MOVEMENT PALEOECOLOGY OF THE EARLY CAMBRIAN CHENGJIANG BIOTA

Nathan et al. (2008, PNAS) have called for a new “movement ecology paradigm for unifying organismal movement research.” This approach views movement as resulting from the interactions of the organism's internal state, its biomechanical ability to move, and its navigation capacity with the external environment. Exceptionally preserved body fossils, such as those from the Early Cambrian Chengjiang biota of southwest China, provide direct information regarding the latter two factors, making the study of their “movement paleoecology” possible. The Cambrian radiation is characterized by profound environmental and biological changes, which markedly increased the spatial complexity of the marine environment. This increased spatial complexity likely drove the evolution of macroscopic sense organs in mobile bilaterians, leading to their first appearance during the Cambrian. In order to explore the distribution of sensory organs during the Cambrian radiation, 5,597 specimens of 31 mobile epifaunal and nektonic genera were examined from the Shankou biota, an assemblage within the Chengjiang biota. Presence/absence data of inferred macroscopic sensory organs (visual and chemosensory) were collected from these genera, and were mapped onto the relative abundance and independent assessments of life mode. The results reveal a difference between the sensory organ distribution of mobile epifaunal and nektonic forms. Antennae are ubiquitous in both mobile epifaunal and nektonic genera (97.8% of mobile epifaunal and 94.2% of nektonic specimens have them), but only nektonic genera are dominated by forms with eyes (94.5%). Only 62.8% of mobile epifaunal specimens possess eyes. Data from a second Chengjiang assemblage, the Haikou biota, shows similar results (100% of nektonic specimens have eyes while only 70.2% of mobile epifaunal specimens do; n = 2,556). These results are compatible with patterns in modern marine ecosystems, in which many large benthic predators (e.g. seastars) may lack eyes but they are ubiquitous in nektonic forms such as sharks.