Movement Palaeoecology of the 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 (“why move?”), its biomechanical ability to move (“how to move?"), and its navigation capacity (“where to move?”), with each other and with the external environment.  Exceptionally preserved body fossils provide direct information regarding the latter two factors through the preservation of limbs and sensory organs, making the study of “movement palaeoecology” possible.  One way to examine this during the Cambrian radiation is through study of the Early Cambrian Chengjiang biota of southwest China.  The Cambrian radiation is characterized by the profound environmental and biological changes associated with the bilaterian radiation.  These include the advent of macroscopic predation, an increase in the size and energy content of organisms, and the transition in seafloors from laminated matgrounds to mixgrounds.  The overall effect of these transitions was to markedly increase 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.  The morphology and distribution of these sense organs should therefore reflect the life habits of the animals that possessed them. 

Presence/absence data of macroscopic sensory organs (eyes and antennae) were collected from 5,597 specimens of 31 genera from the Chengjiang biota, and mapped onto the relative abundance and life mode data.  The results reveal an interesting difference between the sensory organ distribution of mobile epifaunal and nektonic forms.  While the presence of antennae is ubiquitous in both mobile epifaunal and nektonic genera (98% of mobile epifaunal and 94% of nektonic specimens have antennae), only nektonic genera are overwhelmingly dominated by forms with eyes (95%).  In contrast, only 63% of mobile epifaunal specimens possess eyes.  There is a statistically significant (p<0.0001) difference between epifaunal and nektonic eye distribution.  This difference may be attributable to the need of nektonic organisms to visually detect flow direction in order to navigate toward the source of an odour plume.  Epifaunal organisms can achieve similar results through mechanosensory.  These preliminary results support two general hypotheses: that essentially modern sensory systems evolved very early in animal evolution; and that sensory systems differed between epifaunal and nektonic forms.  Even by the Early Cambrian, there does appear to be a difference in the selective pressure to develop certain sensory organs based on the local environment of an organism and its life mode.