Early Terrestrial Environment Research Articles

Algae from the Lower Devonian Rhynie chert: Populations of a probable saccoderm desmid (Mesotaeniaceae, Zygnematales) preserved in a microbial mat

Green algae must have been taxonomically diverse in the Early Devonian Rhynie ecosystem; however, documented evidence of this is rare. Rhyniotaenium velatum gen. et sp. nov. is a unicellular organism that occurs in small populations in a microbial mat from the Rhynie chert. It consists of a straight, oblong cell (up to 14.5 μm long and 9 μm wide) surrounded by a stratified mucilaginous envelope; well-preserved cells contain a single, axile plate-like structure interpreted as a chloroplast. Populations are embedded in confluent mucilage. A high level of morphological correspondence exists between R. velatum and present-day saccoderm desmids of the genera Mesotaenium and Serritaenia (Mesotaeniaceae, Zygnematales, Zygnematophyceae), and thus can be used to interpret the fossil as a member of this grouping of streptophyte green algae. Uniformly aligned chloroplasts in one population suggest that R. velatum was capable of profile-to-face chloroplast movement. Pigmented or otherwise darkened envelopes and peculiar inclusions at the cell poles in several R. velatum specimens resemble a photoprotective adaptation recently described in Serritaenia. Rhyniotaenium velatum broadens our knowledge of the green algal component of early terrestrial environments, but is also significant because fossil evidence of Zygnematophyceae older than Carboniferous is exceedingly scant.

The overlooked aquatic green algal component of early terrestrial environments: Triskelia scotlandica gen. et sp. nov. from the Rhynie cherts

AbstractAs a large majority of modern green algae are freshwater rather than marine, it can be hypothesized that algae were an important component of early terrestrial environments. However, this component remains overlooked as algal affinities are difficult to decipher, due to the lack of distinctive characters observable in standard light microscopy. Herein, we use a new approach to studying microfossils from the Devonian (407 Ma) Rhynie cherts, renowned as one of the earliest exceptionally preserved terrestrial ecosystem. Using data from confocal laser scanning microscopy, the microfossils were reconstructed in 3D allowing us to gather information on morphological and physiological traits. We document Triskelia scotlandica gen. et sp. nov., which is the resting stage of a new species of microalgae. Numerous individuals were discovered preserved inside the remains of a flooded plant aerial axis. They are shown to be spheroidal to ellipsoidal walled cells, with a pseudo‐reticulate surface ornamentation and occasional large openings probably resulting from the germination process. We draw comparison with fossil and modern microorganisms demonstrating morphological convergences in wall ornament and physiological traits, especially with zygospores of modern Chlorophyta. Our description of Triskelia leads us to critically evaluate the Rhynie chert fossils Cymatiosphaera and Pterospermella. We conclude that it is unlikely that the Rhynie cherts contained prasinophycean algae of these types. We suggest that our microfossils are Chlorophyta (green algae) incertae sedis that were highly adapted to the special ecological conditions encountered in a geothermal wetland.

The Cosmochemical Record of Carbonaceous Meteorites: An Evolutionary Story

This account traces a lecture given to El Colegio Nacional last March during a Conference “On the origin of life on the Earth” organized to celebrate Darwin’s Bicentennial. It reports on the extraterrestrial organic materials found in carbon-containing meteorites, their composition, likely origin and possible prebiotic contribution to early terrestrial environments. Overall, this abiotic chemistry displaysstructures as diverse as kerogen-like macromolecules and simpler soluble compounds, such as amino acids, amines and polyols, and show an isotopic composition that verifies their extraterrestrial origin and lineage to cosmochemical synthetic regimes. Some meteoritic compounds have identical counterpart in the biosphere and encourage the proposal that their exogenous delivery to the early Earth might havefostered molecular evolution. Particularly suggestive in this regard are the unique l-asymmetry of a number of amino acids in some meteorites as well as the rich and almost exclusively water-soluble compositions discovered for other meteorite types.

A 4.2 billion year old impact basin on the Moon: U–Pb dating of zirconolite and apatite in lunar melt rock 67955

A sharp rise in the flux of asteroid-size bodies traversing the inner Solar System at 3.9 Ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments. The prevalence of ∼3.9 Ga crystallization ages for lunar impact-melt breccias and U–Pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short-lived, cataclysmic episode of late heavy bombardment at that time. Here we report U–Pb isotopic compositions of zirconolite and apatite in coarse-grained lunar melt rock 67955, measured by ion microprobe, that date a basin-scale impact melting event on the Moon at 4.22±0.01 Ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later. Significant impacts prior to 3.9 Ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish. Other isotopic evidence such as 40Ar–39Ar ages of granulitic lunar breccias, regolith fragments, and clasts extracted from fragmental breccias, and Re–Os isotopic compositions of lunar metal is also suggestive of impact-related thermal events in the lunar crust during the period 4.1–4.3 Ga. We conclude that numerous large impactors hit the Moon prior to the canonical 3.9 Ga cataclysm, that some of those pre-cataclysm impacts were similar in size to the younger lunar basins, and that the oldest preserved lunar basins are likely to be significantly older than 3.9 Ga. This provides sample-based support for dynamical models capable of producing older basins on the Moon and discrete populations of impactors. An extended period of basin formation implies a less intense cataclysm at 3.9 Ga, and therefore a better opportunity for preservation of early habitable niches and Hadean crust on the Earth. A diminished cataclysm at 3.9 Ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance.

Astrobiology: Towards an Understanding of the Emergence of Life in the Universe

Long before the idea of spontaneous generation was incorporated by JeanBaptiste de Lamarck into evolutionary biology to explain the first emergence of life, the possibility that other planets were inhabited had been discussed, sometimes in considerable detail, by scientists and philosophers alike (Lazcano 2001). More often than not, these were speculations that rested on the idea of a uniform universe but with little or no empirical basis. Today our approaches to the issue of life in the Universe have changed dramatically; neither the formation of planets nor the origin of life are seen as the result of inscrutable random events, but rather as natural outcomes of evolutionary events. The interconnection between these two processes is evident: understanding the formation of planets has major implications for our understanding of the early terrestrial environment, and therefore for the origin of living systems.

HCN and HNC in comets C/2000 WM1 (Linear) and C/2002 C1 (Ikeya-Zhang).

Comets have been suggested as a possibly significant source of organic molecules to the early Earth. Hydrogen cyanide (HCN) is important in models of prebiotic chemistry, but may be difficult to form in the early terrestrial environment, while hydrogen isocyanide (HNC) is a 'classical' tracer of interstellar ion-molecule chemistry. We have observed both HCN and HNC in 2 recent comets, bringing the number of comets with published measurements of the HNC/HCN abundance ratio to 6. The HNC/HCN ratio in comet Ikeya-Zhang appears to increase with decreasing heliocentric distance, as was previously observed for comet Hale-Bopp, indicating that the HNC is produced at least in part by processes in the cometary coma (atmosphere) and is not simply a constituent of the nuclear ices. Both comets C/2000 WMI (Linear) and C/2002 C1 (Ikeya-Zhang) exhibit values of the HNC/HCN ratio that appear to be too large (approximately 0.09-0.19) to be matched by current models of coma chemistry. Cometary HNC may be a photodissociation product of organic grains or large organic polymers stored in the nucleus. We have also set a limit on the emission from the NO radical in comet WM1.