In lieu of an abstract, here is a brief excerpt of the content:

149 6 Biological Inferences of the Cranial Microstructure of the Dicynodonts Oudenodon and Lystrosaurus Sandra C. Jasinoski and Anusuya Chinsamy-Turan Dicynodonts were nonmammalian therapsids and the most common herbivores during the Permo-Triassic. The remains of dicynodonts are well documented from therapsid bearing deposits worldwide, and because of their abundance, they have been used as biostratigraphic markers (e.g., Kitching 1977) (see Plate 1, Fig. 2.2). Their derived feeding apparatus, which allowed orthal and propalinal jaw movement, may have accounted for their success as herbivores (Crompton and Hotton 1967). Further details of their morphology and paleobiology are given in the following sections and chapter 5 of this book. Studies of bone microstructure of various extant tetrapods have demonstrated its usefulness in deducing various aspects of the biology of the animal (e.g., Enlow 1969; Starck and Chinsamy 2002) and have also provided insight into biomechanical adaptations of the skeleton (e.g., Bouvier and Hylander 1981; de Margerie 2002). Here we undertake a comparative examination of the cranial histology of the dicynodonts Oudenodon and Lystrosaurus (see Plates J and K) in order to assess whether functional signatures were recorded in their bone microstructure. This study represents the first comprehensive cranial histological analysis of an extinct tetrapod and documents the microstructure as well as the histovariability that exists among various cranial elements. In conjunction with previous quantitative (e.g., finite element analysis [Jasinoski, Rayfield, and Chinsamy 2010a]) and qualitative functional analyses (e.g., King and Cluver 1991), we evaluate to what extent histology provides insight into dicynodont cranial function. Phylogenetic Context Within the Dicynodontia, Oudenodon and Lystrosaurus are both part of the higher taxonomic group Bidentalia (Kammerer and Angielczyk 2009). This group was split into the clade Cryptodontia, which contains the family Oudenodontidae (Oudenodon), and the more derived clade Dicynodontoidea, containing the family Lystrosauridae (Lystrosaurus) (Kammerer and Angielczyk 2009). The genus Lystrosaurus occurred from the Late Permian to the Early Triassic and consists of four species: L. curvatus, L. declivis, L. maccaigi, and L. murrayi (Grine et al. 2006). Among these, Lystrosaurus curvatus Introduction Jasinoski and Chinsamy-Turan 150 is the only confirmed species to have crossed the end-Permian extinction boundary (Smith and Botha 2005). The genus Oudenodon occurred only during the Late Permian and is represented by three species, with O. bainii being the genotype and most common species (King 1988). Paleobiological Context: Dicynodont Cranial Function Except for early dicynodonts, a keratin layer covered the surfaces of the upper and lower jaws and replaced the function of teeth (King 1990). Small foramina on the surface of the bones, similar to the pitting on regions of horn attachment in extant chelonians, were used to infer the presence of a keratin beak (King 1990; Angielczyk 2001). Using this correlation , it was deduced that keratin covered the premaxilla and maxilla, forming an effective shearing blade against the lateral edge of the mandible , and the anteroventral (rugose) part of the palatine, which provided an effective grinding surface during retraction of the mandible (Cluver 1971; Jasinoski, Rayfield, and Chinsamy 2010b). A bony secondary palate was also present, allowing simultaneous food processing and breathing (King 1990), as well as strengthening of the rostrum (Thomason and Russell 1986). Early studies of dicynodont cranial function (Watson 1948; Crompton and Hotton 1967) indicated that the specialized jaw joint of dicynodonts accommodated propalinal, or horizontal, movement of the mandible. The dicynodont masticatory cycle consisted of a beak bite, where shearing of food occurred near the front of the keratin-covered beak, followed by a retraction of the mandible, where food was ground against the rugose palatines (Crompton and Hotton 1967). Jaw movement was facilitated by two large jaw adductor muscles: the M. adductor mandibulae externus lateralis and medialis (Crompton and Hotton 1967). Food processing did not occur during jaw protraction due to the relatively small size of the posterior and internal adductor muscles (Crompton and Hotton 1967). Studies of tooth wear suggested that the tusks of some dicynodonts were used to rake or grub in the substrate (Cluver 1971; Hotton 1986), although not all researchers agree with this conclusion. In Diictodon (see Plate F), the tusks may have been a sexual dimorphic feature, serving a display or armament function (Sullivan, Reisz, and Smith 2003). However, the tusks of other dicynodonts, such as Lystrosaurus (see Ray 2005), may not have been used for sexual display. Changes in cranial structure across Dicynodontia have been correlated to differences in muscle force and masticatory function, which in turn may reflect differences in...

Share