Devonian Period

What happened during this time?

Biological

• 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 the Early Devonian, land plants were small and restricted to the water's edge and moist, lowland habitats.

  • Changes in the stelar arrangement of the vascular system of plants made them more drought-resistant, which opened up new landscapes for exploration (Bouda et al., 2022)

    • 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 appear at this time

  • Plants probably evolved heterospory in the early Emsian, in which there are large "female" spores, and smaller "male" spores (e.g., Aarabia)

• By the Middle Devonian, plants became increasingly larger with the evolution of leaves and complex vascular tissue

  • Plants became tree-like (e.g., Eospermatopteris) during the Eifelian, and early forests ecosystems formed by the Givetian

  • Seed-like structures or proto-seeds appear  (e.g., Runcaria), but are not true seeds since they have an incomplete integument

• By the Late Devonian, true trees (i.e., Archaeopteris) had evolved and dominated floodplain environments. 

  • In addition, early lycophyte trees grew in stands in swamp-like conditions

• • The earliest evidence of true seeds appears in the Late Devonian 

Flora

• Diversification of polysporangiophytes and the vascular plants, including the lycophytes, and the origin of the euphyllophytes

  • Horneophytes, Aglaophyton, Rhyniophytes, and Cooksonia continue from the Silurian into the Devonian

  • There is a diversification of lycophytes

    • Zosterophyllophytes dominate shallow water with first true roots, lateral sporangia, but lacking leaves

    • Drepanophycalean lycopsids have the first leaf-like appendages in the plant world, and probably arose in the Silurian

    • 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 are becoming taller and more complex in anatomy, branching, and reproduction strategies

    • "Trimerophytes" displaying pseudomonopodial growth, but lacking leaves, were successful during the Early and Middle Devonian

    • First appearance of the fern-like cladoxylopsids in the late Early Devonian

      • Iridopterids appear in the Middle Devonian with whorled axes; possible ancestor to horsetail

      • Origin of arborescent plants (pseudosporochnids), although these were not true trees with a bifacial cambium

        • First large tree-like plants appear in the early Middle Devonian (Eospermatopteris/Wattieza)

    • Early ferns appear during the Early to Middle Devonian

      • Rhacophytes and stauropterids appear, which lack leaves but have complex fern-like anatomy

      • Zygopteridales, with true laminate leaves, appear in the Late Devonian

    • Origin of horsetails and the equisetophyte, Sphenophyllum

      • Horsetail ancestors, such as Pseudobornia, appear in the Late Devonian

    • Origin of secondary xylem and robust wood (lignophytes) during the late Early Devonian

      • Progymnosperms appear during the Middle Devonian

        • Aneurophytes, large vine-like plants, with true wood but lacking leaves, appear in the Middle Devonian

        • Appearance of first true trees with a 2-faced, vascular cambium and lateral growth (e.g., Archaeopteris) 

      • Origin of seed plants and early pteridosperms during the Middle Devonian

Fauna

• Diversification of land invertebrates 

• Amphibians originate

Geophysical

• 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 increase 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):

    1. Atmospheric CO2 decline and climatic cooling (permanent transition)

    2. Atmospheric O2 rise and ocean oxygenation (potentially permanent transition)

    3. 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 lie relatively near each other in a single hemisphere, while a vast ocean covers the rest of the globe. 

  • Subduction zones surrounded these supercontinents. 

  • 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 the Late Devonian, environmental conditions changed, causing a large faunal extinction event

• "Biotic crisis" decimates tropical marine environments (Algeo & Scheckler, 1998)

  • Land plants may have increased weathering of rocks and soils, producing an influx of minerals to the marine environment.

  • Both an increase in root depth and penetration, as well as the ability of seed plants to colonize drier, upland environments, 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 drawdown of atmospheric CO2, and results in global cooling and glaciation.

• Contrary to this notion, the repeated evolution of trees is widely thought to have enhanced the capacity of silicate weathering via the impact of deep rooting. 

  • Land plants still may cause reductions in steady-state atmospheric CO2 levels, but via increasing the silicate weathering feedback strength, not silicate weathering rates (D'Antonio et al., 2019)