The Devonian Period

From 417 to 354 Million Years Ago.

Here's where this period fits within the larger geologic time frame – the Paleozoic Era:

Paleozoic Era 543 to 248 mya, “Million Years Ago”	Permian (290 to 248 mya)
	Carboniferous (354 to 290 mya) Pennsylvanian (323 to 290 mya) Mississippian (354 to 323 mya)
	Devonian (417 to 354 mya)
	Silurian (443 to 417 mya)
	Ordovician (490 to 443 mya)
	Cambrian (543 to 490 mya) Tommotian (530 to 527 mya)

The Devonian world, in general, was relatively warm, arid, and dry, with a temperature range from Equator to pole that was less than it is today. As with any point in Earth's history, the temperature changed with time and geologic environments throughout this period.

You can see below the main continental masses of the Devonian:

To the north, a portion of modern Siberia,
In the middle, Euramerica (North America and Europe), and
Gondwana (South America, Africa, Antarctica, India, and Australia) dominating the southern half, as well as the South Pole.

Figure 1.

On these lands and during this period of Earth's history (63 mya) we see

The continuing development of fish from the Silurian
The first bony fish – ray-finned and lobe-finned fish
The evolution of some fish to develop legs and to walk on land – the tetrapods
The continuing development of arthropods: insects, arachnids, crustaceans, etc
The first seed-bearing plants to spread across dry land

So, to get a sense of this development, let's first begin with the oceans, estuaries, fresh water environments and the life discovered within them, starting with the vertebrates – those fish whose flexible column of bones, the vertebra, encased and protected their spinal cord.

Devonian Vertebrates

Jawless Fish

When fish first started to develop, they had no jaws. Their mouths were supported with cartilage, which doesn't fossilize well. Many of these jawless fish also had an outside skin protected by scales and plates made of bony tissue.

Called Ostracoderms (shell-skins), they used their mouths to create suction that pulled prey into their mouths.

Here's a photo of an early jawless fish, Zenaspis pagei, from Scotland.

Figure 2.

Placoderms

Eventually fish developed jaws, gills and paired fins. The first group with these characteristics were called Placoderms, and while they still had “shell skin,” it mainly covered their head and neck area.

The largest and most famous of the Placoderms was Dunkleosteus, a huge predator with large bony plates which stuck down in the front of its mouth and functioned as teeth.

Figure 3.

Photo by Chip Clark, NMNH.

And while the placoderms as a group were diverse and successful animals, by the end of the Devonian, they were gone.

Sharks

Much of the information scientists have about ancient sharks comes from their fossil teeth. Because their skeletons were made of cartilage, very little fossil evidence is available.

While the oldest undisputed shark scales are about 420 million years old, from Silurian deposits in Siberia, the earliest fossil shark teeth come from early Devonian deposits in what was Euramerica.

Figure 4.

Cladoselache
One of the best known early sharks, 3-foot-long. Cladoselache hunted in the warm seas of Euramerica, 375 million years ago.

And, as successful animals, they are still with us today, represented by around 400 species.

Bony Fish

Bony fish evolved from Placoderm ancestors.

Bony fish divide into ray-fin (actinopterygii) and lobe-fin (sarcopterygii) groups. Both had swim bladders (or lung) and paired pectoral (shoulder) and pelvic fins.

Figure 5. From placoderm ancestors, ray-fins (left) and lob-fin (right) diverged.

From Romer, A. S. 1964. The Vertebrate Body. W. B. Saunders. Philadelphia.

Ray-fins – Today, Ray-fin fish constitute the largest class of vertebrates. As a bony fish, ray-fins have a rigid skeleton, and their fins are supported by rays, with webs of skin between the bony spines.

Here's a close up of this structure.

Figure 6.

And here's an illustration of an early ray-fin, Paleoniscoids.

Figure 7. Palaeoniscus.

From Romer, A. S. 1964. The Vertebrate Body. W. B. Saunders. Philadelphia.

Lobe-fins – These fish had pairs of fins, each with a fleshy lobe base with typical fin membranes at the ends. The lobes, however, contained jointed bones.

Here's an illustration:

Figure 8. Onychodus sigmoides, a lob-fin from the Late Devonian of Euramerica.

A famous group of lobe-fin fish, first developed in the Devonian, still has some members alive today. The coelacanth family was thought to have gone extinct, but in 1938 a new, living member of the family was caught. And since then, many other individuals of this family have been documented.

