Hugo Beraldi-Campesi

Early life on land and the first terrestrial ecosystems

Terrestrial ecosystems have been largely regarded as plant-dominated land surfaces, with the earliest records appearing in the early Phanerozoic (<550 Ma). Yet the presence of biological components in pre-Phanerozoic rocks, in habitats as different as soils, peats, ponds, lakes, streams, and dune fields, implies a much earlier type of terrestrial ecosystems. Microbes were abundant by ~3,500 Ma ago and surely adapted to live in subaerial conditions in coastal and inland environments, as they do today. This implies enormous capacities for rapid adaptations to changing conditions, which is supported by a suggestive fossil record. Yet, evidence of “terrestrial” microbes is rare and indirect in comparison with fossils from shallow or deeper marine environments, and its record has been largely overlooked. Consequently, the notion that microbial communities may have formed the earliest land ecosystems has not been widely accepted nor integrated into our general knowledge. Currently, an ample record of shallow marine and lacustrine biota in ~3,500 Ma-old deposits, together with evidence of microbial colonization of coastal environments ~3,450 Ma ago and indirect geochemical evidence that suggests biological activity in >3,400 Ma-old paleosols endorses the idea that life on land perhaps occurred in parallel with aquatic life back in the Paleoarchean. The rapid adaptations seen in modern terrestrial microbes, their outstanding tolerance to extreme and fluctuating conditions, their early and rapid diversification, and their old fossil record collectively suggest that they constituted the earliest terrestrial ecosystems, at least since the Neoarchean, further succeeding on land and forming a biomass-rich cover with mature soils where plant-dominated ecosystems later evolved. Understanding how life diversified and adapted to non-aquatic conditions from the actualistic and paleontological perspective is critical to understanding the impact of life on the Earth’s systems over thousands of millions of years.

70 Ma nonmarine diatoms from northern Mexico

Carbonaceous cherts of the Tarahumara Formation, exposed near Huepac, Sonora, Mexico, contain abundant diatom frustules occurring as benthic filamentous colonies. Stratigraphic and paleontological observations indicate that Tarahumara sediments accumulated in a nonmarine setting; radiometric ages on encompassing volcanic rocks delimit their depositional age to ca. 70 Ma. Tarahumara fossils therefore extend the paleontological record of nonmarine diatoms from middle Eocene to Late Cretaceous. Preserved populations include forms similar to species of the extant genera Amphora and Melosira , as well as filament-forming araphid pennates comparable to species of Fragilaria and Tabellaria . Tarahumara fossils indicate that by 70 Ma, nonmarine diatoms had achieved considerable environmental as well as taxonomic diversity.

Modern terrestrial sedimentary biostructures and their fossil analogs in Mesoproterozoic subaerial deposits

ABSTRACT Microbial communities developing on modern clastic sedimentary surfaces of arid lands are dominated by phototrophic microorganisms that form a variety of characteristic “microbially induced sedimentary structures” (MISS) through their interactions with detrital sedimentary grains, aided by secretions of extracellular polymeric substances and other organic materials. In this study, we describe modern MISS from unvegetated arid topsoils and compare them with fossil MISS found within decimeter- to meter-thick sedimentary sequences of Mesoproterozoic siliciclastic outcrops of the Dripping Spring Quartzite formation of the Apache Group in central Arizona, USA. These sequences contain numerous bedding plane exposures with desiccation surfaces including polygonal cracks, curls, and chips. Repetition of these structures within stratigraphic sequences indicates recurring episodes of subaerial exposure. Some of these MISS contain cellular microfossils that exhibit morphological adaptations for surviving desiccation. The strong similarities between modern and ancient MISS in this study provide additional criteria for recognizing morphological biosignatures of terrestrial microbial communities in ancient deposits. Our results provide compelling evidence for the presence of land-based microbial communities by the Mesoproterozoic (∼1200 Ma). The association of MISS features further suggests that the primary producers that had colonized Mesoproterozoic land surfaces were likely desiccation-adapted photosynthetic microbes, similar to modern desert soil crust communities.

