What happened during this time?
Trends over time...
Major radiation of land plants during this time, in which plants are relatively small at the start of the period and become 100 foot trees by the end
In Early Devonian, land plants small and restricted to the water's edge and moist, lowland habitats.
The cylinder-shaped arrangement in the earliest land plants had initially served them well in their early watery habitats.
As plants moved onto land with fewer water resources, the plants had to overcome drought-induced air bubbles.
Early land plants did this by reconfiguring the ancestral, cylindrical-shaped xylem into more complex shapes that prevented air bubbles from spreading.
The origin of roots in the zosterophylls, and woody growth in the euphyllophytes appears at this time
Plants probably evolve heterospory in the early Emsian, in which there are large "female" spores, and smaller "male" spores (e.g. Aarabia)
By Middle Devonian plants become increasing larger with the evolution of leaves and complex vascular tissue
By the Late Devonian, true trees (i.e. Archaeopteris) had evolved and dominated flood plain environments.
In addition, early lycophyte trees grew in stands in swamp-like conditions
There is a diversification of lycophytes
Zosterophyllophytes dominate shallow water with first true roots, lateral sporangia, but lacking leaves
Protolepidodendrales are larger clubmoss plants with branched, microphyllous leaves
By the Late Devonian, early lycophyte forests were forming (Wang et al. 2019)
Origin of euphyllophytes, with pseudomonopodial growth and spiral branching, but early members lack leaves
Plants becoming taller and more complex in anatomy, branching, and reproduction strategies
"Trimerophytes" displaying pseudomonopodial growth, but lacking leaves are successful during Early and Middle Devonian
First appearance of the fern-like Cladoxylopsids in late Early Devonian
Early ferns appear during Early to Middle Devonian
Horsetail ancestors, such as Pseudobornia, appear in the Late Devonian
Origin of secondary xylem and robust wood (Lignophytes) during late Early Devonian
Progymnosperms appear during Middle Devonian
Origin of seed plants and early pteridosperms during Middle Devonian
Diversification of land invertebrates
Image from Berry (2019), on the ecology and appearance of known early forests throughout the Devonian Period.
Oxygen levels begin at 20% of the atmosphere, and then steadily increase to 27.5%
Carbon dioxide levels begin at ~4,200 ppm and drop to 2,900 ppm in the Middle Devonian, then increasing to ~3,500 ppm by the Period's end
The colonization of land by plants led to the following changes in carbon dioxide and oxygen (Dahl & Arens 2020):
Atmospheric CO2 decline and climatic cooling (permanent transition)
Atmospheric O2 rise and ocean oxygenation (potentially permanent transition)
Ocean fertilization and anoxia (temporary perturbations, ~1 Myr)
Significant changes in the world's geography took place during the Devonian
Earth was divided into two supercontinents, Gondwana and Euramerica.
These vast landmasses lay relatively near each other in a single hemisphere, while a vast ocean covered the rest of the globe.
These supercontinents were surrounded on all sides by subduction zones.
With the development of the subduction zone between Gondwana and Euramerica, a major collision was set in motion that would bring the two together to form the single world-continent Pangea in the Permian.
In addition to global patterns of change, many important regional activities also occurred.
In the Middle Devonian, the continents of North America and Europe collided, resulting in massive granite intrusions and the raising of the Appalachian Mountains of eastern North America (Acadian Orogeny).
Vigorous erosion of these newly uplifted mountains yielded great volumes of sediment, which were deposited in vast lowlands and shallow seas nearby.
During Late Devonian environmental conditions change causing large faunal extinction event
"Biotic crisis" decimates tropical marine environments (Algeo and Scheckler 1998)
Land plants may have increased weathering of rocks and soils, producing an influx of minerals to the marine env't
Both an increase in root depth and penetration as well as the ability of seed plants to conoloize drier, upland env'ts may have contributed to increased soil formation (pedogenesis)
Increased nutrient input on rivers creates eutrophic conditions in epicontinental seaways, "resulting in algal blooms, widespread bottom water anoxia, and high sedimentary organic carbon fluxes".
Increase in plant biomass causes a draw-down of atmospheric CO2, and results in global cooling and glaciation.
Early Land Plants & Earth's Ancient Climate (In Defense of Plants 22Jan2023)
Algeo, T.J. 1998. Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events. Phil. Trans. R. Soc. Lond. B 353: 113-130.
The Root of an Ancient Mass Extinction (In Defense of Plants 18Dec2022)
A new theory emerges for where some fish became four-limbed creatures (Phys.org 27Dec2011)