Eurypterid Fossils

This fossil is a nearly complete specimen of Acutiramus, a pterygotid eurypterid, with its claws folded back towards the body when the animal molted. Note the large eyes and the division of the body into segments, named tergites.

Acutiramus cummingsi
YPM catalog no. 218956
Ridgemont Quarry, Late Silurian deposit near Niagara Falls, Fort Erie, Ontario, Canada


Eurypterids are an extinct group of chelicerates, arthropods whose modern relatives include the horseshoe crabs, scorpions, spiders, mites and ticks. Most common in rocks of Silurian age (444 to 416 million years ago), they ranged from the Ordovician Period (approximately 480 Mya) until the end of the Permian (approximately 250 Mya), when they died out.

They are aptly described by their common name, sea scorpions, because they look much like swimming versions of these living land animals. Yet, apart from living in water, they differ from scorpions in several important respects, most striking being their tendency to grow to enormous sizes!

This specimen is a single tergite (probably the second) of a very large pterygotid. By extrapolating from this single tergite, we estimate that this animal, including its extended claws, was nearly 10 feet (3 meters) in length, larger than an adult human.

Acutiramus macropthalmus
YPM catalog no. 208194
Passage Gulf, New York, USA


The pterygotid eurypterids were the largest arthropods ever to exist, reaching total lengths of more than 11 feet (3.3 meters). They lived all over the world from 430 to 390 million years ago, but the largest of these monsters have been found in the United States in New York State, and in Germany and the Czech Republic. With their formidable claws, unique among the eurypterids, and binocular vision that provided depth perception, the pterygotids would attack and slice into prey, such as primitive fish or the ancestors of squids.

Fossil pterygotids are very common in 425- to 420-million-year-old rocks, but these animals were quite rare during the last 30 million years of their existence. They were also the only eurypterids that were able to swim across open oceans. 

Figure courtesy of Simon Powell.

The Ciurca Eurypterid Collection


Sam Ciurca (at left) and Erik Tetlie hold important specimens of Eurypterus remipes lacustris (YPM.207290) from the Late Silurian Williamsville Formation at Bennett Quarry, Buffalo, New York, USA.


The Yale Peabody Museum recently acquired the largest and most diverse collection of eurypterids ever assembled. Collected over nearly 50 years by Samuel J. Ciurca, Jr. of Rochester, New York, USA, the Ciurca Collection comprises nearly 15,000 specimens. It includes large slabs from over 100 collecting localities in New York, Pennsylvania, Indiana, Ohio, and Ontario, over 60,000 pounds (more than 27,200 kilograms) of rock in all.

Sam Ciurca’s meticulous documentation of the collection localities will enable a greater understanding of the geologic history of New York. The collection’s many examples of associated fossils, including other arthropods, brachiopods, bivalves and plants, as well as sedimentary structures, will also provide important clues to the environment and lifestyle of eurypterids. The availability of the Ciurca Collection in the Peabody’s Division of Invertebrate Paleontology will allow scientists to investigate many aspects of the paleobiology of eurypterids, including changes in their biodiversity, ecology and habitat preferences through time.

Dr. O. Erik Tetlie is a leading expert on the diversity and relationships of eurypterids. Funded by a 3-year postdoctoral fellowship from the Norwegian Research Council, he is working alongside Curator of Invertebrate Paleontology Derek Briggs. The Ciurca Collection has already yielded many new genera and species, as well as beautifully preserved examples of several eurypterids previously only known from partial specimens.