Turonian or lower Coniacian rocks elsewhere. Outside the western interior, the lowest reported occurrence of H. pulchra is middle Coniacian (Bandy, 1967).

If the tiny specimens here referred to H. pulchra are found to be endemic to the western interior. then they should probably be given a new name. On the other hand, they may well be early representatives of true H. pulchra, and this is the hypothesis accepted tentatively here. It would be strengthened were H. pulchra to be found in upper Turonian or lower Coniacian strata of other regions.

Ecology: Although Heterohelix became extinct at the end of the Cretaceous, the evidence is strong that its habitat was planktonic. In the western interior Heterohelix is commonly one of the dominant genera in assemblages that consist otherwise entirely or largely of planktonic specimens.

The habitat of Lunatriella, which apparently descended from Heterohelix, is inferred to have been likewise planktonic. In their early stages. specimens of L. spinifera are indistinguishable from Heterohelix pulchra, and their modes of life must have been similar. It is possible that when individuals developed the characteristic projections and apertural troughs during later growth stages their environment may have changed. However, there is no evidence for this. Inasmuch as six of the 24 samples in which L. spinifera has been found contain only planktonic specimens, L. spinifera is considered to have also been planktonic.

Chamber elongations that are spinelike or clavate in form appear in other genera of planktonic foraminifera. but their adaptive advantages are unknown. In Lunatriella the aboral spine like projections generally accompanied corresponding elongation of the chamber itself and development of flying buttresses from apertural lips. Possibly the flying buttresses may have functioned as support structures.

Comparisons: Apparently. the only previously described heterohelicid that bears pronounced spinelike projections is Heterohelix digitata Massella. which came from a block of Cretaceous marl (referred tentatively to the Campanian or Maestrichtian) in the Miocene "argille scagliose" of Sicily. Massella's specimens lack apertural lips and apertural troughs. and the spine-beanng chambers themselves are not elongate as in Lunatriella spinifera.

Localities: With respect to the Cretaceous succession in western Kansas and eastern Colorado. Lunatriella spinifera ranges throughout the upper three-fourths of the Bridge Creek Limestone Member and the lower one-fourth of the overlying Fairport Shale Member (see text-figure 2). About 300 specimens of L. spinifera have been recovered from scattered sample intervals from the upper 54 feet of the 75-foot Bridge Creek Member of the Greenhorn Limestone

References:

Eicher, D. L. & Worstell, P. (1970). Lunatriella, a Cretaceous heterohelicid foraminifer from the western interior of the United States. Micropaleontology. 16(1): 117-121. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Lunatriella spinifera Eicher and Worstell 1970

Lunatriella spinifera

References:

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