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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 subfamily botanical names with "-oideae", and zoologi ...
of radiodonts
Radiodonta is an extinct 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 last two origi ...
(marine stem-arthropods
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 ...
) from the lower Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
era. It belongs to the larger hurdiidae (peytoiid) family, which were the most diverse and long lasting radiodonts. The members of this subfamily are restricted to the Lower 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.
The ...
-aged Fezouata Formation
The Fezouata Formation or Fezouata Shale is a geological formation in Morocco which dates to the Early Ordovician.
of 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 ...
. Currently only two genera are included: '' Aegirocassis'' and ''Pseudoangustidontus
''Pseudoangustidontus'' (meaning "false ''Angustidontus''") is a genus of hurdiid (peytoiid) Radiodonta, radiodont from the Lower Ordovician of Morocco. This genus is known from two described species, ''P. duplospineus'' and ''P. izidigua'', wit ...
''. These two genera possess large Baleen
Baleen is a 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 the baleen and ...
-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 Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
period. This subfamily shows that following the Great Ordovician Biodiversification Event
The Great Ordovician Biodiversification Event (GOBE), was an evolutionary radiation of animal life throughout the Ordovician period, 40 million years after the Cambrian explosion, whereby the distinctive Cambrian fauna fizzled out to be replaced wi ...
, which saw a rise in the plankton
Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
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 cephalopods, and other arthropod groups like the eurypterids
Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is l ...
, 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 initially only known from frontal appendages from the Buen Formation in Sirius Passet. Further specimens revealed that the frontal appenda ...
'') 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