The osteology and histology of two new actinopterygians, Paphosiscus circulocaudus new genus and species, and P. scalmocristus new species, are described from the Upper Mississippian Serpukhovian Bear Gulch Member of the Heath Formation of Montana (USA). These small fish (ca 50–58mm standard length) exhibit head bones and scales with cosmine, the first report of this in any actinopterygian. Other features of Paphosiscus are dorsal, anal and caudal fin margins round posteriorly, dorsal fin long with horizontal dorsal margin and achieving its greatest height immediately before its rounded posterior margin, tall unpaired predorsal ridge scales/fulcra, premaxillae not sutured to other bones, single postrostral bone, one otic canal bone (dermopterotic), dermosphenotic absent as distinct bone, mandibular articulation level with mandibular tooth row, a coronoid process, marginal teeth pleurodont, coronoid teeth with blunt crowns on a single coronoid bone. Species differ in size and features of cranial bone pore openings, pectinations of scales, shape of maxilla and distribution of teeth, position of first predorsal ridge scales.

Paphosiscus exhibits swimming and feeding specializations that previous work has identified as factors in fine niche partitioning in the Bear Gulch bay community. Its skull bones and scales reveal a distinctive form of cosmine, similar to that of the agnathan Tremataspis but differing from sarcopterygian cosmine. Recent reports of cosmine and ”cosmine-like” tissues in various sarcopterygians or putative stem osteichthyans document a range of architectural variation in integumentary tissues with a pore canal system. With the new Paphosiscus information and recent work on the origin and diversification of the vertebrate integumentary skeleton we offer a unifying hypothesis for this range of variation.