Establishing the timescale of early land plant evolution is essential for testing hypotheses on the coevolution of land plants and Earth's System. The sparseness of early land plant megafossils and stratigraphic controls on their distribution make the fossil record an unreliable guide, leaving only the molecular clock. However, the application of molecular clock methodology is challenged by the current impasse in attempts to resolve the evolutionary relationships among the living bryophytes and tracheophytes. Here, we establish a timescale for early land plant evolution that integrates over topological uncertainty by exploring the impact of competing hypotheses on bryophyte-tracheophyte relationships, among other variables, on divergence time estimation. We codify 37 fossil calibrations for Viridiplantae following best practice. We apply these calibrations in a Bayesian relaxed molecular clock analysis of a phylogenomic dataset encompassing the diversity of Embryophyta and their relatives within Viridiplantae. Topology and dataset sizes have little impact on age estimates, with greater differences among alternative clock models and calibration strategies. For all analyses, a Cambrian origin of Embryophyta is recovered with highest probability. The estimated ages for crown tracheophytes range from Late Ordovician to late Silurian. This timescale implies an early establishment of terrestrial ecosystems by land plants that is in close accord with recent estimates for the origin of terrestrial animal lineages. Biogeochemical models that are constrained by the fossil record of early land plants, or attempt to explain their impact, must consider the implications of a much earlier, middle Cambrian-Early Ordovician, origin.

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The timescale of early land plant evolution

Fieldwork in the Early Cretaceous Jehol Group, northeastern China has revealed a plethora of extraordinarily well-preserved fossils that are shaping some of the most contentious debates in palaeontology and evolutionary biology. These discoveries include feathered theropod dinosaurs and early birds, which provide additional, indisputable support for the dinosaurian ancestry of birds, and much new evidence on the evolution of feathers and flight. Specimens of putative basal angiosperms and primitive mammals are clarifying details of the early radiations of these major clades. Detailed soft-tissue preservation of the organisms from the Jehol Biota is providing palaeobiological insights that would not normally be accessible from the fossil record.

An exceptionally preserved Lower Cretaceous ecosystem

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

/pdf/the-evolution-of-seeds-4do0znoclt.pdf

The evolution of seeds

Being by far the largest family of enzymes to support plant metabolism, the cytochrome P450s (CYPs) constitute an excellent reporter of metabolism architecture and evolution. The huge superfamily of CYPs found in angiosperms is built on the successful evolution of 11 ancestral genes, with very different fates and progenies. Essential functions in the production of structural components (membrane sterols), light harvesting (carotenoids) or hormone biosynthesis kept some of them under purifying selection, limiting duplication and sub/neofunctionalization. One group (the CYP71 clan) after an early trigger to diversification, has kept growing, producing bursts of gene duplications at an accelerated rate. The CYP71 clan now represents more than half of all CYPs in higher plants. Such bursts of gene duplication are likely to contribute to adaptation to specific niches and to speciation. They also occur, although with lower frequency, in gene families under purifying selection. The CYP complement (CYPomes) of rice and the model grass weed Brachypodium distachyon have been compared to view evolution in a narrower time window. The results show that evolution of new functions in plant metabolism is a very long-term process. Comparative analysis of the plant CYPomes provides information on the successive steps required for the evolution of land plants, and points to several cases of convergent evolution in plant metabolism. It constitutes a very useful tool for spotting essential functions in plant metabolism and to guide investigations on gene function.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2011.04529.x

A P450-centric view of plant evolution.

We estimated the dates of the monocot–dicot split and the origin of core eudicots using a large chloroplast (cp) genomic dataset. Sixty-one protein-coding genes common to the 12 completely sequenced cp genomes of land plants were concatenated and analyzed. Three reliable split events were used as calibration points and for cross references. Both the method based on the assumption of a constant rate and the Li–Tanimura unequal-rate method were used to estimate divergence times. The phylogenetic analyses indicated that nonsynonymous substitution rates of cp genomes are unequal among tracheophyte lineages. For this reason, the constant-rate method gave overestimates of the monocot–dicot divergence and the age of core eudicots, especially when fast-evolving monocots were included in the analysis. In contrast, the Li–Tanimura method gave estimates consistent with the known evolutionary sequence of seed plant lineages and with known fossil records. Combining estimates calibrated by two known fossil nodes and the Li–Tanimura method, we propose that monocots branched off from dicots 140–150 Myr ago (late Jurassic–early Cretaceous), at least 50 Myr younger than previous estimates based on the molecular clock hypothesis, and that the core eudicots diverged 100–115 Myr ago (Albian–Aptian of the Cretaceous). These estimates indicate that both the monocot–dicot divergence and the core eudicot’s age are older than their respective fossil records.

Dating the monocot-dicot divergence and the origin of core eudicots using whole chloroplast genomes.

Archaefructaceae is proposed as a new basal angiosperm family of herbaceous aquatic plants. This family consists of the fossils Archaefructus liaoningensis and A. sinensis sp. nov. Complete plants from roots to fertile shoots are known. Their age is a minimum of 124.6 million years from the Yixian Formation, Liaoning, China. They are a sister clade to all angiosperms when their characters are included in a combined three-gene molecular and morphological analysis. Their reproductive axes lack petals and sepals and bear stamens in pairs below conduplicate carpels.

https://science.sciencemag.org/content/sci/296/5569/899.full.pdf

Archaefructaceae, a New Basal Angiosperm Family

The continuing study of early angiosperms from the Yixian Formation (≈125 Ma) of northeastern China has yielded a second early angiosperm genus This report is a detailed account of this early flowering plant and recognizes earlier reports of similar fossils from Russia and China Entire plants, including roots, stems, and branches terminating in fruits are presented and reconstructed Evidence for a possible aquatic nature of this plant is presented The relationship of Hyrcantha (“Sinocarpus”) to the eudicots is discussed The presence of this second early angiosperm genus, now known as a whole plant, is important in the discussion of its systematics and the ecology of the earliest angiosperms

https://www.pnas.org/content/pnas/104/22/9370.full.pdf

Qiang Ji

Papers

Archaefructaceae, a New Basal Angiosperm Family

An early infructescence Hyrcantha decussata (comb. nov.) from the Yixian Formation in northeastern China