Devonian period spans from 416 to 359.2 million years ago.
The Devonian period is named for Devonshire, England where
rocks of this age were first studied (USGS). Many groups continue
their adaptive radiations resulting in many first appearances
in both aquatic and terrestrial environments.
Producers & Reefs
dominant primary producers in the oceans continue to be
cyanobacteria, green and red algae (Knoll, Summons, Waldbauer,
and Zumberge, 2007, p. 148). Tabulate corals and stromatoporoids
continue to be the primary builders of reef systems (Stanley,
1987, p. 75) and (Webb, 2001, p. 175). Michigan’s
state stone is the Petoskey Stone. Petoskey Stones are fossils
of colonial rugose corals that grew in the Devonian seas. Many
Petoskey Stones represent the colonial coral Hexagonaria
Some sources identify the coral making up the Petoskey stones
as the Prismatophyllum genus. Petoskey stones were given their
cobble shape from the action of Pleistocene glaciers.
brachiopods, mollusks, rugose corals, and bryozoans continue
to flourish. Sponges and tabulate corals continue to be
important in reef building. Some of the best brachiopods
come from Devonian sediments in
The "generic" brachiopod
is the state fossil for Kentucky. Trilobites are present,
show a decline in numbers during the Devonian. Phacops
rana is a trilobite with wonderful compound eyes containing
calcite lenses. Phacops rana is Pennsylvania's
state fossil. Ammonoids make their first
appearance in the Devonian and represent an important evolutionary
trend in the Cephalopods. Ammonoids look like Nautiloids, but
can be distinguished by looking at the structure of their shells.
The sutures joining the Ammoniod chambers are folded into complex
patterns. This suturing may have strengthened the shell. The
ammonoid siphuncle is near the outer margin of the whorls.
The septae curve towards the body chamber in Ammonoids.
undergo such a great adaptive radiation during the Devonian
often referred to as “the age of fish.” Cartilaginous
fish and bony fish (ray-finned, lobe-finned, and lungfish)
make their first appearance early in the Devonian.
class of armored jawed fish Placodermi becomes well established
in the early Devonian.
In fact they were the dominant fish
group during this time (Prothero, 1998,
p. 345). Placoderms possessed articulated bony plates over
their head and shoulder regions. A neck joint allowed placoderms
to lift the anterior portion of their head shield. A cartilaginous
skeleton supported the placoderm body. Placoderms had scales
or skin covering the rest of their body and tail. Placoderms
had a long dorsal fin and a heterocercal tail (an asymmetrical
tail in which the dorsal lobe is extended due to an upward
flexion of the spine). Placoderms had paired pectoral fins
and were the first vertebrates to evolve paired pelvic fins
(Benton, 2005, p. 55). The majority of placoderms did not have
true teeth, their head shield and mandibular shield were modified
into sharp-edged cutting surfaces forming broad dental plates. Most
placoderms were small fish reaching lengths of 15 cm; however,
lengths of 4 to 10 meters. Placoderms lived in both marine
and freshwater environments. Placoderms were one of the first
groups of fish to evolve jaws.
Although placoderms reached their greatest diversity during
the Devonian they did not survive the extinction event at the
end of this period.
class Chondrichthyes includes the cartilaginous fish, today
skates, rays, and ratfish. Fish
in this class have skeletons made of cartilage strengthened
by granules of calcium carbonate in a mosaic pattern. A thin
layer of bone covers the cartilage and the skin is covered
with teeth-like scales. The paired fins are reinforced with
rays of cartilage. The jaws contain bony teeth. Finally, males
have “clasping” pelvic fins to aid in sperm transfer
during copulation. Sharks diversified into many forms throughout
the Devonian and Carboniferous after which they decline in
the Permian. Sharks make a rebound during the Mesozoic. In
the Jurassic modern groups of sharks evolve (Dixon, 1988, p.
class Osteichthyes includes the bony fish, represented by
and the lobe-finned, or
sarcopterygians. Bony fish now account for more than half of
all vertebrate species. Lobe-finned fish include the ancestors
of all land vertebrates. Osteichthyes proved to be a very successful
vertebrate class. Fish in this class have a skeleton made of
bone. Ray-finned fish possess parallel bony rays that support
and stiffen each fin. The fin does not have any internal muscles;
muscles in the body of the fish move it. The first primitive
ray-finned fish (Palaeoniscids) had thick bony articulated
scales, a single dorsal fin near the posterior end and an asymmetric
tail. Ray-finned fish evolved to live in both marine and freshwater
environments. The first lobe-finned fish appear in the early
Devonian. The pectoral and pelvic fins of sarcopterygians are
long, fleshy, muscular lobes. Each fin has articulated bones,
muscles, and a fan of bony rays. The bones in the fins can
be related to the bones found in the limbs of terrestrial vertebrates.
