FIGURE 1. Siltstone from Cretaceous Rosario Formation, Mexico, containing Phycosiphoniform burrows with "frogspawn texture" in vertical section. 1.1. Outcrop photograph; 1.2. Photograph of cut and ground surface of the sample examined during three- dimensional reconstruction of burrows. The halo is accentuated by diagenetic pyrite precipitation. Note that the burrow halo is predominantly located below the black mudstone core.