The eurypterids were an extremely successful group of arthropods. These animals are often referred to as “sea scorpions”. However, their taxonomic relationship to extant scorpions remains unclear and they were not confined to marine environments. These invertebrates are members of the Subphylum Chelicerata (claw horns) and are members of the Eurypterida Order. Approximately 250 different species have been named. One of the fascinating aspects of eurypterids is understanding how they breathed.

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How Did Eurypterids Breathe?

Most palaeontologists believe that eurypterids evolved in marine environments. However, fossils have been found in association with estuarine and freshwater environments. Carboniferous trackways suggest that some eurypterids may have been amphibious and able to spend some time on land.

No eurypterid is thought to have been wholly or primarily terrestrial. There is some remarkable fossil evidence to suggest that they did evolve air-breathing organs. When discussing respiration, it is thought that the primary organs of aquatic respiration are likely to have been retained at least in part.

The extant horseshoe crab (Limulus) may provide an analogue. These animals inhabit marine environments although they do venture into shallow water to mate and onto the shore to lay eggs. Horseshoe crabs have five pairs of gills located on their abdomen (Opisthosoma). Each pair of gills consists of flap-like structures covering a series of membranes that resemble the pages from a book. These are called lamellae, but their morphology has given rise to the popular term “book lungs”. Oxygen is absorbed via gaseous exchange on the surface area of the lamellae.

Picture credit: Everything Dinosaur

The image (above) shows a ventral view of the popular CollectA horseshoe crab model.

To view the range of CollectA models in stock: CollectA Prehistoric Animal Models and Figures.

Four Pairs of Book Gills or Five?

Debate still continues as to whether eurypterids had four pairs of book gills or five. A specimen of Onychopterella augusti from Late Ordovician strata located in South Africa preserved four pairs of vertical book gills. The book gills were located in the third to sixth segments of the abdomen. It has been suggested that all eurypterids had four pairs of book gills rather than the five pairs seen in xiphosurans such as horseshoe crabs.

Terrestrial scorpions also have four pairs of vertically orientated book lungs, located in the third to sixth body segments. It is not known whether this is case of synapomorphy (shared characteristic different from that of their latest common ancestor). This anatomical similarity could indicate that scorpions (Order Scorpiones) are more closely related to eurypterids than previously thought. The phylogenetic relationship between eurypterids and other members of the Arthropoda remains controversial.

Pancaked Sea Scorpion Fossils

Unfortunately, most fossil specimens of eurypterids are squashed flat and may only represent exuviae (shed exoskeletons after moulting [ecdysis]).

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Eurypterids (Eurypterida) are often referred to as sea scorpions. Like scorpions these extinct invertebrates are members of the Arthropoda Phylum. They are distantly related to extant scorpions and spiders. It is thought that the first eurypterids evolved during the Ordovician. They thrived in the Silurian and Devonian. Giant forms evolved, animals like Jaekeklopterus, Acutiramus and Pterygotus. However, the number of taxa was severely depleted during the end-Devonian extinction event and although they survived for at least another 100 million years or so, during the Carboniferous and Permian they only made up a very small percentage of the taxa described from fossil deposits.

Picture credit: Everything Dinosaur

The picture (above) shows two Pterygotus anglicus fossil specimens on display at the London Natural History Museum. These Early Devonian fossils come from Arbroath (Scotland).

The Shape of the Telson

Note the broad, flattened, blade-like final segment of the animal. This is the telson and in the Pterygotioidea lineage (as well as in some other Superfamilies), the telson evolved into an organ to help with propulsion and steering. In other eurypterids, the telson is shaped very differently. For example, in the sea scorpion fossil (below), the telson is long and pointed.

The Giant Claws (Chelicerae) Seen in Some Sea Scorpion Fossils

The segmented body of eurypterids consisted of the frontal prosoma (head) and the posterior opisthosoma (abdomen). The prosoma contained the mouth and six pairs of appendages which are usually referred to as appendage pairs I to VI using Roman numerals. The segments that make up the opisthosoma are usually numbered using Arabic numerals 1, 2, 3 etc. The opisthosoma comprised twelve segments in total plus the telson.

The first pair of appendages, the only pair located in front of the mouth opening, is called the chelicerae (pronounced kel-iss-ser-ray). This pair of appendages evolved into a myriad of forms in the Chelicerata (pronounced kel-iss-ser-rat-ah), the Subphylum containing the eurypterids, spiders, mites, scorpions and horseshoe crabs. This pair of appendages form the fangs seen in spiders and form the feeding limbs of horseshoe crabs.

Picture credit: Everything Dinosaur

Powerful Pincers Adapted for Grasping Prey

Some of these appendages, such as the chelicerae of giant pterygotids evolved into powerful pincers armed with strong claws analogous to those seen in crabs and lobsters. These chelicerae seem to be adapted for grasping and subduing prey. This suggests that many eurypterids were predatory.

Note

A single appendage is referred to as a chelicera (pronounced kel-iss-ser-rah). Whereas a pair or more are referred to as chelicerae (kel-iss-ser-ray).

A spokesperson from Everything Dinosaur commented that these arthropods were remarkable animals.

“Some 250 different taxa have been described and some of these sea scorpions show adaptations that indicate they may have been partially terrestrial. Venturing out onto land is supported by trace fossils potentially preserving tracks of eurypterids walking across mud close to bodies of water.”

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