Air sac attachments or tendon scars: the distinction between soft tissue traces in archosaur bone

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