Protaspides of Ptychopariida and taxonomy of Hystricuridae, with discussion of ancestry of Proetida

Species within the order Ptychopariida are considered to have given rise to the family Hystricuridae, the primitive family of the order Proetida. The family is considered to contain ancestors to various younger groups of the Proetida. This evolutionary speculation is investigated by revising taxonomy of the Ptychopariida and the Hystricuridae.;Protaspides of 34 Cambrian ptychopariid species are re-described. All the protaspid specimens are compared with one another and with post-protaspid specimens of all the species. The 34 species are classified into 28 genera, 17 families, and nine groups of superfamilial rank. Each of these nine groups is characterized by a distinctive protaspid form; five of them are assigned to the suborder Ptychopariina and the remaining four to the suborder Olenina.;The taxonomy of the Hystricuridae is revised upon the basis of 173 species that have been referred to the family and newly discovered materials from the Great Basin in the western United States. The revision concludes that the family includes 12 genera and 52 species. Seven genera are newly erected: Carinahystricurus, Glabellosulcatus, Parahillyardina, Paramblycranium, Politohystricurus, Pseudoplethopeltis, and Spinohystricurus. Pygidia are associated for 10 genera of the Hystricuridae and for 15 genera that have been and are considered to be related to the family.;The revised taxonomies and new information, in particular on protaspid and pygidial morphologies, give a new insight into the origin of the Proetida. Comparison of the pygidia leads to division of the hystricurid pygidia into two morphotypes, one with a pygidial fulcral ridge and the other without the structure. The first morphotype, present in Hystricurus, the primitive genus of the Hystricuridae, appears to have transformed from the ptychopariid form and been retained in some younger proetide groups. The second morphotype, present in non-Hystricurus hystricurid genera, is considered to have derived from the first and to have gone extinct.;Protaspid morphologies of the Hystricuridae are considered to have transformed from those of ptychopariid protaspid morphotype B that represents a group consisting of the Cedariidae, Anomocaridae, Crepicephalidae, Marjumiidae, and Llanoaspididae. This suggests that the origin of the Hystricuridae, and subsequently the Proetida, may lie in the taxa possessing morphotype B.