Old and Groovy

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Science  29 Jun 2012:
Vol. 336, Issue 6089, pp. 1646-1647
DOI: 10.1126/science.1223848

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Since Darwin's time, paleontologists and biologists have searched for evidence for the first bilaterian animals, which have a front and a back end as well as an upper and a lower side. Bilateral symmetry alone does not define this group, and confident interpretation of fossil embryos has proven difficult. On the other hand, evidence of sediment furrowing over extended distances has been widely accepted as evidence of bilaterian life: Flatworms may glide along a surface (1) and deep-sea protists can produce short furrowed surface traces (2), but making a long furrowed trace fossil with evidence of backfill requires a bilaterian body plan. On page 1693 of this issue, Pecoits et al. (3) describe furrowed, backfilled trace fossils dated to over 585 million years, which they interpret as the oldest bilaterian trace fossils and thus the oldest evidence of bilaterians.