Evidence of the First Colonization of the Land by Plants

Mineral weathering and soil development in the earliest land plant ecosystems

Authors:

Mitchell et al

Abstract:

Land colonization by plants and their fungal and bacterial symbionts during the Paleozoic was fundamental to the evolution of terrestrial ecosystems, but how these early communities influenced mineral weathering and soil development remains largely unknown. We investigated cryptogamic ground covers (CGCs) in Iceland to identify modern analogous communities and to characterize soil structure and biologically mediated weathering features. Using a novel application of X-ray microcomputed tomography, we show that moss-dominated CGCs and their soils are not adequate analogues of early communities. Comparisons with the 407 Ma Rhynie Chert (Scotland) biota indicate that modern CGCs dominated by lichens, liverworts, and their associated symbionts (fungi, cyanobacteria) are more representative of early soil-forming communities. Liverwort and lichen soils are thin, and their depth and complexity are constrained by the size and growth form of the dominant plants or lichens. They are aggregated and stabilized by cyanobacteria, mycorrhizal and lichenized fungi, rhizoids, and associated exudates. Smectite was associated with liverwort but not with moss CGC soils. Soil grain dissolution features are diverse and attributable to different organisms (e.g., bacteria, fungi) and types of interaction (e.g., symbiosis). We postulate that such features provide a novel indirect means of inferring biotic interactions in paleosols.

How Plants Conquered the Land 500 Million Years Ago

Research at the University of Leeds has identified a key gene that assisted the transition of plants from water to the land around 500 million years ago.

The ANR gene is required to tolerate 'extreme dehydration' in the moss Physcomitrella patens, a land plant that is used as an experimental model.

Researchers at the Centre for Plant Sciences at the University found that the ANR gene - present in the most ancient land plants - was inherited from ancestral fresh water algae.

The ANR gene has since been lost in the evolution of seed plants. The results are published today in the American Society of Plant Biology's journal The Plant Cell.

Dr Andrew Cuming, who led the research, said: "This gene hadn't been identified so far because most research until now has focused on modern flowering plants.

link.

The Origins of Land Plants

The Evolutionary Origin of a Terrestrial Flora

Authors:

Delwiche et al

Abstract:

Life on Earth as we know it would not be possible without the evolution of plants, and without the transition of plants to live on land. Land plants (also known as embryophytes) are a monophyletic lineage embedded within the green algae. Green algae as a whole are among the oldest eukaryotic lineages documented in the fossil record, and are well over a billion years old, while land plants are about 450–500 million years old. Much of green algal diversification took place before the origin of land plants, and the land plants are unambiguously members of a strictly freshwater lineage, the charophyte green algae. Contrary to single-gene and morphological analyses, genome-scale phylogenetic analyses indicate the sister taxon of land plants to be the Zygnematophyceae, a group of mostly unbranched filamentous or single-celled organisms. Indeed, several charophyte green algae have historically been used as model systems for certain problems, but often without a recognition of the specific phylogenetic relationships among land plants and (other) charophyte green algae. Insight into the phylogenetic and genomic properties of charophyte green algae opens up new opportunities to study key properties of land plants in closely related model. This review will outline the transition from single-celled algae to modern-day land plants, and will highlight the bright promise studying the charophyte green algae holds for better understanding plant evolution.

3D Video Reconstruction of Retallack's Problematic "Terrestrial" Fossils

 

The original post about Retallack's latest controversial fossil is here.  This video is from the supplements.

Diskagma buttonii: Evidence of Paleoproterozoic Terrestrial Life?

Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa

Authors:

1. Gregory J. Retallack (a)
2. Evelyn S. Krull (b)
3. Glenn D. Thackray (c)
4. Dula Parkinson (d)

Affiliations:

a. Department of Geological Sciences, University of Oregon, Eugene, Oregon

b. C.S.I.R.O. Land and Water, Waite Campus, Glen Osmond, South Australia 5064

c. Department of Geosciences, Idaho State University, Pocatello, Idaho 83209

d. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720

Abstract:

Small (0.3-1.8 mm long), locally abundant, urn-shaped fossils within surface horizons of a paleosol in the 2.2 Ga Hekpoort Formation near Waterval Onder, South Africa, are here described and named Diskagma buttonii Retallack gen. et sp. nov. The fossils are from fresh rock of a deep highway cutting, and have been metamorphosed to upper greenschist facies like their matrix. Despite metamorphic alteration, total organic carbon of the samples was 0.04% and its isotopic composition (δ13C) was–25.6 ± 0.08 ‰ (two standard deviations) versus Vienna Pee Dee belemnite standard. Organic outlines of the fossils are also accentuated by recystallized berthierine and opaque oxides. The fossils are locally clumped within surface swales of a Vertisol paleosol, identified from characteristic penecontemporaneous deformation (clastic dikes between swales of mukkara structure) and from pronounced geochemical differentiation (phosphorus and copper strain-corrected mass-depletion characteristic of an oxidized biologically active soil). This paleosol's chemical composition is evidence of temperate humid climate (mean annual temperature 11.3 ± 4.4 °C, and mean annual precipitation 1489 ± 182 mm). Associated paleosols indicate atmospheric CO2 of 6640 +12880/-4293 ppm (0.6%) and 0.9-5% atmospheric O2. The best preserved examples of Diskagma are shaped like an urn with a flared rim, and closed below the flare. Observation of hundreds of specimens in thin section reveals substantial variation in growth (elongation) and decay (shredding and deflation). They had a hollow ellipsoidal interior that is unusually devoid of opaque debris, unlike the matrix. Diskagma is superficially comparable with lichens such as Cladonia (Ascomycota) and Geosiphon (Glomeromycota). Definitive reproductive structures remain unknown. They predate the oldest other likely fossil eukaryotes (1.9 Ga) and fungi (1.5 Ga), and current molecular clock estimates for eukaryotes (1.6 Ga) and fungi (1.1 Ga). Lichenized actinobacteria are plausible prokaryotic alternatives permitted by molecular clocks. Although biological affinities of Diskagma are uncertain, these fossils reveal the general appearance of Paleoproterozoic life on land.

I gotta add that I am skeptical.  Not that there is life, but...lichens?  That early?  The paper even notes there are problems with the timing...