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Aegirocassisinae is a
Aegirocassisines are distinguished by their frontal appendages possessing endites that bore long auxiliary spines (the longest of any radiodont). The appendages are composed of around six podomeres that each bore one endite. Although all of the members of this subfamily were suspension feeders, the study notes that other suspension feeding radiodonts (like ''
subfamily
In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
of radiodonts
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 ...
(marine stem-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 ...
s) from the lower Paleozoic
The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
era. It belongs to the larger 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 ...
(peytoiid) family, which were the most diverse and long lasting radiodonts. The members of this subfamily are restricted to the Lower Ordovician-aged Fezouata Formation
The Fezouata Formation or Fezouata Shale is a geological formation in Morocco which dates to the Ordovician, Early Ordovician.
of 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 ...
. Currently only two genera are included: ''Aegirocassis
''Aegirocassis'' is an extinct genus of giant radiodont arthropod belonging to the family Hurdiidae that lived 480 million years ago during the early Ordovician in the Fezouata Formation of Morocco. It is known by a single species, ''Aegirocassis ...
'' and '' Pseudoangustidontus''. These two genera possess large 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 on their frontal appendages, which suggests a suspension feeding lifestyle for the group. These radiodonts are some of the few known from sediments beyond the Cambrian
The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
period. This subfamily shows that following the Great Ordovician Biodiversification Event, which saw a rise in the 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 ...
population in the worlds oceans, suspension feeding became more common in radiodonts then other feeding styles. It also seems that due to the evolution of new predators, like large nautiloid
Nautiloids are a group of cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. Th ...
cephalopods, and other arthropod groups like the eurypterid
Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the Order (biology), order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period, 467.3 Myr, mil ...
s, the radiodonts evolved suspension feeding lifestyles in order to minimize competition for food.
Description
Aegirocassisines are distinguished by their frontal appendages possessing endites that bore long auxiliary spines (the longest of any radiodont). The appendages are composed of around six podomeres that each bore one endite. Although all of the members of this subfamily were suspension feeders, the study notes that other suspension feeding radiodonts (like ''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 ...
'') are not included in this subfamily. This subfamily is named after ''Aegirocassis'' and not ''Pseudoangustidontus'', as even though the latter was named before the former, ''Pseudoangustidontus'' was previously considered enigmatic before the 2023 studies publication. The aegirocassisines were some of the largest radiodonts, with ''Aegirocassis'' reaching a length of up to 2 meters, and ''Pseudoangustidontus'' potentially reaching a similar size.
References