The Burgess Shale-type Lagerstätten of the Early Cambrian Maotianshan Shale record an apparently sudden conquest of pelagic niches by ten phyla of metazoans. One of these phyla is Chaetognatha, a group of predatory marine worms. Given their role as major predators in modern planktonic ecosystems, the chaetognaths discovered in the Maotianshan Shale (Yunnan Province, South China) suggest that the pelagos at the time was already quite complex. Modern chaetognaths, however, can be divided into benthic and pelagic forms; the pelagic nature of Eognathacantha ercainella should therefore be validated by strong morphological evidence.

Knowing that planktonic animals present morphological adaptations that increase their buoyancy, we studied the drag produced during the active phase of chaetognath locomotion for the modern forms Paraspadella gotoi (benthic) and Sagitta elegans (pelagic). By using a motion model developed by Jordan in 1992, we could calculate the resistive force produced by the undulatory movement of chaetognaths' bodies.

This mechanistic approach evaluates the effect of three motion parameters (relative speed, total length, and tail ratio) on the drag force produced during locomotion. Our results show that the increase of size contributes to higher drag while the shorter tail of the pelagic form balances this effect by reducing the wetted surface subject to friction. For chaetognaths, therefore, a bigger body (both in length and width) and a shorter tail indicate a pelagic lifestyle, a finding that can be applied to the study of the fossil Eognathacantha ercainella. A discriminant analysis can confirm that the Early Cambrian chaetognath presents a pelagic morphology with similarities to modern bathypelagic and mesopelagic species.

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