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Roger Weller, geology instructor                     

Elizabeth Cruz
Historical Geology
Spring 2007

Photo Credited to R. Weller/Cochise College


What Are Eurypterids?


Eurypterids also called “sea scorpions” for the similarity to today’s scorpion was a terrifying predator that lived in shallow warm water in the Ordovician time period and the last couple went extinct in the Permian period.  These periods occurred in the Paleozoic Era.  Most Eurypterids have been found in brackish or fresh water.  Some of the earliest may have lived in the sea and fresh water during the Pennsylvanian period.  Some Eurypterids have also been known to have spent short periods of time on land, and have been known as one of the only arthropod predators unlike the thousands of species of Trilobites.  In fact, Trilobites were the prey of Eurypterids.  The most common ancestors are the horseshoe crabs and land scorpions.


Body Structure of Eurypterids


Eurypterids were basically large, flat, and had jointed sections.  The bottom half of Eurypterids included a flexible tail with a long spine.  The spiked tail of the Eurypterids was said to be used by injecting venom into its prey sort of like the modern day scorpions.  The Eurypterid’s segment body included a dorsal plate and ventral plate. 


Some Eurypterids had paddles in order to propel themselves, although some argue the paddles could have been used for digging.  There were also four jointed legs for walking and two claws in the front for grasping. These arthropods also had compound eyes and a smaller pair of eyes as well.  Eurypterids also had smaller legs sort of like today’s crabs for movement out of the water.


The respiratory systems of Eurypterids have been determined by whole body evidence and remaining fragments.  Evidence suggests Eurypterids had a dual respiratory system.  There is also evidence that shows Eurypterids may have had partially terrestrial gills such as those with aerial respiratory organs.  These aerial respiratory organs are related to the organs of terrestrial crabs of today.


There is little information reported about the internal structures of Eurypterids.  There have only been a few cases where the intestines where preserved.


Photo Credited to R. Weller/Cochise College and the Exhibit Museum of Natural History


Movement of Eurypterids

Eurypterids had paired appendages for underwater swimming.  For example, in order for Eurypterids to swim, they had to produce a thrusting motion to overcome the drag of their body.  When the water was at low velocity, the Eurypterid had little problems swimming.  However, at high water velocity, the Eurypterid used a sixth appendage to function as a paddle for steering.  


There were two types of mechanisms used by Eurypterids for swimming:

1)       Drag based- Where the Eurypterid uses a power stroke and drags their appendage.

2)       Lift based- Where the movement of the appendage is up and down.  The down stroke was longer in order to protect the Eurypterid from sinking.


Size of Eurypterids

Eurypterids were one of the largest and fearsome arthropods that ever lived.  Even though the smallest Eurypterids found are approximately ten centimeters long or five inches, records show the largest Eurypterids were more than two meters or six feet in length.  The genus Pterygotus was said to be the largest form of Eurypterids being approximately six to eight feet long.  Below, is a picture of the largest Pterygotus found in North America and maybe the world.  It was found near Spinnerville, New York.

Photo Credited to Samuel J. Ciurca, Jr.


Different Members of Eurypterids

There are more than three hundred species of Eurypterids known today which have been classified in more than sixty genera and approximately twenty different families.  Some species of Eurypterids include:


Mixopterus are said to be one of the largest family members of the Eurypterids.  Some of the largest sizes are up to two meters in length.  It is said that the long tail curls over the back of the animal to pierce its prey such as fish.  Mixopterus were capable of crawling out of water and were able to walk on land because of the two sets of legs near its head.  These forms of Eurypterids are the most common to look like our current day scorpions, and perhaps the ancestor to all scorpions.  Mixopterus are the spiny types of Eurypterids.  Very few specimens of the Mixopterus have been found.


Lepidoderma mazonense

Lepidoderma mazonense were members of the Eurypterids and were found in the oceans of the Pennsylvanian period.  They were a part of the Mazon Creek collections which has preserved remains of various animals.  The Mazon Creek collection can be found in the Illinois State Museum. 

Photo Credited to


Eurypterus Remipes

During the late Silurian Age, these Eurypterids lived on the bottom of the brackish sea in New York.  In 1984, Eurypterus Remipes was selected as the state fossil for New York by Governor Cuomo.  Eurypterus Remipes was the first Eurypterid ever found and was named after DeKay in 1818 only after DeKay realized the arthropod nature of the fossil.  Another name for this Eurypterid is known as Eurypterus remipes DeKay.

Photo Credited to R. Weller/Cochise Geology and the Exhibit Museum of Natural History


Eurypterus lacustris

Eurypterus lacustris are the younger form of Eurypterus remipes.  Even though these Eurypterids do not include past species because of evolution, Eurypterus lacustris have commonly been misidentified.  These misidentifications happen because of preservation features.  Eurypterus lacustris are a common form of Eurypterids.  Many are being found and collected to this day.

Photo Credited to R. Weller/Cochise College



Hunting for Eurypterids

Some of the most common places to find Eurypterids are in the Niagara Peninsula of Ontario, Canada.  A great variety of Eurypterids have been exposed by quarries, ledges, and railroad cuts.  Eurypterids have also been found near Buffalo, New York in the Bertie limestone formation.   In the Bertie limestone formation is where the Pterygotus genus was found which is said to be the largest arthropod ever. Other places Eurypterids have been found in the United States include Pennsylvania and Maryland.  Eurypterids have been found in England and Norway as well.


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