Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
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Chapter 6 - The Devonian Period
Lesson 31: Devonian Period Overview
The Devonian is a geologic period of the Paleozoic era spanning from 416 to 359.2 million years ago. It is named after Devon, England, where rocks from this period were first studied. During the Devonian Period, which occurred in the Paleozoic era, the first fish evolved legs and started to walk on land as tetrapods around 397 Ma. Various terrestrial arthropods also became well-established. 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 ray finned fish appeared. 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 between.
Chapter 6 - The Devonian Period
|Devonian Fast Facts
Started: 416.0 Ma
Ended: 359.2 Ma
Duration: 56.8 Million Years
Preceded By: Silurian Period
Followed By: Carboniferous Period
The period is named after Devon, a county in southwestern England, where Devonian outcrops are common. While the rock beds that define the start and end of the period are well identified, the exact dates are uncertain. The Devonian extends from the end of the Silurian Period 416.0 ± 2.8 Ma, to the beginning of the Carboniferous Period 359.2 ± 2.5 Ma (in North America, the beginning of the Mississippian subperiod of the Carboniferous) (ICS 2004). In nineteenth-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. The Devonian has also erroneously been characterized as a "greenhouse age", due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid but in fact the climate in the Devonian differed greatly between epochs and geographic regions. For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, Australia, North America, and China, but Africa and South America had a warm temperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common.
Period is formally 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 Devonian was a relatively warm period, and probably lacked any glaciers. Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of 30 °C (86 °F) in the Early Devonian. CO2 levels dropped steeply throughout the Devonian period as the burial of the newly-evolved forests drew carbon out of the atmosphere into sediments; this may be reflected by a Mid-Devonian cooling of around 5 °C (9 °F). The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in CO2 concentrations, continental weathering increases (as predicted by warmer temperatures); further, a range of evidence, such as plant distribution, points to Late Devonian warming. The climate would have affected the dominant organisms in reefs; microbes would have been the main reef-forming organisms in warm periods, with corals and stromatoporoid sponges taking the dominant role in cooler times. The warming at the end of the Devonian may even have contributed to the extinction of the stromatoporoids.
The Devonian period was a time of
great tectonic activity, as
Gondwanaland drew closer together.
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.
crinoids were abundant,
and trilobites were still
fairly common. Among vertebrates, jaw-less armored fish (ostracoderms)
declined in diversity, while the jawed fish (gnathostomes) simultaneously
increased in both the sea and
fresh water. Armored
placoderms were numerous during the lower
stages of the Devonian Period and became extinct in the Late Devonian,
perhaps because of competition for food against the other fish species.
Early cartilaginous (Chondrichthyes)
and bony fishes (Osteichthyes)
also become diverse and played a large role within the Devonian seas. The
first abundant genus of shark,
Cladoselache, appeared in the oceans during
the Devonian period. The great diversity of fish around at the time, have
led to the Devonian being given the name "The Age of Fish" in popular
A now dry barrier reef, located in
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
They were composed of calcareous algae and coral-like
tabulate and rugose corals, in that
order of importance.
By 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). Also the first possible fossils of insects appeared around 416 Ma in the Early Devonian. The first tetrapods eviolving from lobe-finned fish, appeared in the costal water no later than middle Devonian, and give rise to the first Amphibians.
The Greening of Land
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 far the
greatest land organism was
Prototaxites, the fruiting body of an
enormous fungus that stood more than 8 meters tall, towering over the low,
carpet-like vegetation. By
Middle Devonian, shrub-like forests of
primitive plants existed:
and progymnosperms had
evolved. Most of these plants had true roots
and leaves, and many were quite tall. The earliest known trees, from the
Wattieza, appeared in
the Late Devonian around 380
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. 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".
Animals and the First Soils
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, arthropods were solidly
established on the land.
End of Reading