Three Chinese fossils have led to the discovery of a new species of ancient sea-scorpions. The extinct dog-sized arthropods, the Terropterus xiushanensis, ruled the oceans and were top predators of their time.
Sea-scorpions date back to the Silurian, mid-Palaeozoic, around 440 million years ago. This period starts after the Ordovician-Silurian extinction, the first of the five great mass extinctions in the history of earth.
A glaciation event followed by global warming caused ocean temperatures and sea-level to oscillate, altering and reducing marine habitats. This led to the loss of 86% of animal species.
In the early Silurian, life was still limited to the oceans. The global warming trends and rising seas submerged land, creating extensive continental shelves. The warmth and abundance of light in these environments provided an ideal habitat for many animals and supported diversification.
Long, lamprey-like fish swam through vast reefs of corals and sponges. The sea floor was covered in ancient shelled creatures, sea snails, and crinoids – flower-like animals with feathery petals. And at the very top of this underwater food chain, were sea-scorpions, or Eurypterids, which fed on most other animals, including smaller individuals of their own species.
Sea-scorpions were named due to their general resemblance to modern scorpions. However they are not directly linked. Both groups belong to the arthropods, along with spiders, insects, and crustaceans. But the closest living relatives of the extinct sea-scorpions are actually horseshoe crabs.
One remarkable sub-division of Eurypterids are Mixopterids. These are perhaps the most scorpion-like, with tails tapering into sharp stings and long front limbs. Until now, the knowledge of this group was limited to four species, with the last one being described 80 years ago.
Previous Mixopterids were found in Estonia, Norway, Scotland, and New York, which would all have been part of the northern silurian continent of Laurasia. The Terropterus xiushanensis fossils were found in southern China, which belonged to the southern continent, Gondwana. The discovery widens the known distribution of the Mixopterid family.
T. xiushanensis differs from other Mixopterids in the size and number of its spines, and in details of the overall shape of its appendages. This variation is important as it could reflect diverse modes of life and prey capture methods.
Much is still unknown about the Eurypterids, which stay in history as the largest invertebrates to ever inhabit earth. But we can look forward to what future discoveries will reveal about these long-lost rulers of the seas.
