Iterative evolution of digitate planktonic foraminifera

Abstract

2007). Iterative evolution of digitate planktonic foraminifera. Paleobiology, 33(4) pp. 495-516.For guidance on citations see FAQs.Abstract.-Digitate shell morphologies have evolved repeatedly in planktonic foraminifera throughout the Cretaceous and Cenozoic. Digitate species are usually rare in fossil and modern assemblages but show increased abundance and diversity at times during the Cretaceous and middle Eocene. In this paper we discuss the morphology and stratigraphic distribution of digitate planktonic foraminifera and establish the isotopic depth ecology of fossil ones to draw parallels with modern counterparts. ␦ 18 O and ␦ 13 C values of six extinct and two modern digitate species, from six time slices (Cenomanian, Turonian, Eocene, Miocene, Pleistocene and Holocene) have similar isotopic depth ecologies, consistently registering the most negative ␦ 13 C and usually the most positive ␦ 18 O compared to coexisting species. These results indicate a similar deep, subthermocline (Ͼ150 m) habitat, characterized by lower temperatures, reduced oxygen, and enrichment of dissolved inorganic carbon. This is consistent with water-column plankton studies that provide insight into the depth preferences of the three modern digitate species; in over 70% of observations digitates occurred in nets below 150 m, and down to 2000 m. The correlation between digitate species and subsurface habitats across multiple epochs suggests that elongated chambers were advantageous for survival in a deep mesopelagic habitat, where food is usually scarce. Increased abundance and diversity of digitates in association with some early and mid-Cretaceous oceanic anoxic events, in middle Eocene regions of coastal and equatorial upwelling, and occasionally in some modern upwelling regions, suggests an additional link with episodes of enhanced ocean productivity associated with expansion of the oxygen minimum zone (OMZ). We suggest that the primary function of digitate chambers was as a feeding specialization that increased effective shell size and food gathering efficiency, for survival in a usually food-poor environment, close to the OMZ. Episodes of increased digitate abundance and diversity indicate expansion of the deep-water ecologic opportunity under conditions that were unfavorable to other planktonic species. Our results provide evidence of iterative evolution reflecting common functional constraints on planktonic foraminifera shell morphology within similar subsurface habitats. They also highlight the potential of digitate species to act as indicators of deep watermasses, especially where there was expansion of the OMZ.

Seasonality and depth distribution of a mesopelagic foraminifer, Hastigerinella digitata, in Monterey Bay, California

“…H. digitata is thought to be strictly carnivorous, while G. adamsi and B. digitata may be relatively more omnivorous (C. Hemleben pers. comm., as cited in Coxall et al [2007]). No selective advantage in maintaining buoyancy has been demonstrated for spines, although it has been hypothesized (Lipps 1979), and living planktonic foraminifera maintain notably stable depth distributions in spite of negatively buoyant tests (Hemleben et al 1989).…”

“…Most species of modern foraminifera are primarily epipelagic, although some descend into mesopelagic waters for reproduction or seasonal survival (Fairbanks et al 1982;Hemleben et al 1989). Modern species with primarily or entirely mesopelagic depth distributions commonly possess digitate chambers (Hemleben et al 1989;Coxall et al 2007). Digitate chambers, defined by the presence of one or more fingerlike extensions from chambers in the final whorl (Fig.…”

“…Digitate chambers, defined by the presence of one or more fingerlike extensions from chambers in the final whorl (Fig. 1A,C-G; see operationalized definition in Coxall et al [2007]), are not observed in extant epipelagic species. There are three living digitate species, Hastigerinella digitata (Rhumbler 1911), Globigerinella adamsi (Banner and Blow 1959), and Beella digitata (Brady 1879); all are generally known from low to mid latitudes worldwide (Bé 1977;Coxall et al 2007).…”

“…There are three living digitate species, Hastigerinella digitata (Rhumbler 1911), Globigerinella adamsi (Banner and Blow 1959), and Beella digitata (Brady 1879); all are generally known from low to mid latitudes worldwide (Bé 1977;Coxall et al 2007). Oblique plankton tows and depth-stratified tows indicate that extant digitate species occur primarily in the water column below 200 m in depth (Rhumbler 1911;Bradshaw 1959;Bé 1977) and comprise a rare component of both planktonic and fossil assemblages (Prell et al 1999;Coxall et al 2007). …”

Depth-habitat reorganization of planktonic foraminifera across the Albian/Cenomanian boundary

“…delrioensis, it draws a parallel with the case of Hd. simplex, which has also been interpreted to prefer oxygendeficient environments (Coxall et al 2007). Nevertheless, habitat preference in the oxygen minimum zone should be illustrated by the most negative d 13 C values within the assemblage (Boersma and Premoli Silva 1989;Coxall et al 2007), the isotopic features that cannot be seen in Hd.…”