Odd, comblike fossils unearthed from 520-million-year-old rocks in northern Greenland are the food-gathering structures of the world’s first known free-ranging filter feeder, a new study suggests. Considering the size of its combs (longer than 12 centimeters, or almost 5 inches) and the overall body proportions of its predatory kin, the shrimplike creature Tamisiocaris borealis (as shown in this artist’s reconstruction) may have grown to reach lengths of 61 centimeters (2 feet)—not large by modern standards but huge for that era. Nevertheless, the spacing of the long, slender spines on the feeding combs suggest that this species had evolved to become a gentle giant, sweeping its feeding appendages through the water (as seen in this video simulation) to collect free-swimming organisms as small as 0.5 millimeters (about the size of today’s brine shrimp) rather than chasing prey further up the food web, the researchers report online today in Nature. This creature, and its free-ranging, filter-feeding lifestyle, evolved during the so-called “Cambrian Explosion,” a burst of diversification that began about 542 million years ago and spawned most of the major groups of animals known today, including vertebrates. The mere presence of filter feeders as large as Tamisiocaris suggests that Cambrian ecosystems were much more productive than previously recognized, the researchers contend: As seen in modern species as diverse as fish, sharks, and whales, large animals can successfully exploit small prey only when they can be sieved from the environment in great concentrations.

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