Conclusions
Desiccation cracks in bone (Fig. 6C, D) are quite distinct from radial microcracks, which occur during fossilization under aquatic conditions (Fig. 6A, B). The stresses in drying bone produce central and peripheral radial cracks in secondary osteons, combined with the separation of the osteons from the surrounding bone by circumferential cracks. Peripheral radial cracks start at the circumference of secondary osteons, but never pass into the adjacent bone. In contrast, radial microcracks in bones from aquatic sites (Pfretzschner, 2000 and Pfretzschner, 2006) are short cracks which are peripheral and bridge the circumference of secondary osteons (Fig. 6B). Due to the stresses caused by swelling of the collagen during hydrolyzation in secondary osteons they start near the circumference of an osteon and penetrate into the surrounding bone material.
Similar patterns of desiccation cracks are present in recent goat bones and Upper Jurassic dinosaur bones from the Junggar Basin. This allows to infer that the dinosaur bones were deposited under dry conditions, which is in agreement with other palaeoclimate proxy data. This type of crack system is characteristic and differs clearly from the radial microcracks cutting through the cement lines of secondary osteons, as often observed in fossil bones deposited in aquatic settings. It is thus a histological indicator for bones fossilized in dry environments in terrestrial settings.
Abundant manganese mineral infillings of pyrolusite in the dinosaur bones from the Upper Jurassic Shishugou Formation of the Junggar Basin commonly preserve histologic bone microstructures such as canaliculi and osteocyte lacunae and also fill early diagenetic desiccation cracks. Under evaporative conditions, dissolved manganese was probably transported by capillary forces with soil solutions into the bones still exposed on the surface during early fossilization, possibly over several months or years. During this time, the capillary system of the bone was connected continuously or with interruptions with water in the sediment. Partial mineral infillings of pyrolusite in the Haversian canals form multiple spirit levels in the dinosaur bones that indicate rotation of the dinosaur bones during fossilization probably during flash floods.