A timeline for terrestrialization: consequences for the carbon cycle in the Palaeozoic

Abstract

The geochemical carbon cycle is strongly influenced by life on land, principally through the effects of carbon sequestration and the weathering of calcium and magnesium silicates in surface rocks and soils. Knowing the time of origin of land plants and animals and also of key organ systems (e.g. plant vasculature, roots, wood) is crucial to understand the development of the carbon cycle and its effects on other Earth systems. Here, we compare evidence from fossils with calibrated molecular phylogenetic trees (timetrees) of living plants and arthropods. We show that different perspectives conflict in terms of the relative timing of events, the organisms involved and the pattern of diversification of various groups. Focusing on the fossil record, we highlight a number of key biases that underpin some of these conflicts, the most pervasive and far-reaching being the extent and nature of major facies changes in the rock record. These effects probably mask an earlier origin of life on land than is evident from certain classes of fossil data. If correct, this would have major implications in understanding the carbon cycle during the Early Palaeozoic.

Figure 1.

Stratigraphic record of types of land plant fragments and taxonomic groups through the Middle Ordovician–Middle Devonian. Solid range bars indicate confidently dated and determined fossils; broken bars indicate fossils with contentious identifications. Symbols in the timescale refer to important plant-bearing horizons: RC, Rhynie chert; Lochkovian*, North Brown Clee Hills charcoalified assemblage; Silurian*, Ludford Lane charcoalified assemblage; small Cooksonia symbol, earliest Cooksonia (Wenlock, Ireland). Dashed horizontal line in the Mid-Llandovery marks change in spores shown in figure 3.

Figure 2.

Early land plant fossils. (a–f) Dispersed cryptospores from the Upper Ordovician (Katian) of Oman. (a) Naked monad. (b) Naked dyad. (c) Naked tetrad. (d) Monad enclosed in an envelope. (e) Dyad enclosed within an envelope. (f) Tetrad enclosed within an envelope. (g) Laevigate trilete spore from the Lower Devonian (Lochkovian) of the Anglo-Welsh Basin. (h–i) Ornamented trilete spores from the Lower Devonian (Late Pragian–Earliest Emsian) sediments associated with the Rhynie chert. (j,k) Dispersed phytodebris. (j) ‘Nematothallus’ cuticle from the Lower Devonian (Late Pragian–Earliest Emsian) sediments associated with the Rhynie chert. (k) Banded tubular structure from the Lower Devonian (Late Pragian–Earliest Emsian) sediments associated with the Rhynie chert. (l) Conducting tissues of the primitive vascular plant Gosslingia exceptionally preserved by iron pyrite from the Lower Devonian (Pragian) of the Brecon Beacons in the Anglo-Welsh Basin. (m) Rooting system preserved intact in Chamaedendron multisporangiatum from the Upper Devonian, Hubei Province, China. (n) Cooksonia mesofossil exceptionally preserved by charcoalification from the Lower Devonian (Lochkovian) of North Brown Clee Hill in the Anglo-Welsh Basin (courtesy Dianne Edwards). (o–q) Sporangium with in situ cryptospores from the Upper Ordovician (Katian) of Oman [15]. (o) SEM image of the contents of an entire sporangium. (p) Close-up of (o) illustrating the in situ cryptospore dyads. (q) TEM image of section from (o) showing multilamellate wall ultrastructure in the in situ cryptospore dyads. (r) Exceptional preservation (silicification) of hair-like absorptive cells in the early plant Horneophyton lignieri from the Lower Devonian (Late Pragian–Earliest Emsian) Rhynie chert. (s) Stem segment of the lycopod Baragwanathia longifolia, from the Late Silurian (Ludlow) of Australia. (t) Bifurcating stems and spore sacs of Cooksonia pertoni from the Lower Devonian of the Anglo-Welsh basin.

Figure 3.

Stratigraphic record of types of plant-dispersed microfossils through the Middle Ordovician–Middle Devonian, with approximate relative abundances. Symbols and abbreviations in the timescale as in figure 1.

Figure 4.

Stratigraphic record of terrestrial arthropod groups through the Middle Ordovician—Middle Devonian. Symbols and abbreviations in the timescale as in figure 1.

Figure 5.

Summary of the colonization of land by plants and myriapods (Arthropoda) against the rock record of Western Europe from Smith & McGowan [139]. Consensus phylogenetic hypotheses (chronograms) are shown for land plants and myriapods. Range bars to right of phylogenetic trees are estimates for the ages of the basal node of the land plant and myriapod lineages: M, molecular estimates; S, spore record; F, macrofossils; T, trace fossils; B, body fossils. Filled bars indicate minimum estimates (youngest molecular estimate or oldest well-established fossil); open bars indicate earlier molecular dates or contentious fossil assignments. Shaded boxes on trees indicate major evolutionary events: lower box (plants), early colonization of land by stem-group land plants; upper box (plants), initial radiation of crown group land plants; initial radiation of myriapods. AA, algal ancestor; LI, liverworts; MO, mosses; HO, hornworts; VP, vascular plants; CH, Chilopoda; SY, Symphyla; PA, Pauropoda; DI, Diplopoda; RC, Rhynie chert.