Date of Award
Master of Science (MS)
Dr. Laura Wilson Brantley
Throughout ammonite evolution, shell suture patterns grew increasingly more complex, but the purpose of these sutures has long been debated. One hypothesis is that suture complexity is related to the structural integrity of the shell under pressure. To test this hypothesis, suture complexity was compared to shell form and stratigraphic formation to determine if there were significant differences in suture complexity, as a proxy for structural integrity, among shell forms or stratigraphic formations. Highly complex sutures might have allowed for the tightly coiled form of many ammonites, an advantage compared to less coiled forms because the pressure is distributed over more points on the shell. If this is the case, coiled forms should have more complex suture patterns. Suture pattern complexities of coiled, straight, and heteromorphic ammonite shell forms from the Pierre, Carlile, Greenhorn, Graneros, and Mowry shales were quantified using box-counting fractal analysis. Results indicate there is a significant difference in the median suture complexity among the defined shell forms (H = 27.9, df = 2, p < 0.001). A Kruskal-Wallis multiple comparisons test confirms there is a significant difference in median suture complexity between coiled and heteromorphic shell forms (p < 0.03), and a significant difference in median suture complexity between heteromorphic and straight shell forms (p < 0.03). However, there is no significant difference in median suture complexity between coiled and straight shell forms (p > 0.03). The most complex suture patterns are typically found in tightly coiled shells, possibly adding structural support as the coiled form evolved. Most of the straight shell forms examined in this project evolved from coiled forms and perhaps retained highly complex sutures to protect against hydrostatic pressure. On the other hand, heteromorphic shell forms may have significantly reduced their suture complexity to loosen the coil of their shell. However, these forms did not require suture patterns as complex as the straight shell forms because the partially coiled shell would have provided more protection against hydrostatic pressure than the straight shell. Results also show no significant difference in median suture complexities among formations (H = 5.24, df = 4, p = 0.264), suggesting that there is no significant change in median suture complexity over time.
Copyright 2017 Darrah Jorgensen
Jorgensen, Darrah, "The Relationship Among Suture Complexity, Shell Form,
and Stratigraphic Formation In Ammonites Of The
Western Interior Seaway" (2017). Masters Theses. 5.