Pseudoangustidontus

''Pseudoangustidontus'' (meaning "false '' Angustidontus''") is a genus of hurdiid (peytoiid) radiodont from the Lower Ordovician of Morocco. This genus includes two described species, ''Pseudoangustidontus'' ''duplospineus'' and ''Pseudoangustidontus'' ''izidigua'', which both belong to the smaller Aegirocassisinae subfamily, however, a third unnamed species is also known. This animal is only known from the Fezouata Formation, a lower Ordovician fossil site in Morocco that is of ''Konservat-Lagerstätten'' status, meaning that the fossils from this site are exceptionally well preserved. This taxon was first described in 2006, but due to the fragmentary remains known at the time, its classification was debated, but with more complete fossils, this taxon was identified as a hurdiid radiodont in 2023. Similarly to its larger relative, ''Aegirocassis benmoulae'', this radiodont is thought to have lived a primarily filter feeding lifestyle, using its numerous auxiliary spines to grab small organisms from the water.

Background

Located in the Draa Valley of southeastern Morocco, the Fezouata Formation is a lower Ordovician (Tremadocian – Floian) aged geologic formation well known for its exceptional preservation of soft-bodied marine organisms. The soft body producing layers of the formation, known as the Fezouata Lagerstätte, were first identified in the late 1990s, when Ben Moula,local fossil collector in the area, showed some uncovered specimens to a PhD student working in the area, and the soon to be discovered ecosystem would come to be known as the Fezouata Biota. The formation is especially important, as it helped bridge the gap between Cambrian-type and Ordovician-type faunas, showing that many groups of organisms once thought to have gone extinct at the end of the Cambrian, survived the transition into the Ordovician. The site is highly productive, with around 50 distinct taxa being represented from over 1,500 specimens, with the majority of said specimens coming from two small intervals of the formation around and thick, compared to the nearly thickness of the whole formation. During the Ordovician, the area that would become Morocco was situated much closer to the South Pole then it is today, and the ecosystem of Fezouata lived in a storm wave base roughly 50 and 150 metres (160 and 490 ft) underwater. The organisms in the environment were preserved in situ, and because of storms in the area helping to facilitate rapid sediment deposition, the majority of known organisms were benthic in nature.

Discovery and etymology

The fossil specimens belonging to ''Pseudoangustidontus,'' more specifically the species ''P. duplospineus'', were discovered in the upper strata of the Fezouata Formation, and were subsequently described by Van Roy & Tetlie, 2006. However, because the fossils were found by local collectors, and were brought to the authors' attention via circulation, the exact locality of origin is uncertain, with several sites located near the town of Zagora being possible candidates. Because of its unique appearance and fragmentary state, the taxon would be considered problematic (in terms of its taxonomy), but would later be identified as a radiodont later on in 2023. In that same year, a second species, ''P. izdigua'', was described based on differences in spine length compared to the type species, and is known from more complete remains ( endites, frontal appendages, and a partial h-element).There is also a currently unnamed species, but due to its fragmentary state, is unassignable aside from genus level classification.

The genus name, ''Pseudoangustidontus'', is derived from the Greek word ''pseudes'', meaning "false", and the genus name '' Angustidontus'', an unrelated pelagic crustacean from the Devonian-Carboniferous periods, due to similarities between the raptorial limbs of ''Angustidontus'', and the endites of ''P. duplospineus'', while still acknowledging the differences between the two taxa. The type species name, ''duplospineus'', is derived from the Latin words ''duplus'', meaning "double", and ''spineus'', meaning "spiny", or "thorny", highlighting the double pair of auxiliary spines seen in the species. The other species name, ''izdigua'', is derived from the Tamazight word ''izdigue'', which means "filter", highlighting the filter feeding nature of the taxon.

Description

File:20230828 Radiodonta frontal appendage Pseudoangustidontus duplospineus.png, Reconstruction of the frontal appendage of ''P. duplospineus''
File:20230828 Radiodonta frontal appendage Pseudoangustidontus izdigua.png, Reconstruction of the frontal appendage of ''P. izdigua''
File:20191229 Radiodonta frontal appendage Aegirocassis benmoulai Aegirocassis benmoulae.png, Reconstruction of the frontal appendage of ''Aegirocassis'' for comparison
''Pseudoangustidontus'' is only known from frontal appendage remains, with an exception of MGL 108047_1, the holotype of ''P. izidigua'', which preserved part of a frontal sclerite (H-element). ''P. duplospineus'' had paired auxiliary spines alternating in length short/long while the frontal appendages of ''P. izdigua'' bore endites with spines that are equal in length. ''P. duplospineus'' is differentiated from its close relative ''Aegirocassis'' due to the paired spines on its frontal appendages, with the shorter spines being equal in length compared to the width of the endites, and the longer spines being nearly twice as long compared to the endite width.

Classification

When described in 2006, the only known fossils of this genus were isolated endites that bore copious amounts of auxiliary spines. Because of this, the taxonomy of this stem-arthropod was debated, with possible suggestions of its affinity being with the eurypterids of the megalograptidae family, radiodonts, or crustaceans such as ''Angustidontus''. In a review article published in 2023, it was questionably placed in the Hurdiidae. And in another paper that was released in the same year, and published by Potin, Gueriau & Daley, 2023, described newly found fossils, and found it to be closely related to ''Aegirocassis'', and both into the subfamily Aegirocassisinae. The family that the Aegirocassisinae subfamily belongs to, the Hurdiidae, were the most diverse and long lasting lineage of radiodonts, usually characterized by subequal endites, a tetraradial oral cone, and large frontal sclerites (which in some species reach, or exceed the total body length).

Paleoecology

''Pseudoangustidontus'' is speculated to be a filter feeder similar to ''Aegirocassis'', that used numerous auxiliary spines on its frontal appendages as mesh to catch microorganisms, similarly to the baleen seen in various cetacean species. The spine and mesh size of the frontal appendages of ''Pseudoangustidontus'' is around 0.5 mm, slightly smaller than that of ''Aegirocassis'' (0.56 mm), both genera probably fed on mesoplankton. However, a specimen of ''P. izdigua'' had an unidentified shell or carapace with a diameter about 3 mm trapped in its setae, suggesting it could have fed on even larger prey.

References

{{taxonbar, from=Q21368832, from2=Q3924863

Fossils of Morocco
Radiodonta
Fossil taxa described in 2006
Fezouata Formation fossils
Ordovician arthropods