Suture Lab Research
Guo et al. 2025: Temporal Implications of the Cerata-Septa Model: Evolutionary Stasis and Adaptive Radiation in Ammonoid Septal Geomtetry Before and After the Appearance of Complex Bidirectional Fluting
ABSTRACT: The current prevailing explanation for septal genesis in ammonoids, the Cerata-Septa Model, compares ammonoid sutures to the branching of recent dendronotid sea slugs. Ceratitids are the earliest order of Mesozoic ammonoids, and they represent the first appearance ofcomplex septal fluting, in which the major folds of the ammonoid suture line (namely, the lobes) begin to subdivide into multiple smaller folds. The onset of fluting marks a key evolutionary shift in septal geometry, before which quantitative analysis of partial sutures was universal to all ammonoids, and immediately after which, the fractal geometry of ammonoid sutures became substantially more diverse and specific to individual genera. We endeavored to determine at what point in ammonoid evolution the Cerata-Septa Model becomes appropriate, given that branching patterns in dendronotids undergo short-term adaptive radiation from ecological instability and are apparently evolutionarily static in early ammonoids. Ceratitids follow an ontogenetic trajectory similar to that of Jurassic and Cretaceous ammonites, indicating that even unidirectional fluting created enough complexity to result in a rapid, irregular increase in complexity. We compared ontogenetic trajectories of fractal dimension for dendronotids versus ceratitids, and compared morphological variance in Jurassic and Cretaceous ammonite families of varying temporal range via Procrustes analysis. We find that later Mesozoic ammonoids (Cretaceous and some Jurassic) show accelerating degrees of variability over time, and therefore, the Cerata-Septa Model may be applied to them. This suggests a shift in septal genesis from stasis in early, less complex ammonoid evolution to possible adaptive radiation in the Late Mesozoic.
Marriott et al. 2024: Bigger on the Inside: South Pole Maastrichtian Ammonites and Bergmann’s Rule
ABSTRACT: The early classification and description of Didymoceras were largely done in the absence of its juvenile sutures, which are rarely accessible through museum collections, which favor larger and more portions of the shell, and of those juvenile stages known, shell is still covering the sutures. Although morphologically dissimilar, available adult Didymoceras suture patterns showed remarkable similarities to Lytoceras in septal development over ontogenetic progression. Upper Cretaceous heteromorphs were once thought to have a close phylogenetic relationship with Lytoceras. However, they are now understood to be merely extremely morphologically convergent despite only a distant taxonomic relationship, and such septal similarities may simply be a result of similar environmental pressures. Our study aims to clarify and contribute to understanding the relationship between Jurassic lytoceratid ammonites and Didymoceras using the first-ever record of intact septal plans in a juvenile Didymoceras. We compared several aspects of the septal development of Didymoceras, including complexity over time, elongation of the external lobe compared with Jurassic and Cretaceous ontogenetic sequences (such as those published by Schindewolf, 1968), and intraseptal distance. We found that the development of fractal complexity over ontogeny is non-linear, as it is for most other ammonites, and that the mature phase in Didymoceras seems to coincide with a plateauing of fractal values, as is common in Jurassic lytoceratids and Cretaceous gaudryceratids.
Zhang et al. 2024: First description of septal development in juvenile Didymoceras (Hyatt, 1894): Implications for lytoceratid-derived ammonoid development
ABSTRACT: The early classification and description of Didymoceras were largely done in the absence of its juvenile sutures, which are rarely accessible through museum collections, which favor larger and more portions of the shell, and of those juvenile stages known, shell is still covering the sutures. Although morphologically dissimilar, available adult Didymoceras suture patterns showed remarkable similarities to Lytoceras in septal development over ontogenetic progression. Upper Cretaceous heteromorphs were once thought to have a close phylogenetic relationship with Lytoceras. However, they are now understood to be merely extremely morphologically convergent despite only a distant taxonomic relationship, and such septal similarities may simply be a result of similar environmental pressures. Our study aims to clarify and contribute to understanding the relationship between Jurassic lytoceratid ammonites and Didymoceras using the first-ever record of intact septal plans in a juvenile Didymoceras. We compared several aspects of the septal development of Didymoceras, including complexity over time, elongation of the external lobe compared with Jurassic and Cretaceous ontogenetic sequences (such as those published by Schindewolf, 1968), and intraseptal distance. We found that the development of fractal complexity over ontogeny is non-linear, as it is for most other ammonites, and that the mature phase in Didymoceras seems to coincide with a plateauing of fractal values, as is common in Jurassic lytoceratids and Cretaceous gaudryceratids.
Tran et al. 2023: Vertical Exaggeration When Using Unconverted LLS Data in Ontogenetic Sequences of Ammonite Sutures
ABSTRACT: Size-independent modifications to the fractal method have been employed by ammonite researchers since the 1990s as a way to exercise consistency when measuring ammonite sutures, and make use of those published without a scale index. In 2021, a new method to extrapolate the full hemisuture’s fractal dimension using just the lateral lobe (L) and second saddle (S2) (the LLS method) further adapted these methods The fractal dimension of the portion defined by the lateral lobe and second saddle (LLS) of an ammonoid suture can be measured by the Richardson method and adjusted to reflect the fractal dimension of the full hemisuture by dividing by a factor consistent at the genus level. Our paper explores the amount of vertical exaggeration exhibited by unconverted LLS data across the ontogeny of individual genera by examining suture ontogeny of 7 genera representing 3 highly disparate families on 8 ontogenetic sequences of very immature ammonites. We tested 4 variables from the spreadsheet, including fractal dimensions in the Richardson and unconverted LLS methods; S(Lmax), the proportion of (L) to the hemisuture; and the calculated conversion value for each suture. We found that conversion values are only necessary for sutures at or nearing the point of maturity, which we define in this study as the first suture that shows 3 levels of infolding. We found that vertical exaggeration increased over ontogeny, although conversion values are only needed after the onset of maturity. Overall, immature sutures, regardless of taxonomic affinity, conversion value (V) is approximately equal to 1. Prior to the onset of maturity, variance around V=1 were not statistically significant. After maturity, conversion values become necessary.