DRUZINSKY,R.E.: Sciuromorphy and Protrogomorphy in extant and fossil Rodentia.
It has long been recognized that sciuromorphy, in which the anterior lateral masseter muscle originates from an expanded bony plane on the rostral surface of the anterior root of the zygomatic arch, evolved more than one time during the evolutionary diversification of rodents. Arguments for multiple origins of the sciuromorphic condition are usually based on the necessity of deriving sciuromorphy independently in disparate clades within the Rodentia. However, it is clear that careful study of the masticatory apparatus also demonstrates that sciuromorphy in the Sciurids is anatomically different from sciuromorphy in Geomyids. Anatomical study of Aplodontia, the only extant Progtrogomorph, indicates that Aplodontia shares charaters of the jaw musculature with the Sciurids that it does not share with the Geomyids. Examination of early rodents also demonstrates that more than one type of sciuromorphy and progtrogomorphy may be found in the fossil record. Oligospermophilus and Cedromus are fossil sciurids that, as described by Korth and Emry (1991) exhibit a “unique zygomatic structure” in-between sciuromorphy and protrogomorphy. These specimens clearly have attachment areas for the superficial masseter and lateral masseter similar to those of the Aplodontia/Sciurid clade. In contrast, the Eocene rodent Sciuravus nitidus, which has been described as a primitive sciuromorph or protrogomorph (Dawson, 1961), has attachment areas for the masseter muscles that are distinctly different from the Aplodontia/Sciurid clade, and more similar to the extant Dipodoids. Thus, anatomically there is more than one kind of sciuromorphy and even more than one kind of protrogomorphy. And classic protrogomorphy, as found in Paramys and Aplodontia, is probably primitive only for Ischyromyids, Sciurids, and Aplodontids.