Biological aspects of the Permian dicynodont Oudenodon (Therapsida: Dicynodontia) deduced from bone histology and cross-sectional geometry

INTRODUCTION The Dicynodontia are recognized as the most successful herbivorous therapsid group of the Late Permian (King 1990). They radiated into several, varied ecological niches and became the most numerous herbivorous tetrapods by the end of this period (Hotton 1986; King 1990). As a result of the highly varied and widespread nature of this group, the dicynodonts have received much attention in the literature, both systematically and morphologically (e.g. Angielczyk 2001; Cluver & Hotton 1983; Cluver & King 1983; Hotton 1986; Keyser 1975; Keyser & Cruickshank 1979; Rubidge & Sidor 2001). The focus of previous studies has been on cranial descriptions, locomotory modifications and masticatory systems (e.g. Cluver 1971; Cluver & Hotton 1983; Cluver & King 1983; Cox 1998; Kemp 1982; Keyser 1975; King 1996; King et al. 1989), while other biological traits, such as growth patterns and lifestyle habits, have been less extensively studied. A few studies, such as those of Chinsamy & Rubidge (1993), Enlow and Brown (1957) and Ricqlès (1972, 1976, 1991), have used bone histology to deduce the growth patterns of several dicynodont genera. However, these studies were not comprehensive examinations and thus provided limited information. In addition, only one type of element was usually examined, making it difficult to deduce generic patterns of growth. The lifestyle habits of a few dicynodonts such as Lystrosaurus (Brink 1951; Broom 1903; Groenewald 1991; King 1991; King & Cluver 1991), Cistecephalus (Cluver 1978) and Diictodon (Ray & Chinsamy 2003; Smith 1987) have been examined using burrow casts and functional anatomy to suggest specific modes of life. Oudenodon was a medium-sized dicynodont (skull length from 100–300 mm) whose skeletal remains have been excavated from Late Permian deposits in South Africa (Cluver & Hotton 1983). The morphology of the skull has been described in detail (Broom 1912; Cluver & Hotton 1983; Cluver & King 1983; Keyser 1975; Owen 1860) and distinctive features include a lack of teeth in both the upper and lower jaws, a deep and relatively narrow secondary palate, a sharp maxillary crest behind the caniniform process, the absence of maxillary tusks and narrow dentary tables on the dentaries (Cluver & Hotton 1983; Cluver & King 1983). Although the cranial morphology of Oudenodon (Cluver & Hotton 1983; Cluver & King 1983; Keyser 1975;) and to a lesser extent the postcranial skeleton (Broom 1901), have been examined, little pertaining to the biology of this genus has been discussed. Thus, there is inadequate information regarding the growth patterns and lifestyle habits of Oudenodon for deducing its overall biology. Bone histology is a well-established technique for examining growth patterns and lifestyle habits of extinct animals (e.g. Enlow & Brown 1957; Reid 1996; Ricqlès 1976, 1980). The bone histology of Oudenodon has previously been described by Ricqlès (1972) and Chinsamy & Rubidge (1993). Ricqlès (1972) examined a humerus and a femur, whereas Chinsamy & Rubidge (1993) used a humerus to interpret the growth patterns of Oudenodon. Although informative, both descriptions were brief and neither study considered inter-elemental histovariability. Although, Ricqlès (1972) examined a humerus and a femur, histological variation between the two elements was not discussed. It is becoming increasingly evident that inter-elemental histovariability should be considered when deducing the overall growth patterns of a genus (Botha 2002; Curry 1999; Horner et al. 1999, 2000; Starck & Chinsamy 2002; Ray et al., in press). A technique known as bone cross-sectional geometry can be used in conjunction with bone histology analysis to