New look at Concavicaris woodfordi (Euarthropoda: Pancrustacea?) using micro-computed tomography

Known from at least the Silurian to the Cretaceous, Thylacocephala is an enigmatic fossil euarthropod ingroup, often allied with Pancrustacea. Previous studies show that thylacocephalans are characterized by a folded protective shield, hypertrophied compound eyes, three pairs of raptorial appendages, a posterior trunk comprised of eight to 22 segments bearing appendages, and eight pairs of gills. Despite this knowledge of their anatomy, many questions remain, especially surrounding the anatomy of Paleozoic representatives. The Upper Devonian Woodford Shale (upper Famennian, Oklahoma, USA) has yielded several fossil euarthropods, including two species of Thylacocephala: Concavicaris elytroides and Concavicaris woodfordi. Here, we use micro-computed Xray tomography to re-explore the anatomy of the holotype of C. woodfordi, illustrating fine details of the shield structure, of the circulatory, digestive and reproductive systems, and of the appendages. A marginal fold of the shield as well as an inner layer are described for the first time in a thylacocephalan. Concavicaris woodfordi shares similarities with Concavicaris submarinus, another Famennian species, including the morphology of the shield and the internal anatomy. It also displays a similar organisation as Mesozoic taxa, such as Dollocaris ingens. All of this provides important information that will be crucial to reconstruct the evolution and the affinities of Thylacocephala.