The muscles can move each fin independently. The sarcopterygians
are divided into two groups the lobe-finned fishes and the
lungfishes. Today only the coelacanth and 6 species of lungfish
represent the sarcopterygians. The best fish candidate as ancestor
to the first amphibians and all tetrapods would be a member
of the Rhipidistia, a group of lobe-finned fish. The characteristics
shared by these lobe-finned fish and the first tetrapods are
remarkable. The skulls of both share the same bone arrangement.
The foreleg and fore limbs have bones equivalent to the humorous,
radius and ulna. The vertebrae and the vertebral body of both
creatures share a similar form (Johnson & Stucky, 1995,
p. 46). Panderichthys lived during the Devonian and
had a tetrapod-like head and paired muscular fins. Fish like Panderichthys were
the ancestors to tetrapods, air breathing terrestrial vertebrates
(Plamer, 1999, p 79).
The first terrestrial vertebrates or tetrapods (Superclass Tetrapoda) appear
during the Devonian period. Tetrapods
include vertebrates with four toe-bearing legs, or descendants
of such a vertebrate. The word tetrapod is used to refer
to vertebrates other than fish. Fossil evidence suggests that
lobefinned fish, such
as Eusthenopteron (Order Osteolepiforemes) and Panderichthyes (Order
Panderichthyida) are among the closest sarcopterygian relatives of tetrapods
(Benton, 2005, p. 80). In fact, Eusthenopteron, Panderichthyes, Tiktaalik, Acanthostega,
and Ichthyostega make up a series of fossils used to model the transition
from aquatic lobe-finned fish to fully four-legged tetrapods.
(2007) explores the transitional features of these fossils
and how they help us to better understand the evolution
of tetrapods from lobefinned fish (pp. 222-230). Eusthenopteron is
a Devonian aged lobefinned fish. The pelvic and pectoral
bones in this fish are homologous to tetrapod limb bones.
The skull bones of Eusthenopteron are the same
as early tetrapods.
the Late Devonian had a flattened body with a straight
tail. This "fishibian" had a skull and brain
case similar to early tetrapods with upward facing eyes.
In fact, the skull of Panderichthyes was classified
as a tetrapod until its fishlike body was discovered. Panderichthyes had
labyrinthodont-like teeth, which characterize later tetrapods. Panderichthyes possessed
both lungs and gills. This fish had lost its dorsal and
anal fins, but retained pectoral and pelvic foot-like lobed
the Late Devonian (375 million years ago) is nicknamed
the "fishapod" by its discoverer Neil Shubin. Tiktaalik
roseae like other fish had gills, scales, and
fins. However, the fins were weight bearing possessing
both wrist bones and finger-like bones. Tiktaalik also
possessed tetrapod characteristics. The head of Tiktaalik ("large
freshwater fish"-taken from the Inuktitut language)
was flat with eyes positioned on the top of the skull (Ridley,
2009, p. 70). The pectoral girdle was separate from the
skull forming a neck and allowing the head to turn. The
ribs were designed to support the body and allow breathing.
The fact that Tiktaalik had spiracles on its skull
and a more robust ribcage indicates this organism had lungs
as well as gills. The appearance of tetrapod characteristics
in a fish that existed 12 million years before the first
tetrapod is very significant (Tiktaalik website, 2008).
a basal tetrapod from the Upper Devonian (365 million years
ago) that shows characteristics intermediate between lobe-finned
fish and the first fully terrestrial tetrapods. Acanthostega had
gills, fins, and a lateral line for sensing vibrations,
but possessed lungs, spine, and limbs like a tetrapod.
Studies of Acanthostega's limbs suggest that they
were used for swimming or crawling along the bottom.
the Late Devonian is yet another basal tetrapod that represents
a transition between fish and amphibians. Ichthyostega possessed
limbs that were longer and more tetrapod-like than Acanthostega. Ichthyostega still
possessed a tail, gill slits, and a lateral line for sensing
movement, but it also had the limbs and spine of a tetrapod. Acanthostega and Ichthyostega represent
important transitional fossils linking fish and amphibians,
but are not classified as amphibians.
amphibian-like fossils are referred to as basal tetrapods. Acanthostega and Ichthyostega from
Greenland are the best known Devonian-aged basal
tetrapods. Ichthyostega and Acanthostega measured
around 1 meter in length. As we have already mentioned,
both had fully developed limbs and limb girdles,
but still retained features associated with their
aquatic life. These animals had a gill apparatus
as shown by their opercular bones. Flanges on their
ribs indicate they also had lungs. They possessed
a strong fin tail used for swimming. Evidence of
lateral lines can be seen on their skulls and a possible
otic notch on the back of the skull may indicate
the presence of an eardrum (Prothero, 1998, p. 362).
