LECTURE 7: THE DEVONIAN PERIOD
The Devonian is a geologic period and system of the Paleozoic era spanning from to million years ago (ICS, 2004)[6]. It is named after Devon, England, where Exmoor rocks from this period were first studied.
During the Devonian Period, which occurred in the Paleozoic era, the first fish evolved legs[7]and started to walk on land as tetrapods around 365 Ma.
The first seed-bearing plants spread across dry land, forming huge forests. In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician, and the first lobe-finned and bony fish. The first ammonite mollusks appeared, and trilobites, the mollusc-like brachiopods, as well as great coral reefs were still common. The Late Devonian extinction severely affected marine life.
The paleogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small supercontinent of Euramerica in the middle.
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 [1].
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).
Devonian rocks are oil and gas producers in some areas.
Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods, the enigmatic hederelloids, and corals. Lily-like crinoids were abundant, and trilobites were still fairly common, but less diverse than in earlier periods due to the abundance of mobile swimming predators such as early sharks and predatory bony fish (Osteichthyes) such as Dunkleosteus. The ostracoderms were joined in the mid-Devonian by the first jawed fishes and were declining in diversity and were being out competed by the jawed fish in both the sea and fresh water, also the great armored placoderms, as well as the first sharks and ray-finned fish. The first abundant species of 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.
The first Ammonites also appeared during or slightly before the early devonian period around 400 million years ago.[2]
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 importanceBy the Devonian Period, life was well underway in its colonization of the land. The moss forests and bacterial and algal mats of the Silurian were joined early in the period by primitive rooted plants that created the first stable soils and harbored arthropods like mites, scorpions and myriapods (although arthropods appeared on land much earlier than in the Early Devonian and the existence of fossils such as Climactichnites suggest that land arthropods may have appeared as early as the Cambrian period). 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. Also the first possible fossils of insects appeared around 416 Ma in the Early Devonian.
By the Devonian, shrub-like forests of small, primitive plants existed: lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants had true roots and leaves, and many were quite tall. In the Late Devonian, the tree-like ancestral fern Archaeopteris and the giant cladoxylopsid trees grew with true wood. These are the oldest known trees of the world's first forests. Prototaxites was the fruiting body of an enormous fungus that stood more than 8 meter tall. 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". 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 rapid evolution of a terrestrial ecosystem containing copious animals opened the way for the first vertebrates to seek out a terrestrial living. By the end of the Devonian, early amphibians and arthropods were solidly established on the land.
The earliest known trees, from the genus Wattieza, appeared in the Late Devonian around 380 Ma.[3]
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.
Reasons for the late Devonian extinctions are still speculative. The Canadian paleontologist Digby McLaren suggested in 1969 that the Devonian extinction events were caused by an asteroid impact; however, little evidence supports the existence of a Devonian crater large enough.
READING FOR THE NEXT LECTURE
