An Age of Transformation
A period of geological history, part of the Paleozoic Era, stretching from about 360 million years ago to about 418 million years ago.
In the 1830's, controversy erupted when Henry Thomas De la Beche found coal in the middle of the Devonshire rocks he was studying. In jumped Adam Sedgwick and Roderick Impey Murchison, who went to Devonshire and found that de la Beche had mistakenly placed the coal in the middle of the sequence instead of the top. On the other hand, plant fossils in the Devonshire rock threatened to relegate Sedgwick's Cambrian system and Murchison's Silurian system to local curiosities1, so the two insisted that there must be an unconformity in the rock separating the coal from the Old Red Sandstone. Eventually, however, Sedgwick and Murchison2 had to admit there was no such unconformity, but they were also able to show that the Devonshire coal was contemporary with other coal from the later Carboniferous period. So, in 1839, they solved the argument3 by setting up the Devonian system, with the Old Red Sandstone at the bottom and the coal safely in the Carboniferous.
During the Devonian period, the Earth transformed completely: At the beginning, Earth resembled a deserted alien planet one might often see in Star
Trek or Lost in Space. By the end, we might have found the Earth very strange-looking, but would nevertheless have recognized it as
home.
The transformation occurred because an eons-long process, the oxygenation of Earth's atmosphere, had reached its culmination. For over two billion
years, cyanobacteria had been producing food by the process of photosynthesis. Towards the end of this period, they had been joined by multicellular life such as algae, and later still, the first land plants. As photosynthetic organisms spread, they produced more and more of a waste product, oxygen. Much of the oxygen was bound with iron during the Proterozoic, Cambrian, Ordovician, and Silurian periods, producing red rocks you will find everywhere. However, oxygen slowly collected in Earth's atmosphere until, during the Devonian, oxygen reached its current level of 21%.
At the beginning of the Devonian, life was confined almost entirely to water, and the land was mostly barren. The only land plants were simple
things that had to remain near open water - lycopods (related to club moss) and rhyniophytes. The only land animals were worms and arthropods
(scorpions, centipedes, mites, and spider-like creatures called trigonotarbids). The oceans contained reef-building sponges, corals, and
bryozoans, was well as brachiopods, echinoderms such as crinoids and asteroids, a few graptolites left over from the
Ordovician mass extinction, and arthropods such as trilobites and scorpion-like Eurypterids. The most common vertebrates were Agnatha
(jawless fishes) such as the conodonts. However, the first jawed fishes, placoderms and ancanthodians, had appeared.
During the Devonian, plants fully colonized the land, with the first swamps and climax forests (composed of giant horsetails, lycopods and fern
s). In the oceans, fishes spread to such an extent that the period is often called the "age of fishes": placoderms and acanthodians evolved into the more
familiar cartilaginous fishes4 and ray-finned fishes5, as well as lobe-finned fishes
which are rarely seen today (except for the ones that evolved into today's land vertebrates. Towards the end, a mass extinction in the oceans wiped out 70% of all species on Earth. At the very end, ferns had evolved
into the very first seed plants, insects evolved to take advantage of them, and the descendants of some funny lobe-finned fishes pulled
themselves onto land, the very first of our tetrapod ancestors to do so.
At the beginning of the Devonian, the continent of Gondwana made up most of the land on the Earth, situated in the Southern Hemisphere. Small
continents such as Laurentia (essentially the Canadian Shield) and Baltica (northern Europe) sat near the Equator. Laurentia was in the process of
colliding with Avalonia, a New Zealand-like minicontinent. This was the Acadian Orogeny whose rocks now appear in Cornwall, Nova Scotia, coastal Maine,
and southeastern New England. Further along in time, Laurentia and Avalonia also collided with Baltica, finally closing the Iapetus Ocean. This was the
culmination of the Caledonian Orogeny, producing a new continent called "Euramerica" or "Laurussia". In between, volcanism from subduction created
batholiths in present-day New South Wales, Ireland, Nova Scotia, coastal Maine (e.g Cadillac Mountain), northern Alaska, and the Sierra
Nevada. Devonian volcanism also produced the necks in northern France and Germany which became quite popular for castles in the Middle Ages.
The Avalonian and Caledonian orogenies produced a spine of mountains running down the new continent, the forerunners of the Appalachian Mountains as
well as those of the Scottish Highlands. Sediments washed into shallow seas on either side of the mountains, producing sedimentary rocks that can be found in
Greenland, Eastern North America, Great Britain, Ireland, and the Northern European Plain. On the west side of the mountains, these ran into a foreland
basin forming what is now called the "Catskill Delta", rocks which are now the mountains of western New York and northeastern Pennsylvania, stretching
down into Maryland and West Virgnia. On the eastern side, the first rocks were formed from red-brown sand eroded from rocks full of iron oxide, forming the
Old Red Sandstone now so identifiable with the British Devonian. In these sediments we find the fossils that let us reconstruct the biological history of
the period.
