Lamellar-zonal bone with zones and annuli in the pelvis of a sauropod dinosaur

Abstract

De Ricqles1–7 distinguishes two principal types of primary (unremodelled) compact bone (compacta), which he terms lamellar-zonal and fibro-lamellar. In the former, bone laid down by accretion at the periosteal (external) surface shows a general coarsely lamellated texture, and vascular canals (for blood vessels) are often sparsely developed and sometimes lacking. Primary osteons, formed by inward growth of bone inside vascular canals, may be present or absent. This type of compacta is seen in bones which grow slowly or only to small sizes, and is the only type seen in most ectotherms. In contrast, fibro-lamellar bone is always rich in primary osteons, set in a matrix of woven bone which is typically laid down rapidly. This type of bone is seen mainly in medium- to large-sized endotherms, and reflects their ability to reach such sizes more quickly than comparable ectotherms. In ectotherms, lamellar-zonal bone may show a cyclical alternation of thin layers of dense bone, termed annuli, with thicker vascular layers known as zones, but it is claimed2 that such features are not developed cyclically in endotherms. The presence of fibro-lamellar compacta, without annuli and zones, in various dinosaurs7 is hence seen as evidence that dinosaurs were endotherms. However, I report here a case in which compact bone from the pelvis of a dinosaur is instead lamellar-zonal, and shows typical zones and annuli.