RECOGNITION OF TRUNCOROTALIA CRASSAFORMIS AS A MODERN PLANKTONIC FORAMINIFERAL MORPHOSPECIES IN THE CARIBBEAN AND EQUATORIAL ATLANTIC OCEAN AND PROPOSAL OF A NEOTYPE

摘要

To clarify patterns of within-population variation in Truncorotalia crassaformis and their bearing on selection of a neotype, the shape of shells from two Caribbean sediment traps was analyzed statistically. These data were compared with those from a Holocene equatorial Atlantic core, from topotypes of Globigerina crassaformis Galloway and Wissler (Lomita Quarry, California), and from type specimens of Globorotalia oceanica. This approach was used because the holotype (now destroyed) was the only designated specimen. Morphometric analyses of shells in axial and spiral orientations use data from equally-spaced coordinates around their outlines recorded from SEM imagery. Special attention is given to the profile of the last-formed chamber and to development of a keel at its periphery. Comparison of data from 150 m and 700 m traps shows that the former population includes only small, possibly pre-adult specimens that are non-crusted. Shell shape, especially in axial orientation is much less variable than in the 700 m sample which includes many encrusted and kummerform specimens. Variation in the Holocene sample is closely similar to that in the 700 m trap. Between group comparisons show that the holotype and two paratypes of Globorotalia oceanica plot within the morphospaces of the trap and Holocene samples. All are referred to Truncorotalia crassaformis on the basis of their shape, calcification patterns, and notably the absence of a keel or carina on the final chamber. It is likely that its holotype had this attribute. The axial shape of the neotype is similar to the modelled mean shape of specimens from the sediment traps. Topotypes with a keel on all chambers of the outer whorl are identified as Truncorotalia aff. crassacarina.