''Aegirocassis'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus of giant

radiodont Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. Radiodonts are distinguished by their distinctive frontal appendages, which are morphologically diverse and were used for a variety ...

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

belonging to the family

Hurdiidae Hurdiidae (synonymous with the previously named Peytoiidae) is an extinct cosmopolitan distribution, cosmopolitan Family (biology), family of radiodonts, a group of Crown group#Stem groups, stem-group marine arthropods, which lived during the Pal ...

that lived 480 million years ago during the

early Ordovician The Early Ordovician is the first epoch (geology), epoch of the Ordovician period, corresponding to the Lower Ordovician series (stratigraphy), series of the Ordovician system. It began after the Cambrian Stage 10, Age 10 of the Furongian epoch o ...

in the

Fezouata Formation The Fezouata Formation or Fezouata Shale is a geological formation in Morocco which dates to the Ordovician, Early Ordovician.

of Morocco. 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 aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a specia ...

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

predatory Predation is a biological interaction in which 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 ...

radiodonts. This animal is characterized by its long, forward facing head

sclerite A sclerite (Greek language, Greek , ', meaning "hardness, hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instea ...

, and the endites on its frontal appendages that bore copious amounts of

baleen Baleen is a filter feeder, filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by th ...

-like auxiliary spines. This animal evolving filter-feeding traits was most likely a result of the Great Ordovician Biodiversification Event, when environmental changes caused a diversification of

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

, which in turn allowed for the evolution of new suspension feeding lifeforms. Alongside the closely related '' Pseudoangustidontus'', an unnamed hurdiid from

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

, the middle Ordovician dinocaridid '' Mieridduryn'', and the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

hurdiid ''Schinderhannes'', this radiodont is one of the few dinocaridids known from post-Cambrian rocks.

Description

''A. benmoulai'' was the largest known radiodont and the largest known animal that existed in this period, and the length was described as exceeding . The fossil was preserved with exceptional

three-dimensional In geometry, a three-dimensional space (3D space, 3-space or, rarely, tri-dimensional space) is a mathematical space in which three values (''coordinates'') are required to determine the position (geometry), position of a point (geometry), poi ...

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 describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

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 The fus ...

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 Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as w ...

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

Peytoia ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...

'' and '' Hurdia'' 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 (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae ...

-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 from the Cambrian period. The taxon was initially described in 2010 based on frontal appendages discovered from the Sirius Passet lagerstat ...

'', 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 a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...

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 (, abbreviated as UGent) is a Public university, public research university located in Ghent, in the East Flanders province of Belgium. Located in Flanders, Ghent University is the second largest Belgian university, consisting o ...

in Belgium.

Phylogeny

Phylogenetic position of ''Aegirocassis'' within Panarthropoda, according to Pates ''et al''. (2022).

Notes

References

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