Description

Aegirocassis is an extinct genus of colossal radiodont arthropod, belonging to the family Hurdiidae, that inhabited the Fezouata Formation in Morocco approximately 480 million years ago during the early Ordovician period. It is distinguished by its sole species, Aegirocassis benmoulai. The scientific exploration of this fossil was initiated by Van Roy, marking the earliest discovery of a "giant" filter-feeder known to date. Aegirocassis is believed to have originated from early predatory radiodonts. It held the distinction of being the most massive radiodont ever recorded and the largest creature of its era, with its length documented as surpassing 2.0 meters (6.6 feet). Aegirocassis also identified ribbon-like formations thought to represent gills. These structures have been observed in other anomalocaridids as well, with previous assumptions suggesting attachment to the rear. However, the researchers studying Aegirocassis pointed out that these potential gills were connected to the upper series of flaps.

Filter feeding

This creature is characterized by a lengthy, forward-facing head sclerite, along with endites on its frontal appendages that carried abundant baleen-like auxiliary spines. The evolution of filter-feeding attributes in Aegirocassis likely occurred due to the Great Ordovician Biodiversification Event. This event saw shifts in the environment that prompted the diversification of plankton, thereby facilitating the emergence of new lifeforms specialized in suspension feeding. The frontal appendages of Aegirocassis were equipped with five endites, each adorned with approximately 80 auxiliary spines resembling setae. These spines are believed to have functioned as a mesh-like structure for the purpose of filter feeding.

Special fossil findings

Unlike many other radiodont fossils, which are typically flattened, the fossil of Aegirocassis benmoulai was remarkably well-preserved in three-dimensional form, revealing intricate details. This high-quality three-dimensional preservation has provided insights into the characteristics of radiodont trunk flaps. Within each segment of the Aegirocassis benmoulai specimen, there exist both ventral and dorsal pairs of flaps. The distinct features observed in this specimen prompted a reevaluation of existing research on similar specimens, leading to a significant conclusion: the ventral pair can be identified as homologous with arthropod endopods (inner limb-like branches) and lobopodian limbs (lobopods), while the dorsal pair corresponds to the flaps of gilled lobopodians and exites (outer gill-like branches) of arthropod biramous limbs.