From morphology to behaviour: Quantitative morphological study of the trace fossil Helminthorhaphe

The ichnogenus Helminthorhaphe is a meandering trace fossil valuable to the study of movement behaviours of the trace-making marine invertebrates. In this work, we studied systematically the morphology of selected examples of Helminthorhaphe from the Upper Cretaceous to Miocene by means of primary morphological constructs, morphometrics, and theoretical morphological models. Eleven morphological constructs encompassing meander arrangement, meander tip, and meander course properties are established to illustrate the essential morphological features of meanders, which cover key aspects of the motion and navigation capacities of Helminthorhaphe makers. Tunnel diameter, meander width, and meander length are extracted to explore the size range of Helminthorhaphe, from which an exceptionally large end member is recognised, H. magna ichnosp. nov. Meander width versus tunnel diameter ratio and the fractal dimension of Helminthorhaphe are two prominent morphometric parameters that characterise the average closeness and space usage of the meanders, respectively, which aid differentiation between ichnospecies. Helminthorhaphe miocenica, H. magna, H. flexuosa, and H. japonica generally conform to a series of increasing closeness and space usage of the meanders. Theoretical morphogenetic analysis of the meander-generating process of Helminthorhaphe based on the bilateral phobo-thigmotactic robot car model reveals that the tempo and hierarchy of bilateral movement differentiation contribute to the formation of primary terminal U-turn at the meander tip, as well as lower-level meander-course undulations. The meander tip property proves to be an important morphological feature with biomechanical and neurobiological implications on the body flexibility and lateralisation in the nervous system of Helminthorhaphe producers. Larger-scale meander arrangement styles suggest that Helminthorhaphe makers potentially possessed certain radial or concentric spatial cognition of the environment. With an emphasis on theoretical morphogenetic analysis, this study provides a preliminary attempt in linking ichnology with behavioural, neurobiological, and intelligence sciences.