Episode 156 – Eurypterids (Sea Scorpions)

Listen to Episode 156 on PodBean, YouTube, Spotify, or wherever you listen to your favorite podcasts!

During the Paleozoic, the so-called “sea scorpions” were among the most successful carnivores in the world’s waters. This episode, we explore the ancient history and claims-to-fame of Eurypterids.

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“Sea Scorpions”

Eurypterids are often colloquially called “sea scorpions,” but this catchy name is somewhat misleading. They are not true scorpions, although they are close cousins of arachnids (including true scorpions as well as spiders) and horseshoe crabs; and while they are aquatic animals, they lived in ancient freshwater and brackish environments more often than the ocean.

Eurypterid anatomy includes a “head” section with eyes, mouthparts, and several pairs of appendages; and a segmented “body” section with gills, genitals, and a tail-like rear end. Across hundreds of species, this body plan came in a variety of different forms for varying lifestyles.
Top row, left to right: Brachyopterus, Megarachne, Eurypterus, Mixopterus
Bottom row, left to right: Hibbertopterus, Stylonurus, Stoermeropterus, Pterygotus
Art by Nobu Tamura, CC BY 3.0 (Source 1, Source 2)

Eurypterids in their many forms were highly successful. Their fossils are found all over the world between the Middle Ordovician and Late Permian (~470-250 million years ago), though they are most common in northern continents and most diverse during the Silurian and Devonian Periods.

By far the most commonly found and well-studied eurypterid is Eurypterus, a group of swimming species known from the Silurian Period. Like most well-known eurypterids, these are known mainly from exceptional fossil sites.
Image of Eurypterus remipes by James St. John, CC BY 2.0
Most eurypterids were relatively small, typically under 20cm (8in). The tiniest could be a mere 2cm (1in).
But multiple lineages grew to preposterous sizes of more than 2m (6 ft). These are the largest arrthropods that ever lived.
Image by Slate Weasel

Eurypterids were hunters and scavengers with varying body shapes linked to different lifestyles. Many had flattened paddle-like legs that made them powerful swimming predators while others’ limbs were built for scuttling along the sediment. Many species had spiny comb-like appendages that could sweep small prey into their mouths while others had crab-like pincers that could be used to crush or slice prey before feeding.

Some eurypterids might also have spent time on dry land. Certain features of their respiratory systems are similar to modern arthropods which can breathe in both air and water, and certain species’ had powerful legs that could support their bodies on land, not to mention at least one record of eurypterid footprints that document a terrestrial sojourn.

The large Ordovician eurypterid Pentecopterus.
Image by Patrick Lynch

The early evolution of eurypterids is not well understood thanks to a lack of early fossils, but by the Silurian and Devonian Periods they were some of the most diverse and important predators in the world’s waters. A major drop in diversity during the Devonian might have been related to competition with the then-increasing diversity of fish, but multiple groups of freshwater eurypterids lingered in the Carboniferous and Permian Periods even after their saltwater cousins disappeared. The very last eurypterids seem to have made it all the way to the end of the Permian before being among the many casualties of the Permian Extinction.

Learn More

The oldest known eurypterid
The youngest known eurypterid

Evidence of air-breathing in a Carboniferous eurypterid

Giant eurypterid trackways at the Carnegie Museum

Listen to our Spotlight episode with Dave Marshall, where he shares some of his own eurypterid research!

Distribution and dispersal history of Eurypterida, 2007 (technical)
Autoecology of Silurian eurypterids, 1984 (technical)

And by the way, the Cambrian organisms we talked about for this episode’s Patron Question are Vetulicolians.


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2 thoughts on “Episode 156 – Eurypterids (Sea Scorpions)

  1. Alexander Baggerød January 12, 2023 / 11:17 am

    Wonderful episode, as always, love you guys! 🙂 But not getting anywhere trying to find out what you talked about at the end, sometimes it’s hard to hear properly? I’m hearing “vetulacouleans”, but turns out that’s not a word? Keep ut the good work! 🙂


    • commondescentpc January 13, 2023 / 2:14 pm

      Good point! We’ve added the word and a link to the post above. They’re Vetulicolians!


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