Name:
Eurypterus
(Wide wing).
Phonetic: You-rip-teh-rus.
Named By: James Ellsworth De Kay - 1825.
Synonyms: Baltoeurypterus. Possibly also
Eurypterus flintstonensis.
Classification: Arthropoda, Chelicerata,
Merostomata, Eurypterida, Eurypteroidea, Eurypteridae.
Species: E. remipes
(type), E. dekayi, E. hankeni, E. laculatus,
E. lacustris, E. leopoldi, E. megalops, E.
pittsfordensis, E. quebecensis, E. serratus, E.
tetragonophthalmus. Also E. cephalaspis
and E. minor,
but there is uncertainty regarding fossils assigned to these species.
Diet: Carnivore.
Size: From as little as around 13 centimetres
long to up to 1.3 meters long. Size depends upon the species and
known individuals.
Known locations: Eastern Canada and the United
States. Across Europe.
Time period: Late Llandovery through to the Pridoli
of the Silurian.
Fossil representation: So common the number of
fossilised individuals is unknown. However, most of the fossils are
of excuviae, the shed exoskeletons of Eurypterus
that moulted as they
grew larger.
When
it comes to the study of eurypterids,
the type genus Eurypterus
is
the one you need to be most familiar with since the vast majority of
known eurypterid fossils belong to this genus. It should be noted
however that many of these fossils are of shed exoskeletons and that
fossils of actual dead Eurypterus are rarer.
Eurypterus
is often noted as having a short temporal distribution, yet the genus
is known to have existed for most of the later stages of the Silurian
period for up to some fourteen million years. The oldest fossils are
attributed to European deposits while North American fossils are of
individuals that lived later in the Silurian. This indicates that
Eurypterus radiated out from their origin in Europe
though they only
seem to have reached North America from the close proximity of what
would become North America to Europe at this time. So far there is no
solid evidence to suggest that Eurypterus managed
to colonise other
parts of the world, though other genera of eurypterids are known.
Like
with other members of the Eurypterida, Eurypterus
was a primarily
aquatic arthropod. Current thinking is that Eurypterus
and other
eurypterids were relatives of the arachnids and indeed the body parts
of eurypterids can match up pretty well to the body parts of
scorpions, even if the proportions are very different. However,
Eurypterus and its relatives are usually treated as
what is termed a
sister group to the arachnids. This means that while they were very
similar and shared ancestors common in form, they were still
different enough to be considered a different evolutionary branch.
Like
with its relatives the body of Eurypterus was
elongated and clearly
divided into two portions. The rear portion is the largest and can be
broken down into two further sections, the mesosoma (segments 1
to 6) and metasoma (segments 7 to 12 and the telson).
The smallest of these is called the prosoma and situated at the
front, it incorporates both the head and thorax. These are actually
made up of six segments, but they are covered by a single carapace
that would have provided both extra support and protection for the
prosoma. There were two crescent shaped compound eyes in the
carapace, and a further two light sensitive eyes. On the underside
of the prosoma was the mouth and six pairs of appendages. The first
pair at the front of the mouth had pincers which were used for
manipulating and breaking apart food so that it could fit into the
mouth. The second, third and four pairs were short legs covered in
spines which could have been used to both kill and hold onto prey as it
was torn apart by the pincers. Moving towards the side, the fifth
pair were the most adapted as legs, however there is some size
variance of these legs between certain individuals. This could be a
sign of sexual dimorphism, but opinions vary as to which sex the
longer legs belong to. The sixth and final pair of appendages are
much larger than the others and are shaped into a paddle-like form at
the end. These seem to have been for the primary purpose of swimming
through the water.
There
are three forms of locomotion associated with Eurypterus
and by
extension other eurypterids. While foraging around on the sea floor,
Eurypterus would have crawled along on their legs,
which although
small, would have probably been good enough to support the body
weight when in the water. These legs however would not have been
suitable for travelling over extended distances, especially since
swimming would have been the superior method of locomotion in terms of
speed and energy efficiency.
There
are two forms of swimming locomotion associated with Eurypterus,
both
are likely, though which one used may have depended upon the size of
the individual. The rear appendages that have paddle-like ends are
interpreted as the primary swimming devices, but these appendages can
only really move forward and backwards, with little up and down
motion taking place. How these paddles were used then would have
actually been similar to how you angle your hands when swimming. On
the down stroke you make sure that the palms of your hands ‘push’
against the water, the increase in surface area allowing for greater
resistance against the water. On the upstroke however you don’t
want this resistance otherwise you counter the push from the down
stroke, so you angle your hands so that the smaller surface area
of just the side moves through the water, cutting like a blade.
Eurypterus likely angled the paddle on the end of
the rear appendage
so that the wider surface pushed against the water on the back stroke,
while turning it so that only the lateral edge cut through the water
on the forward stroke. In this manner forward motion could be easily
maintained.
