THE CORRELATION BETWEEN SUTURE COMPLEXITY AND SHELL FORM IN AMMONITES OF THE PIERRE AND CARLILE SHALE

Throughout ammonite evolution, their shell suture patterns grew increasingly more complex. The purpose of these immovable joints has long been debated among researchers. A common hypothesis is that complex sutures are used to aid structural integrity. In this study, the suture patterns of coiled, straight, and heteromorphic ammonites were quantified using box-counting fractal analysis to determine if there is a significant difference in suture complexity among shell forms. This study aims to examine the relationship between suture complexity and structural integrity. More complex suture patterns should be found in coiled shells as it aids in the coiling and growth of the shells. Ammonites from the Pierre and Carlile Shales, ranging from the Cenomanian to the Campanian, were examined. The suture complexity was compared to shell form and stratigraphy using a Two-Way ANOVA statistical test. Results indicate that there is a significant difference in suture complexity among the defined shell forms (X=24.20, df=2, p<0.001,). Pierre Shale ammonites averaged a complexity of 1.7783 for coiled, 1.7635 for straight, and 1.7337 for heteromorphic. The Carlile Shale ammonites averaged a complexity of 1.7852 for straight, 1.692 for coiled, and 1.7262 for heteromorphic. Results also indicate that there is a significant difference in suture complexity between ammonites from the Pierre and Carlile shales (F=0.08, df=2,385, p<0.001). The difference in variation is caused by the coiling in the shells. As the shells begin to uncoil, the complexity of the sutures begin to be less complex as exhibited in both the Pierre and Carlile Shale. In the Carlile Shale, the straight shells have significantly more complex suture patterns.