Synchrotron X-ray tomography sheds light on the phylogenetic affinities of the enigmatic thylacocephalans within Pancrustacea

Thylacocephala is one of the most puzzling groups of fossil marine euarthropods. They are known from the Silurian to the Cretaceous and are characterized by a univalve shield, hypertrophied compound eyes, three pairs of large, raptorial appendages and an 8 to 22 segmented posterior trunk. Despite this knowledge of their anatomy, numerous questions remain on their phylogenetic affinities. Usually considered as pancrustaceans, they have been tentatively placed within various pancrustacean ingroups such as thecostracans, malacostracans or remipeds. This uncertainty on their phylogenetic relationships is mostly due to a lack of knowledge on their body organization, especially on the number, nature and morphology of their cephalic, raptorial and posterior trunk appendages. We applied synchrotron X-ray microtomography to exceptionally preserved specimens of <i>Dollocaris ingens</i> from the Middle Jurassic La Voulte-sur-Rhône Lagerstätte, Ardèche, France (Callovian, approximately 165 Ma). It revealed unique details of their tagmatization. For the first time, we demonstrate the unambiguous presence of several cephalic appendages (i.e. mandibles, maxillules, maxillae), an anterior trunk bearing the raptorial appendages and a posterior trunk. These new anatomical data allowed us to test the phylogenetic affinities of thylacocephalans among arthropods. The phylogenetic analyses strongly suggest that thylacocephalans form a pancrustacean monophyletic group, being closely related to malacostracans.