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Ammonoid septal formation and suture asymmetry explored with a geographic information systems approach ABSTRACT
Given the centrality of suture patterns to ammonoid systematics, it is remarkable how little we understand about the formation of septa. Various models for septal formation have been proposed, but given the spatial complexity of suture patterns, the means to test and constrain them have been lacking. Here we use Geographic Information Systems (GIS) to test models for septal formation by analyzing spatial constraints and asymmetries in several ammonoid species, including Cretaceous ammonites (the Middle Turonian acanthoceratacean Coilopoceras and the Early Cenomanian hoplitacean Neogastroplites) and Carboniferous goniatites (the dimorphoceratacean Metadimorphoceras and the goniatacean Somoholites). These analyses show that suture lines are not more strongly constrained at lobe tips than at saddles, nor are lobe tips more strongly inflected than saddles. Rather, the entire suture line is similarly constrained and shaped at both lobes and saddles, suggesting that the process controlling septal folding acts along the entire margin of the septal membrane rather than at a few specified tie points. Right and left opposing sutures from the same specimens do not match precisely. More generally, right and left suture patterns show asymmetries both in lengths and degrees of variability. These asymmetries are consistent across individuals sampled from different localities, implying that they are species-level traits, rather than pathologies or taphonomic deformations. The directed asymmetry of suture patterns may reflect soft part asymmetry in ammonoids. These results can be used to place constraints on alternative models for septal growth and function; the GIS-based method will permit further testing of such models.
KEY WORDS: cephalopods; sutures; GIS; morphometrics; growth; constraint
PE Article Number: 14.1.3A |
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