Growth patterns deduced from the bone histology of the cynodonts Diademodon and Cynognathus

Diademodon and Cynognathus were contemporary cynodonts that lived during th e Early to Middle Triassic. Their fossilized remains are found in the same deposits and, in the absence of diagnostic cranial material, the postcrania l skeletons of these animals are considered indistinguishable from one anot her. In this study the bone histology of Diademodon and Cynognathus was exa mined, using thin sections of several long bone elements. Distinctive histo logical patterns were recognized for each genus: the cortex of Diademodon i s zonal, consisting of alternating bands of fibro-lamellar and lamellar bon e tissue, whereas that of Cynognathus consists of uninterrupted fibro-lamel lar bone tissue. The bone histology indicates that Diademodon had a cyclica l growth strategy whereas Cynognathus grew continually, at a constant, rapi d rate throughout the year. Quantification of the vascularization of the co mpacta using image analysis. has shown that the bone tissue of Cynognathus is significantly more vascularized than that of Diademodon. The marked diff erences in their bone histology and vascularization, have permitted the gen eric identification and distinction of postcranial material (limb bones) pr eviously categorized as either Cynognathus or Diademodon. This study of Dia demodon and Cynognathus is the first detailed examination of the bone histo logy of representatives of the Triassic Cynodontia. The characteristic patt erns 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 ma terial is absent.