Why is Opabinia regalis so strange?

Cambrian seas teemed with animals of various sizes, shapes, and ecologies. Strategies for obtaining nutrients expanded to include suspension feeding (filtering particles of food out of the water), deposit feeding (gathering particles of food that settled on the sea floor), predation (actively capturing and devouring other animals), scavenging (finding and eating dead organisms), and grazing (munching algae and microbial mats). Some lived on or in the sea floor (benthic), while others (including Opabinia regalis) actively swam in the water column, but were still dependent on the benthos for foraging (nektobenthic). The rapid appearance of such a wide variety of animals during the Cambrian Explosion led to the development of radical new ecological interactions. As the number and variety of organisms increased, they occupied a variety of newly created marine environments and habitats.

New ecological niches (particular spaces in an ecosystem) would have been created by the organisms interacting with their environment. Moving into previously-unexploited environments would have allowed even a poorly-adapted animal to survive, perhaps with one of the more "exotic" body plans. For example, the emergence of predators like Opabinia might have stimulated the evolution of hardened exteriors for protection, or swimming as a means of escape. Before predation became widespread, early "experiments" in different body plans could have briefly thrived because species interactions were probably more limited.
 

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