Which organisms were free swimmers and found in a great variety of Paleozoic and Mesozoic marine habitats?

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Ammonoids and conodonts were marine organisms that thrived during the Paleozoic and Mesozoic eras. Ammonoids, a group of extinct marine mollusks, were notable for their spiral shells and are often used as index fossils. Conodonts, on the other hand, were eel-like creatures with tooth-like elements, playing a significant role in understanding marine paleoenvironments. Both were free-swimming organisms found in a variety of marine habitats during their respective eras.

Which Organisms Were Free Swimmers and Found in a Great