Throughout the Devonian, most of North America was covered by a shallow sea, as was much of Gondwana. Because of this, limestone layers hundreds of meters
thick were laid down from chemical deposition, as well as organic deposition from crinoids, corals, bryozoans and sponges.
Other terranes known as Armorica (North France) and Iberia (Spain and South France) approached the new continent throughout the Devonian period. Thus,
the Variscan Orogeny (aka Hercynian Orogeny) can be said to have begun during this time.
Some of the most exciting current paleobiological research concerns the Devonian period -- findings at Red Hill in Pennsylvania, the Rhynie Chert of
Scotland, and the Remigolepis Group in east Greenland have completely altered our ideas of plant and tetrapod evolution.
Geologists divide the Devonian into seven stages or epochs:
- "Early Devonian"
- Lochkovian - 418 to 413 million years ago
Jawless and jawed fishes became common. On land, zosterophylls and Trimerophytes evolved from the rhyniophytes.
- Pragian - 413 to 410 million years ago
Plants began to spread across the land: lycopods, and the first fossils of fungi also appear.
- Emsian - 410 to 392 million years ago
The earliest fossils of ammonoids and sharks (Doliodus) appear. Brachiopod communities developed strong regional characters which later
became homogenous.
- "Middle Devonian"
- Eifelian - 392 to 388 million years ago
The Acadian and Caledonian orogenies were complete, with Euramerica migrating south towards Gondawana. The southern tip of the continent part may
have struck South America, but if so, it was brief, and the formation of Pangaea would have to wait until the end of the Carboniferous.
- Givetian - 388 to 384 million years ago
On land, the first forests appeared, composed of giant of lycopods (related to today's club-mosses). The first ray-finned fish fossil,
Cheirolepis, appears. There are far fewer gastropods in the fossil record than in previous epochs.
- "Late Devonian"
- Frasnian - 384 to 376 million years ago
The Frasnian saw the development of many more types of ammonoids, as well as lobe-finned and ray-finned fishes. On land, the first plants classified
as "ferns" appeared. Forests of lycopods and horsetails were augmented by the first true trees, the progymnosperm Archaeopteris.
But at the end of the Frasnian, something happened in the oceans to cause one of the largest mass extinctions in Earth's history. Free -floating
(graptoloid) graptolites and stromatoporoid sponges were wiped out. tabulate and rugose corals were so hard-hit that coral reef
building was curtailed for another 200 million years. Placoderms, agnatha, brachiopods, conodonts, and ammonoids were hit hard. Trilobites, the signature organisms of the Paleozoic, were reduced to one order.
Land life was relatively unaffected, but may have caused the extinction, according to a hypothesis whose acceptance is growing. Large amounts of organic matter, washed into the ocean from the first forests, may have caused an algal bloom that led to anoxic conditions in the habitats favored by the affected organisms. However, there appears to also be evidence that 4 kilometer wide comet struck the shallow ocean covering what is now southern Nevada 370 million years ago.
- Famennian - 376 to 360 million years ago
In the oceans, the survivors of the extinction radiated into abandoned ecological niches. Ray-finned fishes and sharks began to spread out. Lobe-
finned fishes are in decline. Howeverm, some lobe-finned fishes (Eusthenopteron) evolved bones we would recognize as the bones of tetrapod
limbs, evolving into creatures (e.g. Acanthostega) with bendable arms to maneuver in rivers, ponds, and swamps. Some of these may have crawled
onto fallen logs and onto land. The first seed plants evolved from ferns, and the first insects appear at the same time. At the very end of the Famennian,
the last placoderms and progymnosperms died out.
Notes
1The rocks of Devonshire were assumed to be the same age as those in Wales, where the Cambrian and Silurian systems had been developed.
But Sedgwick and Murchison found no plant fossils in the Cambrian and Silurian strata of Wales.
2Murchison admitted he made a mistake!
3One argument at any rate. Sedgwick and Murchison would later have a falling out over the rocks in between the Cambrian and
Silurian.
4There is evidence for shark dermal denticles ("skin teeth") from the early Silurian, but there are no shark fossils until the Devonian.
5You will hear of "ray finned fishes" from the Silurian, but these are actually acanthodians.
Sources
Devonian times - http://www.mdgekko.com/devonian/
Palaeos - The Trace of Life on Earth
http://www.palaeos.com/
Papæobotanical Research Group, University of Müster
http://www.uni-muenster.de/GeoPalaeontologie/Palaeo/Palbot/seite3.html
Museum of Paelontology, University of California, Berkeley
http://www.ucmp.berkeley.edu/devonian/devonian.html
Online review of Martin J. S. Rudwick's The Great Devonian Controversy:
The Shaping of Scientific Knowledge among Gentlemanly Specialists at
http://www.friesian.com/rudwick.htm