Steemans, Le Hérissé, Melvin, Miller, Paris, Verniers and Wellman (Science 324:353, 2009) reported naturally dissociated hilate and trilete spores, several of them ornamented, from tightly age-constrained Late Ordovician (Katian, about 453-445 Ma) marine strata of the Qusaiba-1 core hole in Saudi Arabia, and inferred that vascular plants (tracheophytes) originated and began diversifying earlier than the Silurian megafossil record shows, at high southern latitudes on the Gondwana supercontinent, before migrating and secondarily diversifying elsewhere. The inference rests on dispersed-spore morphology alone: a small minority of living bryophytes produce convergent trilete-like spores, and no Ordovician plant fossil with preserved conducting tissues is known. The literature lineage since 2009 shows that the Late Ordovician trilete/hilate record has been geographically broadened (Algeria, Iran, Argentina, Baltica) and indirectly supported by molecular clocks, mercury isotopes and carbon/phosphorus burial ratios, but the producers' identity has not been established; the Gondwana-first scenario is now contested by a Baltic Sandbian record whose trilete spores other specialists dismiss as folding artifacts; a recent reinterpretation argues trilete spores are a proxy for land plants generally, not for vascular plants; and an unpublished 2022 preprint claiming Ordovician tracheids has not survived to publication. The problem — resolving which plants produced the earliest trilete/hilate spores, and thereby the timing and geography of the origin of the vascular-plant lineage — remains open.

Background

Land plants (embryophytes) evolved from freshwater green algae of the charophycean lineage and colonized the land in a stepwise process beginning in the Middle Ordovician, roughly 470-460 million years ago (Ma). The earliest plants were small and lacked resistant tissues, so they left almost no body-fossil (megafossil) record; their early history is instead written mainly in dispersed spores — microscopic reproductive propagules released in abundance and preserved in marine and terrestrial sediments, recovered by acid maceration of rock samples (the study of such organic-walled microfossils is palynology).

Two dispersal strategies define the early spore record. Cryptospores are spores that remain permanently united in pairs (dyads) or fours (tetrads) after meiosis; they characterize the earliest vegetation, which is inferred to have had bryophyte-grade organization (bryophytes — mosses, liverworts and hornworts — are nonvascular land plants lacking water-conducting tissue). The oldest cryptospore assemblages are Middle Ordovician (Dapingian-Darriwilian), reported from Argentina and Saudi Arabia (Rubinstein et al. 2010). Hilate spores (single spores shed from dyads, bearing a contact scar without a germination opening) and trilete spores (single spores shed from tetrads, bearing a Y-shaped trilete mark that serves as a germination suture) are the derived condition. Nearly all living nonseed vascular plants (tracheophytes), which possess lignified water-conducting cells called tracheids, produce single trilete or monolete spores, as do polysporangiophytes (plants with branched sporophytes bearing more than one spore-producing sporangium). However, a small minority of living bryophytes — on the order of 2.5% of genera (Gray 1985) — also produce trilete spores, in many cases as nonfunctional 'cryptotrilete' scars formed by packing within the tetrad. The trilete condition is therefore a suggestive but not decisive indicator of vascular affinity.

Until 2009 the accepted reading was: a cosmopolitan, morphologically static, cryptospore-producing bryophyte-like vegetation from the Middle Ordovician onward; then, in the Early Silurian (about 443-433 Ma), an abrupt turnover in which cryptospore diversity collapses and trilete spores appear, become abundant and diversify. This turnover coincided approximately with the oldest vascular-plant megafossils — simple dichotomously branched Cooksonia-type plants, first recorded from late Wenlock strata in Ireland (Edwards and Feehan 1980), with late Silurian specimens showing an anatomically preserved vascular conducting strand (Edwards, Davies and Axe 1992) — so the turnover was read as marking the origin and adaptive radiation of vascular plants in the Early Silurian.