Figure 9.

This specimen was caught the in 1974, next to Salimani/Selimani (Grande Comore, Comoros Islands)

Lung fish – Known from the Early Devonian, lungfishes are highly specialized lobe-fins, and like Coelacaths, an Australian member of this group – Neoceratodus fosteri – is the oldest known vertebrate species to still be living (100 million years – from fossils to now).

The notable lungfish feature, lungs, has created speculation and debate on their relationship to tetrapods. However, researchers conclude that the lungs in lungfishes and tetrapods evolved independently from swim-bladders, a primitive feature of all bony fishes (ray-fin and lobe-fin).

Here's an illustration of an ancient and modern lung fish:

Figure 10. A. The oldest Devonian type. B. A living Australian lungfish.

From Romer, A. S. 1964. The Vertebrate Body. W. B. Saunders. Philadelphia.

Tetrapods

Today, these four-legged animals include amphibians (an outmoded term), reptiles, birds, and mammals. But, the earliest tetrapods first appeared in the Late Devonian.

Figure 11.

Life restoration of Tiktaalik roseae, a transitional fossil (“missing link”) between lob-fin fishes and late Devonian tetrapods.

Author: Zina Deretsky, National Science Foundation (Courtesy: National Science Foundation)

Ten million years later, the tetrapod fossils Acanthostega and Ichthyostega were found in rocks deposited in a river environment, which indicated these tetrapods also lived in streams or small rivers.

Because these specimens are well preserved and almost complete, they indicated they died and were buried where they lived.

Here's a beautiful Acanthostega fossil:

Figure 12.

Devonian Invertebrates

Also in the oceans, some invertebrates, like sponges, became more abundant during the Devonian period, while corals continued to evolve through the period. Especially widespread were tabulate and rugose corals.

Here's an illustration of a coral-based Devonian reef.

Figure 13.

From such a reef as this, here's a horn coral fossil, New York:

Figure 14.

Amplexiphyllum hamiltoniae Coral in Windom shale.

Also, brachiopods flourished, like this Spinocyrtia from the Lime Creek Formation, Rockford, Iowa.

Figure 15. Late Devonian.

Photo by Collinson

Or this Spirifer Whitney from the same Formation.

Figure 16. Late Devonian.

Photo by Collinson

Also flourishing were Crinoids and other echinoderms, as well as ammonites.

Figure 17. Middle Devonian (375 Million Years Old) from Madagascar.

Photo by Collinson

Land Plants

Initiated during the Ordovician and Silurian Periods, the development of land plants during the Devonian was the botanical equivalent of the great Cambrian diversification.

These land plants had evolved from freshwater ancestors that adapted to the dry land conditions, and they developed ways to control the loss of water.

This growth of vascular plants in the Early Devonian had a direct relationship to animal life in estuary and freshwater ecosystems.

Some of the key plants were Prototaxites (the fruiting body of a fungus), Parka (an algae), and Aglaophyton and Rhynia (creeping plants with upright branches topped with spore structures).

Figure 18.

Model of Aglaophyton major, sculpted by Stephen Caine for the Rhynie Research Group, University of Aberdeen.

While vegetation started out small, by the late Devonian, ferns, horsetails, and seed plants had appeared, along with the first trees and then the first forests.

To keep this in perspective, the first tetrapods appeared millions of years after these forests, which Archaeopteris was one of the first trees.

Here's an artist's rendition of what this tree looked like.

Figure 19.

And here's a fossil of one of its frond-like leaves.

Figure 20.

Archaeopteris hibernica fossil specimen in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

The special vascular system of a tree enabled it to circulate water and nutrients against the pull of gravity. Developed at the end of the Devonian were advanced root systems and increased hardiness.

With the spread of plants and animals, the first terrestrial arthropods ( insects, arachnids, crustaceans, and others ) expanded into the new ecological niches being created.

However, for the last 20 to 25 million years of the Devonian, there occurred a series of extinction events that cleared away numerous animals and again changed the texture of the evolving world.

Extinction Events

The Devonian Extinction events are considered one of five major extinctions in the Earth's history:

Ordovician
Devonian
Terminal Permian
Terminal Triassic
Terminal Cretaceous

And while most of these lasted less than 1 to 5 million years, the Devonian events ranged from 20 to 25 million years.

