The new order Guildayichthyiformes n. ord. (Actinopterygii) is erected for Guildayichthys carnegiei n. gen. n. sp. and Discoserra pectinodon n. gen. n. sp. from the Upper Chesterian Bear Gulch Limestone of Montana. These marine fish have highly compressed, discoidal bodies, tall rhombic "ganoid" scales with peg-and-socket joints, fins supported by well-jointed and well-spaced rays, small mouths and specialized bones of the operculo-gular series. The skull roof features several large median bones extending from the rostrum to the occiput, three bones in the otic canal series, and several longitudinal series of pre-frontal elements. The cheek and antorbital areas are equally unique. The cheek has highly variable numbers of suborbitals, tall narrow dorsal and ventral preopercular lateral line canal bones, and a series of interopercular rays (Guildayichthys carnegiei n. gen. n. sp.) or bones (Discoserra pectinodo n. gen. n. sp.) between the ventral preopercular bone and the branchiostegal series. There are several small postspiracular bones and eight to ten sclerotic bones in the orbit. The extensive development of median skull roofing bones in the Guildayichthyiformes n. ord. compares to patterns seen in Dipnoi and Placodermi, that also concentrate biting forces anterior to the orbits, suggesting similar adaptive responses to similar types of cranial stress regimes. Cladistic analysis involving 72 characters results in a very stable sister group relationship between Polypterus and the Guildayichthyiformes n. ord. as a crown group within the tested Paleozoic Actinopterygii. The Cladistia are rediagnosed as a superorder to reflect this relationship. The Cladistia appear to be the sister group of Platysomus and Amphicentrum.
Guildayichthys carnegiei n. gen. n. sp., Discoserra pectinodon n. gen. n. sp., Osteichthyes, new order, Guildayichthyiformes n. ord., Cladistia, Namurian, Montana, osteichthyan skulls