Interestingly, it was recently discovered that Acanthostega had
eight toes and Ichthyostega had seven. Evidently,
the standard pentadactyl (five-fingered) condition
in tetrapods developed later.
Acanthostega and Ichthyostega are found in sedimentary deposits
that indicate they lived in meandering rivers that flowed through forests of
lycopods and ferns (Benton, 2005, pp. 82-85). They probably lived in water
choked with vegetation. Being able to swim and step over vegetation would be
a real adaptive advantage. Basal tetrapods and amphibians underwent a great
adaptive radiation during the Carboniferous.
the term amphibian has been used to refer to all tetrapods that
are not amniotes (reptiles, birds, and mammals). Thus, under
the traditional definition the first appearance of amphibians
would be in the Devonian. However, it is now clear that this
is a paraphyletic term. The class Amphibia
now refers to present-day amphibians and their extinct sister
groups. The extinct sister groups of amphibians appear in the
Carboniferous and modern representatives of Amphibia appear in
Devonian is often called the “age of fishes”,
but could just as easily be named the “age of plants”.
Plants with specialized cells to transport water and nutrients
evolved during the Silurian. Early in the Devonian these vascular
plants were simple, small, stick-like structures like Asteroxylon,
Cooksoni, and Swadonia (formerly Psilophyton).
Pertica quadrifaria was one of the first simple plants with
no leaves or roots. Stems functioned as the photosynthetic
organs of this plant and reproductive structures holding spores
lay at the end of branching stems. Pertica quadrifaria is
the state fossil for Main. By the end of the Devonian complex
had evolved creating the first
forests and well-developed soil ecosystems (Kenrick & Davis,
2004, p. 34).
The adaptive radiation of plants during the Devonian would
see the evolution of leaves, roots, wood, and primitive seeds.
The first leaves were spike-like as found on Baragwanathia.
Branching and webbing may have led to the development of fern-like
fronds found in plants such as Rhachophyton.
The appearance of wood represents the evolution of the cambium.
Cambium tissue allows a plant to grow in girth to produce trunks.
The evolution of roots and cambium were critical in creating
the tree form (Kinrick and Davis, 2004, p. 66) Archaeopteris could be referred to as the first modern tree (Murphy, 2006,
Archaeopteris page). Archaeopteris made up a significant portion
of the canopy of early forests. The trunk (Callixylon) of this
tree was constructed of conifer-like wood, while the branches
were adorned with fern-like fronds (Archaeopteris). The underside
of the fronds had sac-like sori that contained spores for reproduction.
It is easy to understand why the fern-like fronds and conifer-like
wood of this progynmosperm were considered to be separate organisms
until the fossils connecting the foliage and wood were discovered.
Archaeopteris is a true missing link between fern-like plants
and conifer-like plants.
or seed ferns are the earliest plants with seeds. The fronds
of Pteridosperms were identical to ferns, but bore
seeds instead of spores. The term pteridosperm is descriptive
but, misleading as these plants were really early gymnosperms
(Cleal & Thomas, 2009, p. 139). The seeds of these early
plants were small and had an incomplete integument
(Kenrick & Davis,
2004, pp. 44-45). The first seeds also had slight wing-like
structures indicating that they were wind dispersed. Seeds
not only protect the embryo, but also provide nutrients, a
vehicle for dispersal, and the possibility of delayed growth.
Seeds also represent a change in reproductive strategy in which
an alternation between plants that produce spores (sporophytes)
and plants that produce sperm and eggs (gametophytes) is abandoned.
