The Invasion of the Land in Deep Time: Integrating Paleozoic Records of Paleobiology, Ichnology, Sedimentology, and Geomorphology

Abstract

Synopsis The invasion of the land was a complex, protracted process, punctuated by mass extinctions, that involved multiple routes from marine environments. We integrate paleobiology, ichnology, sedimentology, and geomorphology to reconstruct Paleozoic terrestrialization. Cambrian landscapes were dominated by laterally mobile rivers with unstable banks in the absence of significant vegetation. Temporary incursions by arthropods and worm-like organisms into coastal environments apparently did not result in establishment of continental communities. Contemporaneous lacustrine faunas may have been inhibited by limited nutrient delivery and high sediment loads. The Ordovician appearance of early land plants triggered a shift in the primary locus of the global clay mineral factory, increasing the amount of mudrock on the continents. The Silurian-Devonian rise of vascular land plants, including the first forests and extensive root systems, was instrumental in further retaining fine sediment on alluvial plains. These innovations led to increased architectural complexity of braided and meandering rivers. Landscape changes were synchronous with establishment of freshwater and terrestrial arthropod faunas in overbank areas, abandoned fluvial channels, lake margins, ephemeral lakes, and inland deserts. Silurian-Devonian lakes experienced improved nutrient availability, due to increased phosphate weathering and terrestrial humic matter. All these changes favoured frequent invasions to permament establishment of jawless and jawed fishes in freshwater habitats and the subsequent tetrapod colonization of the land. The Carboniferous saw rapid diversification of tetrapods, mostly linked to aquatic reproduction, and land plants, including gymnosperms. Deeper root systems promoted further riverbank stabilization, contributing to the rise of anabranching rivers and braided systems with vegetated islands. New lineages of aquatic insects developed and expanded novel feeding modes, including herbivory. Late Paleozoic soils commonly contain pervasive root and millipede traces. Lacustrine animal communities diversified, accompanied by increased food-web complexity and improved food delivery which may have favored permanent colonization of offshore and deep-water lake environments. These trends continued in the Permian, but progressive aridification favored formation of hypersaline lakes, which were stressful for colonization. The Capitanian and end-Permian extinctions affected lacustrine and fluvial biotas, particularly the invertebrate infauna, although burrowing may have allowed some tetrapods to survive associated global warming and increased aridification.

Convergent adaptation of true crabs (Decapoda: Brachyura) to a gradient of terrestrial environments

“…Microbial life began to populate terrestrial habitats in the Precambrian, with eukaryotes potentially originating in non-marine settings around 1.6 Ga (Jamy et al 2022), although major multicellular groups such as animals and plants were ancestrally marine. Their terrestrialization followed in the early Paleozoic (approximately 538–444 Ma), led by arthropods entering coastal and marginal marine settings (e.g., estuaries, lagoons), and plants that transformed the land and its sediments (Buatois et al 2022). Although molecular divergence time estimates infer early Paleozoic ages for terrestrial arthropod crown groups (e.g., Bernot et al 2022; Benavides et al 2023), recognizable body fossils of millipedes, arachnids, and hexapods have recorded their presence on land by the onset of the Silurian–Devonian (443–359 Ma).…”

Was There a Cambrian Explosion on Land? The Case of Arthropod Terrestrialization

“…Before the Palaeozoic, the only terrestrial life was unicellular, which, until recently, could only be deduced from indirect evidence [ 30 ]. It was during the Palaeozoic that plants and animals began to colonize the Earth’s landmasess [ 31 ], with plants appearing in the fossil record in the form of microfossils called cryptospores in the Middle Ordovician, around 470 Ma, with potential vascular land plants appearing shortly at ~458 Ma [ 32 ]. In the case of arthropods, with certain terrestrial myriapods and arachnids from the Silurian–Devonian [ 33 , 34 ].…”

Grain-Size Controls on the Siluro-Devonian Colonization of Non-Marine Substrates by Infaunal Invertebrates

“…Ichnology can provide windows into the behavior and environmental preferences of animals in geological history. Ichnological analyses have proven useful when attempting to unravel the timeline of terrestrialization-that interval of the early Paleozoic during which multiple animal clades moved to occupy non-marine habitats for the first time Minter et al 2016;Buatois et al 2022). The body fossil record of early terrestrial animals is imperfect (Dunlop et al 2013;Edgecombe et al 2020), in part due to taphonomic hurdles related to oxygen diffusion, decay, and the limited preservation potential of soft body-parts.…”

“…Focused investigations of this record have further refined our understanding. Considering the record by sedimentary facies, it can be seen that the colonization of the continents occurred rapidly across different environments (Minter et al 2016;Shillito, 2020;Buatois et al 2022). Coastal plains and delta tops were colonized by the Ludlow Epoch (Davies et al 2006), aeolian interdunes within the Silurian Period (Shillito and Davies 2021), fluvial floodplains and ephemeral lakes by the Pridoli Epoch (Morrissey et al 2012), free flowing rivers by the Early Devonian Period (Morrissey et al 2012), and deeper lacustrine settings by the mid-Carboniferous Period Mángano 1993, 1998).…”