Eurypterids, arachnids, and the arthropod invasion of the land

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Although there is terrific diversity in Crustaceamorpha, ancient crustaceans occupied the same general ecological role as do moderns ones. They are therefore boring. Insects are well known from other sources. Therefore our survey of Arthropoda focuses on two speciose and ecologically significant ancient groups that generally lack modern equivalents, Trilobita and its sister taxon, Chelicerata. Today, Chelicerates:

Major arachnomorph clades
(Info in gray for interest only. Not on exam.)

Trilobites and chelicerates are united in the monophyletic group Arachnomorpha: Synapomorphies are subtle, including the loss of the first antenna (recall that crustaceans have two pairs, trilobites only one and chelicerates none). Indeed, a comparison of very early trilobites and chelicerates shows their similarity to their last common ancestor.

Olenellus a trilobite

Cyamocephalus a chelicerate
From Palaeos

Note, for example the post-anal telson (spine), a feature soon lost in trilobites but retained and elaborated in chelicerates.

Chelicerata. (Cambrian - Recent) Marine and terrestrial. Major synapomorphies:
  • Segments consolidated into:
    • Prosoma with compound or simple eyes, six pairs of uniramous walking limbs.
    • Opisthosoma with numerous breathing appendages or their derivatives.
  • Antennae lost
  • Chelicerae - first pair of walking limbs specialized for feeding.
  • Pedipalps - second pair of walking limbs, alternately specialized for walking, copulation, or prey capture.

From palaeos

As cladists have gone to work on the Burgess Shale fauna, it has emerged that many of the formerly enigmatic arthropods are either proper chelicerates of very close to the chelicerate common ancestor, including:

Within proper chelicerates there are two major monophyletic groups:

Xiphosura. (Silurian - Recent) Marine. Horseshoe crabs.

The living xiphosuran Limulus polyphemus nicely demonstrates the modification of the opisthosomal appendages as elements in Book gills.


Arachnida. (Silurian - Recent) Marine and terrestrial. Major groups include:
  • Scorpiones (Silurian - Recent) Originally marine, but well established on land by the Devonian.
  • Aranae (spiders) (Pennsylvanian - Recent)

From Merck's Nature Photos

But today's focus is on.....

"Eurypterida". (Ordovician - Permian) Marine to fresh water. The name is in quotes to signify that the group may well be paraphyletic. (I.e. some eurypterids are closer to arachnids than others.)

Whatever.... Eurypterids are at least an ecologically distinctive and significant grade of chelicerate evolution.

  • Heyday of diversity and size is in the Silurian and Devonian
  • Predatory: Either chelicerae, pedipalps, or first walking legs could be developed as powerful claws
  • Anteriormost six segments of opisthosoma typically very broad: form mesosoma - Derived book gills enclosed in gill chambers superficially resembling abdominal plates. These chambers have, on their roof, vascularized gill tracts that apparantly functioned to retain water when the creature was on land.
  • Opisthosoma flexible dorsoventrally, with plates of cuticle above and below
  • Sixth appendage pair usually modified as paddle.

Swimming: Eurypterids could clearly walk along the bottom, however almost all had sixth appendages modified as paddles and could surely swim to some extent. In some, the small size of the paddles or presence of other distinctly non-streamlined structures argue against routine powerful swimming. Others, such as Baltoeurypterus (pictured below) were streamlined with large paddles. Swimming was appendicular but beyond that what can we say?

The issue is complicated by ontogenetic increase in lift-drag ratio due to allometry of paddles. Juveniles had high drag and probably used rowing. Adults had relatively lower drag and higher lift, consistent with underwater flight (unclear whether it was asymmetrical or symmetrical). Specimens of juveniles with exceptional three-dimensional preservation have shown that the joints were capable of achieving rowing motions. Based on joint reconstructions this tended to be used in rowing rather than subaqueous flight. Extimated top rowing speed is about 2.5 X body length per second however a large mature Baltoeurypterus using subaqueous flight might have managed 3 - 4 m/sec. - similar to sea turtles.

