What was alive 400 million years ago?

Answer 1

400 million years ago Earth was in the middle of the Devonian period. At this time the first forests were emerging, populated by moss trees. Various invertebrates, including early insects, lived in these forests. Vertebrates had not yet come on to land but some species of lungfish were approaching that level. The ocean was populated by a variety of fish including primitive sharks while a now-extinct group of armored fish dominated. The primitive jawless fish of earlier times were declining while fish with jaws were on the rise. Ammonites, a group of shelled cephalopods related to the octopus appeared around this time. Many other creatures craweled along the sea floor, most notably a class of arthropods called trilobites.