There were plenty of other species living during the Cambrian Period also. Mollusks, worms, sponges and echinoderms filled the Cambrian seas.
Many marine metazoans having mineralized exoskeletons flourish in the Cambrian, including sponges, corals, molluscs, echinoderms, bryozoans, brachiopods and arthropods. It is commonly believed that there were no organisms at the very base of Cambrian that had hard parts, either as an external skeleton or simply spicules. This is, however, remains in dispute. The first shelled metazoans that are characteristic of the Cambrian occur well after the earliest complex trace fossils. This suggests that hard parts evolved later. Hence, trilobites, archaeocyaths, and small shelly animals did not evolve before the middle part of the Early Cambrian. The evolution of shelled metazoans is reflected by the appearence of successively more advanced shelly fossils.
Many marine metazoans having mineralized exoskeletons flourish in the Cambrian, including sponges, corals, molluscs, echinoderms, bryozoans, brachiopods and arthropods. It is commonly believed that there were no organisms at the very base of Cambrian that had hard parts, either as an external skeleton or simply spicules. This is, however, remains in dispute. The first shelled metazoans that are characteristic of the Cambrian occur well after the earliest complex trace fossils. This suggests that hard parts evolved later. Hence, trilobites, archaeocyaths, and small shelly animals did not evolve before the middle part of the Early Cambrian. The evolution of shelled metazoans is reflected by the appearence of successively more advanced shelly fossils.
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