The Devonian period was a time of many “firsts”: the first amphibians, the first
sharks, the first plants with roots, leaves and stomata. Despite
such significant innovations, the Devonian is best known for its diversity
and abundance of fish. Not only did fish with cartilage first appear (such as
sharks), but fish
with bony skeletons developed and it is this type of fish which today comprises a majority of
our modern fishes.
Other fish also appeared whose fins were made of strong nodules (lobes) of bones
that could be used as legs. These lobed-finned fish forever changed Life on
Earth as they became the first amphibians that crawled onto land and breathed in
oxygen with their newly-formed lungs.
It was a period of great drama as oxygen levels fell and carbon dioxide levels
rose causing chaos among all forms of life striving to adapt to the
ever-changing conditions. By the Middle Devonian, one of the Five Great Mass
Extinctions occurred devastating the marine communities and temporarily
impeding the development on land and sea.
- Oxygen levels plummeted during this time, leaving marine environments with
little or no oxygen, decimating or stunting marine communities.
- Carbon dioxide levels rose to their highest levels since the Ordovician.
- The continental masses shifted southward. North America and Australia sat along the equator while most of the other
continents were either south of the equator or resting at the South Pole.
- The supercontinent Pangea began to form at the end of the Devonian.
- The climate was predominantly warm with shallow seas.
- This period is referred to as the "Devonian Explosion"
because of the significant advances in plants, their
reproduction techniques as well as the plant structure
- The first seed plants appear in the form of seed ferns, a primitive version of
the later gymnosperms of the Carboniferous period.
- The advantages of these seed-bearing plants over their
spore-bearing contemporaries are significant for the changing
environment. Seeds are able to withstand a variety of
climate conditions while spores rely on water or watery
- Early Devonian plants were still small and restricted to shores and river banks.
- They lacked roots, leaves and structural strength to grow taller than a few
feet; but all these features would be present by the end of the period enabling
the growth of large trees and other plants.
- The Devonian witnessed the appearance of bark, but it
wouldn't be until the Carbniferous that lignin would appear
which would serve as a tough inner lining of the bark,
giving even greater strength to trees and other plants.
- The production of lignin in plant tissues provided the strength plants needed to
grow straight and tall.
- Lignin appeared late in the Late Devonian or Early
Carboniferous. It will play a significant role in the
- The stomatal cells of plants allowed them to absorb carbon dioxide and exhale
oxygen. Plants thrive in the high carbon dioxide and low oxygen levels that held
sway during much of the Devonian.
- By the end of the Devonian, the CO2 levels had crashed, causing devastation among
the plant life.
- Amphibians appear at the end of the Devonian.
- Fish thrived in the Devonian in both marine and freshwater habitats; so much so
that the Devonian is termed the “Age of Fishes”. The important innovations were
- Fish with cartilage and those with rayed fins (fins supported
by bony structures) appeared for the first time. Both types of fish are extant
today: modern cartiliginous fish include sharks and stingrays while the rayed-fin fish
is the predominant type of fish today.
- Lobed-finned fishes eventually moved onto land becoming the first amphibians.
- Another significant aspect of the lobed fishes was their ability to breathe
oxygen using lungs and nostrils making them perfect candidates for living on dry
- Jawed fish dominated the seas by the Late Devonian. (The
first jawed fish appeared in the Silurian, but by the
Devonian they were dominant - having jaws is a definite
advantage over those without.
Ammonites first appear, descending from the existing
bactrites with their straight-coned shells.
- The mass extinction in the latter portion of the Devonian ranks as one of the Five Great Mass
- As much as 56% of marine life was wiped out.
- Some marine areas show a complete lack of oxygen and unable to support life.
- Theories for the cause of the mass extinction include glaciations, meteor impact
and low oxygen levels.