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Abstract for presentation at 12th North American Paleontological Convention, University of Michigan 17-21 June 2024. Program with Abstracts. University of Michigan Papers on Paleontology No. 39, pp. 429-430. 

The complexity of ammonoid cephalopod sutures has made them useful as a characteristic for distinguishing taxa and may also reflect variations in modes of life. Sutural complexity is due to the unique pattern of saddles and lobes that vary in size, amplitude, and wavelength. These patterns are characterized as having various degrees of complexity that can be assigned numerical values. Two of the most used quantitative sutural complexity metrics are fractal dimension (FD) and sinuosity index (SI). Our research goal was to document differences between suture complexities of various taxa of two Cretaceous ammonoid superfamilies, Desmoceratoidea and Acanthoceratoidea, and assess how these complexities changed over time. Previous workers have used various procedures to quantify suture complexity, including measuring the suture lines by hand or using software such as ImageJ to take measurements. While these methods can provide accurate results, they are time consuming and therefore restrict the realistic sample size. We have developed a semi-automated method using AutoCAD (2023) engineering software to quantify both FD and SI of suture patterns. Using Adobe Illustrator, drawings of suture patterns taken from the literature were first converted from raster images to vector files in order for AutoCAD to read them. AutoCAD then automatically scaled and measured each suture pattern in a short amount of time, which allowed for a large sample of 78 suture patterns to be quickly analyzed. Preliminary results show that desmoceratoids had significantly more complex suture patterns than most acanthoceratoids, and that acanthoceratoid suture complexity may have increased through the Late Cenomanian stage before dropping after the Cenomanian-Turonian mass extinction. Using this new method of quantifying suture pattern complexity will expedite the measuring process and allow larger sample sizes in future analyses. 

Ammonoids of the Cretaceous superfamily Desmoceratoidea are geographically and stratigraphically widespread. However, the group is understudied, with just one prominent Japanese worker, T. Matsumoto, who worked on desmoceratoids extensively in the 1970s-1990s. Since then, few researchers have studied desmoceratoid taxonomy, phylogeny, or ecology. Based on their geographic distributions and typical geologic contexts, desmoceratoids are traditionally interpreted to have been open ocean dwellers. However, details of their paleoecology and mode of life are lacking, limiting our ability to interpret desmoceratoid diversity dynamics through the Cretaceous. The Westermann morphospace method is a way to predict the life mode of coiled (non-heteromorph) ammonoids based on morphological measurements of shell shape. The morphospace has three end-member morphotypes, spherocone, oxycone, and serpenticone, with predicted modes of life for different morphospace regions. Shell shape measurements were made on 112 desmoceratoid specimens from collections at the U.S. National Museum of Natural History and the Natural History Museum in London (UK); these specimens are from every continent except Australia and Antarctica. Westermann parameters were computed and specimens were plotted in Westermann morphospace so that patterns of morphospace occupation could be assessed. Desmoceratoids generally fall towards the oxycone corner of the morphospace, with genera falling into two groups, one more platyconic with predicted good swimming efficiency and one more discoconic with more limited swimming efficiency. Lewesiceras and Desmoceras have specimens in both groups, while Puzosia uniquely occupies a more central position in the morphospace. Temporally and regionally, there were no significant variations in morphology. While often interpreted as uniformly effective open ocean swimmers, these results indicate that desmoceratoid ammonoids had varying modes of life.