Eurypterids
 Eurypterids were the top predators of
the Silurian seas.
Eurypterids were aggressive
predators; large eyes to detect movement, enormous spined claws that could
crush and bring prey towards the mouth.
Abundant Prey species in
the form of armored trilobites and bony skin (ostracoderm) fishes during the
Silurian meant that a lot of crushing was needed to get the soft stuff into its
small mouth.
Just behind the eight small
walking legs were large flattened paddles, which are believed to have had a
swimming function. The paddles may have been used to "row" through
the water somewhat like water boatmen and backswimmer bugs of today.
The broad tail of
Pterygotus ended in a large rounded paddle with dorsal keels. The tail could
flex up and down and may have been the best bet for Pterygotus to cover a
distance or make a quick getaway.
But being one of the larger
predators of the Silurian lagoons, Pterygotus probably caused more fleeing than
anything else.
Here's
a geological map of this time that shows some of the lagoons.
Use this link to compare the Silurian
landmasses with our current Earth.
Side Trip
The Silurian
The
Silurian period lasts from about 440 to 410 mya.
- A relatively short period in geological history.
- A time of stabilization of global temperatures.
- A time of the first terrestrial animals and
The
most predominant organisms during the Silurian were the hard-shelled
Brachiopods.
It is
estimated that these creatures made up about 80% of the total species at the
time.
Tropical Reefs were common in the shallow seas with
- Corals
- Stromatoporid organisms, and
- Bryoza being most common.
- Trilobites
- Graptolites
- Cephalopds
- Gastropods
- Echinoderms
However
the Trilobites were still in a period of decline. During this period the
Graptolites also began declining in numbers.
The Silurian period is the first period in history where fish
begin to appear in the fossil record.
At the
beginning of the period, jawless fishes appeared to have invaded the
freshwaters while at the end of the period, jawed fishes begin to appear. These
animals were not considered to be the dominant creatures, however.
Also, during the Silurian is the appearance of the first terrestrial
ecosystem.
The
first evidence of vascular plants (plants with tissues that carry food) appear
in the fossil record of areas above sea level. The earliest land plant and
possible millipede-like animal fossils were found in modern day Europe.
Back To Eurypterids
Eurypterids resembled giant water scorpions, usually measuring 5 to 17
inches, but the biggest (Pterygotus) reached 6 to 8 feet in length.
The Eurypterids terrorized the shallow
seas, estuaries, and lakes.
In addition to pincers on some of the
preoral appendages, the eurypterid could flex its thorax up and forward, like
the terrestrial scorpion, to attack with its spiny telson, which was probably
poisonous.
Eurypterids first appear in the early
Ordovician, peak in number and diversity by the late Silurian, and then decline
through the Carboniferous.
They meet their ultimate demise along
with the trilobites and 95% of all extant species during the great Permian
extinction.
Their
Prey
Early Eurypterid fossils occur with
marine invertebrates during the Ordovician, but the Silurian, they appear in
sediments from brackish lagoons and bays.
By the Pennsylvanian period, they have
moved to freshwater swamps as seen by associated plant fauna as well as
freshwater mussels and snails.
To understand the rise and decline of
the eurypterids and their change in territories, one must understand their
prey.
 The most primitive of vertebrates, the jawless fish or
Agnatha, appeared and flourished in the Silurian period.
These primitive fish
- Lacked lateral fins
- Slow moving, sitting ducks for a larger predator.
The Agnatha were adapted to bottom
dwelling habits, scraping the bottom and sucking up food (referred to as
benthonic).
The Agnatha developed strong external
armor formed from bony plates, resulting in the alternate name of Ostracoderms
(shell skins).
The armor was the only defense against
the ravages of the Eurypterids. These fish congregated in shallow waters and
were ultimately followed by their predator.
There were two types of armored fish (
Acanthodians and Placoderms -- See Below
) that developed mobile jaws and paired lateral fins. The fins allowed much
faster swimming to elude predators and also allowed adaptation to larger bodies
of water such as oceans and thus evade the more confined and less mobile
Eurypterids.
As these new prototypes dominated in
the Devonian period, the Eurypterids began a steady decline in number of
species and of individuals, never to recover.
Just
One More Thing
Ostracoderm - is an archaic and informal term for a member of the group of armoured,
jawless, fishlike vertebrates that emerged during the early part of the Paleozoic
Era (542-251 million years ago).
Ostracoderms include both extinct groups, such as the heterostracans and
hagfishesand lampreys.
Acanthodians
and Placoderms -
- Acanthodians are a poorly understood group of extinct
jawed fishes that are distinguished by the bony spines projecting in front of
their fins and by minute, diamond-shaped scales.
- Placoderms are arguably the most diverse groups of the early
jawed fishes.
For
more information about these, go here, an outstanding site: Devonian Times
http://www.devoniantimes.org/index.html
Here's an interesting video about
Eurypterids:
http://animal.discovery.com/videos/animal-armageddon-eurypterids.html
The Permian-Triassic-Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 251.4 million years ago, forming the boundary between the Permian and Triassic geologic periods.
It was the Earth's most severe
extinction event, with up to 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects. 57% of all families and 83% of all
Because
so much biodiversity was lost, the recovery of life on earth took significantly
longer than after other extinction events.
Main
 an archaic and informal term for a member of the group of armoured, jawless, fishlike vertebrates that emerged during the early part of the Paleozoic
Era (542-251 million years ago). Ostracoderms include both extinct groups, such as the heterostracans and osteostracans, and living forms, such as hagfishes and lampreys.
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