English: Palmichnium kosinskiorum Briggs & Rolfe, 1983 - sea scorpion tracks in sandstone from the Pennsylvanian of Pennsylvania, USA. (CM 34388, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)

The eurypterids, or sea scorpions, are an extinct group of chelicerate arthropods. They have an elongated, scorpion-like body that could reach enormous sizes (2.5 to 3 meters!), with a nonmineralizing exoskeleton composed of chitinous material. They are generally found in shallow to very shallow water marine and marginal marine facies.

Seen here is the holotype of Palmichnium kosinskiorum, the largest eurypterid trackway known.

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From museum exhibit signage:

A Giant Sea Scorpion Trackway from western Pennsylvania

Palmichnium kosinskiorum CM 34388 (Holotype) Briggs & Rolfe, 1983

Eurypterids, also known as sea scorpions, were one of the fearsome swimming predators of the Paleozoic seas (545-250 million years ago). This fossil was discovered by a former museum employee, James Kosinksi in 1948 along the Clarion River, Elk County, Pennsylvania. In 1983, English paleontologists Derek E.G. Briggs and W.D. Ian Rolfe described and named this specimen Palmichnium kosinskiorum.

This eurypterid trackway is the largest known in the world. The 350 million-year-old fossil impressions record the footprints of an animal estimated to be more than seven and a half feet long. The shallow groove in the center of the trackway was caused by the animal's dragging tail, a possible indication of its amphibious movement between water and land.

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Classification: Animalia, Arthropoda, Chelicerata, Merostomata, Xiphosura, Eurypterida, Eurypteridae

Stratigraphy: Pottsville Group, Pennsylvanian

Locality: outcrop along Spring Creek-Clarion River, Elk County, northwest-central Pennsylvania, USA

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See info. at: en.wikipedia.org/wiki/Eurypterid and

en.wikipedia.org/wiki/Palmichnium