Colonial palynomorphs from the Upper Ordovician of north-eastern Iran: ‘thalli’, coenobial Chlorophyceae (Hydrodictyaceae) or cyanobacteria?

Abstract

This study documents 'colonial' palynomorphs from the Upper Ordovician Ghelli Formation of northeastern Iran. The aggregates of organic-walled microfossils come from the Katian Armoricochitina nigerica and Ancyrochitina merga chitinozoan Biozones of this formation. They can be classified as acritarchs and/or cryptospores, but they cannot be attributed to a particular biological group. Some specimens resemble 'thalli' of putative spores, such as Grododowon orthogonalis Strother 2017. Other clusters suggest an affinity to green algal groups, in particular with colonial chlorophyceaen algae, most probably of Hydrodictyaceae. Some specimens also show affinities with cyanobacterial groups. There are no arguments to relate these colonial palynomorphs to primitive land plants, but they are possibly produced by ancient green algal lineages that may have been experimenting with a subaerial existence.