705 Journal of Vertebrate Paleontology 20(4):705–711, December 2000  2000 by the Society of Vertebrate Paleontology GROWTH PATTERNS DEDUCED FROM THE BONE HISTOLOGY OF THE CYNODONTS DIADEMODON AND CYNOGNATHUS JENNIFER BOTHA 1 and ANUSUYA CHINSAMY 1,2 1 Zoology Department, University of Cape Town, Rondebosch, 7701, Cape Town, South Africa; 2 South African Museum, P.O. Box 61, 8000, Cape Town, South Africa ABSTRACT—Diademodon and Cynognathus were contemporary cynodonts that lived during the Early to Middle Triassic. Their fossilized remains are found in the same deposits and, in the absence of diagnostic cranial material, the postcranial skeletons of these animals are considered indistinguishable from one another. In this study the bone histology of Diademodon and Cynognathus was examined, using thin sections of several long bone elements. Distinctive histo- logical patterns were recognized for each genus: the cortex of Diademodon is zonal, consisting of alternating bands of fibro-lamellar and lamellar bone tissue, whereas that of Cynognathus consists of uninterrupted fibro-lamellar bone tissue. The bone histology indicates that Diademodon had a cyclical growth strategy whereas Cynognathus grew continually, at a constant, rapid rate throughout the year. Quantification of the vascularization of the compacta using image analysis, has shown that the bone tissue of Cynognathus is significantly more vascularized than that of Diade- modon. The marked differences in their bone histology and vascularization, have permitted the generic identification and distinction of postcranial material (limb bones) previously categorized as either Cynognathus or Diademodon. This study of Diademodon and Cynognathus is the first detailed examination of the bone histology of representatives of the Triassic Cynodontia. The characteristic patterns of bone histology in Diademodon and Cynognathus have directly revealed information about their growth strategies and has demonstrated the use of bone histology to distinguish between the genera when associated cranial material is absent. INTRODUCTION The radiation of the Cynodontia is generally regarded as one of the most significant events in therapsid history because it includes the ancestry of the mammals. Diademodon and Cy- nognathus are cynodonts that lived during the Early to Middle Triassic (Scythian to Early Anisian; Kitching, 1995), their re- mains being well represented in the Cynognathus Assemblage Zone in the Beaufort Group of Southern Africa. Diademodon, an omnivore, and Cynognathus, a carnivore, have very distinc- tive cranial and dental morphologies. Yet, their postcranial skel- etons appear to be indistinguishable (Brink, 1955; Jenkins, 1971) except for slight differences, such as those in the neural spines and centra of the vertebral column (Brink, 1955). Be- cause these animals were contemporaries, their remains are fre- quently found in the same fossil assemblages, which poses dif- ficulty in distinguishing their postcrania when no associated cranial material is preserved. The gross skeletal morphology of Diademodon (Seeley, 1894; Watson, 1911; Brink, 1955; Hopson, 1971; Grine, 1977; Grine et al., 1978) and Cynognathus (Seeley, 1908; Broom, 1911, 1913; Gregory and Camp, 1918) is reasonably well stud- ied (Brink, 1955; Jenkins, 1971; Kemp, 1982), though very little is known about their biology. This study documents the bone histology and the histological variation within each genus, as well as ontogenetic changes in the bone. Bone histology has become increasingly significant in paleobiological studies of ex- tinct vertebrates. Assessments regarding ontogeny, growth and various other aspects of the life history of animals can be di- rectly deduced from the bone (Chinsamy and Dodson, 1995). Comparative studies of the bone histology of extinct and extant animals (e.g., Enlow and Brown, 1956; Ricqle `s, 1969, 1976; Peabody, 1961; Castanet et al., 1988; Chinsamy 1991) have provided a wealth of information regarding the ontogeny, in- dividual age, growth and, indirectly, the physiology of fossil animals. This study represents the first detailed account of Triassic cynodont bone histology and provides a direct assessment of the growth strategies of Diademodon and Cynognathus. MATERIALS AND METHODS Long bones (humeri, femora, tibiae, fibulae) were used in this study as they are not extensively remodeled (particularly in the mid-shaft region) and therefore permit a reasonable as- sessment of an animal’s ontogenetic growth pattern (Chinsamy and Dodson, 1995). In addition, Horner et al. (1999) found that LAG (lines of arrested growth) counts were more readily ob- served due to better preservation. All thirteen skeletal elements examined in the study were excavated from the Cynognathus Assemblage Zone, Beaufort Group, South Africa (Table 1). Only postcranial material di- rectly associated with cranial elements permitting identification of the particular genus were used in this study. Ten diagnostic Diademodon limb elements were collected from locations at Lady Frere, Burgersdorp, Aliwal North, and Rouxville, South Africa. Three diagnostic limb elements of Cynognathus were collected from Aliwal North, South Africa. The specimens were photographed and the total length and mid-shaft diameters were measured (Table 1). The width of the proximal and distal regions were also measured for comparison between incomplete and complete specimens (Table 1). Thin sections were prepared according to the methodology of Chin- samy and Raath (1992) and were examined using a petrographic microscope. Image analysis, according to the method of Chin- samy (1993), was used to quantify the degree of vascularization of each section. Vascularization was quantified in the different skeletal elements of each genus, as well as between them. In addition, vascularity was quantified in juveniles and adults of Diademodon. Such measurements were standardized in the mid-cortical region of mid-shaft sections of each bone. Institutional Abbreviations—BP/I/, Bernard Price Institute for Paleontological Research, Johannesburg, South Africa; NMQR, National Museum, Bloemfontein, South Africa; SAM-