''Aegirocassis'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus of

radiodont Radiodonta is an extinct Order (biology), order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the ...

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

belonging to the family

Hurdiidae Hurdiidae is an extinct cosmopolitan distribution, cosmopolitan Family (biology), family of radiodonts, a group of Crown group#Stem groups, stem-group arthropods, which lived during the Paleozoic Era. It is the most long-lived radiodont clade, la ...

that lived 480 million years ago during the

early Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

. It is known by a single species, ''Aegirocassis benmoulai''. Van Roy initiated scientific study of the fossil, the earliest known of a "giant"

filter-feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

discovered to date. ''Aegirocassis'' is considered to have evolved from early

predatory Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

radiodonts.

Description

File:20210221 Aegirocassis size.png, Size estimation File:YPM 527123 Aegirocassis benmoulai frontal appendage.png, Fossil of frontal appendage File:20191229 Radiodonta frontal appendage Aegirocassis benmoulai Aegirocassis benmoulae.png, Reconstruction of frontal appendage ''A. benmoulai'' was the largest known radiodont and largest animal existed in this period, and the length was described as exceeding in the scientific journal ''

Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...

''. The fossil was preserved with exceptional

three-dimensional Three-dimensional space (also: 3D space, 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called ''parameters'') are required to determine the position of an element (i.e., point). This is the informal ...

detail, unlike most other radiodont fossils, in which the animals are flattened. The quality of three-dimensional preservation has shed light on the nature of radiodont trunk flaps. Each trunk segment of the ''Aegirocassis benmoulai'' specimen has both a

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...

and a

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

pair of flaps. Several details seen clearly in the specimen led to a review and reassessment of research of existing specimens and, most importantly, to the conclusion that the ventral pair are homologous with

endopods (limb-like inner branches) and

lobopodian The lobopodians, members of the informal group Lobopodia (from the Greek language, Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may ...

limbs (lobopods), and the dorsal pair are homologous with the flaps of gilled lobopodians and exites (gill-like outer branches) of the arthropod

biramous limb The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, plur ...

. This discovery also found that other hurdiid radiodonts like ''

Peytoia ''Peytoia'' is a genus of hurdiid radiodont that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and ''Peytoia infercambriensis'' from Poland, dating to Cambrian Stage 3. Its two frontal ...

'' and ''

Hurdia ''Hurdia'' is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. As a radiodont like ''Peytoia'' and ''Anomalocaris'', it is part of the ancestral lineage that led to euarthropods. Description ''H ...

'' had a dorsal pair of flaps as well. Frontal appendages of ''Aegirocassis'' had five endites, and each endites had around 80

setae In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. Th ...

-like auxiliary spines. These spines are estimated to have been used as a mesh for filter feeding. It was probably able to consume mesozooplankton roughly the same size as the other filter feeding radiodont, ''

Tamisiocaris ''Tamisiocaris'' (from Latin ''tamisium'', sieve, and Greek ''karis'', crab, shrimp) is a radiodont genus initially only known from frontal appendages from the Buen Formation in Sirius Passet. Further specimens revealed that the frontal appenda ...

'', was able to catch. However, the spines on the frontal appendages of ''Aegirocassis'' are inward-angled, which allowed the spines to overlap to a degree, allowing more control over the size of the filtering mesh. This has led to the estimation that ''Aegirocassis'' may have fed on larger size ranges of zooplankton than ''Tamisiocaris'' could. In addition, the large carapace (H-element) may have helped to guide the feeding current to the frontal appendages. Eyes and mouthpart (oral cone) are not known from ''Aegirocassis''. Since it was probably a filter feeder, ''Aegirocassis'' probably lacked a hard mouthpart and had a flexible one, explaining why mouth structures are not preserved.

Discovery

A fossil of ''A. benmoulai'' from the Fezouata biota,

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to ...

was discovered by and named after Mohamed Ben Moula, a fossil collector who recognized its rare characteristics and brought it to the notice of a professional paleontologist, Peter Van Roy, at the

Ghent University Ghent University ( nl, Universiteit Gent, abbreviated as UGent) is a public research university located in Ghent, Belgium. Established before the state of Belgium itself, the university was founded by the Dutch King William I in 1817, when the ...

in Belgium.

Notes

References

{{Taxonbar, from1=Q19564283, from2=Q19652298 Ordovician arthropods Dinocarida Fossil taxa described in 2015 Fossils of Morocco Fezouata Formation fossils