A freshwater palynological assemblage from the Hirnantian of Saudi Arabia

Abstract

A set of samples from the Hirnantian of Saudi Arabia, all of which contain the freshwater euglenid fossil, Moyeria Thusu, were examined for their organic-walled microfossil (OWM) content. Although these samples are dominated by sphaeromorph acritarchs of unknown phylogenetic and geographic provenance, they also contain members of both the Hydrodictyaceae (Proteolobus, Scenedesmus, Tapetisphaerites) and the Zygnemataceae (?Lecaniella, Stigmozygodites,?Zygnema, and a new form, Stigmatocystia, possibly related to Gelasinicysta). The closest living members of all these green algae are restricted to fresh water settings. Their recovery here extends the geological record of first occurrences for several taxa, including members of the Zygnemataceae, the sister group to the embryophtyes, which are here shown to be coeval as far back as the Late Ordovician. Bona fide marine palynomoprhs (i.e., acritarchs) are extremely rare in the assemblage, reinforcing the idea of a predominantly freshwater setting. Cryptospores (Velatitetras, Abditusdyadus, Rimosotetras and Tetrahedraletes) are common, but other than a single specimen of Imperfectotriletes, and a poorly preserved specimen of Ambitisporites, the lack of trilete spores in this deposit is curious, especially given reports of their prior antiquity. The association of some envelope-enclosed tetrads and dyads with freshwater algae lends support to prior proposals that consider Quadrisporites and related forms to be classified with the chlorophycean green algae, not as cryptospores.