Sciuridae

INTRODUCTION The family Sciuridae comprises about 275 living and numerous extinct species of squirrels – the fourth most diverse extant mammal family (after Muridae, Vespertilionidae, Soricidae; Vaughan, Ryan, and Czaplewski, 2000) – and squirrels inhabit all continents with land mammals except Australia (Nowak, 1999). The family includes nocturnal gliders and a diverse assemblage of diurnal species occupying arboreal and terrestrial niches, with sizes of extant species ranging from approximately 10 g (tropical pygmy squirrels) to approximately 7500 g (some marmots) (Nowak, 1999); the late Neogene Paenemarmota was larger still. Extant generic diversity is centered in southern and eastern Asia and adjacent islands (Nowak, 1999), but a speciose terrestrial clade of chipmunks and the tribe Marmotini (ground squirrels, marmots, prairie dogs, and extinct relatives) is most diverse in North America. The Marmotini exhibit remarkable annual cycles and diverse patterns of social behavior, and they have been intensively studied (e.g., Murie and Michener, 1984). The North American fossil record for the family extends from the late Eocene (Chadronian), documenting over 20 Tertiary genera as reviewed below. DEFINING FEATURES OF THE FAMILY SCIURIDAE CRANIAL Characterizations of Sciuridae provided by Vianey-Liaud (1985), Korth and Emry (1991), and Korth (1994) were primary sources for the cranial, dental, and skeletal descriptions which follow. The skull is relatively broad and variably convex in dorsal profile. The rostrum is typically short and bears small, paired incisive foramina ventrally.