The Early Cambrian Diversification of Burrow Forms
From the very start of the Cambrian period (about 542 million years ago) many new types of traces first appear, including well-known vertical burrows such as Diplocraterion and Skolithos, and traces normally attributed to arthropods, such as Cruziana and Rusophycus. The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard (although not necessarily mineralised) exoskeletons.
Read more about this topic: Cambrian Substrate Revolution
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