The result, however, was immense: 20% of all animal families and 70-80% of all animal species.

Ammonites
Benthic foraminifera
Brachiopods
Conodonts
Rugose and Tablulate corals
Jawless fishes, Placoderms
Stromatoporoid sponges
Trilobites

Devonian reef-builders, the corals, didn't really recover and the reef ecology was devastated.

The reasons for the Devonian Extinctions have been widely researched. Some proposed are asteroid impacts, global anoxia (widespread dissolved oxygen shortages), plate tectonics, sea level changes, and climatic change.

Of these, two are inter-related and worth discussion:

Some researchers – with regard to the large distribution of black shale in Devonian sediments – believe that oxygen depletion in marine waters was one of the causes of these events.

The black shale indicates organic-rich sediments from anoxic (oxygen-deprived) bottom waters. Such water would kill off species intolerant to low-oxygen environments. A form of eutrophication.

For example, eutrophication events are where microscopic animals bloom in response to greatly increased nutrients in the waters. When this happens, these animals boom so much they deplete the nutrients.

This causes a die off and the bodies of these animals fall to the ocean floors. This in turn causes the ocean bottoms to slowly be rendered anoxic. Slowly, but surely, the oceans turn anoxic up to surface depths, even in the absence of high temperatures.

Another line of evidence points to global cooling.

With the expansion of terrestrial plants, there would be a decrease in the amount of carbon dioxide in the atmosphere.

Here's how it worked: Plants store carbon by converting atmospheric carbon dioxide into organic carbon and storing it in their stems, roots, and leaves. When they die, this organic carbon is released to the soils.

This would reduce the amount of carbon dioxide in the atmosphere, thus triggering global cooling and eventually glaciation.

With many of marine species adapted to warm, shallow waters, climate cooling might have pushed narrowly adapted species out, and such cooling would also explain why some terrestrial organisms were affected

Researchers have found evidence of substantial glacier formation at the end of the Devonian period. This would dramatically lower sea levels and radically affect shallow seas and wet lands.

And while this period of earth history ended with much of its marine animals going extinct, during the Devonian, many key animals developed and continued on, while some eventually evolving into us.

I leave you with this wonderful mural from the Chicago Field Museum, created by Karen Carr:

Figure 21.

Illustration by Karen Carr © The Field Museum

Terms

Definition of Eutrophication

n. Excessive nutrients in a lake or other body of water, usually caused by runoff of nutrients (animal waste, fertilizers, sewage) from the land, which causes a dense growth of plant life; the decomposition of the plants depletes the supply of oxygen, leading to the death of animal life.

motherjones.com

Definition of Jawless fish

n. Eel-shaped vertebrate without jaws or paired appendages including the cyclostomes and some extinct forms.

www.historyoftheuniverse.com

Definition of Placoderm

n. Fish-like vertebrate with bony plates on head and upper body; dominant in seas and rivers during the Devonian; considered the earliest vertebrate with jaws.

www.aquaportal.bg

Definition of Ray-Fin Fish

n. Any of the stiff bony spines in the fin of a fish.

Generic synonyms: Spine
Group relationships: Fin

Definition of Tetrapod

n. A vertebrate animal having four feet or legs or leglike appendages.

universe-review.ca

Links

These links will give you more information about the Devonian, or about interesting facts, fossils, and how they were found.

To learn more about Karen Carr and her exquisite illustrations:

www.karencarr.com

Here's ReefQuest Centre for Shark Research

This organization is dedicated to shark and ray conservation through its scientific research and public education programs.

www.elasmo-research.org

From the University of Maryland Honors College, here's a link to Prof. John W. Merck's “On Beyond Dinosaurs: Patterns and Enigmas in Vertebrate Evolution” course, and a lecture about the origins of jaws.

www.geol.umd.edu

Here's two videos with Neil Shubin, discussing his book, “Your Inner Fish, A Journey into the 3.5 Billion-year History of the Human Body.”

Part 1: www.youtube.com

Part 2: www.youtube.com

This explains his research approach, where and how his crew searches for fossils, the Tiktaalik discovery, and how that transitional fossil fits into the development of humans.

Here's a long conversation with Professor Shubin.

