July 28, 2010 : Sea Scorpion (Extinct)

Sea Scorpions

Eurypterids (sea scorpions) are an extinct group of arthropods related to arachnids which include the largest known arthropods that ever lived. They are members of the extinct class Eurypterida (Chelicerata). The word Eurypterid comes from the Greek word eury meaning "broad" or "wide" and the Greek word pteron meaning "wing". They predate the earliest fishes. The largest, such as Jaekelopterus, reached 2.5 metres (8 ft 2 in) or more in length, but most species were less than 20 centimetres (8 in). They were formidable predators that thrived in warm shallow water in the Ordovician to Permian from 460 to 248 million years ago. Although informally called 'sea scorpions', only the earliest ones were marine (later ones lived in brackish or freshwater), and they were not true scorpions. According to theory, the move from the sea to fresh water probably occurred by the Pennsylvanian period. They went extinct during the Permian-Triassic extinction event 251 million years ago, and their fossils have a near global distribution. 

About two dozen families of eurypterids are known. Eurypterus is perhaps the most well-known genus of eurypterid, of which 18 fossil species are known. The genus Eurypterus was created in 1825 by James Ellsworth DeKay, a zoologist. He recognized the arthropod nature of the first ever described eurypterid specimen found by Dr. S. L. Mitchell. In 1984, Eurypterus remipes was named the State Fossil of New York. 

Eurypterids have traditionally been regarded as close relatives of Horseshoe Crabs; together forming a group called Merostomata. Subsequent studies placed eurypterids closer to the arachnids in a group called Metastomata. There has also been a prevailing idea that eurypterids are closely related to scorpions, which they obviously resemble. This hypothesis is reflected in the common name 'sea scorpion'. More recently it has been recognised that a little-known, extinct group called chasmataspids also share features with eurypterids and the two groups were sometimes confused with one another. The most recent summary of relationships between arachnids and their relatives recognised Eurypterida, Xiphosura and Arachnida as three major groups, but was not able to resolve details between them.