Mammalia 72 (2008): 286–296  2008 by Walter de Gruyter • Berlin • New York. DOI 10.1515/MAMM.2008.049 Article in press - uncorrected proof 2008/49 Phylogeny, adaptation and mandible shape in Sciuridae (Rodentia, Mammalia) Jacques Michaux 1, *, Lionel Hautier 2 , Tiennet Simonin 3 and Monique Vianey-Liaud 2 1 Laboratoire EPHE de Pale ´ ontologie des Verte ´ bre ´ s et Institut des Sciences de l’Evolution, UMR 5554, CC 064, Universite ´ Montpellier 2, Place Euge ` ne Bataillon, 34095 Montpellier Cedex 05, France, e-mail: jacques-jules.michaux@univ-montp2.fr 2 Laboratoire de Pale ´ ontologie, Institut des Sciences de l’Evolution de Montpellier, UMR-CNRS 5554, CC 064, Universite ´ Montpellier 2, Place Euge ` ne Bataillon, 34095 Montpellier Cedex 05, France 3 Tiennet Simonin, 9 rue des Clercs, 38000 Grenoble, France *Corresponding author Abstract Morphological diversity in the sciurid family was analysed by applying geometric morphometry (Fourier transform) to the mandible outline. The results obtained showed that phylogeny and ecology had a prominent role in shaping the mandible. Three distinct extreme forms are recognis- ed illustrated by Marmota, dwarf tree squirrels and dwarf flying squirrels, respectively. Contrary to the mandible of the dwarf flying squirrel, the dwarf tree squirrel mandible does not have a developed coronoid process, while Mar- mota mandible is rather elongated. Tooth morphology, as well as chewing direction, studied by microwear analysis were only slightly variable and had no clear impact on the shape of the mandible. A study of the mandible of the Early Oligocene European squirrel Palaeosciurus goti did not contradict a preliminary interpretation (based on its limbs) that it had been a ground dweller, despite the fact that its mandible still exhibits a primitive outline. Keywords: adaptive radiation; elliptic Fourier transform; mandible; Sciuridae. Introduction The sciurid family (or squirrels) includes approximately 51 genera and 278 species and is the third most diverse family of the order Rodentia (Wilson and Reeder 2005). The family also provides a good example of complex evolutionary radiation (Mercer and Roth 2003). Sciurids vary in size from very small dwarf squirrels (approximate- ly 10 g) to large marmots or woodchucks (approximately 3–8 kg) and can be found living in a variety of different environments ranging from steppes to forests. While many sciurids are ground dwellers and some are burrow- ers, most of them (including the flying squirrels) are tree dwellers (Nowak 1999, Wilson and Reeder 2005). The spectrum of their alimentary habits extends from animal dominant to plant dominant diets. It has also been noted that a few species tend to be more clearly animal eaters, while some are more plant eaters. The external mor- phology of sciurids is rather typical, because they are either squirrel-like or marmot-like. The zygomasseteric architecture of their skulls is sciuromorphous, with a small infraorbital foramen associated with a large zygo- matic plate. However, the oldest members of the family, dating from the Late Eocene period (McKenna and Bell 1997), show either protrogomorphous-like zygomasse- teric architecture (e.g., Protosciurus Black, 1965, Cedro- mus Korth and Emry, 1991) or sciuromophous-like architecture (Early Oligocene Palaeosciurus Vianey- Liaud, 1974), which consequently indicates the evolution of this feature in sciurids. Sciurid mandibles exhibit a sciurognathous condition in which the horizontal branch, which bears the incisor and cheek teeth, and the vertical branch with the coronoid, condylar and angular process- es, are in the same plane. The increase in morphological diversity is the most spectacular result of a radiation in response to habitat and geographical changes, along with an increase in the number of species. If the latter is rather easy to measure, the former is evidently more difficult to quantify. In order to quantify the divergence between lineages, measure- ments must be made on a trait or feature, first shared by all the members of the group, and second to have likely changed due to adaptation. As opposed to teeth, where the number, shape and morphology may vary greatly amongst species and therefore prevent any simple meas- urement, the mandible or dentary is much easier to use. The development of the mandible is also heritable and consequently mandible shape can express phylogeny. In fact, the situation is even much more interesting, because squirrel systematics have been abundantly dis- cussed on the basis of molecular data (Mercer and Roth 2003, Herron et al. 2004, Steppan et al. 2004). The phy- logeny of the species studied based on Mercer and Roth (2003; Figure 2) is given in Figure 1A. Opposite this is given the morphological distance tree for the same sam- ples based on the seven elliptic Fourier transform (EFT) harmonics of mandible outline (Figure 1B). This aspect is discussed and developed upon later in this paper. Because the mandible is also a component of the diges- tive system, variation and selection can act upon it and so the shape of the mandible can therefore reveal adap- tation. However, the mandible is only one component of the digestive system, which also includes teeth, stomach and intestine. A forcing factor, such as competition for Brought to you by | University of Chicago Authenticated Download Date | 10/16/14 7:39 PM