Recorded March 19, 2009
globetrotter.berkeley.edu
globetrotter.berkeley.edu

Here's a video of a member of the coelacanth family:

www.youtube.com

Here's the first of a two part video presentation about Coelachanth's:

Part One: www.youtube.com

Part Two: www.youtube.com

Here's the Tree Of Life Project:

tolweb.org

The Tree of Life Web Project (ToL) is a collaborative effort of biologists and nature enthusiasts from around the world. On more than 10,000 World Wide Web pages, the project provides information about biodiversity, the characteristics of different groups of organisms, and their evolutionary history (phylogeny).

More About Ray-Fins

www.devoniantimes.org

This is a must read: The Upstream Progression of Ecosystem Development
during the Devonian Transformation

www.devoniantimes.org

Here's a good reference dictionary:

www.lexic.us

Figures & Acknowledgments

Figures

Figure 1. schools-wikipedia.org

Figure 2. www.btinternet.com

Figure 3. paleobiology.si.edu

Figure 4. school.discoveryeducation.com

Figure 5. bill.srnr.arizona.edu

Figure 6. coris.noaa.gov

Figure 7. bill.srnr.arizona.edu

Figure 8. commons.wikimedia.org

Figure 9. en.wikipedia.org

Figure 10. bill.srnr.arizona.edu

Figure 11. commons.wikimedia.org

Figure 12. commons.wikimedia.org

Figure 13. www.geology.wisc.edu

Figure 14. Charles William Collection, photo: Collinson

Figure 15. Charles William Collection, photo: Collinson

Figure 16. Charles William Collection, photo: Collinson

Figure 17. Charles William Collection, photo: Collinson

Figure 18. www.abdn.ac.uk

Figure 19. www.devoniantimes.org

Figure 20. commons.wikimedia.org

Figure 21. www.fieldmuseum.org

References

Books:

Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.
Edwards, D. S. 1986. Aglaophyton major, a non-vascular land-plant from the Devonian Rhynie chert. Botanical Journal of the Linnean Society, 93: 173-204.
Hallam A. and P.B. Wignall. 1997. Mass Extinctions and Their Aftermath. Oxford: Oxford University Press.
Janvier, P. 1996. Early Vertebrates. Claredon Press. Oxford.
Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
McGhee GR Jr. 1996. The Late Devonian Mass Extinction. New York: Columbia University Press.
Romer, A. S. 1964. The Vertebrate Body. W. B. Saunders. Philadelphia.

Scientific Papers:

Algeo, T.J., R.A. Berner, J.P. Maynard, and S.E. Scheckler. 1995. "Late Devonian oceanic anoxic events and biotic crises: "Rooted" in the evolution of vascular land plants?" GSA Today 5: 45, 64-66.
Algeo, T.J. and S.E. Scheckler. 1998. "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events." Phil. Trans. R. Soc. Lond. B 353: 113-130.
Algeo, T.J., S.E. Scheckler and J. B. Maynard. 2000. "Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate." pp. 213-236. In: P.G. Gensel and D. Edwards (eds.). 2001 Plants Invade the Land: Evolutionary and Environmental Approaches. Columbia Univ. Press: New York.
Campbell, K.S.W. and R.E. Barwick, 1988. "Geological and paleontological information and phylogenetic hypotheses." Geological Magazine 125: 207-227.
Scheckler, S.E., 1986. "Floras of the Devonian-Missipian transition." pp. 81-96. In: T.W. Broadhead (ed.). Land Plants. Univ. Tenn. Dept. Geology. Studies in Geology 15. Knoxville.
Streel, M., M.V. Caputo. S. Lovboziak, and J.H.G. Melo. 2000. "Late Frasnian-Famennian climate: based on palynomorph analyses and the question of the Late Devonian glaciation." Earth-Science Reviews 52: 121-173.<
Woodrow, D.L., R.A.J. Robinson, Anthony R. Prave, A. Traverse, E.B. Daeschler, N.D. Rowe and N.A. DeLaney. 1995. "Stratigraphic, sedimentologic, and temporal framework of Red Hill (Upper Devonian Catskill Formation) near Hyner, Clinton County, Pennsylvania: site of the oldest amphibian known from North America." Field Trip Guide. 60th Annual Field Conference of Pennsylvanian Geologist: 1-8.

Web:

Martin, R. Aidan. 2003. Copyright and Usage Policy. World Wide Web Publication, www.elasmo-research.org/copyright.htm
Thomas Algeo’s web page on the Devonian Mass Extinction:
homepages.uc.edu/~algeot/homepage/research.htm