The earliest vascular land plants from the Upper Ordovician of China

Abstract Land plants (embryophytes) on the Earth were inferred to originate in the Cambrian Period (Morris et al., 2018; Donoghue et al., 2021) and to evolve from algae through nonvascular forms to vascular plants. This evolutionary scenario is verified by progressive fossil records ranging from Middle Ordovician embryophyte-derived cryptospores (Strother et al., 1996, 2015) and spore-containing fragments (Wellman et al., 2003), to vascular plant debris, including Late Ordovician trilete spores (Rubinstein and Vajda, 2019; Steemans et al., 2009), branched axes, and late Silurian tracheids and stomata (Kenrick et al., 2012). Among these key characters of vascular land plants, tracheid crucially distinguishes them from nonvascular groups, yet its fossil record is much later than estimated origination time, and vascular plant sporophytes had fossil records till the Wenlock (c. 430 Ma) of the Silurian Period (Libertín et al., 2018). Here we demonstrate the earliest bona fide vascular plants by specific meso- and microfossils from the Upper Ordovician (early Sandbian, c. 458 Ma) of the Ordos Basin, Inner Mongolia, China. Our fossils include bifurcated axes up to 687 μm wide, stomata-bearing epidermis, laevigate trilete spores, and well-developed tracheids with distinct lateral wall facets. Combined with the small stature of these plants, our finds show an evolutionarily transitional link from pre-Darriwilian, cryptospore-producing, and gametophyte-dominant plants to crown tracheophytes. Late Ordovician might witnesses the first explosion of vascular land plants and there had been a widespread flora of both bryophyte-like and small vascular plants dwelling the seaside by the Late Ordovician Epoch, than which the terrestrialization of vascular plants started much earlier.