The find was announced on Wednesday, and the research team claim that the ancient animal lived around 480 million years ago. The arthropod family includes crabs, scorpions, insects and spiders, as well as this huge sea monster. The findings were published in the journal Nature, writes Will Dunham for Reuters.

Ancient Arthropod Was A Gentle Giant

 

Biggest creature of its time

According to Peter Van Roy, lead researcher and paleontologist at Yale University, it was the largest animal on Earth at the time, measuring at least double the size of any other creature. However its huge size did not make it the ruler of the seas, but rather it was a gentle giant that survived on a diet of plankton.

Aegirocassis benmoulae was around 2.1 meters long, and also has the distinction of being the last known member of the anomalocaridids, a group which counted some of the first top predators among its number.

The majority of other animals in the group were predators, using appendages attached to their heads to capture prey. These same appendages were used by Aegirocassis as a sieve through which it captured huge amounts of plankton.

First example of an evolutionary trend

On multiple occasions over the course of Earth’s history, large filter-feeders have emerged from large predatory animal groups. The first example of this trend is Aegirocassis, which profited from a plankton explosion during the Ordovician Period. Later examples include baleen whales and whale sharks.

“Given the huge size of Aegirocassis and its very alien appearance, I assume most people would probably be terrified if they’d encounter it while swimming. However, contrary to almost all other anomalocaridids which were active predators, our animal would have been a very peaceful guy,” Van Roy said.

The body of Aegirocassis was elongated, and split into 11 segments. Two sets of flaps are thought to be earlier versions of the limbs seen on later arthropods, and the lower flaps were used to propel Aegirocassis through the water.

Its back was covered in ribbon-like structures which are thought to have functioned as gills, while two appendages near its mouth were adapted for successful filter-feeding.

“The discovery informs about the origins and early evolution of arthropods, the most ubiquitous, species-rich, morphologically diverse and successful animal group on Earth,” Van Roy said.

Anomalocaridids first appeared around 530 million years ago, and are a very early form of today’s arthropods.