The evolution of dinoflagellates

Abstract

Recent work on modern dinoflagellates indicates that these organisms occupy a critical position in the evolution of life, being intermediate between prokaryotes and eukaryotes. It further suggests that dinoflagellates acquired chloroplasts through symbiosis with ingested autotrophic organisms. Two models have been proposed to explain the development of the cellulosic theca in the Dinophyceae. The first, the plate increase model, is based primarily on observations of living algae, while the second, the plate reduction model, relies mainly on paleontological data. However, neither satisfactorily reconciles both lines of evidence and so the plate fragmentation model is proposed. This postulates that dinoflagellates with two anterior flagella and a wall consisting of two large valves were successful through much of the Paleozoic. A major evolutionary breakthrough occurred in the Triassic with the development of a transverse-longitudinal flagellar arrangement and change in swimming direction. Associated with these modifications was a fragmentation of the valves into numerous polygonal plates. Subsequent evolution emphasized the influence of the two flagellar furrows over the number and arrangement of thecal plates. This led to a decrease in number and stabilization of the thecal plates as seen in modern dinoflagellates.