The great Rhomboaster controversy - Introduction

(By Jeremy Young, NHM, 2000)

Recently a surprisingly lively, and occasionally acrimonious, controversy has developed surrounding the Palaeogene nannoliths Tribrachiatus and Rhomboaster. Very briefly:

A. The traditional position (e.g. of Romein 1979) has been
1. R. cuspis and T. bramlettei have been regarded as structurally and morphologically discrete.
2. Nonetheless most workers have inferred that T. bramlettei evolved from R. cuspis.
3. The base of NP10 has been placed at the FO of T. bramlettei.

B. Bybell and Self Trail (1995) argued that:
1. R. cuspis and T. bramlettei are structurally identical.
2. Ray length is largely a function of dissolution and so of no biostratigraphic value
3. Therefore the base of zone NP10 should be moved down to the level of FO of any members of the group.

C. Aubry (1998) by contrast argued that:
1. R cuspis and T. bramlettei are structurally different.
2. Ray length is primarily a function of primary variation and so of biostratigraphic value
3. The NP10 base should stay at the level of the FO of T. bramlettei

Additional contributions were made by Angori and Monechi (1997), Aubry (1996), Aubry and Requirand (1999), Bybell and Self Trail (1997), Wei and Zhong (1996).

In an attempt to reach a sensible compromise von Salis et al. (2000) reviewed the available evidence. As part of this review we investigated the conflicting morphological interpretations by constructing computer models of the morphotypes (see example above). This investigation lead us to conclude that:

1. R. cuspis and T. bramlettei are structurally identical.
2. Ray length is a function of both dissolution and primary variation so of only limited biostratigraphic value.
3. In practice the base of NP10 has almost certainly not been consistently placed by nanno workers and should be placed at the base of the lineage.

In addition it should be noted that a new and indisputably discrete morphotype T. digitalis Aubry (1997) has now been described and proposed as the prime nannofossil marker for the base of the Eocene - although this is linked to a further controversy as to whether the Palaeoene-Eocene boundary should be placed at the level of the rapid global change event or at the level of a good biostratigraphic marker

These pages are intended primarily to act as an online appendix to von Salis et al. (2000). They consist of:
The Goteborg conference proceedings appeared as "Early Paleogene Warm Climates and Biosphere Dynamics" (GFF volume 122, p. 138-140). This volume contains both our paper and another, Aubry et al (2000), arguing the alternative opinion.

Aubry M.P., Requirand C., & Cook J. (2000). The Rhomboaster-Tribrachiatus lineage: A remarkable succession of events from 55.5 to 53.2 Ma". GFF, 122, 15-18.

von Salis K., Monechi S., Bybell L.M., Self-Trail J, & Young J.R. (2000) Remarks on the calcareous nannofossil genera Rhomboaster and Tribrachiatus around the Paleocene/Eocene Boundary. GFF, 122, 138-240.

Return to abstract, drawings of models, notes on the models