Bone histology and growth patterns of some nonmammalian therapsids

ABSTRACT

The bone histology of gorgonopsian, therocephalian and cynodont genera, ranging from the Late Permian to Early Jurassic, was examined. The analysis reveals a predominance of cortical fibrolamellar bone tissue in most skeletal elements. The high prevalence of fibrolamellar bone tissue suggests an overall rapid osteogenesis, but interruptions in the form of annuii and/or LAGs do occur. Inter-elemental histovariation within individuals, such as primary tissue type, cortical porosity, LAGs and annuii, extent of secondary reconstruction and endosteal bone deposition, is observed. In general, propodials were found to have grown faster than epipodials. Distinct growth patterns were identified for each of the genera studied. These growth patterns vary from a cyclical growth pattern in the Late Permian gorgonopsian, Scylacops to fast, sustained growth in the Early Jurassic cynodont, Tritylodon. Growth patterns of contemporaneous genera are also noted and discussed. A possible trend towards fast growth, and an independent decrease or eventual loss of developmental plasticity within the therapsids, is proposed.