Diagnoses based upon preservable carapace features are provided for the brachyuran families Carpiliidae, Palaeoxanthopsidae new family, Pseudoziidae, and Zanthopsidae, newly raised to family status. In order to make family and generic level assignments, characters of the dorsal carapace can be used successfully as proxies for soft-part morphology that is not commonly or never preserved in fossils. The identification of carpiliids and pseudoziids in the fossil record yields critical information about the time of divergence of these groups, no later than Eocene, and the relationships between those two families, the Eriphiidae, and the extinct Zanthopsidae. Because the timing of divergence of at least two xanthoid families is now known more accurately, more constrained phylogenetic studies can result. The Eocene to Recent Carpiliidae is restricted to one extant genus and five fossil genera. The Eocene to Recent Pseudoziidae contains the fossil genera Archaeozius new genus, Priabonocarcinus, and Santeezanthus as well as several extant genera; one extant genus, Euryozius, also has a fossil record. The Paleocene-Miocene Zanthopsidae embraces five extinct genera including Neoxanthopsis new genus. The new family Palaeoxanthopsidae includes four extinct genera, including Paraverrucoides new genus and Remia new genus and occurs in Maastrichtian-Eocene rocks. New combinations include Archaeozius occidentalis, Harpactoxanthopsis bittneri, Harpactoxanthopsis souverbei, Neozanthopsis bruckmanni, N. carolinensis, N. rathbunae, N. sonthofensis, N. tridentata, Ocalina straeleni, Palaeocarpilius mississippiensis, Palaeocarpilius valrovinensis, Paraverrucoides alabamensis, and Remia africana.