In seed plants the gametophyte is no longer an entire plant;
it is reduced to pollen and ovules. Seedless plants require
water to carry the sperm to the eggs, while seed plants use
pollen as a vehicle to carry the sperm to the egg. The evolution
of the seed is an important adaptation for dry conditions.
evolution of trees and forests produced a new carbon dioxide
sink. Forest lowered carbon dioxide levels in the air through
the process of photosynthesis, which in turn would lower
global temperatures. As roots evolved during the Devonian
they increased in depth
impact on the weathering and development of
soils (Kenrick & Davis, 2004, pp 50-51). Roots weather
calcium and magnesium silicates allowing carbon dioxide from
the air to form calcium and magnesium carbonates. These carbonates
help to form limestone and dolomite. The evolution of plants
created new ecosystems on the land and in the soil for terrestrial
organisms to exploit. Plants impacted the soil, atmosphere,
rock cycle, and oceans through photosynthesis and the action
there is evidence for insect-like organisms in the Silurian
the first true insect appears in the Devonian. Springtails
belong to the order Collembola but are no longer considered
insects. Springtails are found
in the Rhine Chert. Bristletails (order Archaeognatha)
are the most primitive living insects. Bristletails make
during the Devonian (Grimaldi & Engel, 2005, p. 148). The
first insects were wingless, underwent incomplete metamorphosis
and did not organize
into social groups.
The Rhine Chert
Rhine Chert near the Aberdeenshire villiage of Rhynie in
early Devonian (396 Ma). Plants and animals
of this area lived near a sinter terrace. The area was periodically
flooded with silica rich solution from hot springs and geysers
(Selden & Nudds, 2004, p. 52 and Kenrick & Davis, 2004,
p. 24). Some of the thick chert deposits preserve plants in
their life position (Kerp, 2002, p. 24). Organisms
were permeated with silica before any cellular decay could
occur. Five of the seven land plants described are true vascular
plants (Tracheophytes) with tracheids for water conduction.
All of the land plants have cuticles, stoma, intercellular
spaces, and vascular strands with lignin. Rhine plants are
branching stick-like structures growing off rhizomes and reaching
only knee height. Among the plants are found algae, fungi,
and the oldest known lichen. The fauna of the Rhine Chert is
comprised of crustaceans, trigonotarbid arachnids, the first
mites, collembolans, euthycarcinoids, and
myriapods. Like other early land faunas carnivores and detritivores
dominate the Rhine, while herbivores were absent or rare (Kenrick & Davis,
2004, p. 28). True herbivores, which do not make their appearance
until the Carboniferous, require a symbiotic relationship between
bacteria and fungi in their gut to digest plant material (Selden & Nudds,
2004, p. 57).
Hunsrück Slate in Bundenbach, Germany is lower Devonian (390
ago). Both the Hunsrück Slate and the Burgess
Shale represent marine benthic communities living above a muddy
seabed at a depth of less than 200m. All of the fish groups
except the chondrichthyans (which had not yet evolved) are
represented with agnathans and placoderms being the most common.
Among the Echinoderms the starfish, brittlestars, and crinoids
are the most common. Polychaete worms are a rare find and represent
the Annelids. Among arthropods, trilobites are common, while
crustaceans, and chelicerates are rare. Sponges, corals, brachiopods,
and mollusks are also found in the Hunsrück, but are not numerous.
Small pieces of plant material washed out to sea along with
fish coprolites, burrows, and tracks are also preserved (Selden & Nudds,
2004, pp. 37-46).
Gilboa, New York
Gilboa, New York, Devonian aged limestones, shales, and sandstones
windows into several different habitats.
The first appearance of trees and a forest are found in the
fossils of Gilboa (385 million years ago). Stumps with roots
stretching out into a paleosol (fossil soil) have been preserved
as sandstone casts at a site known as Riverside Quarry. The
stumps were given the name Eospermatopteris and are
now known to be a cladoxylopsid. Cladoxylopsids are thought
to be ancestors
of ferns and horsetails. Recently, two Gilboa trees with trunk
and branching crowns were found lying prostrate. The trunk
of the trees was identified as a cladoxylopsid of the genus
Wattieza (Stein et al, 2007, pp 904-907). Illustrations
combining Wattieza with Eospermatopteris give
paleontologist an idea of what this Gilboa tree looked like
in real life. Carbonized foliage at Riverside Quarry preserves
arboresent and herbaceous lycopods, such as Lepidosigillaria and Leclercqia.
Progymnosperms like Aneurophyton can also be
found. In the Devonian, Riverside Quarry was a swampy forest
near a coastline. A second location near Gilboa, known as Brown
Mountain, is a lagerstatten, which preserves important animal
fossils in what was once a tangled mass of Leclercqia (a
herbaceous lycopod) and mud, which was located in a deltaic
Fossil foliage at Brown Mountain includes lycopods, progymnosperms,
and cladoxylopsids. Animals preserved in the tangled Leclercqia mud
mass at Brown Mountain include: eurypterids, scorpions, trigonotarbids
(spider-like organisms), the first known spider,
the first known pseudoscorpions, millipedes, and centipedes.
