Evolutionary Trends

Anomalocaris Evolutionary Trends
The anomalocaridids were one of the first families in the Cambrian to evolve into successful predators. Anomalocaris, a member of the anomalocaridids, contained many similar adaptations to other anomalocaridid species. The evolution and adaptation of different traits was essential to the development of Anomalocaris and the anomalocaridids.

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Evolved Predatory Traits
Anomalocaris shows many evolutionary adaptations that enabled the species to become a successful predator. Some of the modifications were the evolution of stalked, high-resolution, compound eyes to identify prey, a streamlined body that increased mobility, a complex oral cone for feeding, and large frontal appendages to capture prey. The frontal appendages were essential to the hunting methods of the anomalocaridids, and over time different species evolved different modifications of the frontal appendages. The divergence in types of frontal appendages allowed species to feed on different prey and take advantage of different parts of the ecosystem. This division of the ecosystem allowed multiple species of the anomalocaridids to co-exist without competition driving any to extinction.

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Two Main Types of Frontal Appendages
The different types of frontal appendages are linked to the diet and predatory habits of the different species of the anomalocaridids. There were two main types of frontal appendages among the anomalocaridids. Frontal appendages usually either had long spines or short spines. The long spines were very effective at trapping slower moving benthic or shallow infaunal prey, but the long spines restricted some movement and were not as efficient for capturing all types of prey. The shorter spines were considerably more agile, and they allowed more movement. These spines enabled the anomalocaridids to trap faster and more agile prey, and maintain that grasp while transferring the prey into their oral cones. The types of frontal appendages directly related to the predatory habits of the anomalocaridids.

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Special Modifications to Frontal Appendages
Other species evolved special modifications to these generic frontal appendage formats that enabled them to be successful predators in specific niches. The anomalocaridid Peytoia had blade-like frontal appendages, which could be raked over the sediment surface and turn up organisms in the ocean floor in order to help Peytoia find prey.
Another anomalocaridid, Hurida, had similar frontal appendages that may have been used to sift through shallow sediment and find organisms that were shallow infaunal burrowers.
Anomalocaris briggsi had spinules, or very small spines, on its frontal appendages. These spinules allowed the organism to capture prey through a method similar to filter feeding, where Anomalocaris caught prey in its frontal appendages as it swam along, but allowed water and other particles to pass through.
Another species, Anomalocaris Canadensis, had short stout spines on the frontal appendages. These short stout spines enabled the predator to grab and eat prey with an exoskeleton. The frontal appendages were a necessary part in the process because they were flexible and, therefore, able to bend and flex the exoskeletons of prey, such as trilobites, until their skeletons snapped. The bending and flexing was necessary because of Anomalocaris’s oral cone, which due to size and placement of plates was not able to bite through biomineralised prey.

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The evolution of different frontal appendages in the anomalocaridids allowed them to successfully inhabit many different levels of ecosystems, by feeding on different prey. The entire anomalocaridid family evolved over millions of years to become the successful group of predators they were, and the adaptation and modifications of the frontal appendages were essential for that success.

References
Briggs, D. E., 1994, Giant Predators from the Cambrian of China. American Association for the Advancement of Science. Science. 264.5163 (May 27, 1994): p1283.
Nedin, C. 1999. Anomalocaris Predation on Nonmineralized and Mineralized Trilobites. Department of Geology and Geophysics, University of Adelaide. Nov. 1999. p987-990.
Daley, A. C., and Edgecombe, G. D. 2014. Morphology of Anomalocaris Canadensis from the Burgess Shale. The Paleontological Society. Journal of Paleontology, 88(1), 2014, p. 68–91
Daley, A. C., Paterson, J. R., Edgecombe, G. D., Garcia-Bellido, D. C., and Jago, J. B. 2013. New Anatomical Information on Anomalocaris from the Cambrian Emu Bay Shale of South Australia and a Reassessment of its Inferred Predatory Habits. The Palaeontological Association. Palaeontology, Vol. 56, Part 5, 2013, pp. 971–990.
Roy, P. V., and Briggs, D. E. 2011. A Giant Ordovician Anomalocaridid. Nature. Vol. 47. pp. 510-513.
Daley, A. C., and Budd, G. E. 2010. New Anomalocaris Appendages from the Burgess Shale, Canada. The Palaeontological Association. Palaeontology, Vol. 53, Part 4, 2010, pp. 721–738.

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