Neil S. Davies

Paleozoic vegetation and the Siluro-Devonian rise of fluvial lateral accretion sets

A long-standing hypothesis links the increased prominence of meandering rivers in the middle Paleozoic to the colonization of terrestrial environments by vegetation. This hypothesis is tested using a data set of Cambrian to Devonian fluvial literature and field examination of key stratigraphic units. According to some researchers, Cambrian to mid-Silurian river systems were braided in planform, with a sharp increase in the abundance of meandering rivers during the Silurian–Devonian. Although meandering systems were largely identified on the basis of thick mudstones and organized channel deposits, the data set record of lateral accretion sets appears to be a robust proxy for the abundance of meandering river point bars. Lateral accretion is first recorded from Pridolian–Lochkovian strata, but is noted in nearly 40% of fluvial case studies by the Famennian. This trend matches the known record of rooted vegetation, suggesting that vegetation progressively stabilized river banks and promoted single-thread channels. However, the presence of Precambrian and extraterrestrial meandering systems indicates that vegetation is not essential for meandering, and the lack of evidence for Cambrian to Silurian (Ludlow) point bars is surprising. If originally present, they may largely have been destroyed by extreme floods, chute cut-offs in coarse-grained meandering systems, and eolian activity.

Trace Fossils and Paleoenvironments of a Late Silurian Marginal-Marine/Alluvial System: the Ringerike Group (Lower Old Red Sandstone), Oslo Region, Norway

Abstract The Late Silurian Ringerike Group of southern Norway is a lower Old Red Sandstone megasequence that marks the regressive culmination of Cambro–Silurian marine deposition in the Oslo Region. The basal Sundvollen Formation represents deposition in a number of sub-environments of a broad, muddy coastal-plain setting, and is succeeded by fluvial deposition of the Stubdal and Store Arøya Formations. The terminal formation of the Ringerike Group, the Holmestrand Formation, represents deposition in sub-environments of a sandy beach setting. Both the marginal-marine Sundvollen and Holmestrand formations contain a wide variety of trace fossils (Arenicolites, Cruziana, Didymaulichnus, Diplichnites gouldi, Diplocraterion, Gordia marina, Margaritichnus, “Merostomichnites,” Oniscoidichnus, Paleohelcura, Palmichnium stoermeri, ?Polarichnus garnierensis, Rusophycus, Siskemia bipediculus, Skolithos, Steinsfjordichnus brutoni, Taenidium) that are discussed and analyzed, comprising the first complete Late Silurian study from the Baltic area. The trace fossils, which occur in facies-controlled assemblages, are combined with sedimentologic evidence to perform a high-resolution paleoenvironmental analysis of the Ringerike Group. Multivariate cluster analysis of the bulk Ringerike ichnofauna with similar-aged ichnofaunas from other paleocontinents supports the hypothesis that localized environmental factors outweighed provincialism as the dominant control on the composition of arthropod-trackway-dominated trace-fossil assemblages during the Siluro–Devonian.

MARINE INFLUENCE IN THE UPPER ORDOVICIAN JUNIATA FORMATION (POTTERS MILLS, PENNSYLVANIA): IMPLICATIONS FOR THE HISTORY OF LIFE ON LAND

Abstract The Juniata Formation comprises Upper Ordovician sandstone and mudstone that crops out in the Appalachian region of the eastern United States from Pennsylvania to Tennessee. An outcrop at Potters Mills, central Pennsylvania, has previously been attributed to a terrestrial environment. Because this outcrop contains numerous sub-vertical burrows and evidence for pedogenesis, it has regularly been cited as the oldest evidence for several aspects of continental ecosystem development, including the first evidence for terrestrial infauna and animal-plant interactions. We present evidence from both original fieldwork and published literature that collectively sheds considerable doubt on previous interpretations. The evidence suggests that the Juniata Formation at Potters Mills was deposited in a marginal marine setting and, as such, no evidence for early life on land can be inferred from its strata. This has significant implications for the numerous studies that have cited the Juniata Formation as providing a key record of early terrestrial evolution. Removing it from the dataset of studies that deal with the history of life on land, we conclude that currently the majority of fossil evidence from localities worldwide supports the appearance of terrestrial infauna and animal-plant interactions in the Silurian–Devonian.

