In the following diagram, four different index fossils are labeled: which of the labeled fossils comes from the oldest organism? a b c d

D. Fossils of the Cambrian Period

Explanation:

The Cambrian Period (541–485.4 million years) is the period known for the explosion of life on Earth, when all the major phyla of multicellular organisms originated, and almost all the groups we find today, in a period of time geologically short.

The primitive faunas of this period had numerous morphological patterns, an immense variety of species, mostly soft-bodied animals, and new ecological strategies such as predation, burying themselves deep in the sediment and building complex, branched tunnels. They also featured some relatively large skeletal animals, such as trilobites, brachiopods, and

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