What life-forms lived during the mississippian period?

The Mississippian Period is an American term for what is known as the Lower Carboniferous Period in the rest of the world. It covers a time period extending from about 375 million years ago to the beginning of the Upper Carboniferous (Pennsylvanian) about 345 million years ago.

During the Mississippian period the first winged insects appeared and tetrapods were beginning to spread across the heavily-watered landscapes. Amphibians were beginning to occupy the land and insects, many of which grew to enormous sizes, were beginning to swarm everywhere. Because of the warm, damp climate that prevailed on most of the land areas, the world was heavy with plant life, no grasses had yet evolved, but there were others, mainly ferns, scale trees, cycads, horsetails, and a group of plants known as Archaeopteris. This latter group produced thick bodied trees that grew up to 20 feet in height and flourished in the marshes and heavily watered land areas - which were abundant. Plants, in fact, were so abundant that their photosynthetic mechanisms pumped oxygen into the air in enormous quantities, and estimates are that oxygen gas may have occupied as much as 30-35 percent of the atmosphere (today it's closer to 21 percent). That much oxygen in the air permitted insects and other land-dwelling invertebrates to grow to enormous sizes. Dragonflies with wingspans of as much as two feet have left their imprints in the fossil record, ancestral arachnids (Spiders, scorpions) as big as melons, cockroaches a foot long and other crawling insects were everywhere, all of which served as food for each other as well as for the amphibians and fishes that swarmed in the rivers, oceans, lakes and swamps. There were sharks in the oceans, bony fishes, clams, brachiopods, ammonites, and the swamps were loaded with lobe-finned fishes (Coelacanths and Rhipidistians amongst them). Invertebrate animals abounded in the shallow marine Coastlines and in the rivers and swamps.