Origin and evolutionary trends of the Neogene genera Amaurolithus and Nicklithus (calcareous nannofossils)

Abstract

Sediment samples from Ocean Drilling Program (ODP) Sites 999 and 1237 in the Caribbean Sea and Eastern Pacific Ocean were studied to monitor the evolution of ceratoliths from 7.4 Ma to 6 Ma. Orthorhabdus rugosus shows high variability at the end of the Tortonian to the Early Messinian (7.35 Ma to 6.91 Ma), resulting in the Amaurolithus (7.354 Ma) and Nicklithus branches (6.985 Ma). Orthorhabdus rugosus is an ortholith with three blades (sinistral, median, and dextral). The first ceratholith, A. primus, has two arms and a horseshoe shape with marked laths, and is stable and concave upwards. Its sinistral arm is formed from the sinistral blade of Orthorhabdus rugosus and the right arm is formed from the other blades. Early robust A. primus evolved into stylised forms and then to Amaurolithus delicatus (7.226 Ma), an almost plain horseshoe ceratolith with two arms. The left arm, usually the longer one, comes from the sinistral wing of A. primus and has a characteristic flattened omega section (Ω) without laths. There is a distribution overlap between A. primus and A. delicatus; the highest occurrence of the first one, at 6.282 Ma, is a newly proposed bioevent for the Messinian. After this, Amaurolithus does not have laths on its longer left arm and should be included in A. delicatus. The second branch has only one species, Nicklithus amplificus, which became extinct at 6.049 Ma. The dextral and median blades of O. rugosus form the N. amplificus dextral arm. The sinistral arm, which has the characteristic beak, hook, or hawk's bill, is the main difference between the development of this horseshoe and that of the previous one. The descriptions of A. primus, A. delicatus, and N. amplificus were specified based on the proposed evolutionary genesis.