Uses for Incomplete Ammonite Sutures: Lateral Lobe and Second Saddle as Markers of Sutural Complexity

Abstract

Ammonoid sutures are geometric patterns formed by the intersection of the septa and the shell wall, and have long been a diagnostic tool for ammonite researchers for such applications as species identification, taxonomic relationships, ontogenetic change, functional and evolutionary morphology, determination of ecological niche, and other aspects of ammonoid paleobiology. Researchers interested in a variety of paleobiological questions related to ammonoids have almost always required access to the entire hemisuture. Without access to specimens in museum or institutional collections, researchers must rely on previously published illustrations and photographs of ammonoid sutures. However, due to the perspective in photographs, distortion of the marginal elements of suture geometry occurs due to shell curvature near the venter and umbilicus when photographed in profile. The revised approach described here, which we refer to as the Lateral Lobe Saddle, or LLS approach, makes use of only the lateral lobe and second saddle S2 (lateral lobe-second saddle pairs, or LLS) which lie in the central, mid-whorl undistorted sector of a suture line as viewed in lateral, profile shell photos and illustrations. The factors by which fractal dimension of LLS data convert to fractal dimension of the standard hemisuture measurements are largely consistent within genera. The LLS method’s non-requirement of a full hemisuture also facilitates comparisons among sutures within an ontogenetic sequence, or sutures from multiple ammonite taxa where ventral and umbilical sutural elements are hidden by whorl overlap or poor preservation.

Ontogenetic Trends of Sutural Complexity in Jurassic Ammonites

“…Because the degree of accuracy changes based on the rule size, size-independent applications of fractal geometry are especially useful for ammonite shells or any fractal-mimicking object that is subject to growth. The LLS method described in [11] addresses the process by which partial ammonite sutures can be used to collect ontogenetic data from a single specimen. When taking a fractal dimension of a portion of the suture, the fractal dimensions are higher than they would be for the full suture.…”

“…These values were calculated for incomplete sutures using only the lateral lobe (L) and second saddle (S 2 ), or LLS. However, Marriott and Chamberlain [11] noted the separate value of leaving fractal values unconverted. Because of the wider range of "natural fractal" values required for unconverted LLS data, a more nuanced examination of the differences between ontogenetic data points is possible.…”

“…We measured the ontogenetic trajectories for suture complexity, expressed as a fractal dimension, for the photographed ammonite specimens used in this study. The Kruskal-Wallis analyses from [11] indicated that LLS tracings from the photographs could be used for the analysis of suture complexity without significant distortion. Photographs were shared from the research collection of Rene Hoffmann and were also taken from the Museum National d'Histoire Naturelle in Paris and the online repository JSD Ammonites, the private collection of Jean-Stephane David, which is based in France (File S2 in the Supplementary Materials).…”

“…Similar plots for the complexity of the ammonite suture over ontogeny have been difficult to procure in large part due to the phenomenon of whorl overlap, by which the marginal portions of the suture are covered up by new revolutions of shell growth, and only the medial portions of the hemisuture are easily accessible. In 2021, Marriott and Chamberlain [11] devised a mathematical modification to normalize data taken from the medial part of the hemisuture, namely the lateral lobe (L) and second saddle (S 2 ), or LLS, which is visible and uncovered by successive whorls in most sutured ammonites. The LLS data greatly elongated the portion of a single shell that can be used to plot the fractal dimensions of sutures within a single specimen's ontogenetic sequence.…”

“…The other more abstract driver of complexity appears to be the shape and configuration of the suture pattern itself. For example, in baculitids measured in [11], more geometrically orthogonal saddles and lobes offered lower complexity, and with it, lower conversion values. The saddles and lobes of the more complex baculitids had narrow bases and wide, branching septal fluting configurations.…”