Biomechanical Analysis of Fish Swimming Trace Fossils (Undichna): Preservation and Mode of Locomotion

This chapter includes a morphological analysis of sinusoidal swimming trails of the ichnogenus Undichna and inferences on likely producers, mode of swimming, and preservation. A total of 166 Undichna specimens were measured, including selected examples from the literature and unpublished material from different basins of Argentina. These specimens belong to seven ichnospecies, including U. bina, U. britannica, U. consulca, U. insolentia, U. quina, U. simplicitas, and U. unisulca. The morphology of these ichnospecies is used in conjunction with that of the presumed producer to infer the mode of swimming of the fish. Most Undichna ichnospecies are interpreted as produced by a fish swimming with subcarangiform locomotion. U. insolentia, U. bina, and some specimens of U. britannica are interpreted as reflecting anguilliform locomotion. The essential measurements used in this analysis are wavelength and wave amplitude. Maximum wavelength (in most cases interpreted as the trail produced by the caudal fin) is used to infer the length of the producer for each specimen by comparison with experiments using extant fishes from the literature. Estimated length of fish producing Undichna is in the range 24-800 mm, but most values are less than 250 mm. By estimating the Reynolds number (Re) for each specimen, it is inferred that a fish larger than 650 mm will produce a flow disturbance and bottom sediment suspension that will preclude the preservation of trails recognizable as Undichna. Larger fish may leave identifiable Undichna provided that the sediment underwent early cementation or the fish was swimming at a speed lower than maximum sustained speed.