[14] Briggs & Rolfe, working on fossils from Australia's Devonian deposits were unable to attribute certain specimens to a known group, and created an order of uncertain affinities, the Concavicarida, to accommodate them.

[4] It was apparent the three groups were in fact working on a single major taxon (Rolfe noted disagreements over interpretation and taxonomic placement largely resulted from a disparity of sizes and differences in preservation.

Some efforts have been made at further classification: Schram split currently known taxa into two orders:[13] The accuracy of this scheme has been questioned in recent papers,[5] as it stresses differences in the eyes and exoskeletal structure, which – in modern arthropods – tend to be a response to environmental conditions.

More reliable anatomical indicators would include segmentation and appendage attachments (requiring the internal anatomy, currently elusive as a result of the carapace).

As this feature has evolved independently numerous times within the Crustacea and other arthropods, it is not a very reliable pointer, and such evidence alone remains insufficient to align the class with the crustaceans.

[13] Secrétan also mentions – with caution – possible mandibles in serial sections of Dollocaris ingens, and traces of small limbs in the cephalic region (not well preserved enough to assess their identity).

A study in 2022 describing a new arthropod from Wisconsin, Acheronauta, found that the Thylacocephalans occupied a position more primitive than the crustaceans and myriapods as basal stem-group mandibulates.

[1] †Parioscorpio venator †Cascolus ravitis †Tanazios dokeron †Captopodus poschmanni †Acheronauta stimulapis †Thylacocephala †Occacaris oviformis Myriapoda †Ercaicunia multinodosa †Clypecaris pteroidea †Waptia fieldensis †Perspicaris dictynna †Canadaspis perfecta †Tokummia katalepsis †Branchiocaris pretiosa †Nereocaris exilis †Odaria alata †Euthycarcinoidea †Fuxianhuia †Chengjiangocaris †Shankouia Argulus Ostracoda †Lepidocaris Triops Artemia Remipedia Cephalocarida Hexapoda Multicrustacea This cladogram represents the placement of the Thylacocephalans within the arthropoda as suggested by Pulsipher, 2022.

Based on poorly preserved Ostenocaris cypriformis fossils from the Osteno deposits of Lombardy, Pinna et al. erected the class Thylacocephala.

This opinion arose from the misinterpretation of the stomach as a reproductive organ (its contents included vertebral elements of fish, thought to be ovarian eggs).

[5] Such an arrangement is reminiscent of cirripede crustaceans, leading the authors to suggest a sessile, filter feeding mode of life, the 'cephalic sac' used to anchor the organism to the seabed.

Secrétan suggested Dollocaris ingens was too large to swim,[20] so inferred a predatory 'lurking' mode of life, lying in wait on the sea bed and then springing out to capture prey.

Another notable proposal is that, like hyperiids, the class could gain oil from their food source for buoyancy, an idea supported by their diet (known from stomach residues containing shark and coleoid remains, and other Thylacocephala).

[4] The authors speculate that due to the terracing of the carapace an infaunal mode of life is possible, or the ridges could provide more friction for hiding in crevices of rock.

Schram suggests a dichotomy in size of the class results from different environments;[13] larger Thylacocephala could have lived in a fluid characterized by turbulent flow, and relied on single power stroke of trunk limbs to position themselves.

He suggests that smaller forms may have resided in a viscous medium, characterized by laminar flow, and used a lever to generate the speed necessary to capture prey.