Biogeographic and morphological variation in Late Pleistocene to Holocene globorotalid foraminifera

Planktonic foraminifera are marine, calcite secreting protists. They have a long history of study in both industry and academia. Individual species show distinct biogeographical distributions and ecological tolerances. Traditionally species concepts are based on the gross morphology of the foraminiferal test. The closer the morphology of two species, the closer they are related. This has resulted in a single species being named by several authors from differing global locations and also, in long lived species, differing time intervals. This work investigates morphological variation of Late Pleistocene – Holocene menardiform globorotalids, and links this morphological variation to different ecological and environmental conditions. To achieve this 70 global sample sites are investigated covering a range of differing environmental conditions, but within constrained time limits. Where possible samples dated as Holocene have been used, where absolute dating was unavailable samples from about the Emiliani huxleyi acme zone, giving an upper age is given of 65 – 70 thousand years. Analysis of morphological variation allowed identification ofintergrading morphoclines and a total of six distinct morphotypes (e.g. the menardi-form morphotypes α, β, χ and η and the two tumid form morphotypes ε and φ). The morphotypes are shown to have distinct though overlapping biogeographic distributions. In the bivariate morphospace of spiral height versus axial diameter the equation y = 2.07x –15 separates morphocline(G. menardii morphologies) from morphocline(G. tumida morphologies). Within morphoclinethe line with equation y = 3.2x –160 separates morphotypes α (G. menardii menardii) from morphotype β (G. menardii cultrata). Morphotype β is interpreted as G. menardii cultrata and is seen to dominate environments with mean annual sea surface temperatures over 25°C. Morphotype α is interpreted as G. menardii menardii and becomes more dominant as sea surface temperatures become cooler. In areas where both morphologies are present in a sample we interpreted the situations a vicariant trophic depth adoption. G. menardii cultrata lives at shallow depths, while G. menardii menardii occurs deeper
within the water column. This interpretation is supported by stable isotope studies carried out on samples from the Gulf of Mexico and Caribbean region where the two morphologies show significantly different isotopic signals. G. menardii cultrata morphologically has a flattened smooth test with little secondary encrusting, while isotopically it has a shallow depth habitat and possible symbiotic relationship. G. menardii menardii morphometrically shows greater inflation and encrusting of the test and isotopically it shows a deeper and colder depth habitat. The presence of all ontogenetic stages within the two recognized morphological groups with distinct isotopic signatures, suggests that G. menardii may have two distinct subpopulations living at different depths within the Caribbean.
Ultrastructural studies on adult forms of morphotypes α and β from the same size fractions taken from a single sample, show that differences are present even in juvenile growth stages. Prolocular size and rate of growth suggest that morphotype α has a r-selected (rapid growth, opportunistic) mode of life. While morphotype β is k-selected (longer living, symbiont bearing, specialist) mode of life.
Morphotype η is interpreted as G. menardii gibberula this is the highest spired morphotype within the G. menardii group and is found only at the southerly extent of the sample set. Specimens have been identified in sample sites from the Western Pacific, which extends its known biogeographic range. It also has the highest spire of all the menardii forms and shows a correlation to the coldest sea surface temperatures.
Morphotype χ is only found in the northern part of the Indian Ocean and is interpreted as G. menardii neoflexuosa. It has a distinct flexure of the final chamber, but with removal of the final “flexed” chamber, the morphotype falls within morphotype β morphospace, to which is shows similar textural structure. The cause of the flexing is not clear, but as it is found in increased numbers during the summer monsoon, it has been suggested that it is a response to lowered salinity and an increase in turbidity of the surface waters.
Within morphocline 2 morphotype ε (G. tumida) is seen to intergrade the morphologically similar
but texturally different morphotype φ (G. ungulata). The diminutive size and delicate structure of G. ungulata is suggestive of it being the shallow dwelling juvenile form with being G. tumida the deeper dwelling more robust adult form. However, isotopic studies show differing depth habitats for the two morphotypes, with the heavier encrusted G. tumida showing a constantly deeper signal than the smoother more delicate form of G. ungulata, when comparing size equivalent specimens from the same sample sites. The first occurrence of G. ungulata is unclear but is believed to occur during the late Pleistocene. Because of this the results are interpreted as indicating ecophenotypic variation within a species, rather than just ontogenetic variation, with morphotype φ representing the shallow dwelling morphology, and morphotype ε the deeper dwelling morphology. Secondary encrusting of all specimens used in this present study indicates that encrusting is a function of which the foraminifera lived an not an indication of its stage of ontogeny or gametogenesis.