Ordovician vertebrate habitats: A Gondwanan perspective

Abstract The Middle–Upper Ordovician represents a significant period in the early evolution of fishes. During this time, many of the major lineages, including jawless and putative jawed taxa, made their first appearance in the fossil record, marking a series of diversification events. As a number of studies have focused on the habitat of Laurentian fish during this interval, work has been undertaken at a number of known Gondwanan vertebrate localities in order to provide new perspectives on the ecological preferences of early fish from the Southern Hemisphere. Ichnological and sedimentological data collated from these localities enable reconstructions of the habitats of a number of Ordovician fish, most notably those of the arandaspid-bearing successions of the Anzaldo Formation of Bolivia, the Stairway Sandstone of central Australia, and the Amdeh Formation of Oman, from which articulated or macroscopic fragmentary fossil remains are recorded. These data indicate that the arandaspids were constrained to very shallow marine habitats and prone to seasonal influxes of freshwater and terrigenous sediment. It is proposed that this narrow paleoecological range may be used as a prospecting tool to search for other Ordovician vertebrate-bearing horizons.

The establishment of continental ecosystems

The colonization of land was a major evolutionary transition. Following a protracted prelude to the terrestrial invasion during the Ediacaran to Ordovician, the remainder of the Paleozoic experienced an explosion of diversity and the expansion of benthic biotas into new environments through the creation of new niches. This expansion progressed from coastal settings into rivers, floodplains, deserts, and lakes, as well as increasing colonization of infaunal ecospace. A pattern emerges in which colonization of a new environment is followed by rapid filling of available ecospace, after which animals establish new behavioral programs. These programs are represented initially by the creation of original architectural designs, and subsequently modified by a proliferation of ichnogenera representing variation upon these established themes. The overall pattern is consistent with the early burst model of diversification that has been identified for various animal and plant clades, wherein there is a decoupling as an initial expansion in disparity is followed by an increase in diversity.

Ichnofacies of the Stairway Sandstone fish-fossil beds (Middle Ordovician, Northern Territory, Australia)

The Stairway Sandstone is a 30–560 m thick succession of Middle Ordovician siliciclastic sedimentary rocks within the Amadeus Basin of central Australia, deposited in the epeiric Larapintine Sea of northern peri-Gondwana. The Stairway Sandstone is significant as one of only two known Gondwanan successions to yield articulated arandaspid (pteraspidomorph agnathan) fish. Herein we use the ichnology of the Stairway Sandstone to reveal insights into the shallow marine habitat of these early vertebrates, and compare it with that of other known pteraspidomorph-bearing localities from across Gondwana. The Stairway Sandstone contains a diverse Ordovician ichnofauna including 22 ichnotaxa of Arenicolites, Arthrophycus, Asterosoma, Cruziana, Didymaulichnus, Diplichnites, Diplocraterion, ?Gordia, Lockeia, Monocraterion, Monomorphichnus, Phycodes, Planolites, Rusophycus, Skolithos and Uchirites. These ichnofauna provide a well-preserved example of a typical Ordovician epeiric sea assemblage, recording the diverse ethologies of tracemakers in a very shallow marine environment of flashy sediment accumulation and regularly shifting sandy substrates. New conodont data refine the age of the Stairway Sandstone to the early Darriwilian, with ichnostratigraphic implications for the Cruziana rugosa group and Arthrophycus alleghaniensis.

Discussion on 'Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation'. Journal of the Geological Society, https://doi.org/10.1144/jgs2016-063

The first-order importance of tectonic and environmental controls for terrigenous sediment supply has rarely been questioned, but the role of vegetation in the modification of ancient alluvial signatures has been observed since the mid-20th century (Vogt 1941). Studies of sparsely vegetated rivers (Schumm 1968) and alluvial stratigraphic variation (Cotter 1978; Davies & Gibling 2010) led to observations of (1) plant modulation of alluvial signatures and (2) Palaeozoic facies shifts (PFS): unidirectional changes to facies diversity and frequency, in stratigraphic alliance with the plant fossil record. One PFS is the Siluro-Devonian appearance of mud-rich, architecturally complex alluvium, traditionally ascribed to meandering rivers, and sedimentologically distinct from pre-vegetation strata (Davies & Gibling 2010; Long 2011). Using selected secondary data, Santos et al. (2017) dispute the correlation of these observations using three key points, as follows. (1) The mid-Palaeozoic was typified by orogenic assembly of low-gradient equatorial continents and elevated sea-level, which led to tropical weathering (abundant fine sediment) and extensive alluvial plains. This drove the PFS by promoting river meandering independently of vegetation. (2) Meandering does not require vegetation; this is shown by examples in Precambrian deposits, on other planets, and in ‘non-vegetated’ deserts. Meandering rivers were more abundant than the pre-vegetation rock record suggests, owing to selective bypass and deflation of fine material. (3) Early Siluro-Devonian (meaning Ludlow–Early Devonian) land plants were too small, their biomass and cover too limited, and their wetland habitat too narrow to have stabilized meandering channels, influencing landscape little more than earlier microbial communities. We contest the conclusions and method of the paper, and deal with each point in turn.

Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation: COMMENT

A recent study has suggested that early Cambrian trace-fossil–bearing strata in the Wood Canyon Formation (California) provide the oldest evidence for metazoans in continental environments ([Kennedy and Droser, 2011][1]). Although the search for superlatives in the history of terrestrial life is

Evolution of alluvial mudrock forced by early land plants.

Mudrocks get a vegetative assist Mudrocks such as slate and shale are rarely found in stratigraphy older than about 500 million years. McMahon and Davies compiled a large database of mudrock occurrence over the past 3.5 billion years to help assess the origin of this ubiquitous rock type (see the Perspective by Fischer). Mudrocks appeared at the same time as did deep-rooted land plants. The interplay between plants and sedimentary rocks suggests that a change in erosion rate and the chemistry of sediments delivered to the oceans occurred around 500 million years ago. Science, this issue p. 1022; see also p. 994 Rooted land plants appear to help reduce erosion rate and promote mudrock lithification. Mudrocks are a primary archive of Earth’s history from the Archean eon to recent times, and their source-to-sink production and deposition play a central role in long-term ocean chemistry and climate regulation. Using original and published stratigraphic data from all 704 of Earth’s known alluvial formations from the Archean eon (3.5 billion years ago) to the Carboniferous period (0.3 billion years ago), we prove contentions of an upsurge in the proportion of mud retained on land coeval with vegetation evolution. We constrain the onset of the upsurge to the Ordovician-Silurian and show that alluvium deposited after land plant evolution contains a proportion of mudrock that is, on average, 1.4 orders of magnitude greater than the proportion contained in alluvium from the preceding 90% of Earth’s history. We attribute this shift to the ways in which vegetation revolutionized mud production and sediment flux from continental interiors.

SHORT-TERM EVOLUTION OF PRIMARY SEDIMENTARY SURFACE TEXTURES (MICROBIAL, ABIOTIC, ICHNOLOGICAL) ON A DRY STREAM BED: MODERN OBSERVATIONS AND ANCIENT IMPLICATIONS

Abstract: A wide variety of sub-ripple-scale sedimentary surface textures are known from bedding planes in the sedimentary rock record. Many of these textures were traditionally ascribed an abiotic origin (e.g., due to rain drop impact, adhesion, etc.), but in recent decades the role of microbial mats and biofilms in sculpting and mediating some forms has become increasingly recognized. Microbial sedimentary textures are now well-described and understood from modern tidal environments and biological soil crusts, but descriptions from fluvial settings are less common, despite their known occurrence in ancient alluvium. This paper reports a suite of primary sedimentary surface textures which were observed forming in discrete bodies of standing water in the lower reaches of the ephemeral Murchison River, Western Australia. Microbial sedimentary signatures included bubble impressions (burst and intact) and roll-ups, in addition to reduced horizons. Many of these features exhibited rapid temporal evolution of their morphology in the dry days following an interval of heavy rain. Significantly, these microbial features were witnessed in close spatial proximity to other abiotic and biotic sedimentary surface textures including raindrop impressions, adhesion marks, desiccation cracks, and vertebrate and invertebrate traces. Such proximity of abiotic and microbial sedimentary surface textures is rarely reported from bedding planes in the rock record, but these modern observations emphasize the fact that, particularly in non-marine environments, such structures should not be expected to be mutually exclusive. An appreciation of the fact that primary sedimentary surface textures such as these develop during intervals of stasis in a sedimentation system is crucial to our understanding of their significance and diversity in the rock record.