Rarely Seen Bear Gulch Horseshoe Crab Paleolimulus longispinus

Part and Counterpart from the U.S. Bear Gulch Limestone

Paleolimulus longispinus

Subphylum Chelicerata, Class Xiphosura, Family Paleolimulidae

Geological Time: Mississippian (~320 m.y.a.)

Size (25.4 mm = 1 inch): Fossil is 90 mm long (including 40 mm telson) by 52 mm wide on a 100 mm by 155 mm and 200 mm by 180 mm matrix pair

Fossil Site: Heath Shale Formation, Bear Gulch Limestone, Fergus County, Montana

Fossil Code: BGF704

Price: Sold

Bear Gulch Horseshoe Crab Paleolimulus longispinusThe Bear Gulch Limestone is a deposit of some 70 square km in extent and 30 m in depth that has been a source of one of the most diverse assemblages of fossil fish with some 110 species having been described over the past 30 years. Most were new to science, and provided a unique view of the marine environment of Mississippian times. Fine preservation of both fish and invertebrates is a hallmark of these deposits, presumably due to an anoxic depositional environment. This fine specimen is an example of the horseshoe crab Paleolimulus longispinus. The Xiphosurida (horseshoe crabs) are related to the extinct Eurypterids, and more distantly to spiders and scorpions. They trace their ancestry back to the Cambrian, and proceed into the modern day as the genus Limulus which is termed a “living fossil”. The genus Paleolimulus is also known in the US from the carboniferous deposits of Illinois and Kansas. The species longispinus is known from some 15 odd examples as of the publication of the referenced work below. This is a fine articulated counterpart example with a complete telson (tailspine). Several limbs are also visible through the prosoma, making for a fine example of an interesting ancestor of the common horseshoe crab seen along shorelines today.

Also see: Bear Gulch Fossils


  • LETHAIA, 33, 2000, pp 129-141
  • Paleontology, Vol 50, Part 4, 2007, pp 1013-1019

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