During Silurian-Devonian times, the progressive colonisation of the land by plants ("terrestrialization") marked a period of major global change on the planet. The increase in areal extent of the vegetation cover together with the evolution in plant architecture and physiologies, appear to closely correlate to climate changes, to evolution of atmospheric composition and soil chemical differentiation, and to profound modification of weathering rates and nutrient input into the oceans. This study, based chiefly on cryptospores and miospores from North Africa (Ghadamis Basin, southern Tunisia), is a contribution to the understanding of the terrestrialization process, its timing and its palaeoenvironmental consequences across the Silurian-Devonian transition in Gondwana. Well preserved and diversified palynological assemblages throughout the Silurian-Devonian boundary have been documented from borehole MG-1 in the northern part of Ghadamis Basin (southern Tunisia). Over 60 species of cryptospores and trilete spores have been identified and their relative abundance estimated.
Trilete spores of simple morphology such as Ambitisporites constitute the dominant component of the palynological spectrum in almost all the studied levels. Miospores as Archaeozonotriletes, like Ambitisporites but with a thin proximal contact, mainly characterise the lower part of the succession of Ludlow (Ludfordian) age. In this portion, other trilete spores such as Emphanisporites, Synorisporites, and Lophozonotriletes in association with a relatively well diversified cryptospore assemblage, also occur. The cryptospores consist of permanent tetrads (e.g. Tetrahedraletes and Rimosetras), dyads (Dyadospora), pseudodyads (Pseudodyadospora), and monads (e.g. Gneudnaspora). Going upward along the succession, the relative abundance of trilete spores increases relatively to cryptospore abundance. Miospores as Apiculeretusispora and tripapillate trilete spores co-occur with Retusotriletes, Brochotriletes, and Chelinospora. The incoming of these forms approximately suggests a Pridoli age. The uppermost studied succession is constituted by Lochkovian assemblages. These are mainly characterised by a diversification of tripapillate spores with Emphanisporites spp. and Streelispora, with permanent tetrads, pseudodyads, dyads.
The increase in trilete spore abundance is accompanied by an increase in diversity. The recognised miospore assemblages can be correlated with miospore zonal schemes defined in the type sequences of the Welsh Borderland and those described from North Africa. The results of this study are discussed in terms of their importance relatively to the identification of the Silurian-Devonian boundary in the study area, and to their bearing on the analysis of patterns of colonization of terrestrial environments by the early land plants.