Terrace ridges in trilobites
Brown, Abigail Mary
Many trilobites have cuesta-like structures, known as terrace ridges, on both the dorsal and ventral surfaces of the exoskeleton. Although terrace ridges all appear to have the same basic construction, they are highly variable and several types are known. These structures are poorly understood and there are many, varied and sometimes contradictory theories as to their function, which are discussed herein.Terrace ridge shape variation was explored across Class Trilobita, first qualitatively and then using a novel geometric morphometric technique, extended (landmarkregistered) eigenshape analysis (EEA) (MacLeod, 1999). A database containing details of over 6000 images of trilobite terrace ridges in the literature was compiled from over 450 scientific papers, from which a resource of 1600 scanned images of terrace ridges within the Asaphida was produced. A successful heuristic analysis technique was developed using EEA, analysing approximately 400 of these images. Trends in the variation of simplified terrace ridge arrays on several parts of the trilobite were successfully identified. The analysis of these terrace ridge arraysachieved good taxonomic separation and, in particular, this analysis appeared to separate pelagic and benthic terrace ridge-bearing forms, potentially providing an independent cryptic test for trilobite mode of life hypotheses based on exposed morphologies. Both qualitative and quantitative strands of research contributed to a phylogenetic discussion of terrace ridges across Class Trilobita as well as informing an analysis of the suggested functions of terrace ridges. The mapping of terrace ridge character states clarified patterns of acquisition and secondary loss of terrace ridges across the Class. Secondary losses were suggested to be related to the adoption of specialised feeding behaviours and the development of alternative types of sculpture. Some support was found for theories of frictional interaction and species recognition as roles for terrace ridges from the morphometric analyses.