Two problems are examined in this paper: (1) the identification of flying squirrels in the fossil record by means of their teeth; and (2) the identification of features of the limbs that indicate that the animals were gliders. Dental features vary widely among flying squirrels and among other squirrels, and a thorough survey demonstrates that most features that have been used to distinguish fossil flying squirrels are also found in some tree squirrels. A review of the descriptions of fossil flying squirrels reveals few convincing arguments that these animals actually belong to the Ptero-myini and none to support the hypothesis that they were gliding animals. Recent flying squirrels exhibit a number of distinguishing morphological features in their carpal and tarsal bones and at the proximal and distal ends of their long bones. Some of these morphological structures are obligatory features required for gliding locomotion in squirrels and hence are diagnostic of flying squirrels. INTRODUCTION There is a long-standing disagreement on the relationship of flying squirrels to other squirrels. Some paleontologists have argued that flying squirrels evolved from paramyids independent of other squirrels (Major, 1893; Mein, 1970; de Bruijn and Unay, 1989). Most others have classified flying squirrels within the Sciu-ridae (Simpson, 1945; Hoffmann et al., 1993; McKenna and Bell, 1997), and recent molecular studies support the view that they evolved from one particular group of tree squirrels, long after the Sciuridae evolved from paramyids (Mercer and Roth, 2003; Step-pan et al., 2004). In this paper, we address two problems related to this disagreement. The first is the problem of identifying fossils that belong to the lineage of the Recent Pteromyini, the ancestors of modern flying squirrels or their close relatives. The second problem is determining whether the fossil animals display morpho-logic features that clearly suggest that they were gliders. In the paleontological literature, the main focus has been on the first problem, and it has been almost completely based on dental morphology, for the obvious reason that teeth are all we have for most taxa. We propose to examine the criteria that have been used to distinguish flying squirrels in the fossil record, and to determine if they are supported by the dental morphology of Recent squirrels, both flying squirrels and all others. Second, we submit a list of postcranial characters that should be retrievable and recognizable in the fossil record and that would demonstrate that the fossils were derived from squirrels that could actually glide. Fossils have long been identified as belonging to flying squirrels. Major (1893) described Sciuropterus albanensis (= Al-banensia albanensis), comparing it with Pteromys phaeomelas (= Aeromys tephromelas), and suggested that several other fossil squirrels were in fact flying squirrels. He reviewed a number of Eocene rodents that showed "analogies" to the dental morphology of Recent flying squirrels, but he included rodents of several...