Cracks in fossil enamels resulting from premortem vs. postmortem events.

Vertebrate enamel preserves a record of fracture-producing strain. Fracturing during the life of the individual is potentially a source of selection for stronger enamel in the course of evolution. To determine if it is possible to recognize such fractures in fossil enamel, cracks in a variety of fossil materials, including enamel-covered holostean scales, crocodilian teeth, theropod and hadrosaurid dinosaur teeth, and mammalian teeth were examined. Cracks that occurred during the life of the individual could be recognized by abrasive wear on edges exposed at the surface of the enamel in areas worn by oral or locomotor abrasion. Certain distinctive crack patterns were identified as results of specific stress states occurring during life. Transverse cracks on the anterior parts of Lepisosteus scales were probably caused by external loading. Hertzian cracks and shallow, arcuate, lateral cracks on the occlusal edges of tooth enamel appear to be caused by stress concentrating impacts. Horizontal cracks arranged asymmetrically on the sides of conical teeth were reproduced in models subjected to bending stresses. Oblique cracks near the tips of conical fossil teeth were produced in models by oblique loads near the tip. Vertical cracks around cylindrical or conical tooth surfaces may be caused by several different sources of stress, including lateral "wind" loads and vertical "snow" loads. Of the postmortem causes of fracturing of fossil enamel, drying cracks seem to be the most important. Experimental drying produced from 25% to 50% of the cracks in dry teeth.