417-354Ma

<< | Silurian | Year Index | Carboniferous | >>

Devonian Period

In the Devonian period, fish have become abundant and begun to evolve to exploit the growing and untapped resources on land. Devonian lobe finned fish such as Eusthenopteron and Panderichthys evolved limb bones in their fins as a first step toward a migration to land. The first amphibians, Ichthyostega and Acanthostega were most likely descended from Panderichthys. This period also saw the evolution of many land arthropods in addition to plants such as horsetail ferns and liverworts.

The remains of a 415-million-year-old fish skull from Siberia — though miniscule in size — offer hints about the origins of all jawed vertebrates, ranging from reptiles to humans. It is named Janusiscus schultzei, for "Janus," the Roman god of doorways and transitions, who is often depicted with two faces, and the Latin "piscis" for fish. The species is also named for Hans-Peter Schultze, of the University of Kansas, who first described the fossil in 1977.

Human sexual intercourse evolved from the activities of a bony fish - Microbrachius dicki - in Scottish lakes 385 million years ago.

Devonian subdivisions

The Devonian is usually broken into Early, Middle, and Late subdivisions. The rocks corresponding to these epochs are referred to as belonging to the lower, middle, and upper parts of the Devonian System. The faunal stages from youngest to oldest are:

Late (most recent)

  • Famennian/Chautauquan/Canadaway/Conneaut/Conneautan/Conewango/Conewangan
  • Frasnian/Senecan/Sonyea/Sonyean/West Falls

Middle

  • Cazenovia/Cazenovian
  • Givetian/Erian/Senecan/Tioughniogan/Tioughnioga/Taghanic/Taghanican/Genesee/Geneseean
  • Eifelian/Southwood

Early (oldest)

  • Helderberg
  • Emsian/Sawkill/Deer Park
  • Pragian/Siegenian
  • Lochkovian/Gedinnian

Devonian rocks are oil and gas producers in some areas.

Devonian palaeogeography

The Devonian period was a time of great tectonic activity, as Laurasia and Gondwanaland drew closer together.

The continent Euramerica (or Laurussia) was created in the early Devonian by the collision of Laurentia and Baltica, which rotated into the natural dry zone along the Tropic of Capricorn, which is formed as much in Paleozoic times as nowadays by the convergence of two great airmasses, the Hadley cell and the Ferrel cell. In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidized iron (hematite) characteristic of drought conditions.

Near the equator, Pangaea began to consolidate from the plates containing North America and Europe, further raising the northern Appalachian Mountains and forming the Caledonian Mountains in Great Britain and Scandinavia.

The west coast of Devonian North America, by contrast, was a passive margin with deep silty embayments, river deltas and estuaries, in today's Idaho and Nevada; an approaching volcanic island arc reached the steep slope of the continental shelf in Late Devonian times and began to uplift deep water deposits, a collision that was the prelude to the mountain-building episode of Mississippian times called the Antler orogeny.

The southern continents remained tied together in the supercontinent of Gondwana. The remainder of modern Eurasia lay in the Northern Hemisphere. Sea levels were high worldwide, and much of the land lay submerged under shallow seas, where tropical reef organisms lived.

The deep, enormous Panthalassa (the "universal ocean") covered the rest of the planet. Other minor oceans were Paleo-Tethys, Proto-Tethys, Rheic Ocean, and Ural Ocean (which was closed during the collision with Siberia and Baltica).

Marine biota

Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods and corals. Lily-like crinoids were abundant, and trilobites were still fairly common, but less diverse than in earlier periods. The ostracoderms were joined in the mid-Devonian by the first jawed fishes, the great armored placoderms, as well as the first sharks and ray-finned fish. The first shark, the Cladoselache, appeared in the oceans during the Devonian period. They became abundant and diverse. In the late Devonian the lobe-finned fish appeared, giving rise to the first tetrapods.

Reefs

A great barrier reef, now left high and dry in the Kimberley Basin of northwest Australia, once extended a thousand kilometers, fringing a Devonian continent. Reefs in general are built by various carbonate-secreting organisms that have the ability to erect wave-resistant frameworks close to sea level. The main contributors of the Devonian reefs were unlike modern reefs, which are constructed mainly by corals and calcareous algae. They were composed of calcareous algae and coral-like stromatoporoids, and tabulate and rugose corals, in that order of importance.

360 million years ago in the Late Devonian period, the environment that had clearly nurtured reefs for at least 13 million years turned hostile and the world plunged into the second mass extinction event.

Terrestrial biota

By the Devonian Period, life was well underway in its colonization of the land. The bacterial and algal mats were joined early in the period by primitive plants that created the first recognizable soils and harbored some arthropods like mites, scorpions and myriapods. Early Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall.

By the Late Devonian, forests of small, primitive plants existed: lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many were quite tall. The tree-like ancestral fern Archaeopteris, grew as a large tree with true wood. These are the oldest known trees of the world's first forests. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". The primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed-plants that characterizes a recognizably modern world had its genesis in the late Devonian. The development of soils and plant root systems probably led to changes in the speed and pattern of erosion and sediment deposition.

The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped. This may have cooled the climate and led to a massive extinction event. see Late Devonian extinction.

Also in the Devonian, both vertebrates and arthropods were solidly established on the land.

Late Devonian extinction

A major extinction occurred at the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when all the fossil agnathan fishes, save for the psammosteid heterostracans, suddenly disappeared. A second strong pulse closed the Devonian period. The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota, more drastic than the familiar extinction event that closed the Cretaceous.

The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems .

Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms. Freshwater species, including our tetrapod ancestors, were less affected.

Reasons for the late Devonian extinctions are still speculative.

Sources

<< | Silurian | Year Index | Carboniferous | >>