A third site, called South Mountain, represents a deltaic environment
like that found at Brown Mountain. Fossil foliage at South
Mountain is a mix of lycopods, progymnosperms, and cladoxylopsids.
Two types of fish are found at South Mountain, placoderms and
acanthodians. Gilboa, like other Devonian aged fossil ecosystems,
represents a primitive trophic structure that lacks herbivory;
only predator-detritivore food chains existed at this time
(Nudds & Selden, 2008, pp. 95-113).
The mass extinction that occurred in the Late Devonian affected
mainly the marine environment with terrestrial plants escaping
the crises. It is estimated that up to 75% of marine species
and 50% of marine genera were lost (Prothero, 2004, p.
90). Brachiopods, trilobites, conodonts, ammonoid, corals,
and stromatoporoids were hit hard. Reef building communities
were decimated. Tabulate corals and stromatoporoids would
never again be major reef builders after the Devonian crises.
The rest of the Paleozoic would see very little reef building.
Reef building would recover in the Mesozoic with the appearance
of modern corals (Stanley, 1987, pp. 78-79). The Devonian
crisis seems to be correlated with cooling. Coral reefs
were in decline as cold water glass sponges expanded. Shallow,
warm water marine species declined. Freshwater fish that
were adapted to seasonal environments survived, while warm
water marine fish experienced heavy extinction. As these
shallow warm water species declined the stromatolites had
a small resurgence in reef building. There is evidence
of glaciation and lower sea levels. Both the Ordovician
and Devonian cooling events may be tied to the movement
of Gondwanaland over the South Pole (Stanley, 1987, pp.
86-89). The cooling event may also explain Late Devonian
carbon and oxygen isotope anomalies. A severe cooling would
trigger a massive overturn within the ocean. This overturn
would bring deep ocean water to the surface. The deep ocean
water is nutrient rich, but cold and oxygen poor. The Devonian
crises lasted for 4 million years (Prothero, 2004, p. 90).
(2005) Vertebrate Palaeontology [3rd Edition]. Blackwell
Publishing: Main, USA.
C.J. & Thomas, B.A. (2009). Introduction
to Plant Fossils. United Kingdom: Cambridge
D., Cox, B., Savage, R.J.G., & Gardiner, B. (1988). The
Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric
Animals: A Visual Who’s Who of Prehistoric Life.
New York: Macmillan Publishing Company.
D. & Engel, M.S., (2005). Evolution of the Insects.
New York: Cambridge University Press.
K.R. & Stucky R.K. (1995). Prehistoric Journey:
A History of Life on Earth. Boulder, Colorado: Roberts Rinehart
P. & Davis,
P. (2004). Fossil Plants. Washington: Smithsonian Books.
H. (2002). The Rhynie Chert: The Oldest and Most Completely
Preserved Terrestrial Ecosystem. In Dernbach, U. & Tidwell,
W.D. Secrets of Petrified Plants: Fascination from
Millions of Years (pp. 23-27). Germany: D’ORO
Summons, Waldbauer, and Zumberge. (2007). The Geological
Succession of Primary Producers in the Oceans. In Falkowski,
P.G. Knoll, A.H. [Eds] Evolution of Primary Producers
in the Sea. (pp. 133-163). China: Elsevier Academic
Murphy, D. (2006). Devonian
Times: See: http://www.devoniantimes.org/index.html
J.R. & Selden
P.A. (2008). Fossil Ecosystems of North America: A Guide
to the Sites and Their Extraordinary
Biotas. Chicago: The University of Chicago Press.
D. (1999). Atlas of the Prehistoric World. New
York: Discovery Books.
D.R. (2007). Evolution: What Fossils Say and Why
It Matters. New York: Columbia University Press.
M. (2009). The Darwin Bicentennial Part II: Modern Darwins. National
Geographic, February 2009, Vol. 215, No. 2.
Stanley, S.M., (1987). Extinction. New York: Scientific American
W.E., Mannonlini, F, VanAller Hernick, L., Landing, E.
& Berry, C.M. (2007). Giant cladoxylpsid trees resolve
the enigma of the Earth's earliest forest stumps at Gilboa.
Nature, vol 446: pp. 904-907.
P. & Nudds,
J. (2004). Evolution of Fossil Ecosystems. Chicago: The University
of Chicago Press.
roseae (2008): http://tiktaalik.uchicago.edu/
G.E. (2001). Biologically Induced Carbonate Precipitation
in Reefs through Time. In Stanley, G.D. Jr. [Ed] The
History and Sedimentology of Ancient Reef Systems (159-203).
New York: Kluwer Academic/Plenum Publishers.