Baltica cradle of early land plants? Oldest record of trilete spores and diverse cryptospore assemblages; evidence from Ordovician successions of Sweden

Abstract

The origin of land plants is one of the most important evolutionary events in Earth's history. The mode and timing of the terrestrialization of plants remains debated and previous data indicate Gondwana to be the center of land-plant radiation at~470-460 Ma. Here we present the oldest occurrences of trilete spores, probably the earliest traces of vascular land plants yet recorded. The spores occur in Ordovician, Sandbian (455 million years old) successions in central Sweden, once part of the paleocontinent Baltica. These strata are independently dated by marine microfossils (conodonts) and 206 Pb/ 238 U dating of volcanic ash deposits. Our discovery extends the record of trilete spores globally by~8 million years, and for Baltica by~25 million years. Additionally, cryptospore assemblages are identified revealing a diverse and stable mid-Ordovician (Darriwilian:~460 Ma) vegetation of free-sporing plants. The formation of regolith substrates on land as a consequence of permanent plant cover must in turn have affected the marine biota. We link these early land plant spore occurrences to the extensive, nutrient-rich volcanic ash deposits and propose Baltica as the possible original region of the radiation of early land plants. ARTICLE HISTORY

<strong>Fossils and Plant Evolution: Structural Fingerprints and Modularity in <strong>t</strong><strong>he Evo-Devo Paradigm</strong></strong>

“…By the beginning of the Silurian (444 Ma ago), basal members of the clade characterized by branched sporophytes (i.e., polysorangiophytes, including all vascular plants) had emerged (Salamon et al 2018) out of a plexus of basal embryophytes whose earliest bryophyte-grade representatives go as far back as the Middle Ordovician (468 Ma ago; Rubinstein et al 2010). If trilete spores are, indeed, exclusively characteristic of vascular plants and not of all the embryophytes, as has been proposed by Steemans et al (2009), then vascular plants and, by extension, polysporangiophytes may have evolved as early as 455 Ma ago, around the beginning of the Late Ordovician (Wellman & Strother 2015;Rubinstein & Vajda 2019). Direct information on these plants is available exclusively from fossils, which provide multiple structural fingerprints that, combined, allow us to reconstruct the morphology and life history of these tracheophyte ancestors.…”

Revision of thelodonts, acanthodians, conodonts, and the depositional environments in the Burgen outlier (Ludlow, Silurian) of Gotland, Sweden

“…The knowledge on the terrestrialization of Baltica is scarce and patchy, with the oldest known records of embryophyte (land plant) spores from the Eoplacognathus lindstroemi conodont subzone of the Furudal Limestone (mid-Ordovician: Darriwillian) of central Sweden (Rubinstein & Vajda, 2019). Definite trilete spores, produced by tracheophytes, occur in the Sandbian Dalby Limestone within the Baltonidus gerdae conodont sub-zone (Rubinstein & Vajda, 2019). Non-marine palynological data from Gotland is sparse and results have so far only been published from the nearshore deltaic successions of the Burgsvik Formation, southern Gotland (Gray et al 1974, Sherwood-Pike & Gray 1985, Hagström, 1997Hagström & Mehlqvist, 2012).…”

Divergent evolutionary trajectories of bryophytes and tracheophytes from a complex common ancestor of land plants

“…sp.) from the Ordovician successions of central Sweden [60]. Spores were sampled from the Borenshult-1 drillcore, comprising a well-dated succession of the Middle to Upper Ordovician.…”