Walking: Depositional settings of eurypterid fossils indicate that they ranged from fresh to marine water, that individual animals seemed to be tolerant of a broad range of salinity, and that different groups tended to have preferences for a particular environment. Some have limbs that seem robust enough to allow excursions onto land.

We have trackways that appear to have been made by brackish-fresh water eurypterids, indicating that, like crabs, they could emerge from the water.


A Rogue's Gallery of Eurypterid Diversity:

Baltoeurypteridae. (Ordovician - Devonian)
  • Estuarine and lagoonal
  • Small chelicerae
  • Limbs 2 - 5 small and undifferentiated
  • Large paddle
  • Small - average adult length ~ 20 cm.


Hughmilleriidae. (Ordovician - Permian)
  • Brackish - fresh water
  • Small-medium chelicerae
  • Limbs 2 - 5 small and undifferentiated
  • Small paddle
  • Small - average adult length ~ 10 cm.

  • Brackish - fresh water
  • Small chelicerae
  • Limbs 3 - 6 elongate and robust
  • Limb six is not developed as a paddle

Pterygotidae. (Ordovician - Devonian)
  • Marine
  • Large chelicerae adapted for grasping prey
  • Limbs 2 - 5 unspecialized
  • Limb six is a large paddle
  • Telson is broad and plate-like
  • Very large size, exceeding two meters. These are the largest arthropods on record.

Mixopteridae. (Ordovician - Pennsylvanian)
  • Marine
  • Small chelicerae
  • Pedipalps and third limbs specialized for prey capture.
  • Limb six is a large paddle
  • Telson is hollow, possibly for transmitting venom.
  • Large size approaching one meter.

Carcinosomatidae. (Ordovician - Devonian)
  • Marine
  • Small chelicerae
  • Pedipalps and third limbs specialized for prey capture.
  • Limb six is a large paddle
  • Telson is hollow, possibly for transmitting venom.
  • Medium ~ 0.5 m.

The Chelicerate Invasion of the Land

We have seen that some eurypterids could walk on land. Starting in the Silurian, we see members an another group that we would call small eurypterids if they had gone extinct in the Devonian. Instead, we recognize them as the first scorpions.

From Palaeos

Their features:

Like the walking eurypterids, scorpions made frequent excursions onto land, and, being mostly small (the 1.5 m. Silurian Brontoscorpio notwithstanding) and having relatively large legs, they were better adapted to it. Note that the Silurian was when vascular plants began invading the land in large numbers. Thus, scorpions were taking a place in the terrestrial ecosystem almost as soon as it existed. By the Mississippian, we see the remains of scorpions that were fully equipped to breathe and feed on land, along with other terrestrial arachnids.

Problems with Arachnid monophyly: All of this rasies some creepy questions. Everyone knows that arachnids are the "land-chelicerates," but strictly speaking, Arachnida is diagnosed as monophyletic by only a handful of rather subtle morphological synapomorphies including:

These would be hard things to identify in eurypterids even if they were there. Indeed, some workers claim to have seen slit sensory organs in some. This, combined with the general similarity of scorpions and eurypterids inclines us to be suspicious of whether Arachnida is really monophyletic. Could scorpions and their relatives actually be kinds of eurypterid? Possibly. Alternately, virtually no one is confident that eurypterids are monophyletic. Perhaps some (E.G. the ones with the hollow telson "stingers") are actually nested inside Arachnida. The issue is unresolved.

Whatever the case, it seems clear that if scorpions hadn't emerged from the water, they would probably have been extinguished at the end of the Permian along with everyone else. What happened to the marine eurypterids? We can break their career down into this timeline:

Of course, maybe just as eurypterids are called 'sea-scorpions," maybe scorpions are "land-eurypterids." Stay tuned.