New insights into the appendage morphology of the Cambrian trilobite-like arthropod Naraoia compacta

摘要

For understanding the life habits of extinct organisms, functional morphology is of crucial importance. In arthropods, the morphology of the appendages in particular gives insights into the mode of life. Yet, in the fossil record of arthropods mainly dorsal structures are preserved, as they are often more sclerotised or even mineralised. Exceptions are, for example, the species of Naraoia from the Cambrian Burgess Shale (about 508 million years old), of which the appendages are also preserved. We present here new details of the appendages of Naraoia compacta, which were made visible with modern imaging methods that were not yet available during former investigations. According to our re-study, the appendage shows a division into basipod, endopod and exopod. The basipod-body joint has not been visible to earlier investigators, and both basipod and endopod are significantly more strongly adorned than previously interpreted. The most important difference to earlier studies regards our interpretation of the exopod morphology. According to our new data, the exopod is composed of a series of similar, small paddles attached to a shaft, and not feather-like in appearance. This morphology has a strong impact on the swimming behaviour of N. compacta, as the paddles can be closed to form a functionally single large paddle during the power stroke, and will be opened during the recovery stroke to reduce the water drag. In this way, our new data provide important insights into the life habits of this long extinct species.