Natalie R. Aubet

Bilaterian burrows and grazing behavior at >585 million years ago

Early Burrowers Direct fossil evidence of animals from Ediacaran period—the time in Earths 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. Neoproterozoic trace fossils from Uruguay indicate that early animals appeared at a time between global glaciations. 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.

Response to Comment on "Bilaterian Burrows and Grazing Behavior at >585 Million Years Ago"

Gaucher et al. suggest that their field observations and petrographic analysis of one thin section do not support an Ediacaran age for the trace fossils–bearing strata of the Tacuarí Formation. We have strengthened our conclusion of an Ediacaran age for the Tacuarí Formation based on reassessment of new and previously presented field and petrographic evidence.

Textural and geochemical features of freshwater microbialites from Laguna Bacalar, Quintana Roo, Mexico

ABSTRACT Microbialites provide some of the oldest direct evidence of life on Earth. They reached their peak during the Proterozoic and declined afterward. Their decline has been attributed to grazing and/or burrowing by metazoans, to changes in ocean chemistry, or to competition with other calcifying organisms. The freshwater microbialites at Laguna Bacalar (Mexico) provide an opportunity to better understand microbialite growth in terms of interaction between grazing organisms versus calcium carbonate precipitation. The Laguna Bacalar microbialites are described in terms of their distinct mesostructures. Stromatolites display internal lamination, attributed to the precipitation of calcite and the upward migration of cyanobacteria during periods of low sedimentation. Thrombolitic stromatolites show internal lamination in addition to internal clotting. The clotting is seen as a result of binding and/or trapping of micritic peloids by cyanobacteria and attributed to periods of high sedimentation. The carbonates in both microbialites had similar C- and O-stable–isotopic signatures, both enriched in 13C relative to bivalves, suggesting photosynthetic CO2 uptake was the trigger for carbonate precipitation. This implies that the rate of microbialite growth is largely a function of ambient carbonate saturation state, while the texture is especially dependent on accretion rates and sediment deposition on their surface. Importantly, the coexistence with grazing animals suggests no significant inhibition on microbialite growth, thereby calling into question the decline of microbialite as a result of metazoan evolution. Varying sedimentation rates are likely important in controlling the distribution of thrombolite–stromatolite packages in the geological record, given the importance of this factor at Bacalar.

Earliest evidence of bilaterians

Life has existed on Earth for almost 4 billion years, but most major groups of animals only appear i…