Paleo Profiles: Pterygotus

A Pterygotus in the National Museum of Scotland

It has been sometime since our last Paleo Profile so I thought we should look at an extinct animal closer to home. Throughout the planet's history the arthropods have been the most diverse type of animal including in their numbers the insects, arachnids, and crustaceans with all their diversity. One interesting order among the arthropods, now long extinct, were the eurypterids, or as they are better known as, sea scorpions. As you can probably tell eurypterids were named thanks to their similarity to scorpions although we now know that they are most closely related to horseshoe crabs; ironically modern scorpions are also more closely related to horseshoe crabs than they are eurypterids. Nevertheless, eurypterids and scorpions were so similar, likely thanks to convergent evolution, that it is not uncommon for them to be mistaken as one another (or their cousins); a giant spider called Mesothelae was depicted in the BBC documentary Walking with Monsters only for them to find out close to the end of production that palaeontologists had made a mistake, Mesothelae was actually a eurypterid called Megarachne. Making matters even more confusing not all sea scorpions lived in the sea, and there are many examples of freshwater eurypterids. Today, we'll be looking at one of the best known eurypterids: Pterygotus. Pterygotus was one of the most common and largest eurypterids being found everywhere. Currently, I attend the University of Edinburgh which has its main campus next to the National Museum of Scotland which contains various Pterygotus fossils - some of the photos on this post today are ones which I took in the museum.

Discovery and Fossils

It is no mistake to say that Pterygotus is a common fossil. We are lucky to have several full specimens of Pterygotus, as well as parts of their bodies including legs, tails, and chelicera (front appendages). They have also been found across the world in four different continents: Europe, Australia, North America and South America. In particular, New York and Scotland have produced some of the richest deposits of Pterygotus fossils. However, when Pterygotus was first discovered the remains were not well preserved so it was believed to be a fish - Pterygotus is Latin for 'Finned One' or 'Winged Animal' depending on the translation. The first Pterygotus, as often with fossils, were not discovered by palaeontologists but by workers in an English quarry. Swiss-American naturalist Louis Agassiz, whom we encountered when we discussed the giant shark megalodon, named the fossil Pterygotus problematicus in 1839. After finding more fossils from the Old Red Sandstone in Scotland Aggasiz realised that Pterygotus was really an arthropod so reclassified it in 1844 - P. problematicus was renamed P. anglicus in 1849 after the discovery of the Eurypterus. Initially both Eurypterus and Pterygotus were believed to be crustaceans, although as more and more fossils were discovered the realisation came that eurypterids were related to, but different from, crustaceans. In 1859 James Hall discovered a new species of Pterygotus from New York - this was important as it was the first Pterygotus to be discovered outside Scotland. Since then New York has become one of the most important places to find Pterygotus fossils. Palaeontologists have continued to find new species across the world from Ukraine to Bolivia to Czechia. 

Biology

A sea scorpion is the best way to describe the overall body shape of a Pterygotus, and most over eurypterids for that matter. Naturally there are several key differences - Pterygotus had more limbs. It had eight 'traditional' legs used for walking along the sea bed, two legs which whose ends were flattened for propulsion, and two very large chelea (claws). These claws were large, some exceeding 30 cm, around 11 inches, in length and were powerful. Pterygotus had a surprisingly small mouth so its claws were designed to crush and tear up prey into smaller pieces to devour.

A Pterygotus claw in the National Museum of Scotland

Like many arthropods Pterygotus was covered in a hard scale-like exoskeleton which they would likely shed as they grew. Tracks from eurypterids have been found in Pennsylvania indicating that eurypterids may have came onto land to shed their skin out of the reach of potential predators. However, as we'll later see this was not an option for Pterygotus. Pterygotus had compound eyes and a 2015 paper by a series of palaeontologists, including Victoria McCoy, used this to determine how they lived. Traditionally, eurypterids in the family pterygotidae were seen as being active hunters with McCoy and others wanting to challenge this interpretation. Their results found that different pterygotids had different hunting styles - Pterygotus had many compounds in its eyes and the way its eyes were angled gave it very good eyesight letting it be an active predator. To help catch their prey Pterygotus had a paddle-like tail. Many arthropods have a division on their posterior called a telson which can vary in size, shape, and purpose - they form the sting of scorpions and tail fans of lobsters as examples. The Pterygotus telson was used as both a rudder and a paddle (as well as the two front appendages) - they helped steer the eurypterid while also giving it a boost in speed. Pterygotus was adapted to a life underwater.

A sketch of the paddle by Henry Woodward

What makes Pterygotus stand out is its size. Arthropods could get very large when Pterygotus swam the seas and Pterygotus showed this. The smallest species, like P. kopaniensis, measured 50 cm (20 inches) in length to put into perspective the size of the largest Pterygotus. The largest grew to over 1.75 metres in length, or 5 foot 7 inches, rivalling the size of an alligator. Edinburgh's National Museum of Scotland has models of P. anglicus which are around the size of a labrador or retriever. Pterygotus was not the largest eurypterid around, Jaekelopterus averaged 2.5 metres, 8.2 feet, in length. However, Pterygotus was likely the top predator in most habitats it happened to inhabit.

