Figure 1.

Generalized stratigraphy of the Proterozoic and Cambrian contrasting the timing of bilaterian appearance as deduced from body fossils, trace fossils, and molecular clock data. The oldest trace fossils accepted here are ≈555 million years old. Reports of older traces including more than 1-billion-year-old traces from India (37) and Australia (38) are considered doubtful. The numbers refer to bilaterian divergence dates based on selected molecular clock studies: 1, appearance of first bilaterians; 2, deuterostome-protostome split; circle, ref. 1; triangle, ref. 11; square^§ ped Z, T. pedum zone; ava Z., R. avalonensis zone.