The Benthic Foraminiferal Genus Gyroidinoides: Early Taxonomic or Morphological Variability?

Gyroidinoides is a common genus among Cretaceous and Paleogene deep-sea benthic foraminifera, and its various species have been often used in paleoceanographic reconstructions, yet there are uncertainties regarding their early diversification. Species such as G. globosus, G. nitidus, G. primitiva, G. sokolovae, G. subglobosus and the most common one, G. infracretaceous, have been reported from Lower Cretaceous (Aptian and Albian) deep ocean records. Differences among species are based on size, number of chambers, apertural face, degree of convexity, inflation of the chambers and the test, and sutural depression. However, these features show high intra-specific variability, as evidenced by original descriptions and illustrations. This variability has been clarified for some species through emendation reports, but there is still a wide range of variability for many species that has not been formally addressed. Here we analyse the morphological variation of Gyroidinoides at Ocean Drilling Program (ODP) Site 511 in the South Atlantic Ocean and at ODP Site 766 in the Indian Ocean. All specimens of Gyroidinoides have been previously assigned to the species G. infracretaceous at both sites, and they have been described as highly morphologically variable. We reviewed the key features of 58 specimens from the NC7 and NC8 biozones at Site 511, and from the CC7a, CC8, and CC9 biozones at Site 766, to recognize the main morphotypes and assess their taxonomic identification. At least three different morphotypes were identified based on the sutural depression and the general shape of the test. These morphotypes could be associated to three different species. Specimens with flush sutures and less inflated chambers are assigned to G. nitidus, and those with depressed sutures and inflated chambers are identified as G. globosus or G. infracretaceous,depending of their apertural face (which is larger, and four times wider than high in G. globosus). These results point to a higher species diversity within the genus Gyroidinoides during the Aptian-Albian, rather than a highly intra-specific variability of a single species. More detailed studies on the variability and morphotypes of Gyroidinoides are needed to obtain robust conclusions, but this review opens the debate about the morphological vs. taxonomic diversification of the genus during the Early Cretaceous.