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What were a group of hard-shelled animals that lived 65 to 500 million years ago that are used as index fossils?

Answer 1

That is the Paleozoic Era.

These fossils include solitary corals and Syringopora , tubular colonial corals. Prolecanites gurley, lophophilllidium proliferum, and more on the referenced chart. A great graphical chart here: https://tutor.hix.ai

https://tutor.hix.ai

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Answer 2

Audrey Adams

The group of hard-shelled animals that lived 65 to 500 million years ago and are used as index fossils are called ammonites.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.