Pentecopterus: The oldest Sea Scorpion yet

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A pair of Pentecopterus exploring together
Image Credit: Patrick Lynch,

In the heart of London a famous food critic, sporting a sharp suit and meticulously polished shoes, sits down at a table. He’s about to review a wildly popular new restaurant, where the signature dish is “The Sea-Food Surprise“. After a disappointing starter that he could only describe as “tasting of old, dried rock” the Surprise arrives. To the critics astonishment the dish is absolutely splendid! Cooked to perfection in a piquant sauce and with a texture and taste reminiscent of the best lobster ever! The next day he returns to the restaurant, demanding to be be shown the “lobster” he had had last night. “I’ll get the chef” the waiter replies. Eventually the chef emerges from the kitchen and asks the food critic to follow him. The critic passes a tank with a pair of North Atlantic Lobsters silently observing him with their beady stalked eyes. However the chef continues on past them. “If they aren’t the lobster then what is?” The critic wonders to himself. He is taken through the kitchen and down a set of stairs. “Surely you don’t keep lobster in the wine cellar?!” the critic joked. “You’re right, we don’t” the chef replies, opening a hatch in the wine cellar that leads to second, hidden cellar. In this cellar is a giant pool, and in this pool is a group of the “lobster” he had eaten the night They are huge, as long as a man and possessed a flat tail, smooth semi-circular head, spines sticking out all over the place and large claws which they were using to tear into the fish that were being used to feed them. It truly was unlike any seafood he had ever seen. “What in the world is that?!” The critic exclaimed! “The Sea-Food Surprise” The chef replied. “Pentecopterus, a Sea-Scorpion”.

An illustration of a Pentecopterus swimming between vegetation.
Image Credit: Apokryltaros,

While a story like this could only be from the realm of science fiction, Pentecopterus decorahensis (Greek for “Decorah’s Penteconter Wing”) was a very real animal. It belonged to an order known as the Eurypterids, although they’re more commonly known as “Sea-Scorpions”. While it is another example of a cool paleontological nickname, “Sea Scorpion” is misleading. While most (but not all) members of this order did live in the sea they were not scorpions, but instead were part of a completely separate group. That being said modern scorpions and other arachnids such as spiders, along with horseshoe crabs, are the Sea Scorpions closest living relatives. This is unsurprising as Sea Scorpions looked like what would happen if a scorpion and a crab decided that they wanted to settle down and start a family! Sea Scorpions were a highly successful order of animals during the Palaeozoic era, having a timespan from 467 Million Years ago (during the Mid-Ordovician period) to 252 Million Years ago, succumbing to extinction during the “great dying” at the end of the Permian period. This means that Sea Scorpions were alive for longer than the dinosaurs!

What makes Pentecopterus special among this already interesting group is that it is the oldest species of Sea Scorpion discovered so far. Their fossils are estimated to be about 467 Million Years Old. These fossils were first formally described in 2015 by a team led by James C Lamsdell, who named the animal after the penteconter, an Ancient Greek galley ship that, like the animal, possessed a sleek shape. The original fossils were discovered in 2010 in the Winneshiek Lagerstätte near Decorah in Iowa, USA (Hence its full name). Winneshiek is an interesting place as it is located within the remnants of an old meteorite crater! When Pentecopterus was alive this 5 km wide meteorite crater was filled with sea water, creating a unique shallow marine environment. The relatively still waters and lack of oxygen on the crater floor resulted in the perfect conditions for fossilisation, and the relatively low salinity of the water meant that marine animals typical of the Ordovician oceans couldn’t live here, creating a unique ecosystem for its time. As a result of these conditions multiple Pentecopterus individuals (ranging from juveniles up to fully grown adults) were perfectly preserved, so perfectly in fact that the outline of the tiny sensory hairs can be seen.

As you can see in this comparison, a Pentecopterus was one large arthropod!
Image Credit: Slate Weasel,

So what exactly did Pentecopterus look like and how would it have behaved? Pentecopterus, like all arthropods (a group of invertebrates containing all insects and arachnids), would have had an exoskeleton divided into a series of distinct segments. At the front was a round, smooth and sleek head with two pairs of eyes; (one pair of compound-eyes at the front and a pair of smaller eyes at the top). Its body had 3 pairs of legs attached to it, that it used for walking on the sea floor (and maybe occasionally on land), and behind them a pair of broad paddles (that were uniquely shaped to this Sea Scorpion) used to help it swim and turn in the water. Finally at the back end there was a long, broad tail (known as a telson) that it used to power itself through the water. Pentecopterus also possessed a pair of vicious claws at the front of its body that it used to grab prey, (consisting of early jawless fish species and other arthropods). Pentecopterus belonged to a larger family of Sea Scorpions known as the Megalograptids. These are characterised by possessing many more spines on their legs compared to other Sea Scorpions, giving Pentecopterus a very spiny look. This would have been an unnervingly large animal to look at, growing as long as a human is tall (1.8 metres). However there were later Sea Scorpions that could grow even larger than this, with the largest being the 2.5 metre long Jaekelopterus!

Such a magnificent animal would have certainly been very eye catching and, if modern day crabs and lobsters are anything to go by, would potentially have made for a unique sea food delicacy, as the food critic in our story found out!

References/Extra Reading

Lamsdell et. al. 2015 paper describing Pentecopterus

Lamsdell, J.C., Briggs, D.E.G., Liu, H.P. et al. The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa. BMC Evol Biol 15, 169 (2015).

Briggs et. al. 2018 paper reviewing the Winneshiek biota, home to Pentecopterus and other animals

Derek E.G. Briggs, Huaibao P. Liu, Robert M. McKay, Brian J. Witzke; The Winneshiek biota: exceptionally well-preserved fossils in a Middle Ordovician impact crater. Journal of the Geological Society ; 175 (6): 865–874. doi:

A blog written by Palaeontologist Dave Marshall, and published in BMC Series blog, about the discovery of Pentecopterus and its significance

Marshall, Dave, “The oldest and the scariest sea scorpion: a giant discovery”, BMC Series blog, Sep. 1, 2015,

Another blog, written by Jim Shelton and published in YaleNews, on the discovery of Pentecopterus

Shelton, Jim, “Meet the newest ‘sea scorpion’: Pentecopterus, a predator from prehistoric seas”, YaleNews, Aug. 31, 2015,