Steemans et al. (2009) examined core samples from the Qusaiba-1 shallow core hole in northern Saudi Arabia (then at high southern latitudes on the Gondwana supercontinent), in marine shelf strata of the Qasim Formation dated by chitinozoan and acritarch biostratigraphy (organic-walled marine microplankton) to the Katian stage of the Late Ordovician, directly beneath glaciomarine Hirnantian deposits of the end-Ordovician glaciation. These mid- to late-Katian samples unexpectedly yielded, alongside typical cryptospores, naturally dissociated spores: unornamented and ornamented hilate spores and seven trilete spore species, five of them ornamented with verrucae or muri — morphologies not otherwise known from strata older than the Silurian (the only prior Ordovician record being rare laevigate trilete spores of Hirnantian age from Turkey). On this basis the authors proposed that the vascular plant lineage emerged and began diversifying earlier than the Silurian megafossil record indicates, at high southern latitudes on Gondwana, and subsequently migrated to other continents where it secondarily diversified.

The proposal carries caveats. Attribution of the spores to the vascular lineage rests on dispersed-spore morphology alone, because some extant bryophytes produce convergent trilete-like spores, so the Qusaiba-1 producers could have been nonvascular plants; and no Ordovician megafossil or mesofossil (millimetre-scale plant fragments recovered by the same maceration techniques) preserving conducting tissues, or sporangia containing in situ spores attributable to the vascular lineage, is known from these intervals. The megafossil record — material exceptionally preserved by charcoalification and retaining exquisite anatomical detail including in situ spores — is one evidence class bearing on the vascular interpretation. Two further limitations follow from the state of the record: the geographic and stratigraphic extent of pre-Silurian hilate/trilete occurrences — and hence the Gondwanan-origin-then-migration scenario, as against independent or earlier origins elsewhere — has still to be established by broader sampling and correlation; and the end-Ordovician glacial deposits contain demonstrably reworked older microfossils, a feature documented in the overlying Sarah Formation, so the ages of critical horizons depend on independent dating.

Problem Statement

Determine the biological source and macroevolutionary meaning of the earliest trilete and hilate spore record. Specifically: do the Late Ordovician (Katian, about 453-445 Ma) hilate and trilete spores first documented from the Qusaiba-1 core hole of Saudi Arabia — and coeval or slightly younger occurrences elsewhere — derive from the vascular-plant lineage (tracheophytes or stem polysporangiophytes), from bryophyte-grade nonvascular plants that acquired dissociated trilete/hilate spores convergently, from some other grade of early land plant such as a clade uniting hornworts with polysporangiophytes or the stem-embryophyte producers envisaged by the 'spores before sporophytes' framework, or are the putative trilete marks not functional germination sutures at all but cryptotrilete ontogenetic scars or folding artifacts in acritarch-like vesicles, so that no pre-Silurian vascular signal exists? And, if the producers do belong to the vascular lineage, did vascular plants originate and begin their adaptive radiation in the Ordovician at high southern latitudes on Gondwana and only later migrate and secondarily diversify on other continents, or did they appear earlier or independently elsewhere — on Baltica or Laurentia, or in several regions?

Attribution of these spores to the vascular lineage rests on dispersed-spore morphology alone, because a small minority of extant bryophytes produce convergent trilete or cryptotrilete spores; no Ordovician plant fossil preserving conducting tissues, and no Ordovician sporangium containing in situ spores attributable to the vascular lineage, is known; and the affinity of the dissociated-spore condition remains open between the polysporangiophytes (protracheophytes plus tracheophytes) and a clade consisting of hornworts plus polysporangiophytes. Because end-Ordovician glaciogenic deposits demonstrably contain reworked older palynomorphs, age assignments for critical horizons depend on independent dating and on screening for reworking.

Which plants produced the earliest trilete/hilate spores, on what evidential basis, and what does this imply for the timing, geography and mode of the origin and adaptive radiation of vascular plants?

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