When and Where

Pterygotus lived in a time long before the dinosaurs - it lived even before there was life on the land. When they appeared the first jawed fish and land plants had yet to appear. Eurypterids had arrived on the scene during the Ordovician 467 million years ago, (to put it into context one of the first land plants, Cooksonia, evolved around 433 million years ago), and Pterygotus arrived during the Late Silurian - around 428 million years ago. During the Late Silurian the Pterygotus lived in a very warm world - the mean CO2 concentration in the atmosphere was sixteen times that of the atmosphere when humans arrived, and the surface temperature was three degrees Celsius above modern levels. Sea levels were also higher so warm, shallow seas covered large tracts of the Earth - including Scotland and New York. The Pterygotus managed to survive into the Early Devonian - around 391 million years ago - which had similar warm temperatures. Many of the Scottish fossil sites containing Pterygotus fossils actually come from this time period. The temperature was very hot - there may have been no glaciers and the mean surface temperature was double that of the Silurian. Due to these warm temperatures and shallow seas the Pterygotus managed to spread across the world producing around 20 species. Scotland, Wales, New York, Ohio, Ukraine, Prague, Australia, and Bolivia are just some of the regions where they have been discovered, and in each region they adapted to their local environment.

Habitat and Neighbours

A model in the National Museum of Scotland of a Pterygotus in its habitat

With around 20 species the Pterygotus lived in a wide range of habitats as a result. Due to their large size and adaptation to a purely marine life they could afford to not live in shallow water - they could safely mate, breed, and shed their exoskeleton in the ocean. An Australian species has been found to have even lived in particularly deep water instead of the reefs that others have been found in. Pterygotus lived in a diverse habitat alongside other eurypterids, even other species of Pterygotus along the Silurian sites on the Welsh border, molluscs, fish, trilobites, cephalopods, and the first ammonites and sharks. During the Devonian 'armoured' fish evolved, perhaps as a defence against the claws of eurypterids, and McCoy found that Jaekelopterus had managed to puncture the armour of these fish, so it is likely that Pterygotus could have as well. As a side note Jaekelopterus was once thought to be a species of Pterygotus. Warm waters with ample prey allowed Pterygotus to be a widespread and common animal. Why then did it go extinct? Palaeontologists are not entirely sure - for many extinct animals we cannot pinpoint why (and sometimes when) they went extinct. With Pterygotus we have a few ideas. One possible reason is climate change. By the mid-Devonian the first forests had managed to evolve which reduced the planet's CO2 levels, and with it, reducing global temperatures. Habitats are dependent on climates remaining the same so even the slightest changes can have drastic consequences - we can see this playing out right now with the extreme weather of the last decade thanks to human caused climate change. There are other theories, and possible contributions, about the extinction. The Devonian has been nicknamed the 'Age of Fishes' thanks to the explosion of fish diversity. Armoured fish and the first sharks emerged offering fast-swimming rivals, and also rivals who could bite back. Possibly, large, jawed fish and climate change led to the extinction of Pterygotus. The eurypterids lived on. Unlike the armoured fish they managed to survive the Devonian extinction between 376 and 360 million years ago (although just barely). It took until the planet's worst mass extinction marking the end of the Permain around 251 million years ago for the eurypterids to finally vanish.

Thank you for reading. For other Paleo Profiles we have a list. The sources I have used are as follows:

-'Pterygotus', Prehistoric-Wildlife.com, [Accessed 30/11/2018]

-Erik N. Kjellesvig-Waering, 'A Synopsis of the Family Pterygotide Clarke and Ruedemann, 1912 (Eurypterida)', Journal of Palaeontology, 38:2, (1964), 331-361

-Derek Briggs and W.D. Ian Rolfe, 'A Giant Arthropod Trackway from the Lower Mississippian of Pennsylvania', Journal of Palaeontology, 57:2, (1983), 377-390

-Simon Braddy, 'Eurypterids from the Early Devonian of the Midland Valley of Scotland', Scottish Journal of Geology, 36, (2000), 115-121

-Victoria E. McCoy, James C. Lamsdell, Markus Poschmann, Ross P. Anderson, and Derek E. G. Briggs, 'All the better to see you with: eyes and claws reveal the evolution of divergent ecological roles in giant pterygotid eurypterids', Biology Letters, 11:8, (2015)

-Randall Miller, 'Pterygotus anglicus: Agassiz (Chelicerata: Euryptida) From Atholville, Lower Devonian Campbellton Formation, New Brunswick, Canada', Palaeontology, 50:4, (2007), 981-999

-W.J. Salter, 'Description of Pterygotus problematicus, Agass', Quarterly Journal of the Geological Society, 8:1-2, (1852), 386-388

-Trey the Explainer, 'Paleo Profiles - Sea Scorpions', Youtube, 13/09/2016, [Accessed 30/11/2018]

Thank you for reading and I hope you found it interesting. Paleo Profiles is an interest of mine instead of a field of expertise so if I got anything wrong, or missed something out, please feel free to mention it. For future blog updates please see our Facebook or catch me on Twitter @LewisTwiby.