The Devonian Period was characterized by major changes in both the terrestrial biosphere, e.g. the evolution of trees and seed plants and the appearance of multi-storied forests, and in the marine biosphere, e.g. an extended biotic crisis that decimated tropical marine benthos, especially the stromatoporoid-tabulate coral reef community. Teleconnections between these terrestrial and marine events are poorly understood, but a key may lie in the role of soils as a geochemical interface between the lithosphere and atmosphere/hydrosphere, and the role of land plants in mediating weathering processes at this interface. The effectiveness of terrestrial floras in weathering was significantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were (1) arborescence (tree stature), which increased maximum depths of root penetration and rhizoturbation, and (2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats and allowed colonization of drier upland and primary successional areas. These developments resulted in a transient intensification of pedogenesis (soil formation) and to large increases in the thickness and areal extent of soils. Enhanced chemical weathering may have led to increased riverine nutrient fluxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottomwater anoxia, and high sedimentary organic carbon fluxes. Long-term effects included drawdown of atmospheric pCO2 and global cooling, leading to a brief Late Devonian glaciation, which set the stage for icehouse conditions during the Permo-Carboniferous. This model provides a framework for understanding links between early land plant evolution and coeval marine anoxic and biotic events, but further testing of Devonian terrestrial-marine teleconnections is needed.

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Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events

Summary The evolution of the seed represents a remarkable life-history transition for photosynthetic organisms. Here, we review the recent literature and historical understanding of how and why seeds evolved. Answering the ‘how’ question involves a detailed understanding of the developmental morphology and anatomy of seeds, as well as the genetic programs that determine seed size. We complement this with a special emphasis on the evolution of dormancy, the characteristic of seeds that allows for long ‘distance’ time travel. Answering the ‘why’ question involves proposed hypotheses of how natural selection has operated to favor the seed life-history phenomenon. The recent flurry of research describing the comparative biology of seeds is discussed. The review will be divided into sections dealing with: (1) the development and anatomy of seeds; (2) the endosperm; (3) dormancy; (4) early seed-like structures and the transition to seeds; and (5) the evolution of seed size (mass). In many cases, a special distinction is made between angiosperm and gymnosperm seeds. Finally, we make some recommendations for future research in seed biology. Contents Summary 817

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The evolution of seeds

Roots, as organs distinguishable developmentally and anatomically from shoots (other than by occurrence of stomata and sporangia on above‐ground organs), evolved in the sporophytes of at least two distinct lineages of early vascular plants during their initial major radiation on land in Early Devonian times (c. 410–395 million years ago). This was some 15 million years after the appearance of tracheophytes and c. 50 million years after the earliest embryophytes of presumed bryophyte affinity. Both groups are known initially only from spores, but from comparative anatomy of extant bryophytes and later Lower Devonian fossils it is assumed that, during these times, below‐ground structures (if any) other than true roots fulfilled the functions of anchorage and of water and nutrient acquisition, despite lacking an endodermis (as do the roots of extant Lycopodium spp.). By 375 million years ago root‐like structures penetrated almost a metre into the substratum, greatly increasing the volume of mineral matter subject to weathering by the higher than atmospheric CO2 levels generated by plant and microbial respiration in material with restricted diffusive contact with the atmosphere. Chemical weathering consumes CO2 in converting silicates into bicarbonate and Si(OH)4. The CO2 consumed in weathering ultimately came from atmospheric CO2 via photosynthesis and respiration; this use of CO2 probably accounts for most of the postulated 10‐fold decrease in atmospheric CO2 from 400–350 million years ago, with significant effects on shoot evolution. Subsequent evolution of roots has yielded much‐branched axes down to 40 μm diameter, a lower limit set by long‐distance transport constraints. Finer structures involved in the uptake of nutrients of low diffusivity in soil evolved at least 400 million years ago as arbuscular mycorrhizas or as evaginations of ‘roots’ (‘root hairs’).

Roots: evolutionary origins and biogeochemical significance

We present molecular dating analyses for land plants that incorporate 33 fossil calibrations, permit rates of molecular evolution to be uncorrelated across the tree, and take into account uncertainties in phylogenetic relationships and the fossil record. We attached a prior probability to each fossil-based minimum age, and explored the effects of relying on the first appearance of tricolpate pollen grains as a lower bound for the age of eudicots. Many of our divergence-time estimates for major clades coincide well with both the known fossil record and with previous estimates. However, our estimates for the origin of crown-clade angiosperms, which center on the Late Triassic, are considerably older than the unequivocal fossil record of flowering plants or than the molecular dates presented in recent studies. Nevertheless, we argue that our older estimates should be taken into account in studying the causes and consequences of the angiosperm radiation in relation to other major events, including the diversification of holometabolous insects. Although the methods used here do help to correct for lineage-specific heterogeneity in rates of molecular evolution (associated, for example, with evolutionary shifts in life history), we remain concerned that some such effects (e.g., the early radiation of herbaceous clades within angiosperms) may still be biasing our inferences.

