Associate Professor New York Insititute of Technology Old Westbury, New York, United States
Abstract Body : The stapes is part of the middle ear elements that have undergone marked transformation in mammaliaform evolution. Integral to understanding the evolutionary changes associated with this transformation has been the basal-most mammaliaform Morganucodon. In 1981, Kermack and colleagues published the cranial morphology of Morganucodon watsoni from the Early Jurassic St Brides fissure fills of South Wales, including descriptions of isolated stapes recovered through screen washing. However, we now describe two stapes from the St Brides fissure suite that, for the first time, are preserved in association with the petrosal.
The two petrosals and stapes, accessioned at the Natural History Museum in London, were µ-CT scanned and manually segmented and digitally measured using the visualization software Amira. Comparisons are made to previously described stapes of Morganucodon and other Mesozoic mammaliaforms.
The stapes are displaced into the cochlear cavity and with near certainty belong to the same individuals as the petrosals. Both stapes are fragmentary and preserve the stapedial footplate and partial crura. Despite the preservation, the morphology is near identical and differs in crucial ways from the previously published stapes: (1) the stapedial footplate is oval rather than near circular; (2) the anterior crus is in a more marginal in position, and not central; (3) the anterior and posterior crus are widely separated and near parallel, rather than diverge from the center of the footplate; (4) the anterior margin of the stapedial footplate is bend upward rather than straight; (5) the internal aspect of the footplate lacks a central depression. The difference between the two morphotypes cannot be explained by preservation or inter- or intraspecific variation. We believe that the newly described stapedial morphology represents the stapes of Morganucodon watsoni, and that the isolated stapes described in 1981 were erroneously assigned to Morganucodon by Kermack and colleagues. The petrosal associated with the two stapes is congruent with the more than 30 petrosals that we µ-CT scanned, and that have been assigned to M. watsoni from the St Brides fissures. Further, they are in line with the morphology of the petrosals of M. oehleri from the Jurassic of China. Based on our data we revise the stapedial morphology of the basal-most mammaliaform Morganucodon. Notably, the revised stapes presented here differ from those of other basal mammaliaforms such as docodontans, and are in various features (including the marginal anterior crus, oval footplate, and anterior bend of the footplate) more similar to those of early crown mammals including multituberculates, eutriconodontans and spalacotheroids.
