The evolution of Radiodonta (Arthropoda)

Radiodonta is an emblematic order of arthropods from the lower Palaeozoic,

particularly from the Cambrian to the Lower Ordovician. The first radiodont discovered was

the famous Anomalocaris canadensis, but today, 46 species are known, distributed among 28

genera and four families. Depending on the taxon, radiodonts could be active raptorial, benthic

feeders, or suspension feeders, all considered predators. The study of their diversity and feeding

strategies relies mainly on the frontal appendages, generally the best-preserved parts of the

body, which were used to seize prey and bring it to the mouth. This thesis focuses primarily on

the radiodonts of the Fezouata Shale, dated to the Lower Ordovician, which represent the last

known community of radiodonts. Four new species have been described: Pseudoangustidontus

izdigua, Falciscaris mumakiana, Peytoia sp. nov. 2, and Stanleycaris sp. nov. 3. Two species

have also been revised: Aegirocassis benmoulai and Pseudoangustidontus duplospineus. In

addition, a new morphotype of Hurdia, another unidentified appendage, and two distinct oral

cones were studied. A new subfamily, the Aegirocassisinae, has been established, comprising

Aegirocassis and Pseudoangustidontus, all interpreted as suspension feeders. In contrast, the

other species and morphotypes from Fezouata are interpreted as benthivores, and although they

are more diverse, they are far less abundant. The dominance of suspension-feeding radiodonts

coincides with the “Ordovician Plankton Revolution”. Furthermore, the Aegirocassisinae and

Falciscaris represent the largest radiodonts described to date, acting as giants within the

Fezouata ecosystems. The revision of Hurdia, Peytoia nathorsti, and the description of Peytoia

sp. nov. 1 from the Burgess Shale enabled the identification of these genera in the Fezouata

Shale. The study of 3,753 published specimens from 35 localities provides a global

understanding of radiodont evolution across their 50-million-year history. Radiodonts first

appeared and reached their diversity peak in Cambrian Stage 3, before gradually declining until

their extinction, with a notable rebound in the Lower Ordovician with the Fezouata Shale

radiodonts. The evolution of feeding strategies reveals three major phases: an active raptorial

phase in Cambrian Series 2, a benthivore phase in the Miaolingian, during which faunas were

also more homogeneous across paleocontinents, and a filter-feeding phase in the Lower

Ordovician.