A drill core from Champaign Co., Ohio, produced evidence for an early Silurian (Llandoverian) terrestrial biota. Mesofossils preserved as carbonaceous compressions and impressions, as well as microfossils, were recovered from a shale lamina within carbonaceous dolomite at the base of the Brassfield Formation. These principally carbonatic rocks were deposited in subtidal to supratidal environments during the pulsatory transgression of the epeiric sea that covered the area in the Llandoverian. One of the mesofossils forms tufts of flat, elongated, branching lobes that diverge from a central zone generating a palmate outline. Lobes divide twice producing long, narrow segments separated by rounded sinuses, with the first division anisotomous, and the second, apical division isotomous. The resulting segments narrow significantly immediately distal to each branching and divide producing acute sinuses. The affinities of the mesofossil are uncertain, but the morphology is reminiscent of the dissected leaves of some liverwort species. Fragments of tissue representing the mesofossils, some of them two cell layers thick, were recovered from palynological preparations, together with hilate monads assignable to Cymbohilates disponerus Richardson, and multiseptate ascomycete spores. These represent the earliest occurrence of spores assignable to Ascomycetes, a principally terrestrial group, and the earliest occurrence of Cymbohilates, a cryptospore probably produced by Salopella-type land plants. This study consolidates and improves a search image for putative terrestrial organisms first defined by the discoveries of Pratt et al. (1978) in Llandoverian alluvial deposits of Virginia. This image consists of millimetric-scale, patchy mesofossils preserved mainly as carbonaceous compressions, but also having left impressions on the rock surface, amorphous or sometimes exhibiting submillimetric morphological detail. This study is also the first to document a second type of lower Silurian setting that preserves compression mesofossils of putative terrestrial origin – the subtidal to supratidal deposits of carbonate shorelines.

Key words: ascomycete, cryptospore, Llandoverian, mesofossil, Silurian, terrestrial