The
above is a very plausible method for Eurypterus of
all sizes, however
it would not have been as energy efficient for larger bodied Eurypterus
since their larger bodies would have caused greater resistance through
the water because of their larger size. The increased weight and mass
of the body would have also required greater effort to stroke through
the water. It is believed that larger Eurypterus
would have utilised
what is technically known as subaqueous flight (also known as
underwater flying). In essence this is where forward propulsion is
actually achieved by undulating the main body, while the paddle-like
rear appendages are used like hydrofoils. When the paddles are
pitched so that the anterior (front) edge is high and the
posterior (rear) edge is down, the paddle would create an upwards
lift as it passed though the water, meaning that a Eurypterus
would
swim up. By contrast when the paddles were pitched so that the
anterior edge was down and the posterior edge was up, a Eurypterus
would swim down.
The
above method is more energy efficient and would have allowed for a
greater top swimming speed, but even large Eurypterus
may have still
used the paddle-stroking method occasionally as well. This could have
especially been the case when an individual first set off since the
paddle stroking method offered a greater rate of acceleration than the
subaqueous flight method. Once a large Eurypterus
had finally got
going and cleared the sea floor it might have then switched to the
subaqueous flight method, especially if it intended to swim for some
way.
The
paddle-like rear appendages may have also been used for steering since
if the paddles could be independently controlled from one another then
they could have been used to provide varying degrees of resistance and
push. This is the same as if you were in a row boat or canoe, you
turn yourself around by only paddling on one side.
Juvenile
Eurypterus are similar to the adults in form but
with a few
differences. The carapaces were longer and parabolic in cross
section, more of a smooth curve. The carapaces of adults by
contrast were trapezoidal meaning that they were more box like. The
eyes of juveniles were located in more lateral positions when young,
eventually shifting towards a more forward position in later life,
possibly signifying a shift to a more actively predatory lifestyle.
The swimming legs are also noted as being thinner while the telsons
are shorter.
The
primary organs through which Eurypterus breathed
are believed to have
been the book gills that were located in between the segments of the
mesosoma, the anterior section of the rear body that connects to the
forward prosoma. As the name suggests they were made up of layers of
folded tissues that looked like the pages of a book when it is left
slightly open, and these organs are believed to have been the
forerunners of the book lungs seen in later arthropods. However
Eurypterus also had a second respiratory system
represented by five
pairs of ovals on the preabdomen which had small spines within them as
well as a connection to blood vessels, similar to how blood vessels
connect up with lungs. These two systems combined are popularly
thought to not only allow for the processing of oxygen from sea water,
but also to allow for respiration for short periods when actually on
land.
Eurypterus
are thought to have behaved in a similar manner to horseshoe crabs,
spending most of the time living in the sea, yet returning to the
shore to spawn. In addition to this a large number of fossils of
moulted exoskeletons have been found in what were intertidal zones
(the part of the coast that is exposed at low tide but submerged at
high tide) back in the Silurian. Although primarily aquatic
creatures, Eurypterus would have still been able
to move about on
land to a certain extent, the rear swimming appendages also probably
being developed enough to push an individual across the sand. During
these gatherings Eurypterus probably approached at
high tide before
spawning at low tide and then returning to the sea as the time came in
again. This would have been the most energy efficient form of
approach.
It
is quite easy to appreciate however that as time went own, some
arthropods that differed slightly with more developed limbs would have
had an easier time moving around on the land. In time, over the
course of millions of years, their descendants would have steadily
evolved into the first terrestrial (fully land based) arthropods
which in turn would develop into the land invertebrates that we know
today.
Fossil evidence suggests that Eurypterus spent most of the time living in shallow seas where they hunted for other marine organisms as well as scavenging the bodies of other creatures. When small, Eurypterus would have been restricted to hunting only small creatures, but larger Eurypterus may have also hunted other eurypterids, possibly even smaller relatives of their own genus. Eurypterus would have been most vulnerable during the time just after moulting and before their ‘new’ exoskeleton hardened. This might explain the huge abundance of moulted exoskeleton in intertidal zones. The only predators known at this time lived in the sea, in fact only rudimentary plants seem have been the only life forms living on the land. By staying out of the water as their exoskeleton hardened, as well as possibly shedding en masse, Eurypterus could reduce the danger to themselves since even if other eurypterid predators followed them onto shore, they would have been far more cumbersome than they would have been in the water.
Further reading
- Two new Silurian species of Eurypterus
(Chelicerata:
Eurypterida) from Norway and Canada and the phylogeny of the genus,
O. Erik Tetlie - 2006.
- Eurypterids, arachnids, and the arthropod invasion of the land,
John W. Merck 2011.
- Specimens of Eurypterus (Chelicerata,
Eurypterida) in the
collections of Museo Geominero (Geological Survey of Spain),
Madrid, O. E. Tetlie & I Rabano - 2007.
- Autecology of Silurian Eurypterids, Paul A. Seldon - 1999.
- The genera, species and subspecies of the family Eurypteridae,
Burmeister, 1845, Erik N. Kjellesvig-Waering - 1958.
- Eurypterid respiration, Paul A. Seldon - 1985.
- The respiratory organs of Eurypterids, Philip J. Manning
& Jason A. Dunlop - 1995.
- Distribution and dispersal history of Eurypterida
(Chelicerata), O. Erik Tetlie - 2007.
- The biomechanics of swimming, John W. Merck - 2011.