https://www.pnas.org/content/pnas/107/13/5897.full.pdf

An uncorrelated relaxed-clock analysis suggests an earlier origin for flowering plants

Spore stratigraphy and correlation with faunas and floras in the type marine devonian of the Ardenne-Rhenish regions

Durant les dernieres 25 annees, nos connaissances sur le Dinantien de la Belgique se sont accrues de maniere substantielle, notamment grâce a l'etude detaillee de nombreuses coupes et a l'utilisation systematique de la micropaleontologie. Recemment, plusieurs travaux de synthese ont ete publies sous la forme de tableaux montrant la succession des fossiles-guides, les rythmes sedimentaires majeurs et les correlations suggerees avec les sections les plus connues de la Republique Federale Allemande, de Grande-Bretagne et d'Irlande, d'Europe Orientale et de la region du Mississippi en Amerique du Nord. La richesse des nouvelles donnees aujourd'hui disponibles reclamait une mise au point de ces tableaux, en particulier dans la mesure ou elles concernent la lithostratigraphie des principales regions de reference du Dinantien en Belgique. Les stratotypes de limite des cinq etages (Hastarien, Ivorien, Moliniacien, Livien et Warnantien) sont illustres pour la premiere fois. Les correlations de ces etages avec ceux qui ont ete proposes en Grande-Bretagne et en Republique Federale Allemande, ont ete ameliorees. Foraminiferes et conodontes permettent maintenant des correlations precises en Belgique meme mais aussi avec des pays voisins, voire des pays eloignes. Les tableaux mentionnent aussi des donnees sur les autres fossiles-guides utiles tels que spores, acritarches, algues, coraux, cephalopodes, trilobites et brachiopodes. Les formations lithologiques ont ete reexaminees en fonction de cette trame biostratigraphique afin de mieux connaitre leurs limites verticales et laterales. Elles illustrent a la fois l'influence des deformations synsedimentaires et des phases transgressees de grande ampleur qui ont marque de leur empreinte l'histoire sedimentaire et paleogeographique de l'Epoque dinantienne. De nouvelles unites lithostratigraphiques cartographiables sont aussi proposees.

https://popups.uliege.be/0037-9395/index.php?id=3180&file=1&pid=3173

Bio- and lithostratigraphic subdivisions of the Dinantian in Belgium, a review

The emergence of the seed habit in the Middle Paleozoic was a decisive evolutionary breakthrough. Today, seed plants are the most successful plant lineage, with more than 250,000 living species. We have identified a middle Givetian (385 million years ago) seed precursor from Belgium predating the earliest seeds by about 20 million years. Runcaria is a small, radially symmetrical, integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the multilobed integument. This extension is assumed to be involved in anemophilous pollination. Runcaria sheds new light on the sequence of character acquisition leading to the seed.

https://science.sciencemag.org/content/sci/306/5697/856.full.pdf

Runcaria, a Middle Devonian Seed Plant Precursor

Lorophyton goense gen. et sp. nov. from the Lower Givetian of Belgium and a discussion of the Middle Devonian Cladoxylopsida

Abundant and well-preserved material of the ligulate lycopsid genus Leclercqia is reported from a new Middle Devonian locality in northeastern Queensland (Australia). The plants occur in a chert horizon in the Storm Hill Sandstone of the Dosey-Craigie Platform. Lithological data and conodont analyses combined with information from in situ spores provide an age for the plant levels ranging from Eifelian, possibly Middle Eifelian, to Early Givetian. Plant taxonomic identification is based on vegetative and fertile stems that display both external morphology and anatomy. This material represents the best documented occurrence of Leclercqia outside Laurussia and possibly the earliest in Gondwana; it provides evidence that colonization of Gondwana by the species L. complexa was contemporaneous to that of Siberia and Kazakhstan. Analysis of the distribution patterns of L. complexa suggests that it was adapted to a wide range of environments, but within certain limits which we hypothesize to be those of a climatic belt. Such considerations support previous studies using other biological data, such as faunas and palynomorphs, for reconstructing Devonian palaeogeography. They favour a close proximity of Laurussia and Gondwana rather than the occurrence of a wide ocean separating the two palaeocontinents in Middle Devonian times.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756802007276

Muriel Fairon-Demaret

Papers

Bio- and lithostratigraphic subdivisions of the Dinantian in Belgium, a review

Runcaria, a Middle Devonian Seed Plant Precursor

Lorophyton goense gen. et sp. nov. from the Lower Givetian of Belgium and a discussion of the Middle Devonian Cladoxylopsida

The plant Leclercqia (Lycopsida) in Gondwana: implications for reconstructing Middle Devonian palaeogeography

Dinantian rocks in the subsurface North of the Brabant and Ardenno-Rhenish Massifs in Belgium, The Netherlands and the Federal Republic of Germany.