14th Swiss Geoscience Meeting, Geneva 2016
The ecology and evolution of the anomalocaridids
Allison C. Daley*
*Institut des sciences de la Terre, Faculté des géosciences et de l'environnement, University of Lausanne, Bâtiment Géopolis, CH-1015 Lausanne ([email protected])
The Cambrian Explosion was a major biodiversification event that saw the rise of nearly all animal phyla in a rapid burst over 500 million years ago. The anomalocaridids (Radiodonta) are iconic members of these early animal ecosystems, owing to their large size, bizarre morphology and complicated history of description. These pelagic animals had a segmented body bearing swim flaps, and a head with a prominent pair of grasping appendages, stalked eyes, and circular mouthparts (Whittington & Briggs 1985). Taxa such as Anomalocaris were first described from the Burgess Shale in Canada, but have since been found globally at many other Cambrian fossil lagerstätten. Much work has been undertaken on the anomalocaridids in the last 10 years, and these animals have proved important for understanding Cambrian ecology and the early evolution of arthropods.
The anomalocaridids were originally interpreted as highly specialised apex predators attacking trilobites, but the diversity of appendage and mouthpart morphologies instead suggests that they actually employed a diverse range of feeding strategies. While taxa such as Anomalocaris may have been highly specialised predators, other taxa such as Hurdia were more generalised in their feeding approach, employing both scavenging and predation (Daley & Budd 2010). The early Ordovician taxon Aegirocassis is a suspension feeder and reached a gigantic body size of 2 metres, equivalent to the ecological niche of whales today (Van Roy et al. 2015).
Anomalocaridids occupied a basal position in the stem lineage leading to Arthropoda, the phlyum that today includes chelicerates, crustaceans, myriapods and insects (Daley et al. 2009). Some aspects of anomalocaridid morphology are remarkably similar to characteristics of extant arthropods, such as the multi-faceted compound eyes consisting of over 15,000 lenses seen in Anomalocaris (Paterson et al. 2014). Other features provide insight into the early evolutionary stages of key arthropod features, such as the biramous limb, head appendages and exoskeleton (Daley et al. 2009). For example, the two branches of the arthropod biramous limb were once separate structures protruding from the body wall, which in the Ordovician anomalocaridid taxon Aegirocassis can be seen at two pairs of body flaps (Van Roy et al. 2015). These separate body flaps eventually fused to form the arthropod biramous limb, but the anomalocaridid condition allows us to see an important early stage in the evolution of this morphological feature. Ultimately, understanding the ecology and evolution of the anomalocaridids can inform on how arthropods achieved such great success both in the earliest animal ecosystems of the Cambrian, and in the modern day.
14th Swiss Geoscience Meeting, Geneva 2016
Figure 1. Anomalocaridids from the Burgess Shale, Canada. A: Circular mouthparts of Peytoia nathorsti. B: Full body specimen of Anomalocaris canadensis, with the prominent grasping appendage indicated by the white arrow. C: Fossil image and reconstruction of Hurdia victoria. Reconstruction by Marianne Collins.
REFERENCES
Daley, A.C. & Budd G.E. 2010: New anomalocaridid appendages from the Burgess Shale, Canada. Palaeontology 53, 721–738.
Daley, A.C., Budd, G.E., Caron, J.-B., Edgecombe, G.D. & Collins D. 2009: The Burgess Shale anomalocaridid Hurdia and its significance for early euarthropod evolution. Science 323, 1597–1600.
Paterson, J.R., García-Bellido, D.C., Lee, M.S.Y., Brock, G.A., Jago, J.B. & Edgecombe G.D. 2011: Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes. Nature 480, 237–240.
Van Roy, P., Daley, A.C. & Briggs D.E.G. 2015: Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps. Nature 522, 77-80.
Whittington, H.B. & Briggs D.E.G. 1985: The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia. Philos. T. Roy. Soc. B. 309, 569-609.