Benthic bacterial diversity from freshwater tufas of the Iberian Range (Spain)

Aiming to characterize the bacterial diversity of modern tufa systems of the Iberian Range (Spain), we surveyed the 16S rRNA gene sequence diversity from 24 sites within three rivers (Añamaza, Mesa and Piedra). These tufas record substantial calcareous growth under different physicochemical conditions and are part of an important, regional landscape-building system. The bacterial community structure and composition, richness and diversity were quantified from denaturing gradient gel electrophoresis fingerprints. Retrieved DNA sequences could be assigned to 10 bacterial phyla and included a variety of phototrophic and heterotrophic groups. Cyanobacteria, mainly filamentous taxa, constituted 43% of all the retrieved sequences, followed by Firmicutes (11%), Gammaproteobacteria (10%), Alphaproteobacteria (7%), Acidobacteria (6%), Bacteroidetes (5%), Betaproteobacteria (4%), Planctomycetes (4%), Actinobacteria (3%) and Deltaproteobacteria (2%). Diatom and Xanthophyceae chloroplast sequences were also detected. Physicochemical variables measured at each site were modelled with multivariate statistics. Principal component analyses yielded the highest variance for salinity-related variables (conductivity; Na(+) , Cl(-) and SO4(2-) concentrations), which correlated negatively and significantly with diversity indices. However, the highest variance explained by individual principal components was relatively low (< 34%). Overall, we show that these young fluvial tufas are inhabited by a large variety of bacteria in diverse and widespread communities.

Terrestrial Ecosystems in the Precambrian

Although Precambrian (>550 Ma old) landscapes have been largely considered devoid of life and do not yield obvious traces indicative of terrestrial fossils (e.g., plant roots), there is now ample evidence for pervasive and diverse microbial communities on land billions of years before the Phanerozoic. Modern “biological soil crusts” or “biocrusts” are excellent analogs for terrestrial Precambrian ecosystems, especially because they develop in plantless deserts, and analogs of Precambrian “barren” lands. They provide an understanding on how ancient, land-based microbial communities could have functioned. Furthermore, biocrust microbes have a variety of adaptations to desiccation and high UV light radiation, which likely evolved well before macroscopic plants and animals. In this chapter, we present evidence for well-developed terrestrial ecosystems during the Precambrian and question conventional narratives that early life on Earth was restricted to aquatic environments. This evidence suggests that microbial life was abundant on the land since the Archean, and that such communities could have been quite complex in structure and functioning. The advancement on this knowledge is essential to better understand the evolution of the biosphere.

Structural evidence of enhanced active subsidence at the bottom of a maar: Rincón de Parangueo, México

Abstract Rincón de Parangueo is a Quaternary maar that has been recently desiccated. The crater was partially occupied by a soda lake, and near the shoreline microbialites have formed. Evaporites (mainly trona and halite) precipitated as the water level dropped. Active subsidence of the lake floor (c. 24 m since 1980) produced countless structures close to the lakeshore, where deformation is extensional. Closer to the depocentre, in the western half of the basin, gliding/spreading produced folds and mud-injection domes. The most remarkable structure throughout the basin is a monocline that forms a ring-like, nearly continuous scarp, approximately 15 m high, which in the eastern half of the basin was produced as a fault-propagation fold developed above the buried diatreme–country rock boundary. A more diffuse (wider) monocline, locally associated with compressive structures, occurs in the western half of the basin. These structures are interpreted as having developed above a gently inclined, irregular lake sediment–country rock (andesite) interphase. The monocline was modified by high-angle extensional faults/fractures with large heaves/apertures. In the eastern half of the basin, there is a second (outer) scarp, approximately 13 m high, formed by a high-angle, listric, normal fault. Rollover antiforms occur in the hanging wall of this structure. Rincón is an example of centripetal gravitational gliding/spreading.

Preliminary palaeobotanical notes on the Early Cretaceous Cintura Formation (Sonora, Mexico)

The Cintura Formation of Albian–Cenomanian age in the Cabullona Basin yielded an important fossil flora, including palynomorphs, leaf impressions and fossil trunks. At the base of the Marquechi Member a poorly preserved palynomorph assemblage is recognised. The palynological assemblage is dominated by gymnosperms, mainly by cheirolepidiacean genus Classopollis. A noteworthy feature is the presence of angiosperm pollen grains of the genera Clavatipollenites, Retimonocolpites and Tucanopollis. A rich, diverse and well-preserved macroflora of leaf impressions has been observed in the uppermost part of this formation belonging to the San Juan Member. There are at least eight morphospecies of leaves including an abundance of taxa with possible botanical affinities to the family Sapindaceae. The angiosperm pollen grains together with the sapindalean leaves constitute the oldest record of such remains in Mexico.