Pseudoangustidontus

• 
  Reconstruction of the frontal appendage of P. duplospineus
• 
  Reconstruction of the frontal appendage of P. izdigua
• 
  Reconstruction of the frontal appendage of Aegirocassis for comparison

Pseudoangustidontus is only described from frontal appendage remains, with an exception of MGL 108047_1, holotype of P. izidigua which preserved part of carapace. P. duplospineus had paired setae alternating in length short/long while frontal appendage of P. izdigua bears endites with setae that are all the same length.[3]

Originally morphology of a fossil of P. duplospineus (top left) was compared to the appendages of Angustidontus (top right).
However, Pseudoangustidontus was later found to be a hurdiid radiodont like Aegirocassis (bottom left), while Angustidontus (bottom right) is a crustacean.

When described in 2006, the only known fossils of this genus were isolated appendages that bore copious amounts of spines. Because of this, the taxonomy of this arthropod was debated, with possible suggestions of this creatures affinity being with the eurypterids, radiodonts, or crustaceans.[1] In a review article in 2023, it was questionably placed in the Hurdiidae.[5] And in another paper that was released in the same year, describing newly found fossils, it was found to be closely related to Aegirocassis, a coexisting radiodont, and classified to the subfamily Aegirocassisinae.[3]

Pseudoangustidontus is speculated to be a filter feeder similar to Aegirocassis, that used numerous setae on its frontal appendages as mesh to catch microorganisms. The setae 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.[3]