The stapes of Lambdopsalis bulla (Multituberculata) and transformational analyses on some stapedial features in Mammaliaformes

ABSTRACT

The stapes of the Paleocene multituberculate Lambdopsalis bulla, recovered from a unique intra-vestibuli circumstance, is small and columelliform. It has a slim shaft, a slit-like stapedial foramen, a distinct head, and a well-developed, nearly circular footplate. With new evidence of the stapes from Lambdopsalis bulla and those reported recently from Sinoconodon, Haldanodon, and Pachygenelus, stapedial characters are analyzed in a selected group consisting of Placentalia, Marsu-pialia, Monotremata, Multituberculata, Morganucodontidae, Sinoconodon, Haldanodon, and the tritheledontid Pachygenelus. Stapedial characters discussed include relative size of the stapes, shape of the stapes, development of stapedial footplate, shape of the stapedial footplate, the stapedial foramen, position of the stapedial artery, and orientation of the stapes. Conclusions are: 1) the relative size of the stapes does not differentiate multituberculates from morganucodontids; 2) stapes are divided into three basic types: rod-like, columelliform, and arched, and the latter two are derived for the selected group, characterized by a restricted distal end, and recorded only in a triossicular ear; 3) an imperforate stapes is not diagnostic of a group including multituberculates plus monotremes and therians; 4) a columelliform stapes and an extrastapedial course of the stapedial artery may represent shared and derived features for monotremes and multituberculates; 5) an arched stapes penetrated by a stapedial artery is probably primitive for the group that includes multituberculates, monotremes, and therians; 6) a well-defined footplate is a shared derived feature diagnostic of the selected group except for the tritheledontids; and 7) the stapes articulating anteriorly with the incus is probably an autapomorphy for multituberculates.