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179 7 Bone and Dental Histology of Late Triassic Dicynodonts from North America Jeremy L. Green Dicynodonts were the dominant terrestrial herbivores during the Late Permian (Hotton 1986). Although diversity declined at the end of the Permian, at least four dicynodont lineages managed to survive the greatest mass extinction of the Phanerozoic (Fröbisch 2007; chapter 1 of this book), and this group continued to thrive until its eventual disappearance near the end of the Triassic (King 1990; although see also Thulborn and Turner 2003). Late Triassic dicynodonts were the last and largest (skull length [SL] > 400 mm) members of this clade (King 1990) and lived in a world of ever-changing climatic and environmental conditions associated with continental rifting in Pangaea (Simms and Ruffell 1990; Parrish 1993; Fraser 2006; Golonka 2007). Studying the life history (e.g., growth, lifestyle adaptations) of these animals may elucidate the strategies dicynodonts used to adapt to this dynamic setting and further expand our understanding of paleobiology in nonmammalian therapsids. Analyses of growth in dicynodonts have focused mainly on small- to medium-sized (SL ≤ 400 mm) taxa from Permian and Early to Middle Triassic deposits (Gross 1934; de Ricqlès 1972; Chinsamy and Rubidge 1993; Ray and Chinsamy 2004; Germain and Laurin 2005; Ray, Chinsamy, and Bandyopadhyay 2005, Ray, Bandyopadhyay, and Bhawal 2009; Botha and Angielczyk 2007; Kriloff et al. 2008; Botha-Brink and Angielczyk 2010), and our knowledge of growth in Late Triassic dicynodonts is poor relative to that of their geologically older, smaller ancestors. Additionally, previous studies have focused primarily on dicynodonts from Africa (Chinsamy and Rubidge 1993; Ray and Chinsamy 2004; Ray, Chinsamy, and Bandyopadhyay 2005; Botha and Angielczyk 2007; Botha-Brink and Angielczyk 2010) and India (Ray, Chinsamy, and Bandyopadhyay 2005, Ray, Bandyopadhyay, and Bhawal 2009), leaving North American taxa relatively unstudied. A recent study on growth in Placerias from Arizona (Green, Schweitzer, and Lamm 2010) has helped address these deficiencies. The current study expands on this previous work by conducting two separate analyses: (1) comparison of bone microstructure between Placerias (Green, Schweitzer, and Lamm 2010) and a previously unsampled, non-Placerias dicynodont (unnamed species) from North Carolina; (2) examination of the thickness and periodicity of growth increments in dicynodont tusks from North Carolina. Collectively, these studies will improve our knowledge of life history in Late Triassic dicynodonts from North America. Introduction Green 180 Institutional Abbreviations The following institutions have been abbreviated in this chapter: North Carolina Museum of Natural Sciences, Raleigh, North Carolina (NCSM); University of California Museum of Paleontology, Berkeley, California (UCMP). Our understanding of the diversity and evolution of Late Triassic dicynodonts from North America has changed drastically over the past hundred years. Before the phylogenetic context of this group can be discussed, it is necessary to review the taxonomy of dicynodonts from the Late Triassic of this continent. In the early part of the twentieth century, scientists recognized three distinct species from the Late Triassic of North America: Brachybrachium brevipes (Williston 1904); Eubrachiosaurus browni (Williston 1904); and Placerias hesternus (Lucas 1904). However, the first taxon is recognized as a nomen dubium because its holotype has been lost, and E. browni is now considered a junior synonym of P. hesternus (Lucas and Hunt 1993). Camp and Welles (1956) described another Late Triassic species, Placerias gigas, from the Placerias Quarry in Arizona, a locality so-named for its relative abundance of Placerias material (i.e., a minimum number of 41 individuals represented by over 1,600 skeletal elements [Fiorillo, Padian, and Musikasinthorn 2000]). However, this species has now also been synonymized with P. hesternus (Lucas and Hunt 1993). Although Placerias is the most commonly recovered taxon from North America, remains of non-Placerias dicynodonts have also been identified from Upper Triassic deposits of this continent; Lucas and Hunt (1993) described skeletal material they identified as cf. Ischigualastia sp. However, subsequent researchers argue that this material differs from Placerias and refute its tentative assignment to Ischigualastia. These investigators suggest this material may instead belong to Stahleckeria or Dinodontosaurus (Long and Murry 1995; Langer 2005), whereas Fröbisch (2009) recommends it be classified as Kannemeyeriiformes gen. et sp. indet. Green et al. (2005) reported a partially complete, semi-articulated postcranium of an unnamed species of dicynodont (differentially diagnosed from Placerias) from the Late Triassic of North Carolina (NCSM 21719) (Fig. 7.1), but a full description and phylogenetic analysis of this specimen has yet to be completed. Similarly, fossils of another nonPlacerias dicynodont have been recently reported from the Late...

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