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Origin and evolution of the Neogene calcareous nannofossil Ceratolithus

Sediment samples of deep marine oceanic ODP boreholes from sites 999 in the Caribbean Sea and 1237 in the Eastern Pacific Ocean covering the period between 6 and 4.5 Ma have been studied with a focus on ceratolith evolution. Orthorhabdus rugosus is a nannolith with three blades (sinistral, median, and dextral) that first appeared during the Serravallian, it is not-birefringent in its stable orientation. It shows a high morphological variability time-interval at the end of the Messinian to the basal Pliocene (5.5 to 5 Ma) during which Ceratolithus (5.484 Ma) evolved. Changes occurred in the sinistral and median blades, whilst the dextral blade was reduced. Ceratolithus finifer n. comb is the first species of the evolutionary line. The nannolith stable position changed during its evolution, resulting in the older forms showing low birefringence and the younger ones moderate to high birefringence in the most stable orientation. Ceratolithus acutus, with an arrowhead shape, Ceratolithus armatus, and the morphologically distinct C. larrymayeri evolved from C. finifer with all three species showing high birefringence. The previous O. rugosus and C. finifer continued. Finally, C. armatus gives rise to C. cristatus. Ceratolithus atlanticus and C. tricorniculatus also evolved from C. finifer. All the species mentioned become extinct during the Pliocene except Ceratolithus cristatus that lives today. Detailed observations permit the analysis of the evolutionary trends of the group, possible mechanisms, patterns, and processes of speciation, and establish new criteria to define the species that, by their relative abundance and short geologic range, have permitted adjustment of biostratigraphic markers for this period.

Development of birefringence in the family Ceratolithaceae. Upper part, the first variability interval when Amaurolithus primus, Amaurolithus delicatus, and Nicklithus amplificus evolved from Orthorhabdus rugosus. As the median wing (in red) does not change its orientation, remaining perpendicular to the most stable position, pointing upwards, those species do not show birefringence. The lower part shows the O. rugosus second variability interval when Ceratolithus developed. The early forms, Ceratolithus finifer, show low birefringence as the c-axis/median wing (in red) incline towards the dextral wing (in green). In the later morphotypes the c-axis/ median wing rotates, tilting the nannolith most stable position to the right, and so showing moderate to high birefringence. Finally, when the lath-end of the sinistral wing (in blue) of C. acutus rotates upwards, the nannolith most stable position tilts further to the right orienting the c-axis parallel to it, producing the high birefringence characteristic of Ceratolithus.

Cite as: Lancis, C., Tent-Manclús, J.-E., Flores, J.-A., 2024. Origin and evolution of the Neogene calcareous nannofossil Ceratolithus. Mar. Micropaleontol. 186, 102310. https://doi.org/https://doi.org/10.1016/j.marmicro.2023.102310

Origin of the ceratolith genus Amaurolithus

 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.

The sudden appearance of A. primus in the sedimentary record at around 7.35 Ma was observed in both studied sections . The  Figure shows the set modifications of O. rugosus producing the early A. primus. The modifications are not a gradual evolutionary sequence of intermediate progressively modified forms but occur simultaneously as they are present from the beginning of the first recorded specimens. Amaurolithus primus coexisted with the O. rugosus ancestor in the studied samples. 

Modification on O. rugosus: Shortening of the specimen; Sinistral blade (blue): Lateral lengthening and its end portion curved upwards, concave to the observer; Dextral blade (green): Rotates backward; Median blade (red): Elevates and becomes more robust showing more marked teeth.

Cite as: Lancis, C., Tent-Manclús, J.-E., Flores, J.-A., 2022. Origin and evolutionary trends of the Neogene genera Amaurolithus and Nicklithus (calcareous nannofossils). Mar. Micropaleontol. 175, 102156. https://doi.org/https://doi.org/10.1016/j.marmicro.2022.102156