Abstract

It can be arduous to discriminate between the lamellar bone fibres, the Sharpey’s fibres (tendon insertions), and the pneumosteum (air sacs diverticula interaction with the bone) during histological observations. We analysed thin sections of several avemetatarsalians from the Cretaceous of Brazil and brought characters from optical mineralogy to differentiate those biomineralized tissue types, resulting in a solid criterion for histological comparison. The observation scale is one of the best guides to differentiate both tissues. Lamellar bone fibres are only perceptible at very high magnifications (100 X) whilst Sharpey’s fibres are detectable at low magnification (5 X). Pneumosteal bone fibres are discernible at 40-50 X. Hence, fibre length is longer in Sharpey’s ones than in the pneumosteum. High optical relief is a good proxy to detect connective tissue insertion scars (Sharpey’s fibres). Finally, the texture is an excellent proxy for identifying pneumosteal bone. The asbestiform aspect of this structure is so far unique among several bone tissue types.