Eurypterids were a dominant group of aquatic chelicerates, related to scorpions, spiders, and horseshoe crabs.
They’re commonly called “sea scorpions” due to a long-recognised superficial similarity to scorpions, as can be seen above. This has led to several studies stating that eurypterids are scorpions (Versluys & Demoll, 1920), that scorpions are eurypterids (Sharov, 1966), or that scorpions and eurypterids share a unique last common ancestor (Dunlop & Webster, 1999). All these conclusions are pretty unlikely: eurypterids seem to merely be more basal chelicerates than scorpions, not least because the eurypterids that resemble scorpions are those that are highly derived, not the basal ones.
On the phylogeny of Recent chelicerates above, they would come out as more closely related to the Arachnida than the Xiphosura (Lamsdell, 2013), a conclusion that was reached by Weygoldt & Paulus (1979a, b) and generally agreed on since then. However, the precise relationships in that area of the tree are still unclear (Shultz, 2007). Traditionally, the Xiphosura (horseshoe crabs) were put together with the Eurypterida in a clade of aquatic chelicerates called the Merostomata, contrasted to the terrestrial Arachnida, but this more ecological distinction is superseded by the Weygoldt & Paulus phylogenetic arrangement, since eurypterids do share more traits with the terrestrial Arachnida, even down to their mating style using spermatophores (Kamenz et al., 2011). An additional problem is posed by the highly-similar Chasmataspidida, a group of also-extinct aquatic chelicerates that share similarities with both xiphosurans and eurypterids; Shultz (2007) compromised by putting the chasmataspidids within a more loose eurypterid clade.
The common name is also misleading because they also weren’t necessarily in the sea: later forms moved towards freshwater habitats, such as lakes and rivers (Lamsdell & Braddy, 2010). Even more exciting is the possibility of amphibiousness allowed by their respiratory system (Braddy, 2001) which, in addition to their standard aquatic gills, may have had pseudotracheae allowing air-breathing (Størmer, 1976). The presence of enigmatic fossil trackways is a further source of evidence. Even so, they don’t appear to have succeeded in colonising land, unlike the majority of their cheliceratan relatives. It’s hard to model a good way of eurypterid walking: they would have had to lurch forward using their paddle leg for propulsion, hardly an efficient mode of transport.
They existed for at least 210 million years. The earliest eurypterids known from the middle of the Ordovician, ~460 Ma (Størmer, 1951), and they went extinct along with most of marine life at the end-Permian extinction, with the latest eurypterid known from ~251 Ma, Russia (Ponomarenko, 1985). The best fossil record we have of them comes from the Silurian and Devonian localities that preserve more than just hard parts – eurypterids had an unmineralised organic cuticle made of chitin, making regular fossilisation a difficult affair (Gupta et al., 2007), although the thickness of their cuticle gives them better chances at fossilisation than most arthropods (Kluessendorf, 1994).
This is why we know of only around 244 species (Dunlop et al., 2010), but expect more. Most eurypterid fossils are found in Europe and the USA, and there is a current debate as to whether this represents a true biogeographic distribution (Tetlie, 2007), or whether this just a sampling bias due to less exploration of other continents. I personally fall in the latter camp due to some findings of poorly dispersing early eurypterids in the Soom Shale of South Africa (Braddy et al., 1995). South Africa was part of Gondwana, which was pretty far away from Laurentia (Europe, USA) at the time, so that’s a pretty strange occurrence that, in my opinion, provides ample justification for the view that we need to dig around in other places.
The best place to find eurypterid fossils is New York State – that’s why a eurypterid species is New York’s designated state fossil. The particular species that has that honour is Eurypterus remipes, pictured above, notable for also being the very first eurypterid to be recognised, by DeKay (1825). Previously, those fossils were interpreted as catfish (Mitchill, 1818). “Eurypterida” as a group were first formally diagnosed by Burmeister (1843).
Eurypterids are fairly easily recognisable when relatively complete and undistorted fossils are found. They had a regular segmented body with a characteristically flat, keeled telson (posterior) that tapered to a spine or sometimes a paddle. Many species also had a grossly enlarged first limb pair, probably used as a grasping claw when hunting. Their head had a pair of compound eyes.
Workers simplifyingly split eurypterids into two groups, unreflective of the true phylogeny: the Eurypterina, and the basal and paraphyletic Stylonurina. The easiest way to distinguish them is to look at their last “leg”, as can be seen in the more precise diagram above. The Stylonurina (top cluster) have a long, thin leg, while Eurypterina (bottom cluster) have modified it to a true swimming paddle. For those of you teaching phylogenetics and tree-thinking, eurypterids are a great example to use to debunk the common idea that basal animals are “primitive” and thus “less successful”: the basal stylonurines are the eurypterids that survived the longest, not the eurypterines who couldn’t compete with the rising dominance of the fishes.
All eurypterids were predators, and quite large ones at that. With sizes of 1+m common across all eurypterid lineages, they were pretty high up on the food chain, eating animals such as trilobites and even the contemporaneous placoderms. In fact, it was suggested by Romer (1933) that such armoured fishes evolved as a response to eurypterid predation pressure; eurypterids then attained larger sizes to counteract the enhanced defences, as in a classic coevolutionary “arms race” (Lamsdell & Braddy, 2010) that began in the Ordovician Radiation. The largest ever arthropod (as of time of writing) is the pterygotid Jaekelopterus rhenaniae from the Devonian of Germany, a eurypterid that was almost 2.5 meters long, with a staggering 46 cm claw (Braddy et al., 2008).
Also notable is the “giant spider” Megarachne popularised by the otherwise excellent BBC documentary, Walking With Monsters. At the time of production, the fossil was identified as a giant spider, but it turned out to be nothing more than a misidentified eurypterid.
Eurypterids were not invincible though: the oldest known fossil barnacle, Cyprilepas holmi from the Silurian of Estonia, is found parasitising eurypterids (Wills, 1962).
If you want to know more about eurypterids, a somewhat older but still very detailed review can be found in Tetlie (2007).
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