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Bilaterian Burrows and Grazing Behavior at >585 Million Years Ago

Science  29 Jun 2012:
Vol. 336, Issue 6089, pp. 1693-1696
DOI: 10.1126/science.1216295

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Early Burrowers

Direct fossil evidence of animals from Ediacaran period—the time in Earth's history just before extensive animal diversification in the Cambrian—is scant. However, the remains of animal activity in sediment, which remain intact through geologic time can provide clues about animal behavior and evolution. Pecoits et al. (p. 1693; see the Perspective by Droser and Gehling) found a suite of fossil animal burrows in sedimentary rocks in Uruguay. Radiometric dating places the age of the structures at ∼585 million years old, coinciding with the likely emergence of stem-group bilaterians. The complex morphologies of the fossil burrows suggest that these animals actively grazed and had the ability to burrow deep within sediments.

Abstract

Based on molecular clocks and biomarker studies, it is possible that bilaterian life emerged early in the Ediacaran, but at present, no fossils or trace fossils from this time have been reported. Here we report the discovery of the oldest bilaterian burrows in shallow-water glaciomarine sediments from the Tacuarí Formation, Uruguay. Uranium-lead dating of zircons in cross-cutting granite dykes constrains the age of these burrows to be at least 585 million years old. Their features indicate infaunal grazing activity by early eumetazoans. Active backfill within the burrow, an ability to wander upward and downward to exploit shallowly situated sedimentary laminae, and sinuous meandering suggest advanced behavioral adaptations. These findings unite the paleontological and molecular data pertaining to the evolution of bilaterians, and link bilaterian origins to the environmental changes that took place during the Neoproterozoic glaciations.

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