Thylacocephala is an enigmatic ingroup of Euarthropoda. Thylacocephalans, only known from Palaeozoic and Mesozoic fossils, are characterized by a particular anatomy: a prominent folded shield enveloping most of the body, large compound eyes, three pairs of large, presumably raptorial appendages and a trunk with 8-22 stout segments bearing swimming appendages. However, lifestyle(s) and phylogenetic relationships of Thylacocephala are still largely unknown. This study is focused on thylacocephalans from the Mazon Creek Lagerstätte (c. 307 Ma, Middle Pennsylvanian, Carboniferous). A new species is described from a siderite concretion of Mazon Creek: Eodollocaris keithflinti n. gen., n. sp. The new species displays a particular mixture of characters typical for Palaeozoic species on the one hand and Mesozoic species on the other hand. Additionally, new details of the appendages and of the trunk are provided for already known species from the same locality, namely Concavicaris georgeorumSchram, 1990 and Convexicaris mazonensisSchram, 1990. These new informations are helpful to better understand the possible lifestyle of these representatives of Thylacocephala.